U.S. patent application number 16/538359 was filed with the patent office on 2019-11-28 for method and system for passive electroseismic surveying.
The applicant listed for this patent is ES XPLORE, L.L.C.. Invention is credited to Naga P. Devineni, Robert England, Alan Katz, Mohammad Rahman, Arthur Thompson.
Application Number | 20190360312 16/538359 |
Document ID | / |
Family ID | 46022001 |
Filed Date | 2019-11-28 |
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United States Patent
Application |
20190360312 |
Kind Code |
A1 |
Thompson; Arthur ; et
al. |
November 28, 2019 |
Method and System for Passive Electroseismic Surveying
Abstract
A method of passive surveying comprises generating one or more
detected signals by passively detecting a signal generated within a
subsurface earth formation due to a seismoelectric response or an
electroseismic response in at least one porous subsurface earth
formation containing at least one fluid, and processing the one or
more detected signals to determine at least one property of the
subsurface earth formation.
Inventors: |
Thompson; Arthur; (Houston,
TX) ; Katz; Alan; (Dallas, TX) ; England;
Robert; (Flower Mound, TX) ; Rahman; Mohammad;
(Dallas, TX) ; Devineni; Naga P.; (Irving,
TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ES XPLORE, L.L.C. |
Dallas |
TX |
US |
|
|
Family ID: |
46022001 |
Appl. No.: |
16/538359 |
Filed: |
August 12, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14044469 |
Oct 2, 2013 |
10378316 |
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16538359 |
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13712177 |
Dec 12, 2012 |
8589079 |
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14044469 |
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13674666 |
Nov 12, 2012 |
8478533 |
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13712177 |
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13431735 |
Mar 27, 2012 |
8347658 |
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13674666 |
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61469498 |
Mar 30, 2011 |
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61528421 |
Aug 29, 2011 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01V 3/38 20130101; E21B
43/16 20130101; G01V 3/16 20130101; Y10T 70/375 20150401; E21B
47/00 20130101; G01V 3/12 20130101; G01V 11/007 20130101; G06F
19/00 20130101; E21B 43/00 20130101; E21B 41/0092 20130101; G01V
2210/6163 20130101 |
International
Class: |
E21B 41/00 20060101
E21B041/00; E21B 43/00 20060101 E21B043/00; G01V 3/38 20060101
G01V003/38; E21B 47/00 20060101 E21B047/00; G01V 3/16 20060101
G01V003/16; G01V 3/12 20060101 G01V003/12; E21B 43/16 20060101
E21B043/16; G01V 11/00 20060101 G01V011/00 |
Claims
1-30. (canceled)
31. A system for subsurface surveying comprising: one or more of an
electromagnetic sensor and a seismic sensor located near or above
the surface of the earth, wherein the electromagnetic sensor is
configured to detect an electromagnetic signal generated by a
subsurface earth formation in response to a passive-source
electromagnetic signal, wherein the electromagnetic signal is
generated by an electroseismic or seismoelectric conversion of the
passive-source electromagnetic signal and wherein the seismic
sensor is configured to detect a seismic signal generated by the
subsurface earth formation in response to a passive-source
electromagnetic signal, wherein the seismic signal is generated by
an electroseismic or seismoelectric conversion of the
passive-source electromagnetic signal; a processor; and a memory,
wherein the memory includes executable instructions that, when
executed, cause the processor to: receive data corresponding to one
or more of the electromagnetic signal generated by the subsurface
earth formation in response to the passive-source electromagnetic
signal and the seismic signal generated by the subsurface earth
formation in response to the passive-source electromagnetic signal;
and determine at least one property of the subsurface earth
formation, based, at least in part, on one or more of the detected
electromagnetic signal generated by a subsurface earth formation
and the detected seismic signal generated by a subsurface earth
formation.
32. The system of claim 31, wherein the instructions that cause the
process to determine at least one property of the subsurface earth
formation, based, at least in part, on the detected electromagnetic
signal generated by a subsurface earth formation, further cause the
processor to: perform an autocorrelation on the data corresponding
to an electromagnetic signal generated by a subsurface earth
formation.
33. The system of claim 31, wherein the executable instructions
that cause the processor to determine the at least one property of
the subsurface earth formation, based, at least in part, on the
detected seismic signal generated by the subsurface earth formation
further cause the processor to: perform an autocorrelation on the
data corresponding to a seismic signal generated by the subsurface
earth formation.
34. The system of claim 31, wherein the electromagnetic signal
generated by a subsurface earth formation is detected at a first
time, the seismic signal generated by the subsurface earth
formation is detected at a second time, the first time and the
second time are different, and wherein the executable instructions
that cause the processor to determine the at least one property of
the subsurface earth formation, based, at least in part, on the
detected seismic signal generated by the subsurface earth formation
further cause the processor to: determine the at least one property
of the subsurface earth formation, based, at least in part, on a
correlation of the detected electromagnetic signal generated by a
subsurface earth formation and the detected seismic signal
generated by the subsurface earth formation.
35. The system of claim 31, wherein the executable instructions
further cause the processor to: demodulate a portion of the
electromagnetic signal generated by a subsurface earth formation to
identify a signal property that is indicative of the presence of
the hydrocarbons.
36. The system of claim 31, wherein the at least one property of
the subsurface earth formation comprises the existence of a
subsurface earth formation containing at least one fluid.
37. The system of claim 36, wherein the at least one fluid includes
at least one hydrocarbon.
38. The system of claim 36, wherein the at least one fluid includes
helium.
39. The system of claim 36, wherein the at least one fluid includes
one or more fluids selected from the group consisting of: water and
carbon dioxide.
40. The system of claim 31, wherein the at least one property of
the subsurface earth formation comprises a depth of a subsurface
formation containing at least one fluid.
41. The system of claim 31, wherein the at least one property of
the subsurface earth formation comprises a porosity of the
subsurface formation containing at least one fluid.
42. The system of claim 39, wherein the at least one fluid includes
at least one hydrocarbon.
43. A method of survey analysis comprising; receiving data
corresponding to one or more of a electromagnetic signal generated
by the subsurface earth formation in response to the passive-source
electromagnetic signal and a seismic signal generated by the
subsurface earth formation in response to the passive-source
electromagnetic signal, wherein the electromagnetic signal
generated by the subsurface earth formation in response to the
passive-source electromagnetic signal is detected by an
electromagnetic sensor located at or near the surface of the earth
and wherein the seismic signal generated by the subsurface earth
formation in response to the passive-source electromagnetic signal
is detected by an seismic sensor located at or near the surface of
the earth; and determining at least one property of the subsurface
earth formation, based, at least in part, on one or more of the
detected electromagnetic signal generated by a subsurface earth
formation and the seismic signal generated by the subsurface earth
formation in response to the passive-source electromagnetic
signal.
44. The method of claim 43, wherein determining at least one
property of the subsurface earth formation, based, at least in
part, on the detected electromagnetic signal generated by a
subsurface earth formation comprises: performing an autocorrelation
on the data corresponding to an electromagnetic signal generated by
a subsurface earth formation.
45. The method of claim 43, wherein determining the at least one
property of the subsurface earth formation, based, at least in
part, on the detected seismic signal generated by the subsurface
earth formation comprises: performing an autocorrelation on the
data corresponding to a seismic signal generated by the subsurface
earth formation.
46. The method of claim 43, wherein the electromagnetic signal
generated by a subsurface earth formation is detected at a first
time, the seismic signal generated by the subsurface earth
formation is detected at a second time, the first time and the
second time are different, and wherein determining the at least one
property of the subsurface earth formation, based, at least in
part, on the detected seismic signal generated by the subsurface
earth formation further comprises: determining the at least one
property of the subsurface earth formation, based, at least in
part, on a correlation of the detected electromagnetic signal
generated by a subsurface earth formation and the detected seismic
signal generated by the subsurface earth formation.
47. The method of claim 43, wherein the at least one property of
the subsurface earth formation comprises the existence of a
subsurface earth formation containing at least one fluid.
48. The method of claim 47, wherein the at least one fluid includes
at least one hydrocarbon.
49. The method of claim 47, wherein the at least one fluid includes
helium.
50. The method of claim 43, wherein the at least one property of
the subsurface earth formation comprises a depth of a subsurface
formation containing at least one fluid.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a non-provisional of and claims
priority to U.S. Provisional Application No. 61/469,498 filed on
Mar. 30, 2011, by Thompson et al., and titled "Method and System
for Passive Electroseismic Surveying," which is incorporated herein
by reference in its entirety. The present application is also a
non-provisional of and claims priority to U.S. Provisional
Application No. 61/528,421 filed on Aug. 29, 2011, by Katz et al.,
and titled "Passive Electromagnetic Tool for Direct Hydrocarbon
Indication," which is incorporated herein by reference in its
entirety.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not applicable.
REFERENCE TO A MICROFICHE APPENDIX
[0003] Not applicable.
BACKGROUND
[0004] Conventional geophysical surveying techniques employ several
distinct technologies, which can include seismology,
controlled-source electromagnetics, (CSEM), magnetotellurics,
microseismology, gravity, magnetics, and controlled source
electroseismology and seismoelectric surveying. Gravity and
magnetics technologies survey large areas, such as whole geological
basins. These technologies generally identify prospective regions
with attractive geological features but do not generally identify
the detailed location of hydrocarbon reservoirs or mineral
resources. Microseismology generally relies on small, localized
seismic events generated in the earth by naturally occurring earth
movements or by well-drilling operations. Microseismology may
locate the source of fracturing events such as encountered in
fracturing reservoirs. Magnetotellurics uses the low-frequency
portion of the earth's background electromagnetic fields to
estimate the subsurface electrical conductivity but the method does
not provide a detailed location or shape of target reservoir
structures. Seismology produces information indicative of reservoir
structures and CSEM provides electrical resistivity information
that indirectly indicates the presence of hydrocarbons. Controlled
source electroseismology and electroseismic surveying use
high-power seismic or electromagnetic sources to create images of
the subsurface that provide both structural and fluid property
information. These methods are seldom used because they are
significantly limited by the requirement for high-power
sources.
[0005] In gravity surveying, sensitive gravity detectors are placed
on the earth or above the earth. Reservoirs typically have smaller
mass density than non-reservoir rock. The sensitive gravity meter
detects a minimum in local gravitational acceleration over a
reservoir. Gravity studies have several limitations. Local gravity
values reflect an average of the mass densities from all materials
in the neighborhood of the sensor. Whereas reservoirs of low
density reduce the measured gravitational acceleration, the
presence of high-density rock reduces the spatial resolution of the
measurement and may obscure the presence of a low-density
formation. The spatial resolution of gravity measurements is
limited to length scales comparable to the depth and lateral extent
of the reservoir. The amplitude of the identifying gravity
signature depends on the volume of the reservoir. Little
information is provided regarding the reservoir structure,
pore-fluid properties, or the permeability.
[0006] In magnetic surveying, magnetic-field sensing devices
measure the magnetic field of the earth, typically from aircraft.
Hydrocarbon reservoirs and mineral deposits, such as iron ore, may
alter the local earth's magnetic field. Measured data can be used
to indicate the presence of reservoir structures. Magnetic
surveying is limited because it measures neither properties related
to the reservoir spatial extent and structure, nor the fluid
identity and flow properties.
[0007] Seismic prospecting techniques generally involve the use of
a seismic energy source and a set of receivers spread out along or
near the earth's surface to detect seismic signals reflected from
subsurface geological boundaries. These signals are recorded as a
function of time, and subsequent processing of these signals is
designed to reconstruct an appropriate image of the subsurface
formation. In generic terms, this conventional process has seismic
energy traveling down into the earth, reflecting from a particular
geologic layer at a seismic impedance contrast, and returning to
the receiver as a reflected seismic wave.
[0008] The seismic energy may be so-called shear waves (S-waves) or
so-called compressional waves (P-waves). Shear waves and
compressional waves differ with respect to their velocities, angles
of reflection, vibrational directions, and to some extent the types
of information that may be obtained from their respective types of
seismic data. However, both types of waves suffer similar
attenuation by earth formations; that is, the earth formations tend
to attenuate the higher frequency components and allow the lower
frequency components to pass through the earth relatively
unattenuated. This means that, for deeper formations, the low
frequency content of the reflected seismic energy contains the
information about the underlying subsurface formations. However,
because of the low frequency of the detected reflected seismic
energy, the resolution of the reflected seismic energy may be
insufficient to allow for detection of very thin geologic
layers.
[0009] Further, if the seismic impedance contrast between adjacent
but distinct geologic layers is small, little seismic energy is
reflected and the distinctness of the geologic layers may not be
discernible from the detected or recorded seismic data.
Additionally, seismic studies might provide information about the
structure of rock formations in the subsurface but generally are
not able to distinguish between pore fluids, such as an aqueous
fluid, oil, or gas.
[0010] Magnetotelluric surveying generally involves the use of the
natural electromagnetic fields that originate in the earth's
atmosphere. Naturally-occurring electromagnetic fields propagate
into the subsurface where they encounter rock formations of
differing electrical conductivity. When the electromagnetic fields
contact a formation of low conductivity, such as is typical of
hydrocarbon reservoirs, the electromagnetic field measured at the
surface of the earth changes. Spatially-dependent electromagnetic
fields measured on the earth's surface can be used to indicate the
presence of low-conductivity formations that might contain
hydrocarbons. Magnetotelluric surveying has several limitations.
Only low-frequency, long-wavelength electromagnetic stimulation may
reach prospective reservoirs because the high-frequency
electromagnetic fields are rapidly attenuated by the conducting
earth. Long-wavelength electromagnetic waves limit the spatial
resolution of magnetotellurics making reservoir delineation
difficult. Additionally, magnetotelluric surveying only provides
information about formation electrical conductivity and does not
yield data revealing information about porosity, permeability, or
reservoir structure.
[0011] Seismic energy may be so-called micro-seismic energy.
Seismic waves are generated in the earth by tectonic forces, by
ocean tides and other natural phenomena. Seismic waves are also
created when drilling or earth fracturing operations are conducted
in hydrocarbon exploration, production, or in water well services.
The events created by these natural and man-made events are called
microseismic events. Generally, micro-seismic studies yield
qualitative information about the location of subsurface structures
or positional information about drilling operations. In these
studies the location of the seismic source is imperfectly known so
that only poor quality images of the subsurface are possible.
[0012] Controlled-source electromagnetic surveying involves the use
of a source of electrical power and a set of electromagnetic
receivers typically deployed on the seafloor in deep water.
Although CSEM surveying may be done on land or in shallow water,
recent work finds particularly useful applications in deep water.
In CSEM surveying, the power source drives an electrical current
into the earth that passes through the various subsurface rock
formations. The electrical current follows a path of low electrical
resistance through the most conducting rock masses. Hydrocarbon
reservoirs contain insulating gas or oil fluids so the applied
electrical current tends to flow around the resistive reservoir
structures. The deflection of current around reservoirs is detected
as a change in electromagnetic response on the detectors deployed
on the seafloor. The measured signal properties can be used to
reflect the presence of resistive reservoir structures.
SUMMARY
[0013] In an embodiment, a method of passive surveying comprises
generating one or more detected signals by passively detecting a
signal generated within a subsurface earth formation due to a
seismoelectric response or an electroseismic response in at least
one porous subsurface earth formation containing at least one
fluid, and processing the one or more detected signals to determine
at least one property of the subsurface earth formation. Generating
the one or more detected signals may comprise generating a detected
electromagnetic field by detecting earth's electromagnetic field
using an electromagnetic field detector and processing the one or
more detected signals may comprises demodulating a portion of the
signal to identify an envelope of the signal, wherein the envelope
is indicative of the presence of the hydrocarbons. Processing the
one or more detected signals may also comprise analyzing the
envelope to determine a value correlated to a depth of the
hydrocarbons in the subterranean formation. Demodulating the
portion of the signal may comprise extracting a frequency
modulation, a phase modulation, an amplitude modulation, or any
combination thereof from the signal. The envelope may comprise at
least one of the frequency modulation, the phase modulation, or the
amplitude modulation. Generating the one or more detected signals
may comprise generating a detected electromagnetic field by
detecting earth's electromagnetic field using an electromagnetic
field detector, and processing the one or more detected signals may
comprise processing the detected electromagnetic field to determine
the at least one property of the subsurface earth formation.
Generating the one or more detected signals may comprise generating
a detected seismic signal by detecting a seismic wave generated
within the subsurface earth formation using a seismic sensor, and
processing the one or more detected signals may comprise processing
the detected seismic signal to determine the at least one property
of the subsurface earth formation. In another embodiment,
generating the one or more detected signals may comprise generating
a detected electromagnetic field by detecting earth's
electromagnetic field using an electromagnetic field detector, and
generating a detected seismic signal by detecting a seismic wave
using a seismic sensor. Processing the one or more detected signals
may comprise processing the detected electromagnetic field and the
detected seismic signal to determine the at least one property of
the subsurface earth formation. The detected electromagnetic field
may be generated at a different time than the detected seismic
signal. The detected electromagnetic field may be generated at a
different location than the detected seismic signal. The at least
one fluid may comprise at least one component selected from the
group consisting of: an aqueous fluid, a hydrocarbon, a petroleum,
carbon dioxide, and any combination thereof.
[0014] In an embodiment, a system for identifying hydrocarbons in a
subterranean formation comprises one or more sensors coupled to a
processor that detect one or more signals generated within a
subsurface earth formation due to a seismoelectric response or an
electroseismic response in at least one porous subsurface earth
formation containing at least one fluid, and an analysis tool, that
when executed on the processor, configures the processor to receive
the one or more signals from the one or more sensors, and process
at least a portion of the one or more signals to determine at least
one property of the subsurface earth formation. The one or more
sensors may comprise one or more electromagnetic field detectors
that measures the earth's electromagnetic field and produces a
signal indicative of the detected electromagnetic field, and the
analysis tool may configures the processor to receive the signal
from the one or more sensors, demodulate a portion of the signal to
identify an envelope of the signal, analyze the envelope to
determine one or more properties indicative of the presence of the
hydrocarbons, and analyze the envelope to determine the depth of
the hydrocarbons in the subterranean formation. The one or more
sensors may comprise an electromagnetic field detector that
measures the earth's electromagnetic field and produces a signal
indicative of the detected electromagnetic field, and the analysis
tool may receive the signal and determine the at least one property
of a subsurface earth formation. The one or more sensors may
comprise a seismic sensor that detects a seismic signal related to
earth's electromagnetic field and produces a first signal
indicative of the detected seismic signal, and the analysis tool
may receive the first signal and process the first signal to
determine the at least one property of the subsurface earth
formation. The one or more sensors may comprise a seismic sensor
that detects a seismic signal related to earth's electromagnetic
field and produces a first signal indicative of the detected
seismic signal and an electromagnetic field detector that measures
the earth's electromagnetic field and produces a second signal
indicative of the detected electromagnetic field. The analysis tool
may receive the first signal and the second signal and determine
the at least one property of a subsurface earth formation. The
first signal may be generated at a different time than the second
signal, and/or the first signal may be generated at a different
location than the second signal. The electromagnetic field detector
may comprise a plurality of pairs of porous pot electrodes, and
each pair of porous pot electrodes may be electrically coupled. The
electromagnetic field detector may comprise a plurality of pairs of
conductive electrodes, and each pair of conductive electrodes may
be electrically coupled. The electromagnetic field detector may
comprise a conductive electrode coupled to a porous pot electrode.
The electromagnetic field detector may comprise an antenna disposed
on or above the surface of the earth. The antenna may comprise at
least one antenna selected from the group consisting of: a
parallel-plate capacitor antenna comprising two or more parallel
conducting plates, a single-plate capacitor antenna comprising one
electrode electrically coupled to the earth, a monopole antenna
comprising a conducting element, a dipole antenna comprising two
conducting elements, a multi-pole antenna comprising a plurality of
conducting elements, a directional antenna comprising conducting
elements arranged to augment a signal amplitude in a particular
direction, a coil antenna comprising one or more coils of wire, a
concentric electric dipole, and any combination thereof. The
seismic sensor may comprise at least one sensor selected from the
group consisting of: a hydrophone, a single-component geophone, a
two-component geophone, a three-component geophone, a single-axis
accelerometer, a two-axis accelerometer, a three-axis
accelerometer, and any combination thereof.
[0015] In an embodiment, a system for identifying hydrocarbons in a
subterranean formation comprises: a memory comprising a
non-transitory computer readable media, a processor, and an
analysis tool, that when executed on the processor, configures the
processor to: receive one or more signals from one or more sensors,
wherein the one or more sensors detect one or more signals
generated within a subsurface earth formation due to a
seismoelectric response or an electroseismic response in at least
one porous subsurface earth formation containing at least one
fluid, and process at least a portion of the one or more signals to
determine at least one property of the subsurface earth formation.
The one or more sensors may comprise one or more electromagnetic
field detectors that measure the earth's electromagnetic field and
produce a signal indicative of the detected electromagnetic field.
The analysis tool may also configure the processor to: receive the
signal from the one or more electromagnetic field detectors;
demodulate a portion of the signal to identify an envelope of the
signal; analyze the envelope to determine one or more properties
indicative of the presence of the hydrocarbons; and analyze the
envelope to determine the depth of the hydrocarbons in the
subterranean formation. The one or more sensors may comprise an
electromagnetic field detector that measures the earth's
electromagnetic field and produces a signal indicative of the
detected electromagnetic field, and the analysis tool may configure
the processor to receive the signal and determine the at least one
property of a subsurface earth formation. The one or more sensors
may comprise a plurality of seismic sensors that detect a seismic
signal related to earth's electromagnetic field and produce a first
signal indicative of the detected seismic signal. The analysis tool
may configure the processor to receive the first signal, and
process the first signal to determine the at least one property of
the subsurface earth formation. The one or more sensors may
comprise a plurality of seismic sensors that detect a seismic
signal related to earth's electromagnetic field and produce a first
signal indicative of the detected seismic signal and an
electromagnetic field detector that measures the earth's
electromagnetic field and produces a second signal indicative of
the detected electromagnetic field. The analysis tool may also
configure the processor to receive the first signal and the second
signal and determine the at least one property of a subsurface
earth formation. The one or more sensors may comprise a seismic
sensor that detects a seismic signal related to earth's
electromagnetic field and produces a first signal indicative of the
detected seismic signal, and an electromagnetic field detector that
measures the earth's electromagnetic field and produces a second
signal indicative of the detected electromagnetic field. The
analysis tool configures the processor to receive the first signal
and the second signal and determines the at least one property of a
subsurface earth formation.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] For a more complete understanding of the features and
advantages of the present invention, reference is now made to the
detailed description of the invention along with the accompanying
figures in which corresponding numerals in the different figures
refer to corresponding parts and in which:
[0017] FIG. 1 is a cross-sectional view of a passive surveying
system according to an embodiment.
[0018] FIG. 2 is a cross-sectional view of a porous subsurface
earth formation containing at least one fluid according to an
embodiment.
[0019] FIG. 3 is a cross-sectional view of a passive surveying
system according to an embodiment.
[0020] FIG. 4 is a perspective view of a passive surveying system
according to an embodiment.
[0021] FIG. 5 is a perspective view of a passive surveying system
according to embodiment.
[0022] FIG. 6 is another cross-sectional view of a passive
surveying system according to an embodiment.
[0023] FIG. 7 is still another cross-sectional view of a passive
surveying system according to an embodiment.
[0024] FIG. 8 is another perspective view of a passive surveying
system according to an embodiment.
[0025] FIG. 9 is a schematic view of a computer useful with an
embodiment.
[0026] FIG. 10 illustrates passive seismoelectric test results
using an embodiment of the system and methods disclosed herein.
[0027] FIG. 11 illustrates passive electroseismic test results
using an embodiment of the system and methods disclosed herein.
[0028] FIG. 12 illustrates combined passive electroseismic and
seismoelectric test results using an embodiment of the system and
methods disclosed herein.
[0029] FIG. 13 illustrates another passive seismoelectric test
results using an embodiment of the system and methods disclosed
herein.
DETAILED DESCRIPTION
[0030] It should be understood at the outset that although
illustrative implementations of one or more embodiments are
illustrated below, the disclosed systems and methods may be
implemented using any number of techniques, whether currently known
or not yet in existence. The disclosure should in no way be limited
to the illustrative implementations, drawings, and techniques
illustrated below, but may be modified within the scope of the
appended claims along with their full scope of equivalents.
Overview
[0031] In controlled-source seismoelectric surveying, generally a
seismic source that might be dynamite or a seismic vibrator,
creates a seismic wave that propagates into the subsurface where
its seismic energy is partially converted to an electric field at a
boundary between rock types or at fluid interfaces. The produced
electric field then propagates to the surface of the earth where it
is detected with electric and/or magnetic field sensors. In
controlled-source electroseismic surveying, a source of electrical
power is connected to electrodes in contact with the earth's
surface. The voltage applied to the electrodes causes electrical
current to flow in the subsurface. When that current passes through
a rock boundary or a fluid interface, a portion of the electrical
energy is converted to seismic energy. The resulting seismic energy
propagates to the earth's surface where it is detected with seismic
detectors that might be selected from geophones, accelerometers, or
hydrophones. Both seismoelectric and electroseismic conversion
amplitudes depend on the presence of hydrocarbon fluids so both
methods yield information about rock fluid content that is of use
in hydrocarbon exploration and production. Both methods also yield
high resolution images of rock formations that are typical of
seismic surveying. However, the high power sources that are
required in these methods limit their usefulness.
[0032] The systems and methods disclosed herein utilize naturally
occurring electromagnetic fields (e.g., the earth's background
electromagnetic field) and their interactions with a subsurface
formation through electroseismic and/or seismoelectric conversions
to employ passive surveying. Previous surveying methods differ from
the methods and systems described herein in several ways. Some
novel elements of the present methods and systems include: the
creation of seismic waves in a passive measurement where the source
is the earth's electromagnetic field; the subsequent conversion of
that seismic energy to electromagnetic energy; the detection of the
generated seismic energy and/or detection of the further conversion
of the seismic energy to electromagnetic energy; identifying the
generated seismic energy by detecting the characteristic time lags
or frequencies associated with the seismic travel time using a
frequency-selective method; and determining the depth of origin of
the seismic signal from said frequency selective method.
[0033] As described in more detail herein, passive surveying may
involve detecting a seismic and/or electromagnetic signal generated
in a subterranean formation, and processing the resulting signal to
determine the presence of one or more fluids (e.g., a hydrocarbon,
an aqueous fluid such as potable water, fresh water, brine, etc.)
in the subterranean formation. The one or more signals may be
detected using a variety of sensors and systems as described below.
The detection may involve 1) detecting an electromagnetic signal
alone, 2) detecting a seismic signal alone, 3) detecting both an
electromagnetic signal and a seismic signal at different times
and/or locations, and/or 4) detecting both an electromagnetic
signal and a seismic signal at overlapping time intervals to allow
for cross correlation of the two detected signals. The detected
signals may then be processed using a variety of techniques to
determine if one or more fluids are present in a subterranean
formation, and optionally one or more additional properties of the
one or more fluids in the subterranean formation (e.g., an
existence of a subsurface earth formation containing at least one
fluid, a depth of the subsurface formation, a porosity, a fluid
permeability, a composition of at least one fluid within the
subsurface earth formation, a spatial extent of the subsurface
earth formation, an orientation of the boundaries of the subsurface
earth formation, a resistivity, and any combination thereof).
[0034] Each of these surveying techniques described herein can
utilize a passive source to provide the energy for generating the
electroseismic and/or seismoelectric conversions. As used herein, a
"passive source" includes any source that is not being actively
used to generate a seismic and/or electromagnetic signal for use
with surveying a subterranean formation. The passive source
generally comprises a natural source of electromagnetic energy such
as the earth's natural electromagnetic field, though other man-made
source of electromagnetic radiation such as electrical power lines
may also be included. While certain man-made sources may induce an
electromagnetic field, they are distinguishable from an "active
source" (e.g., a seismic generator, explosives, electric field
generators, etc.) in that they are generally stationary, not used
for surveying a subterranean formation, and/or not associated with
the detectors being used to detect the seismic and/or
electromagnetic signal. As used herein, "passive surveying,"
"passive electroseismic surveying," and "passive seismoelectric
surveying" may then utilize a passive source (e.g., the earth's
natural electromagnetic field as the source field). The passive
surveying can then make use of the generation of secondary seismic
waves through coupling of the electromagnetic source field to
various rock formations (electroseismic effect) and subsequent
generations of secondary electromagnetic fields through coupling of
the generated seismic waves with various rock formations
(seismoelectric effect) to probe those formations and the fluids
they contain. Generation of tertiary and higher order
electromagnetic fields and seismic waves can also result from
additional couplings.
[0035] Having generally described the systems and methods for
passive surveying, the detection systems, signals, and processing
techniques will now be described in more detail.
Electroseismic and Seismoelectric Conversions
[0036] The systems and methods disclosed herein advantageously
utilize signals that have heretofore been neglected and/or not
detected. Magnetotelluric surveying generally involves the use of
the natural electromagnetic fields that originate in the earth's
atmosphere. In magnetotelluric surveying, naturally-occurring
electromagnetic fields propagate into the subsurface where they
encounter rock formations of differing electrical conductivity.
When the electromagnetic fields contact a formation of low
conductivity, such as is typical of hydrocarbon reservoirs, the
electromagnetic field measured at the surface of the earth changes.
Spatially-dependent electromagnetic fields measured on the earth's
surface can be used to indicate the presence of low-conductivity
formations that might contain hydrocarbons. Magnetotelluric
surveying has several limitations. Only low-frequency,
long-wavelength electromagnetic stimulation may reach prospective
reservoirs because the high-frequency electromagnetic fields are
rapidly attenuated by the conducting earth. Long-wavelength
electromagnetic waves limit the spatial resolution of
magnetotellurics making reservoir delineation difficult.
Additionally, magnetotelluric surveying only provides information
about formation electrical conductivity and does not yield data
revealing information about porosity, permeability, or reservoir
structure.
[0037] In contrast to the magnetotelluric surveying method, the
passive electroseismic surveying method makes explicit use of the
generation of seismic waves through the electroseismic effect, and
the subsequent generation of secondary electromagnetic fields and
seismic waves through combinations of the electroseismic and
seismoelectric effects to determine one or more properties of a
subsurface earth formation of interest that cannot be learned from
the magnetotelluric surveying method. In addition, the passive
electroseismic surveying method addresses the spatial resolution
issue of the magnetotellurics surveying method. Similarly, the
seismic signals generated due to the interactions between the
electromagnetic field and the subterranean formation may be used
alone or in combination with the electromagnetic field measurements
to determine one or more properties of the subsurface earth
formation.
[0038] An understanding of the electroseismic and seismoelectric
effects useful in passive surveying begins with an understanding of
the electromagnetic field within the earth, at least a portion of
which may comprise the naturally occurring background
electromagnetic field of the earth. The earth's naturally occurring
electromagnetic field comprises a broad spectrum of frequencies,
from sub-hertz frequencies to tens of thousands of hertz
frequencies, having a broad coverage over the surface of the earth.
This broad spectrum allows for a broad range of penetration depths
from tens of meters to tens of kilometers. The various frequencies
in the earth may result from various natural events such as
electromagnetic fluctuations in the ionosphere and/or naturally
occurring electromagnetic discharges in the atmosphere (e.g.,
lightning). Further information concerning the earth's background
electromagnetic field including the sources of the earth's
electromagnetic field and the power and frequency spectrum
associated with the earth's electromagnetic field can be found in
PRACTICAL MAGNETOTELLURICS by Simpson et al., published by
Cambridge University Press (2005), which is incorporated by
reference herein in its entirety. Other sources of electromagnetic
radiation may include cultural sources of electromagnetic radiation
that may contain electromagnetic radiation with sufficiently low
frequencies to reach and interact with a subterranean formation.
Power transmission lines are one example of a cultural source of
electromagnetic radiation that can interact with a subterranean
formation.
[0039] The earth's electromagnetic field propagates as an
electromagnetic modulation that unlike an acoustic wave travels at
the speed of an electromagnetic wave in the subsurface, which is
less than the speed of an electromagnetic wave in a vacuum or air.
The electromagnetic wave may typically travel in the subsurface of
the earth at a speed of about one hundred times greater than the
speed of propagation of an acoustic wave in the seismic frequency
band of about 1-100 Hz. Due to the relative speed of the
electromagnetic wave when compared to the seismic signal, the
travel time of the electromagnetic wave into the subsurface earth
formation is generally neglected for purpose of processing the
detected electromagnetic field and/or the detected seismic signal
in the methods disclosed herein.
[0040] As shown in FIG. 1, the earth's electromagnetic field 106
may encounter fluid dipoles 114 associated with at least one fluid
in a porous earth formation layer. A portion 116 of this formation
layer is depicted in an exploded view in FIG. 1. In general, the
earth's electromagnetic field 106 causes a change in the
polarization of the dipoles 114 in the pore fluid, which in turn
causes the fluid to flow or to generate a pressure pulse 118. The
flowing fluid (or pulse) produces a time varying pressure gradient,
which is then propagated into the earth formation (or rock). The
pressure gradient then propagates through the subsurface to the
surface as a seismic wave 120. The term "seismic wave" is used
herein to mean any mechanical wave that propagates in the
subsurface of the earth and includes, but is not limited to, P- and
S-waves.
[0041] In order to more clearly illustrate the basis for the
passive electroseismic surveying system and method disclosed
herein, the conversion of the earth's electromagnetic field to
seismic energy in at least one porous subsurface earth formation
containing at least one fluid will be described in more detail with
reference to FIG. 1 and FIG. 2. The formations of interest may
generally be porous, as is more clearly illustrated in FIG. 2. A
porous formation generally comprises a solid rock portion 201
interspersed throughout with channel-like porous spaces 202. The
term "porous" is used herein to mean some earth substance
containing non-earthen volume or pore space, and includes, but is
not limited to, consolidated, poorly consolidated, or
unconsolidated earthen materials. Where a polarizable fluid (e.g.,
aqueous or other polarizable fluid) exists, an electrochemical
interaction may form between the polarizable fluid and the solid
rock portions 201. The polarizable fluid may include, but is not
limited to, an aqueous fluid, hydrocarbons, petroleum, or any
combinations thereof. The electrochemical interaction is
represented by the "+" symbol in the fluid portion and the "-"
symbol in the rock portion of the formation. In general, the rock
portion 201 may have an existing natural surface charge over at
least a portion of the rock surface. This electrochemical
interaction may result in a local pore fluid dipole that causes a
local background electromagnetic field. It may be noted that,
overall, there is no net dipole field in unperturbed rock and its
associated fluids. The sign of the background electromagnetic field
or field polarity direction depends on the surface charge on the
solid and the way the fluid screens out that charge. In clays, the
charge is typically as shown in FIG. 2. However, in other materials
(e.g., carbonates), the charge could be reversed (i.e., with the
"+" charge on the solid).
[0042] When the earth's electromagnetic field 122 interacts with
the formation, as illustrated for a region of the formation, the
background electromagnetic field that acts upon the established
fluid dipole 114 or the charges associated with that dipole
changes, causing fluid movement. This is illustrated by the earth's
electromagnetic field 106 shown in FIG. 1. Although the earth's
electromagnetic field 106 may be a time varying field, the earth's
electromagnetic field 106 is illustrated as a static electric field
122 in FIG. 2 for ease of discussion. This electric field 122 has
the effect of modifying the electrochemical bonds or moving the
charges (depicted by the small arrows above and below the "charges"
in the fluid in FIG. 2), thereby effectively creating a pressure
pulse where the interactions are distorted or broken. This electric
field 122 exists throughout the fluid area and primarily affects
the charges of the dipoles 114 which are at or near the fluid
surface or interface of the rock. Thus, a pore fluid pressure pulse
is generated from the resulting charge movement and this resulting
pressure pulse is transmitted to the solid rock portions 201 of the
formation. In turn the pressure pulse is transmitted through the
rock portions 201 as a seismic wave 120. This effect may be
referred to as the electroseismic effect or the electrokinetic
effect. Thus, an appropriate subsurface formation layer may be a
subsurface source of seismic energy. A more detailed description of
the electroseismic effect and the electrokinetic effect can be
found in U.S. Pat. No. 5,877,995 entitled "Geophysical Prospecting"
and issued to Thompson et al., and which is incorporated herein by
reference in its entirety. Additional information on seismoelectric
surveying may be found in U.S. Pat. No. 4,904,942 entitled
"Electroseismic Prospecting by Detection of an Electromagnetic
Signal Produced by Dipolar Movement" and issued to Thompson, which
is incorporated herein by reference in its entirety.
[0043] Seismoelectric and electroseismic effects are also known to
generate harmonic responses where the coupling of electromagnetic
and seismic modulations creates new modulations at frequencies that
are harmonics of the generating seismic and electromagnetic
signals. Nonlinear electroseismic conversions may produce signals
useful during processing. In an embodiment, nonlinear, harmonic
signals having frequency components at higher frequency harmonics
of the source's fundamental frequency (i.e., those frequencies
present in the earth's background electromagnetic field) may be
detected as a result of distortions of the background
electromagnetic field interacting with the subsurface earth
formation containing at least one fluid. The harmonic signals may
be processed alone or in conjunction with the fundamental
frequencies of the detected seismic signal and/or the detected
electromagnetic field to determine one or more properties of the
subsurface earth formation. In an embodiment, the harmonic signals
may be present, detected, and/or isolated in both the detected
electromagnetic field and the detected seismic signal. A detailed
description of the nonlinear, harmonic generation of electroseismic
and seismoelectric effects can be found in U.S. Patent Application
Publication No. 2002/0181326 A1 entitled "Nonlinear Electroseismic
Exploration" and is issued to Hornbostel et al., and which is
incorporated herein by reference in its entirety.
[0044] A portion of this seismic energy generated by the conversion
of the earth's electromagnetic field to seismic energy may travel
upwardly from the formation (seismic wave 120) towards the surface,
where it may be detected by one or more seismic sensors, which may
be arranged in an array. When conventional seismic reflection
boundaries exist between the formation and surface, seismic
reflections may occur and may be detected by the detector array,
also in a conventional fashion. The detection of the resulting
subsurface-generated seismic wave may occur whenever there is fluid
in a porous formation, including formations of low
permeability.
[0045] With reference to FIGS. 2 and 3, a seismoelectric effect may
also be observed based on the same principles as the electroseismic
effect. Rather than an electromagnetic wave interacting with the
fluid in a pore to produce a seismic wave, a seismic wave that
originated due to the electroseismic effect, may interact with the
fluid in a pore 202 near the surface of the earth to generate a
modification of the charges present within the pore 202. This pore
202 could be filled with fresh water as is present in the water
table. The resulting modification of the charges may generate an
alternating current field, which may lead to the emission of an
electromagnetic signal 124. This electromagnetic signal may be
directly detected by one or more electromagnetic sensors, which may
be arranged in an array. In addition, the resulting time-variant
electromagnetic field resulting from the seismoelectric effect may
be referred to as a modulating signal since it may modulate an
electromagnetic field within the earth (e.g., the near surface of
the earth). As used herein, the term modulation can refer to
frequency modulation, phase modulation, and/or amplitude
modulation. The seismic wave that originated due to the
electroseismic effect may travel to the near surface of the earth
where it directly modulates an electromagnetic field within the
earth (e.g. the near surface of the earth). This modulation is
believed to occur because the passage of the seismic wave causes a
change in the electrical impedance that results in a time-dependent
variation of the electromagnetic field and/or the passage of the
seismic wave interacts with a fluid or rock boundary to produce an
electromagnetic field. The modulated signal may then be detectable
in the near surface or just above the surface of the earth. A
portion of the electromagnetic field generated by the conversion of
the seismic waves to electromagnetic energy and/or the modulating
signal generated by the conversion of the seismic waves to
electromagnetic energy or the seismic wave generated by the
electroseismic effect may be detected by one or more sensors. The
detection of the resulting subsurface generated electromagnetic
signal, the modulating signal, and/or the seismic signal may occur
whenever there is fluid in a porous formation, including formations
of low permeability. The indication of a fluid in a porous
formation may be used to locate or potentially locate a hydrocarbon
in a subterranean formation and/or locate or potentially locate an
aqueous fluid (e.g., fresh water, potable water, brine, etc.) in
the earth (e.g., detecting water or the water level in a subsurface
water table).
[0046] The earth's electromagnetic field may propagate into the
subsurface of the earth as an approximate plane wave, including
over a subsurface formation of interest. As used herein, a "plane
wave" refers to a wave with a substantially uniform amplitude on a
plane normal to the velocity vector. The velocity vector may be
generally vertical and may not necessarily be perpendicular to the
surface of the earth above the subsurface formation of interest.
For example, the velocity vector would be approximately vertical
rather than inclined from the vertical axis where the surface of
the earth above a subsurface formation of interest was inclined to
the vertical (e.g., on a hill or slope). As a result of the
electroseismic effect and/or seismoelectric effect, the seismic
and/or electromagnetic waves resulting from the earth's
electromagnetic field may be generated somewhat uniformly across
the subsurface formation containing at least one fluid, resulting
in a seismic and/or electromagnetic wave that is approximately a
plane wave traveling back to the surface of the earth. The
discovery of this result may be useful in processing the various
detected signals, as described in more detail below.
Passive Electromagnetic Field Detection Systems
[0047] Various sensors and detection mechanisms may be used to
detect the electromagnetic field within and/or above the surface of
the earth. Referring now to FIG. 3, a system 300 of geophysical
surveying is illustrated in a simplified manner for the purpose of
clarity. In system 300, a sensor 126 for detecting an
electromagnetic field 124 is disposed in contact with or near the
surface of the earth 108. The electromagnetic field 124 may be
emitted above the surface of the earth 108 as a detectable
electromagnetic field 128. It can be noted that the electromagnetic
field comprises both an electric field and a magnetic field, and
the reference to a sensor 126 for detecting the electromagnetic
field refers to a device for detecting an electromagnetic field, an
electric portion of the electromagnetic field, and/or a magnetic
portion of the electromagnetic field. A magnetic field detector may
refer to a device for detecting a magnetic field. In an embodiment,
a magnetic field detector may also be present at each location at
which an electromagnetic field detector is placed, though its
presence may not be specifically mentioned throughout the remainder
of the description for the purpose of clarity. The magnetic field
may be detected and processed using similar processing to that
described herein for the detected electromagnetic field.
[0048] The sensor 126 may be used to detect a signal of interest
124 within the earth 108 while being disposed above the surface of
the earth 108. The signal of interest may include the vertical
component of the electromagnetic field and/or the modulated,
vertical component of the electric field 124 in the near surface of
the earth 108 resulting ultimately from the coupling of an
electromagnetic field 106 with a formation containing a fluid of
interest. In an embodiment, the signal of interest may comprise
and/or be indicative of a time-varying electromagnetic field within
and/or above the surface of the earth 108. The electromagnetic
signal 106 interacting with the formation may generally include the
earth's background electromagnetic field, though other sources of
electromagnetic radiation may result in interactions with the
formation of interest. In order to measure the signal of interest,
the sensor 126 may measure the vertical portion of an electric
field in the near surface of the earth. As a result, the sensor 126
may comprise any type of sensor capable of measuring the vertical
electric field 128 in the near surface of the earth. Additional
signals may also be measured including the background vertical
portion of the earth's electromagnetic signal, the background
cultural source electromagnetic radiation, one or more components
of the magnetic field, one or more horizontal components of the
electromagnetic signal and/or one or more components of the seismic
amplitude.
[0049] The sensor 126 may be placed at or on the surface of the
earth 108 or at any distance above the surface of the earth (e.g.,
up to about one hundred feet) so long as the sensor 126 is within a
detectable range of the vertical electric field in the earth. In an
embodiment, the sensor 126 may be placed up to about ten feet from
the surface of the earth. The sensor 126 may be maintained in one
location during the detection of a signal and subsequently moved to
provide another detection period. Alternatively, a plurality of
sensors 126 may be used to provide a plurality of simultaneous
measurements at multiple locations. For example, a plurality of
sensors may be disposed in an array to allow for a plurality of
measurements at multiple locations.
[0050] Still further, the sensor 126 may be moved during a
measurement to detect the signal at multiple locations. Moving the
sensor 126 may result in a degradation of the spatial resolution of
the resulting analysis, as described below, but may still provide
information that can be useful as a screening or first look at an
area of interest. In an embodiment, the sensor 126 may be disposed
in a moving vehicle. Suitable vehicles may include, but are not
limited to, an airplane, an automobile, and/or a boat in shallow
water (e.g., a river). A recording apparatus (e.g., element 304 of
FIG. 4) may be used to record the signal detected by the sensor
126, as described in more detail herein. In an embodiment, the
sensor 126 may be used to continuously detect the signal of
interest (e.g., while the sensor is moved), and the detected signal
may be recorded. Desired portions of the recorded signal may then
be used in the systems and methods described below.
[0051] In an embodiment as shown in FIG. 4, the sensor 126 may
comprise one or more capacitive plates 301, 302 arranged parallel
to the surface of the earth 108. The sensor 126 may be contained in
suitable housing, and may comprise a capacitor consisting of two
parallel conductive plates 301, 302. The conductive plates 301, 302
may be oriented generally parallel to the surface of the earth 108
to measure the vertical electric field. The conductive plates 301,
302 are preferably made from a conductive metal such as copper,
aluminum, or stainless steel, and can be about several square
inches in area to about several square feet in area. The time
varying, vertical electric field may produce a charge on the area
of one of the conductive plates 301. The other plate of the
conductive plates 302 may be grounded to produce a capacitive
charge for the sensor 126. In an embodiment, a resistor may be
coupled in series with the charged conductive plate 301 and a
recording apparatus 304 to discharge the charged plate 301 and
allow a time varying field to be measured. The parallel plate
design may detect only the vertical electric field due to the
capacitive nature of the sensor 126, which may not detect the
horizontal magnetic component of the electromagnetic signal. While
two plates 301, 302 are shown, the sensor 126 may include a single
plate 301 appropriately grounded through one or more resistive
devices and coupled to a recording apparatus 304.
[0052] In an embodiment, the sensor 126 as shown in FIG. 3 may
comprise one or more antennas disposed on or above the surface of
the earth. Suitable antennas may include, but are not limited to, a
parallel-plate capacitor antenna comprising two or more parallel
conducting plates, a single-plate capacitor antenna comprising one
electrode electrically coupled to the earth, a monopole antenna
comprising a conducting element, a dipole antenna comprising two
conducting elements, a multi-pole antenna comprising a plurality of
conducting elements, a directional antenna comprising conducting
elements arranged to augment a signal amplitude in a particular
direction, and a coil antenna comprising one or more coils of wire,
and/or any combination thereof.
[0053] In an embodiment, the antenna may comprise a concentric
electric dipole (CED). The CED comprises two electrodes in a
concentric configuration. For example, the electrodes may comprise
generally circular dipoles with an inner circular electrode
disposed concentrically within an outer circular electrode. The
electrodes may generally be aligned in a plane that is parallel
with the plane of the surface of the earth. The CED may then
preferentially detect the vertical portion of the earth's
electromagnetic field that is substantially perpendicular to the
plane of the CED, where the vertical portion of the earth's
electromagnetic field may create a detectable potential difference
between the two electrodes.
[0054] In an embodiment illustrated in FIG. 1, an electromagnetic
field detector 104 may be disposed near or within the surface of
the earth 108 to measure the earth's naturally occurring
electromagnetic field 106 within the earth 108. The electromagnetic
field detector 104 may be used to detect the earth's naturally
occurring electromagnetic field 106 within the earth 108, or any
component of the naturally occurring electromagnetic field 106. The
electromagnetic field detector 104 may be configured to measure the
vertical component of the earth's electromagnetic field within the
earth. In an embodiment, one or more electromagnetic field
detectors may be configured to measure a horizontal component of
the earth's electromagnetic field in one or two dimensions. For
example, FIG. 5 illustrates additional electrode pairs 406, 408
disposed in a horizontal alignment to measure one or more
horizontal components of the earth's electromagnetic field. In an
embodiment, a single electromagnetic field detector may be
configured to measure a plurality of components of the
electromagnetic field (e.g., a two-axis electromagnetic field
detector, or a three-axis electromagnetic field detector).
[0055] Returning to FIG. 1, the electromagnetic field detector 104
disposed within the earth may be above or below the water table. In
an embodiment, the electromagnetic field detector 104 may be
disposed at any depth in the earth above the formation of interest.
In an embodiment, the electromagnetic field detector 104 may be
disposed at any depth in the earth within and/or below the
formation of interest. Combinations of these depths may also be
used, for example with electromagnetic field detectors above,
within, and/or below the formation of interest.
[0056] In an embodiment, the electromagnetic field detector 104 may
comprise a pair of electrodes 110, 112 in contact with the earth
and disposed within the earth. For example, a first electrode 112
may be disposed in a hole drilled into the earth ranging from about
10 feet to about 15 feet. A second electrode 110 may be disposed
within about 1 foot to about 3 feet of the surface of the earth,
and the pair of electrodes 110, 112 may be electrically coupled. In
an embodiment, the pair of electrodes 110, 112 may be disposed
within the earth at varying depths as needed to form an electrical
coupling with the earth.
[0057] In an embodiment, the electrodes may take the form of porous
pot electrodes, which may use an appropriate salt and/or aqueous
solution to form an electrical coupling with the earth. Suitable
salts useful with the electrodes 110, 112 may include, but are not
limited to, copper sulfate, silver chloride, cadmium chloride,
mercury chloride, lead chloride, and any combination thereof. In an
embodiment, the electrodes may include, but are not limited to,
conductive electrodes such as rods that are driven into the ground
and/or sheets of metal, mesh sheets, and/or wires buried in
trenches or in shallow pits. The conductive electrodes may comprise
a variety of conductive materials including, but not limited to,
copper, stainless steel, aluminum, gold, galvanized metal, iron,
lead, brass, graphite, steel, alloys thereof, and combinations
thereof. In an embodiment, the electrodes may comprise a conductive
electrode in contact with the earth and electrically coupled to a
porous pot electrode.
[0058] In an embodiment as shown in FIG. 5, a plurality of suitable
electromagnetic field detectors 104 may be employed. The
electromagnetic field detectors 104 may be disposed in a pattern or
array. For example, an array of suitable electromagnetic field
detectors may comprise from about 2 to about 1,000, or from about
10 to about 100 electromagnetic field detectors. In an embodiment
as shown in FIG. 6, one or more of the electromagnetic field
detectors 104 may be disposed within a wellbore 604. The wellbore
604 may be disposed below an array 606 of sensors 608 (e.g.,
electromagnetic field detectros and/or seismic sensors as described
in more detail below) or outside an array 606 of sensors 608. In an
embodiment, a plurality of electromagnetic field detectors 104 may
be used with one or more of the electromagnetic field detectors 104
disposed in a wellbore 604, and one or more of the electromagnetic
field detectors disposed above and/or within the earth outside the
wellbore (e.g., using porous pot and/or conductive electrodes).
[0059] As shown in FIG. 5, suitable additional electric and/or
magnetic field detectors 402 may be located near one or more
detectors used for passive surveying. In an embodiment, the
electric and/or magnetic field detectors 402 may be located within
about 10 yards, alternatively within about 100 yards, or
alternatively within about 500 yards of one or more of the
detectors. In an embodiment, additional electromagnetic field
detectors 412, 414, 416 configured to measure the electromagnetic
field within the earth and/or electric and magnetic field detectors
418 configured to measure the electromagnetic field above the
surface of the earth may be located at a distance away from the
seismic sensors 102 to identify the electric and/or magnetic fields
that occur over large distances. In an embodiment, the additional
detectors 412, 414, 416, 418 may be located more than about 500
yards, alternatively more than about one half mile, alternatively
more than about one mile, or alternatively more than about 5 miles
from the one or more detectors.
[0060] One or more recording devices may record the resulting
electromagnetic field produced by the one or more sensors and/or
detectors disposed above and/or within the earth. A suitable
recording device may include digital and/or analog recording
devices and/or non-transitory media that may, in an embodiment, be
contained in a weather resistant enclosure capable of recording
data over days to weeks without human intervention. The recording
apparatus may be contained in a recording vehicle, a housing
structure, or a weather resistant enclosure located proximate the
sensor. In an embodiment shown in FIG. 1, a suitable recording
device may be contained in a dedicated recording vehicle 130 for
the electromagnetic field detector 104. In an embodiment shown in
FIG. 4, the recording device 304 may be coupled to the sensor 126,
and the recording apparatus 304 may be internal or external to the
sensor 126 housing. In an embodiment shown in FIG. 6, the recording
device 602 may be one of a plurality of recording devices 130, 602
used to record one or more electric and/or seismic signals.
[0061] The signals may be recorded over a length of time sufficient
to allow for analysis of the signals including a sufficient time to
allow for averaging and signal integration to ensure an adequate
signal-to-noise ratio (SNR). In an embodiment, the signal may be
recorded for at least about 2 times, at least about 3 times, at
least about 4 times, at least about 5, times, at least about 10
times, at least about 20 times, at least about 50 times, at least
about 100 times, or at least about 1,000 times the period of the
signal of interest. In an embodiment, the signal may be recorded at
least about 0.1 seconds, at least about 5 seconds, at least about
20 seconds, or at least about 30 seconds. Longer recording times
may allow for increasing reliability, but at the same time, the
amount of data that must be processed is increased. In general, a
recording time of from about 5 to about 6,000 seconds, about 200 to
about 1,200 seconds, or alternatively about 300 to about 900
seconds will be sufficient to analyze the data without generating
excessive data to be processed. Longer recording times may be used,
for example, when the sensor is moved during recording as when
disposed in an airplane or automobile. The signal may be converted
into a digital signal using an analog-to-digital converter with an
appropriate sampling rate. The sampling rate stored by the
recording apparatus may vary, though some consideration of the
frequencies of interest may determine the minimum sampling
rate.
[0062] The signals from the sensor/detector and/or the signals
stored in the recording apparatus may be processed in real time. As
used herein, "real time" refers to a nearly contemporaneous
processing of the detected data though some processing delays may
be present (e.g., due to latency in the sensors and processing
equipment). Thus, real time may refer to the handling and/or
processing of a signal within about five minutes of the generation
of the corresponding signal. Alternatively, the storage of the
signal in the recording apparatus may allow for subsequent
processing and interpretation. As described above, the signal may
be processed using an analog-to-digital converter to allow the
signal to be processed by one or more digital signal processing
components, such as a digital computer as described in more detail
herein. For example, a signal processor 306 as illustrated in FIG.
4 may be used to process the signal. In general, the sampling rate
for the analog-to-digital conversion should be at least twice the
highest frequency of interest in order to properly represent the
recorded waveform. Higher sampling rates are also possible and may
include about 3 times, about 4 times, or about 5 times the highest
frequency of interest.
Passive Seismic Signal Detection Systems
[0063] As shown in FIG. 1, a seismic wave 120 may be detected by
one or more appropriate seismic sensors 102 positioned on and/or in
the surface of the earth 108. In an embodiment, a plurality of
seismic sensors 102 may be used to detect the seismic wave 120 at
multiple locations. As shown in FIG. 7, one or more seismic sensors
702 may be disposed within a wellbore 704 (e.g., an existing
wellbore, a new wellbore, a water well, a hydrocarbon well, etc.)
for detecting a seismic wave within the subsurface earth formation.
The wellbore may extend through and/or below the formation of
interest to allow for detection of the seismic wave at varying
depths in the earth. In an embodiment, the wellbore 704 may be
disposed below the array 706 of seismic sensors 102 and/or electric
sensors/detectors or outside the array 706 of sensors. In an
embodiment, one or more of the electromagnetic field detectors 104
shown in FIG. 6 may be included in the same wellbore 704 as the one
or more seismic sensors 702 as shown in FIG. 7 (e.g., the
embodiments of FIG. 6 and FIG. 7 may be combined). The seismic
sensors 102, 702 may include, but are not limited to, a geophone, a
hydrophone, and/or an accelerometer (e.g., a digital
accelerometer). The geophone may comprise a single-component
geophone, a two-component geophone, or a three-component geophone,
and the accelerometer may comprise a single-axis accelerometer, a
two-axis accelerometer, or a three-axis accelerometer. In an
embodiment, the seismic sensor may comprise a three-component
accelerometer. Combinations of these types of seismic sensors may
be used when a plurality of seismic sensors are used to detect the
seismic wave. The seismic sensors 102, 702 may measure a seismic
wave in multiple directions, for example in one or two directions
parallel to the surface of the earth, in a direction perpendicular
to the surface of the earth, and/or in a vertical direction.
[0064] Returning to FIG. 1, a detected seismic signal may be
generated by each seismic sensor 102. The detected seismic signal
may represent or be indicative of the seismic wave 120 and may be
suitably recorded, for example, using a conventional seismic field
recorder. In an embodiment, a suitable recording device may be
contained in a recording vehicle 130, which may comprise a
dedicated recording vehicle for the detected seismic signal.
Additional or alternative recording devices for the detected
seismic signal may also be used and may include digital and/or
analog recording devices and/or non-transitory media that may, in
an embodiment, be contained in a weather resistant enclosure
capable of recording data over days to weeks without human
intervention. In an embodiment, each seismic sensor may have its
own recording device, and each recording device may be internal or
external to the seismic sensor. The detected seismic signal may
then be processed along with the detected electromagnetic field to
determine at least one property of the subsurface earth formation.
In an embodiment, the detected seismic signal and the detected
electromagnetic field may be processed in real-time without first
being recorded on a non-transitory medium.
[0065] When a plurality of seismic sensors is used to measure the
seismic wave, the seismic sensors may be arranged in a variety of
patterns. In an embodiment as shown in FIG. 5, a grid pattern may
be used, and the spacing between the seismic sensors 102 may be
less than about one half of the wavelength of the highest frequency
surface seismic waves expected to be encountered, which may include
higher frequencies than those expected to be produced by the
electroseismic effect within the subsurface earth formation.
Surface seismic waves may be due to various sources including heavy
equipment (e.g., construction equipment, trains, etc.), vehicular
traffic, and/or natural sources (e.g., natural seismic activity
such as earthquakes, thunder, etc.).
[0066] In an embodiment as shown in FIG. 5, one or more seismic
sensors 410 may be used to measure one or more components of a
seismic wave at a distance away from the seismic sensors 102 above
the subsurface earth formation 420 of interest. In an embodiment,
an additional seismic sensor 410 may be located at a distance away
from the one or more seismic sensors 102 to identify seismic waves
from sources other than the subsurface earth formation and/or that
occur over large distances. In an embodiment, the additional
seismic sensors 410 may be located more than about 500 yards,
alternatively more than about one half mile, alternatively more
than about one mile, or alternatively more than about 5 miles from
the one or more seismic sensors above the subsurface earth
formation 420.
[0067] In an embodiment, the electromagnetic field detectors and
seismic sensors disclosed herein may be used to perform passive
surveying. The sensors may be combined in any combination in order
to obtain the desired signals. For example, when a detected seismic
signal is desired, a single seismic sensor or a plurality of
seismic sensors may be employed. Similarly, when a detected
electromagnetic signal is desired, a single electromagnetic field
detector or a plurality of electromagnetic field detectors may be
employed. When both a detected seismic signal and a detected
electromagnetic signal are desired, a single seismic sensor and a
single electromagnetic field detector may be employed,
alternatively a single seismic sensor and a plurality of
electromagnetic field detectors may be employed, alternatively, a
plurality of seismic sensors and a single electromagnetic field
detector may be employed, or alternatively, a plurality of seismic
sensors 120 and a plurality of electromagnetic field detectors may
be employed. The electromagnetic field detectors 104, 126 may be
disposed in a pattern or array, which in an embodiment may overlap
with a pattern or array of seismic sensors 102.
[0068] In an embodiment, any of the electromagnetic field detectors
and/or seismic sensors disclosed herein may be used to perform
long-term passive surveying. In this embodiment, one or more of the
signals may be detected a plurality of times using the sensors. The
signals may be detected periodically (e.g., every week, every
month, every year, etc.) or aperiodically over a period of hours,
days, weeks, months, years, and/or decades. The long-term survey
results may provide a time-based indication of a subsurface earth
formation of interest with respect to time, including any changes
in the formation over the time period in which the signals are
detected. Such a system may be used to monitor the development
and/or depletion of a hydrocarbon field and/or water well or
aquifer, for example during production of a fluid from the
formation.
Passive Electrical Surveying and Processing: Detecting a Modulating
Signal
[0069] In an embodiment, passive surveying may be carried out by
detecting an electromagnetic signal alone, and processing the
detected electromagnetic signal to determine if one or more fluids
are present in a subterranean formation of interest. The
electromagnetic signal may comprise any of the electromagnetic
signals described above including a nonlinear electromagnetic
response. In this embodiment, an electromagnetic signal may be
detected using any of the electromagnetic field detector
configurations described above. In the broadest sense, the signal
may be optionally pre-processed using one or more filters, followed
by demodulation of the signal to determine if a modulation of a
carrier signal (e.g., the earth's background electromagnetic field,
electromagnetic radiation from a cultural source, a generated
reference signal, etc.) exists. The existence of a modulation of
the carrier signal may be taken as a direct indication that a
coupling has occurred due to the interaction of the carrier signal
with a modulating signal produced in a subterranean formation below
the sensor. Additional processing may be performed including power
spectral analysis and relative power ratios of the modulating
signal relative to a background signal to determine the frequency
characteristics of the modulating signal. The frequency
characteristics may be used to derive depth and location
information about the source and strength of the modulating signal,
thereby revealing information about the location of a subterranean
formation. A variety of models may be used to correlate the
spectral analysis results with the depth of the modulating
signal.
[0070] Having generally described the various types of signal
analysis that can be performed on the detected electromagnetic
signal, each signal processing method will now be described in
greater detail. As discussed above, the source signal interacting
with the formation of interest may comprise a broad spectrum of
frequencies, from sub-hertz frequencies to tens of thousands of
hertz frequencies. While not intended to be limited by theory, the
modulating signal arising from the interactions within the
formation of interest may have a narrower frequency-band spectrum
than the source signal with recognizable and extractable
characteristics. Even with the narrower frequency-band
characteristics, some pre-processing of the signal may be used to
analyze the detected signal. The optional pre-processing of the
signal may include use of noise reduction, separation of a direct
current component of the signal, data reduction, and/or noise
filtering/frequency band-passing.
[0071] A noise reduction scheme may be used to generate a signal
that may have an increased signal-to-noise ratio relative to the
full spectrum of the electromagnetic field in the earth. In an
embodiment as shown in FIG. 8, a reference signal 806 may be
generated by a reference signal generator 802 and introduced into
the near surface of the earth, for example, by transmitting the
reference signal into the earth from a location near to the ground
108. The modulating signal 124 may act to modulate the reference
signal 806 in the same way as the vertical portion of the electric
field within the earth. Upon detecting the modulated reference
signal 808 with the sensor 104, various detection methods may then
be used to compare the detected signal 808 with the known reference
signal 806 and isolate the modulating signal 124 for further
processing. The detected, modulated reference signal 808 may, in
some embodiments, be filtered or otherwise pre-processed prior to
being compared and isolating the modulating signal 124. In an
embodiment, a lock-in amplifier 804 may be used to isolate the
modulating signal 124 from the detected signal 808. The lock-in
amplifier 804 may be of any type known in the art and may receive
as inputs the reference signal 802 and the detected signal 808 from
the sensor 104. For example, the reference signal generator 802 may
be coupled to the lock-in amplifier 804 or may form a part of the
lock-in amplifier 804. The lock-in amplifier 804 may produce a
signal comprising the modulating signal 124 with a reduced
signal-to-noise ratio as compared to the signal detected by the
sensor 104. The produced signal may then be sent to one or more
additional, optional pre-processing steps before being passed on
for further analysis.
[0072] Depending on the type of sensor used to detect the signal,
the signal may comprise both an alternating current (AC) portion
and direct current (DC) portion. The DC portion of the signal may
result from the detection one or more portions of the earth's
electromagnetic field and may not contain data indicative of the
modulating signal. The DC portion may then be thought of as noise
that may be filtered out prior to analysis of the signal. Any
suitable DC filtering methods and/or equipment may be used to
remove the DC portion from the detected signal. When a capacitive
plate sensor is used, the DC portion may be filtered out based on
the sensor design. Alternatively, a capacitive filter and/or a
digital filter implemented in software on a computer may be used to
remove the DC portion from the detected and/or stored signal.
[0073] As discussed above, the time period over which the signal is
detected and recorded and/or the sampling frequency of the signal
may result in a significant quantity of data to analyze. Greater
amounts of data generally require more time to analyze and greater
storage and computing costs. As a result, the amount of data may be
limited and/or reduced to allow for faster processing, especially
when real time processing is desired. While a reduced amount of
data may lead to a less accurate analysis, processing of a
relatively small portion of the data may provide a faster, cheaper
first look at the results of the survey.
[0074] The data may be limited in a number of ways. In an
embodiment, the signal data may be decimated. Decimating the data
reduces the amount of data that is processed in the analysis steps
and, therefore, reduces processing time and costs. Excessive
decimation may reduce the reliability of the analysis to a certain
extent, and thus the savings in processing times and costs must be
weighed against reduced reliability. The signal data may typically
be decimated down to an effective sampling rate approximating two
times the highest frequency of interest while allowing for an
identification of the frequency characteristics in the data. Higher
decimation rates may be used, for example, when a faster, and
possibly less accurate first look at the data is desired.
Decimation of the data may be used alone or in addition to any of
the other pre-processing techniques described herein.
[0075] The signal data may be filtered to reduce the amount of
noise and thereby increase the SNR of the signal and/or the signal
may be band-pass filtered to isolate one or more frequency bands of
interest for use in the further analysis of the data. A noise
filter may comprise any type of noise filter known in the art. For
example, a high pass filter, a low pass filter, any suitable wide
band frequency filter, and/or any suitable narrow band frequency
filter may be used to eliminate noise. The earth's background
electromagnetic field, a cultural source of electromagnetic
radiation, and/or the reference signal, if used, may be used to
determine the frequency range, amplitude range, and/or other
parameters of a desired noise filter.
[0076] In some embodiments, a plurality of electromagnetic
detectors may be disposed in an array and may be used to detect one
or more components of the electromagnetic signal. In an embodiment,
a horizontal component of the detected electromagnetic signal may
be used with a predictive filter to remove noise from the vertical
component of the detected electromagnetic field. The predictive
filter may utilize one horizontal component or two horizontal
components as detected by the one or more electromagnetic field
detectors. This predictive filter may be applied to each
electromagnetic field detector of the plurality of electromagnetic
field detectors.
[0077] In addition to, or in place of, the noise filter, a
band-pass filter may be used to isolate one or more frequency
ranges of interest for purpose of extracting the envelope or
demodulating the signal over this frequency range. A set of
predetermined frequencies may be used as the basis for filtering
the detected signal for further processing. The frequency filters
may improve the SNR of the signal. A variety of band-pass filters
may be used to produce one or more frequency filtered signals
useful for further processing. For example, band-pass filters
implemented in hardware are known and may be suitable.
Alternatively, band-pass filters may also be implemented in
software, for example, using a digital processing environment on a
computer encoded for processing of the data. Any of a number of
known rational polynomial functions may be used alone or in
combination with other functions to separate the data for a
particular frequency and/or frequency range from the data set as a
whole, and in general those functions and processes may be used as
frequency filters. For example, linear phase filters, finite
impulse response filters, forward infinite impulse response
filters, reverse infinite impulse response filters, and the like
may be used. When a plurality of band-pass filters are used to
generate a plurality of signals, each band pass filter may have the
same or may have different bandwidths. The correlation between the
frequency of the modulating signal obtained from the envelope
extracted from the signal and the depth of the formation of
interest may be determined in a number of ways, as described in
more detail below.
[0078] In an embodiment, the detected signal amplitude may be
averaged over one or more frequencies and/or frequency ranges. In
an embodiment, the averaging of the detected electromagnetic field
signal amplitude may comprise averaging the signal amplitude at one
or more fixed frequencies. The frequencies may generally be
selected from those frequencies present in the detected electric
signal. In an embodiment, the one or more frequencies at which the
detected signal can be averaged may be selected from a range of
frequencies between about 0.01 Hz and about 10,000 Hz. The
averaging process may comprise measuring the signal amplitude for a
length of time that is longer (e.g., greater than at least twice as
long) than the period of oscillation of the signal at the selected
frequency, and then averaging the signal amplitude over the
detection time period. The averaging may reduce the momentary
fluctuations in the detected electromagnetic field resulting from
noise fluctuations that are not related to signals originating in
the earth. Processing may further consist of averaging the signal
amplitude at many frequencies over a range of frequencies. For
example, the selected frequencies might be chosen to be separated
by 1 Hz between 1 Hz and 10,000 Hz.
[0079] When the signal has been pre-processed using any optional
pre-processing steps including band-passing the signal, the signal
may pass to a signal envelope extraction step to determine the
envelope of the signal in the band of interest. The envelope of the
signal may refer to the shape of the modulation of the signal. As
discussed above, the modulation, and therefore the envelope, can
comprise one or more of a frequency modulation, a phase modulation,
or an amplitude modulation. An envelope detector as known in the
art may be used to extract the envelope of the signal. The envelope
detector may be implemented in hardware or software and may be used
to demodulate the band-passed signal to extract the signal
envelope. The envelope detector may be used to extract the signal
envelope for a plurality of band-passed signals. Suitable
demodulation techniques and methods are known and may include the
Hilbert transform method.
[0080] Once the signal envelope has been obtained, the envelope may
be analyzed to determine the spectral properties. The spectral
properties may allow for comparison with one or more additional
envelopes for additional signal bands from the originally detected
signal. The spectral properties may be determined in the frequency
domain through well-established methods for calculating the Fourier
Transform and the power spectral density. For example, the power
spectral density for various bands of frequencies may be calculated
to give the power carried by the envelope expressed in units of
power per frequency. Alternatively or in addition to the power
spectral density, a Fourier Transform (e.g., Fast Fourier Transform
(FFT), complex FFT, etc.) may provide an indication of the
frequency characteristics of the envelope (e.g., a frequency
distribution). Further, the power spectral density and FFT
calculations may provide relative amplitudes of each of the
frequencies identified. As used herein, the "spectral properties"
may include amplitude and frequency characteristics of a signal
and/or envelope along with other characteristics of the signal
and/or envelope such as the phase characteristics, etc. Calculation
of the spectral properties may be implemented in hardware and/or
software. Calculation methods for determining the power spectral
density and/or the FFT are known and may be carried out using
processing implemented on a digital computer comprising a
processor, as described in more detail below. In an embodiment, one
or more of the spectral properties may be determined using a
lock-in amplifier and/or a spectrum analyzer.
[0081] The spectral properties of one or more additional envelopes
resulting from one or more corresponding additional frequency
ranges selected from the detected (and optionally pre-processed)
signal may be calculated using the methods described herein. When a
plurality of spectral properties has been calculated, the
corresponding values may be compared to generate one or more ratios
of the spectral properties (e.g., power spectral density, FFT
amplitudes, phase comparisons, etc.) in certain frequency bands to
the corresponding spectral properties in other frequency bands. The
spectral properties of the base detected signal containing various
white noise portions may also be calculated using a variety of
methods to provide a base spectral property (e.g., a power density,
a frequency-amplitude correlation, etc.) for comparison. For
example, the base spectral property may be used to normalize the
other calculated ratios, though other mathematical transformations
may be used to produce similar results.
[0082] The ratios obtained by comparison of the various signal
bands may be analyzed and correlated as a function of the band-pass
frequencies of the original signal and/or the frequency band of the
extracted envelopes. The resulting analysis may provide information
about the frequency characteristics of the modulating signal and/or
an amplitude correlation relating the strength of the modulating
signal for each frequency. Variations within the analysis may be
used to adjust the analysis criteria such as increasing the
bandwidth of the band-pass filters, which may be expected to
increase the amplitude of the ratio of the power spectral
properties. The properties of the analysis may be tailored based on
the quality and amount of data obtained, the type of signals
present and interacting with a formation of interest, and a desired
processing speed and cost.
[0083] The existence of hydrocarbons in a formation may be
indicated by the existence of a modulating signal. In terms of the
signal analysis described in this section, the modulating signal
may be identified by demodulating a portion of the detected signal
to determine if an envelope can be identified. If no envelope is
found that is distinguishable from white noise, for example, or
some other suitable reference signal, then this result may be taken
as evidence that there are no hydrocarbons in a formation of
interest below the sensor location. If a suitable envelope is
identified, then the analysis described herein may be carried out
to identify the spectral properties of the envelope and correlate
the results with the presence of various fluids as well as a
frequency-depth function. In an embodiment, a further survey as
described below may be performed when an envelope is
identified.
[0084] Correlation of the spectral properties of the envelope and
the presence of various fluids in subterranean pore spaces may be
based on a variety of classification methodologies. For example,
statistical regression analysis, and statistical classifiers such
as neural networks, decision trees, Bayes-based classifiers, fuzzy
logic-based classifiers, and conventional statistical classifiers
may all be used to determine a frequency-depth relationship. For
example, the analysis may be performed with the system and methods
described herein at locations with known properties and formation
characteristics to train and/or determine the correlation
parameters. Once the parameters have been determined (e.g., once a
neural net has been trained), the system and methods may be
repeated in a new location.
[0085] While it is known that a correlation generally exists
between the frequency of the modulating signal and the depth at
which it originates, the exact relationship may not be evident from
the analysis of the signal detected by the sensor. A
frequency-depth function may be established using known locations,
parameters, and/or calculations to allow the depth values for
similar locations to be determined once the spectral
characteristics of the signals are analyzed and determined. The
frequency-depth relationship for electric signals may depend on the
Earth's resistivity, formation properties, types of components
present, and, more generally, the electrical properties for a
particular area and corrections may need to be applied to the
spectral properties from location to location. In some instances,
the frequency-depth correlation variation from area to area may not
be so great that for many purposes an approximate or a more or less
typical frequency-depth function may be used. For example, a
frequency-depth function may be derived from empirical data taking
into consideration various locations that have been reported in the
literature. This function may be considered more or less
representative of the "typical" relationship between frequency and
depth. Other suitable sources of data may be considered including,
for example, an approximate frequency-depth function derived from
conventional skin effect conductivity analyses. The resulting
typical frequency-depth function may be used to derive depth
information for the corresponding modulating signal of interest
based on the analysis described herein.
[0086] In still another embodiment, the various properties of the
subterranean formation including the frequency-depth function may
be determined by developing a geological model of the subterranean
formation. Various modeling programs may be used to develop the
model of the subterranean formation and can provide predicted
outputs based on the model. The predicted outputs can then be
compared with the detected signals (e.g., a detected envelope) to
determine if the model is accurate. When a discrepancy is detected,
the geological model can be altered and the process repeated. Such
a process may result in a match between the geological model and
the detected signal, thereby providing one or more properties of
the subterranean formation.
[0087] The processing described herein may be used to determine the
existence of a fluid in a subterranean formation as well as other
properties of the subterranean formation. In an embodiment, the
processing of the electromagnetic signal may provide an indication
of at least one property of a subterranean formation including, but
not limited to, an existence of the subsurface earth formation
containing at least one fluid, a depth of the subsurface earth
formation, a porosity, a fluid permeability, a composition of at
least one fluid within the subsurface earth formation, a spatial
extent of the subsurface earth formation, an orientation of the
boundaries of the subsurface earth formation, a resistivity, and
any combination thereof. Further, the processing may detect one or
more fluids within the subterranean formation, where detectable
fluids may include, but are not limited to, an aqueous fluid, a
hydrocarbon, a petroleum, carbon dioxide and/or other reservoir
gases (e.g., various acid gases, helium, and the like), and any
combination thereof.
[0088] While the measurement of a signal at a single location
followed by an analysis of that signal has been described herein,
the detection and analysis steps may be repeated any number of
times. For example, multiple measurements may be made at a single
location over several time periods. The results may be
statistically analyzed to provide an improved accuracy correlation
and/or survey. In addition, one or more samples may be taken at
varying locations sequentially in time or concurrently in time
using one or multiple sensors. For example, multiple measurements
may be made at varying locations around a site of interest. Various
grid patterns and/or random sample locations may be chosen to
generate a plurality of measurements across an area. For example,
the grid and/or array of detectors described above may be used to
generate a plurality of detected signals for use with the
processing techniques described herein. The multiple measurements
may be performed sequentially or concurrently (e.g., using multiple
sensors) at a single location, and/or the measurements may be
performed sequentially and/or concurrently in the various locations
around a site of interest when a plurality of locations are used to
measure the signal of interest. The resulting hydrocarbon
indications and resulting depth measurements may be used to
generate a two dimensional, a three dimensional, and/or a four
dimensional model (e.g., where the fourth dimension is time) of a
subterranean formation and the one or more fluids contained
therein.
Passive Electrical Surveying and Processing: Directly Detecting an
Electromagnetic Field
[0089] In an embodiment, passive surveying may be carried out by
detecting an electromagnetic signal alone, and processing the
detected electromagnetic signal to determine if one or more fluids
are present in a subterranean formation of interest. The
electromagnetic signal may comprise the electromagnetic signals
described above including a nonlinear electromagnetic response. In
this embodiment, an electromagnetic signal may be detected using
any of the electromagnetic field detector configurations described
above. Once the electromagnetic field has been detected by one or
more sensors and a signal has been generated, the signal may be
analyzed to extract information concerning the existence of a
formation of interest. In the broadest sense, the signal may be
optionally pre-processed using one or more filters, followed by
correlation analysis of the detected electromagnetic signal in
either the time or frequency domain. The temporal or frequency
characteristics of the correlation analysis may be used to derive
depth and location information about the source and strength of the
detected signal, thereby revealing information about the location
of a subterranean formation. A variety of models may be used to
correlate the correlation analysis results with the depth of the
detected signal.
[0090] Having generally described the various types of signal
analysis that can be performed on the detected electromagnetic
signal, each signal processing method will now be described in
greater detail. As discussed above, the source signal interacting
with the formation of interest may comprise a broad spectrum of
frequencies, from sub-hertz frequencies to tens of thousands of
hertz frequencies. In an embodiment, the signal of interest may
comprise the signal directly arising from the electroseismic and
seismoelectric effects within a formation of interest. While not
intended to be limited by theory, the electromagnetic signal
arising from the interactions within the formation of interest may
have a narrower frequency-band spectrum than the source signal with
recognizable and extractable characteristics. Even with the
narrower frequency-band characteristics, some pre-processing of the
signal may be used to analyze the detected signal. The optional
pre-processing of the signal may include use of a noise reduction
detection scheme, separation of a direct current component of the
signal, data reduction, and/or noise filtering/frequency
band-passing.
[0091] Depending on the type of sensor used to detect the signal,
the signal may comprise both an alternating current (AC) portion
and direct current (DC) portion. The DC portion of the signal may
result from the detection of one or more portions of the earth's
electromagnetic field and may not contain signal of interest. The
DC portion may then be thought of as noise that may be filtered out
prior to analysis of the signal. Any suitable DC filtering methods
and/or equipment may be used to remove the DC portion from the
detected signal. When a capacitive plate sensor is used, the DC
portion may be filtered out based on the sensor design.
Alternatively, a capacitive filter and/or a digital filter
implemented in software on a computer may be used to remove the DC
portion from the detected and/or stored signal.
[0092] As discussed above, the time period over which the signal is
detected and recorded and/or the sampling frequency of the signal
may result in a significant quantity of data to analyze. Greater
amounts of data generally require more time to analyze and greater
storage and computing costs. As a result, the amount of data may be
limited and/or reduced to allow for faster processing, especially
when real time processing is desired. While a reduced amount of
data may lead to a less accurate analysis, processing of a
relatively small portion of the data may provide a faster, cheaper
first look at the results of the survey.
[0093] The data may be limited in a number of ways. In an
embodiment, the signal data may be decimated. Decimating the data
reduces the amount of data that is processed in the analysis steps
and, therefore, reduces processing time and costs. Excessive
decimation may reduce the reliability of the analysis to a certain
extent, and thus the savings in processing times and costs must be
weighed against reduced reliability. The signal data may typically
be decimated down to an effective sampling rate approximating two
times the highest frequency of interest while allowing for an
identification of the frequency characteristics in the data. Higher
decimation rates may be used, for example, when a faster, and
possibly less accurate first look at the data is desired.
Decimation of the data may be used alone or in addition to any of
the other pre-processing techniques described herein.
[0094] The signal data may be filtered to reduce the amount of
noise and thereby increase the SNR of the signal and/or the signal
may be band-pass filtered to isolate one or more frequency bands of
interest for use in the further analysis of the data. A noise
filter may comprise any type of noise filter known in the art. For
example, a high pass filter, a low pass filter, or any suitable
wide band frequency filter, and/or any suitable narrow band
frequency filter may be used to eliminate noise. The earth's
background electromagnetic field, a cultural source of
electromagnetic radiation, and/or the reference signal, if used,
may be used to determine the frequency range, amplitude range,
and/or other parameters of a desired noise filter.
[0095] In some embodiments, a plurality of electromagnetic sensors
may be disposed in an array and may be used to detect one or more
components of the electromagnetic signal. In an embodiment, a
horizontal component of the detected electromagnetic signal may be
used with a predictive filter to remove noise from the vertical
component of the detected electromagnetic field. The predictive
filter may utilize one horizontal component or two horizontal
components as detected by the one or more electromagnetic field
detectors. This predictive filter may be applied to each
electromagnetic field detector of the plurality of electromagnetic
field detectors.
[0096] In addition to, or in place of, the noise filter, a
band-pass filter may be used to isolate one or more frequency
ranges of interest for purpose of extracting the signal over this
frequency range. A set of predetermined frequencies may be used as
the basis for filtering the detected signal for further processing.
The frequency filters may improve the SNR of the signal. A variety
of band-pass filters may be used to produce one or more frequency
filtered signals useful for further processing. For example,
band-pass filters implemented in hardware are known and may be
suitable. Alternatively, band-pass filters may also be implemented
in software, for example, using a digital processing environment on
a computer encoded for processing of the data. Any of a number of
known rational polynomial functions may be used alone or in
combination with other functions to separate the data for a
particular frequency and/or frequency range from the data set as a
whole, and in general those functions and processes may be used as
frequency filters. For example, linear phase filters, finite
impulse response filters, forward infinite impulse response
filters, reverse infinite impulse response filters, and the like
may be used. When a plurality of band-pass filters are used to
generate a plurality of signals, each band pass filter may have the
same or may have different bandwidths.
[0097] In an embodiment, the detected signal amplitude may be
averaged over one or more frequencies and/or frequency ranges. In
an embodiment, the averaging of the detected electromagnetic field
signal amplitude may comprise averaging the signal amplitude at one
or more fixed frequencies. The frequencies may generally be
selected from those frequencies present in the detected electric
signal. In an embodiment, the one or more frequencies at which the
detected signal can be averaged may be selected from a range of
frequencies between about 0.01 Hz and about 10,000 Hz. The
averaging process may comprise measuring the signal amplitude for a
length of time that is longer (e.g., greater than at least twice as
long) than the period of oscillation of the signal at the selected
frequency, and then averaging the signal amplitude over the
detection time period. The averaging may reduce the momentary
fluctuations in the detected electromagnetic field resulting from
noise fluctuations that are not related to signals originating in
the earth. Processing may further consist of averaging the signal
amplitude at many frequencies over a range of frequencies. For
example, the selected frequencies might be chosen to be separated
by 1 Hz between 1 Hz and 10,000 Hz.
[0098] When the signal has been pre-processed using any optional
pre-processing steps including band-passing the signal, the signal
may be analyzed to determine the spectral properties. The spectral
properties may be determined in the frequency domain through
well-established methods for calculating the power spectral
density. For example, the power spectral density for various bands
of frequencies may be calculated to give the power carried by the
signal expressed in units of power per frequency. Further, the
power spectral density calculations may provide relative amplitudes
of each of the frequencies identified. As used herein, the
"spectral properties" may include amplitude and frequency
characteristics of a signal. Calculation of the spectral properties
may be implemented in hardware and/or software. Calculation methods
for determining the power spectral density are known and may be
carried out using processing implemented on a digital computer
comprising a processor, as described in more detail below. In an
embodiment, one or more of the spectral properties may be
determined using a lock-in amplifier and/or a spectrum
analyzer.
[0099] Further processing of the obtained power spectral density
may be done, including de-trending of the power spectral density
and integration of the power spectral density. A correlation
analysis of the detected electromagnetic field may then be
performed in the time domain, the frequency domain, or both. For
example, a Fourier Transform or Fast Fourier Transform (FFT) of the
power spectral density, perhaps after processing steps such as
de-trending and integration, can be performed to yield correlations
between the source electromagnetic field and secondary
electromagnetic fields generated by seismic signals through the
seismoelectric effect near the earth's surface, which in turn are
generated by electroseismic effects at the formations of interest.
The properties of the analysis may be tailored based on the quality
and amount of data obtained, the type of signals present and
interacting with a formation of interest, and a desired processing
speed and cost.
[0100] The existence of hydrocarbons in a formation may be
indicated by the existence of strong correlations between the
source electromagnetic signal and the secondary electromagnetic
field generated by seismic signals through the seismoelectric
effect near the earth's surface, which in turn are generated by
electroseismic effects at the formations of interest, at
correlation times that correspond to known seismic transit times
between hydrocarbon formations and the surface of the earth. These
seismic transit times can be obtained explicitly from seismic data
obtained in the area of interest or can be estimated based on rock
acoustic properties.
[0101] The spectral properties may be correlated with the presence
of various fluids in subterranean pore spaces based on a variety of
classification methodologies. For example, statistical regression
analysis, and statistical classifiers such as neural networks,
decision trees, Bayes-based classifiers, fuzzy logic-based
classifiers, and conventional statistical classifiers may all be
used to determine a frequency-depth relationship. For example, the
analysis may be performed with the system and methods described
herein at locations with known properties and formation
characteristics to train and/or determine the correlation
parameters. Once the parameters have been determined (e.g., once a
neural net has been trained), the system and methods may be
repeated in a new location.
[0102] The correlations may also be used to develop a
frequency-depth function. A frequency-depth function may be
established using known locations, parameters, and/or calculations
to allow the depth values for similar locations to be determined
once the spectral characteristics of the signals are analyzed and
determined. The frequency-depth relationship for electromagnetic
signals may depend on the Earth's resistivity, formation
properties, types of components present, and, more generally, the
electrical properties for a particular area and corrections may
need to be applied to the spectral properties from location to
location. In some instances, the frequency-depth correlation
variation from area to area may not be so great that for many
purposes an approximate or a more or less typical frequency-depth
function may be used. For example, a frequency-depth function may
be derived from empirical data taking into consideration various
locations that have been reported in the literature. This function
may be considered more or less representative of the "typical"
relationship between frequency and depth. Other suitable sources of
data may be considered including, for example, an approximate
frequency-depth function derived from conventional skin effect
conductivity analyses. The resulting typical frequency-depth
function may be used to derive depth information for the
corresponding detected electromagnetic signal of interest based on
the analysis described herein. In an embodiment, the
frequency-depth relationship is determined based on a
classification method comprising at least one method selected from
the group consisting of: a neural network, a decision tree, a
Bayes-based classifier, a fuzzy logic-based classifier, and a
conventional statistical classifier.
[0103] In still another embodiment, the various properties of the
subterranean formation including the frequency-depth function may
be determined by developing a geological model of the subterranean
formation. Various modeling programs may be used to develop the
model of the subterranean formation and can provide predicted
outputs based on the model. The predicted outputs can then be
compared with the detected signals (e.g., a detected
electromagnetic signal) to determine if the model is accurate. When
a discrepancy is detected, the geological model can be altered and
the process repeated. Such a process may result in a match between
the geological model and the detected signal, thereby providing one
or more properties of the subterranean formation.
[0104] The processing described herein may be used to determine the
existence of a fluid in a subterranean formation as well as other
properties of the subterranean formation. In an embodiment, the
processing of the electromagnetic signal may provide an indication
of at least one property of a subterranean formation including, but
not limited to, an existence of the subsurface earth formation
containing at least one fluid, a depth of the subsurface earth
formation, a porosity, a fluid permeability, a composition of at
least one fluid within the subsurface earth formation, a spatial
extent of the subsurface earth formation, an orientation of the
boundaries of the subsurface earth formation, a resistivity, and
any combination thereof. Further, the processing may detect one or
more fluids within the subterranean formation, where detectable
fluids may include, but are not limited to, an aqueous fluid, a
hydrocarbon, a petroleum, carbon dioxide, and any combination
thereof. Other fluids know to be present in subsurface earth
formations can include hydrogen sulfide, sulfur dioxide, helium,
and the like.
[0105] While the measurement of a signal at a single location
followed by an analysis of that signal has been described herein,
the detection and analysis steps may be repeated any number of
times. For example, multiple measurements may be made at a single
location over several time periods. The results may be
statistically analyzed to provide an improved accuracy correlation
and/or survey. In addition, one or more samples may be taken at
varying locations sequentially in time or concurrently in time
using one or multiple sensors. For example, multiple measurements
may be made at varying locations around a site of interest. Various
grid patterns and/or random sample locations may be chosen to
generate a plurality of measurements across an area. For example,
the grid and/or array of detectors described above may be used to
generate a plurality of detected signals for use with the
processing techniques described herein. The multiple measurements
may be performed sequentially or concurrently (e.g., using multiple
sensors) at a single location, and/or the measurements may be
performed sequentially and/or concurrently in the various locations
around a site of interest when a plurality of locations are used to
measure the signal of interest. The resulting hydrocarbon
indications and resulting depth measurements may be used to
generate a two dimensional, a three dimensional, and/or a four
dimensional model (e.g., where the fourth dimension is time) of a
subterranean formation and the one or more fluids contained
therein.
Passive Seismic Surveying and Processing
[0106] In an embodiment, passive surveying may be carried out by
detecting a seismic signal alone, and processing the detected
seismic signal to determine if one or more fluids are present in a
subterranean formation of interest. In this embodiment, a seismic
signal may be detected using any of the seismic sensor
configurations described above, including the use of a plurality of
seismic sensors configured in an array. The seismic signal may
comprise any of the seismic signals described above including a
nonlinear seismic response.
[0107] Once the seismic signal has been detected by one or more
seismic sensors and a detected seismic signal has been generated,
the signal may be analyzed to extract information concerning the
existence of a formation of interest. The analysis of the seismic
signal may be similar to the analysis of the detected
electromagnetic signal described above. In the broadest sense, the
signal may be optionally pre-processed using one or more filters,
followed by analyzing the detected seismic signal to determine if a
seismic signal resulting from an electroseismic conversion is
present. The existence of an a seismic signal resulting from an
electroseismic conversion may be taken as a direct indication that
a coupling has occurred due to the interaction of an
electromagnetic source with a fluid in a subterranean formation
below the sensor. The frequency characteristics may be used to
derive depth and location information about the source and strength
of the detected seismic signal, thereby revealing information about
the location of a subterranean formation. A variety of models may
be used to correlate the spectral analysis results with the depth
of the modulating signal. The correlation between the frequency of
the filtered signal obtained from the raw seismic signal and the
depth of the formation of interest may be determined in a number of
ways, as described in more detail below.
[0108] Having generally described the various types of signal
analysis that can be performed on the detected seismic signal, each
signal processing method will now be described in greater detail.
As discussed above, the detected seismic signal may be
pre-processed using one or more filters. The optional
pre-processing of the signal may include use of a noise reduction
detection scheme, separation of a direct current component of the
signal, data reduction, and/or noise filtering/frequency
band-passing.
[0109] As discussed above, the time period over which the seismic
signal is detected and recorded and/or the sampling frequency of
the seismic signal may result in a significant quantity of data to
analyze. Greater amounts of data generally require more time to
analyze and greater storage and computing costs. As a result, the
amount of data may be limited and/or reduced to allow for faster
processing, especially when real time processing is desired. While
a reduced amount of data may lead to a less accurate analysis,
processing of a relatively small portion of the data may provide a
faster, cheaper first look at the results of the survey.
[0110] The data may be limited in a number of ways. In an
embodiment, the seismic signal data may be decimated. Decimating
the data reduces the amount of data that is processed in the
analysis steps and, therefore, reduces processing time and costs.
Excessive decimation may reduce the reliability of the analysis to
a certain extent, and thus the savings in processing times and
costs must be weighed against reduced reliability. The signal data
may typically be decimated down to an effective sampling rate
approximating two times the highest frequency of interest while
allowing for an identification of the frequency characteristics in
the data. Higher decimation rates may be used, for example, when a
faster, and possibly less accurate first look at the data is
desired. Decimation of the data may be used alone or in addition to
any of the other pre-processing techniques described herein.
[0111] The signal data may be filtered to reduce the amount of
noise and thereby increase the SNR of the signal and/or the signal
may be band-pass filtered to isolate one or more frequency bands of
interest for use in the further analysis of the data. A noise
filter may comprise any type of noise filter known in the art. For
example, a high pass filter, a low pass filter, or any suitable
wide band frequency filter, and/or any suitable narrow band
frequency filter may be used to eliminate noise. The earth's
background electromagnetic field and/or local sources of seismic
noise may be used to determine the frequency range, amplitude
range, and/or other parameters of a desired noise filter for the
detected seismic signal.
[0112] In addition to, or in place of, the noise filter, a
band-pass filter may be used to isolate one or more frequency
ranges of interest for purpose of filtering the detected seismic
signal over a desired frequency range. A set of predetermined
frequencies may be used as the basis for filtering the detected
signal for further processing. The frequency filters may improve
the SNR of the signal. A variety of band-pass filters may be used
to produce one or more frequency filtered signals useful for
further processing. For example, band-pass filters implemented in
hardware are known and may be suitable. Alternatively, band-pass
filters may also be implemented in software, for example, using a
digital processing environment on a computer encoded for processing
of the data. Any of a number of known rational polynomial functions
may be used alone or in combination with other functions to
separate the data for a particular frequency and/or frequency range
from the data set as a whole, and in general those functions and
processes may be used as frequency filters. For example, linear
phase filters, finite impulse response filters, forward infinite
impulse response filters, reverse infinite impulse response
filters, and the like may be used. When a plurality of band-pass
filters are used to generate a plurality of signals, each band pass
filter may have the same or may have different bandwidths.
[0113] In some embodiments, a plurality of sensors that may be
disposed in an array may be used to detect the seismic signal. In
this embodiment, various optional pre-processing steps may use data
from the plurality of sensors to improve the signal-to-noise ratio
and/or filter the resulting detected signal. In an embodiment,
local sources of noise may be present in the vicinity of the
seismic sensors, and the local sources of noise may be rejected in
the detected seismic signal using several methods. First, the
detected seismic signals from one or more of the plurality of
seismic sensors may be filtered in the spatial domain to reject
surface waves traveling across the plurality of seismic sensors.
The filter may use at least a portion of the horizontal component
of the detected seismic signal across the plurality of seismic
sensors to remove at least a portion of a noise signal from the
detected seismic signal at each seismic sensor. In an embodiment,
only a portion of the seismic sensors may be configured to measure
a horizontal component of the seismic wave, and these seismic
sensors may be used to generate the horizontal components used in
the spatial filter. In an embodiment, each of the plurality of
seismic sensors may be configured to measure a horizontal component
of the seismic wave, and each of the plurality of seismic sensors
may be used with the filter in the spatial domain to reject surface
waves traveling across the plurality of seismic sensors.
[0114] In an embodiment, a horizontal component of the detected
seismic signal may be used as a predictive filter to remove local
noise from the vertical component of each seismic sensor. The
filter may utilize one horizontal component or two horizontal
components as detected by the seismic sensor. This predictive
filter may be applied to each seismic sensor of the plurality of
seismic sensors that is configured to measure a horizontal
component of the seismic wave.
[0115] In an embodiment, local noise waves may propagate across the
plurality of seismic sensors in expected spreading patterns, which
may be analogous to water waves on a pond. The propagating noise
waves may be suppressed by determining the direction of travel and
speed, and applying a spatial filter that makes use of the
spreading symmetry of the noise wave. The spatial filter may remove
the local noise from the detected seismic signals at each sensor
being affected by the noise. In an embodiment, a predictive filter
may be employed to predict the arrival and amplitude of the local
noise wave at a seismic sensor and remove the local noise wave
during the generation of the detected seismic signal. In an
embodiment, only a portion of the seismic sensors may be configured
to measure a horizontal component of the seismic wave, and these
seismic sensors may be used to determine the spreading geometry of
the local noise wave. The spatial filter may then be applied to
each of the plurality of seismic sensors, including those that are
not configured to measure a horizontal component of the seismic
wave. In an embodiment, each of the plurality of seismic sensors
may be configured to measure a horizontal component of the seismic
wave, and each of the plurality of seismic sensors may be used to
predict and verify the spreading geometry of the local noise wave
used with the spatial filter. In an embodiment, one or more
additional seismic sensors at a distance away from the plurality
seismic sensors may be used to measure a seismic wave, which may
include the local noise wave. The ability to measure the local
noise wave at a distance from the plurality of seismic sensors may
provide better prediction of the local noise wave and an
improvement of the reduction of the local noise wave in the
detected seismic signal.
[0116] In an embodiment, the detected seismic signals from the
plurality of seismic sensors may be cross-correlated, and the
cross-correlated data from all of the seismic sensors may be
summed. The resulting summed data may be used as a predictive
filter to enhance spatial continuity across the plurality of
seismic sensors. The summing of the data may result in an increase
in the amplitude of the seismic waves arriving at the same time,
for example from a plane wave. Any seismic components resulting
from a noise source traveling across the plurality of seismic
sensors and/or any components that are not traveling as a plane
wave will tend not to add. As a result, the summed data may be used
to identify the seismic waves resulting from an electroseismic
conversion and identify the other seismic components as local noise
sources.
[0117] In an embodiment, the assumption that the seismic wave
resulting from the electroseismic conversion can be represented as
a plane wave may be used to remove at least a portion of a noise
signal from the detected seismic signal from one or more of the
seismic sensors. In an embodiment, a dip filter can be used to
reject detected seismic signals arriving at a non-normal angle to
the plurality of seismic sensors. In an embodiment, the dip filter
may be applied after cross-correlating the detected seismic signals
from two or more of the seismic sensors.
[0118] In an embodiment, the detected seismic signal amplitude may
be averaged over one or more frequencies and/or frequency ranges.
In an embodiment, the averaging of the detected seismic signal
amplitude may comprise averaging the signal amplitude at one or
more fixed frequencies. The frequencies may generally be selected
from those frequencies present in the detected seismic signal. In
an embodiment, the one or more frequencies at which the detected
signal can be averaged may be selected from a range of frequencies
between about 0.01 Hz and about 1,000 Hz, alternatively between
about 1 Hz and about 500 Hz. The averaging process may comprise
measuring the signal amplitude for a length of time that is longer
(e.g., greater than at least twice as long) than the period of
oscillation of the signal at the selected frequency, and then
averaging the signal amplitude over the detection time period. The
averaging may reduce the momentary fluctuations in the detected
seismic field resulting from noise fluctuations that are not
related to signals originating in the earth. Processing may further
consist of averaging the signal amplitude at many frequencies over
a range of frequencies. For example, the selected frequencies might
be chosen to be separated by 1 Hz between 1 Hz and 500 Hz. Those
skilled in the art of seismology will recognize that the
propagation of seismic waves from a subsurface structure to the
earth's surface results in certain characteristic times associated
with the time required for a seismic wave that originates at a
subterranean formation to reach the Earth's surface where it is
detected. Processing passive seismoelectric data by the averaging
process may include identifying the characteristic times of seismic
propagation from the subterranean formation. These characteristic
times may then be processed by methods known to those skilled in
the art to determine the depth of subterranean structures.
[0119] When the seismic signal has been pre-processed using any
optional pre-processing steps including band-passing the signal,
the seismic signal may be analyzed to determine the spectral
properties. The spectral properties may be determined in the
frequency domain through well-established methods for calculating
the power spectral density. For example, the power spectral density
for various bands of frequencies may be calculated to give the
power carried by the signal expressed in units of power per
frequency. Further, the power spectral density calculations may
provide relative amplitudes of each of the frequencies identified.
As used herein, the "spectral properties" may include amplitude and
frequency characteristics of a seismic signal. Calculation of the
spectral properties may be implemented in hardware and/or software.
Calculation methods for determining the power spectral density are
known and may be carried out using processing implemented on a
digital computer comprising a processor, as described in more
detail below. In an embodiment, one or more of the spectral
properties may be determined using a lock-in amplifier and/or a
spectrum analyzer.
[0120] Further processing of the obtained power spectral density of
the seismic signal may be done, including de-trending of the power
spectral density and integration of the power spectral density.
Finally, a Fourier Transform or Fast Fourier Transform (FFT) of the
power spectral density, perhaps after processing steps such as
de-trending and integration, is performed to yield correlations
between the seismic signal generated by seismoelectric effects near
the surface of the earth and the seismic signals generated by
electroseismic effects at the formations of interest. The
properties of the analysis may be tailored based on the quality and
amount of data obtained, the type of signals present and
interacting with a formation of interest, and a desired processing
speed and cost.
[0121] The existence of hydrocarbons in a formation may be
indicated by the existence of strong correlations between the
seismic signal generated by seismoelectric effects near the surface
of the earth and the seismic signals generated by electroseismic
effects at the formations of interest, at correlation times that
correspond to known seismic transit times between hydrocarbon
formations and the surface of the earth. These seismic transit
times can be obtained explicitly from seismic data obtained in the
area of interest or can be estimated based on rock acoustic
properties.
[0122] The spectral properties may be correlated with the presence
of various fluids in subterranean pore spaces based on a variety of
classification methodologies. For example, statistical regression
analysis, and statistical classifiers such as neural networks,
decision trees, Bayes-based classifiers, fuzzy logic-based
classifiers, and conventional statistical classifiers may all be
used to determine a frequency-depth relationship. For example, the
analysis may be performed with the system and methods described
herein at locations with known properties and formation
characteristics to train and/or determine the correlation
parameters. Once the parameters have been determined (e.g., once a
neural net has been trained), the system and methods may be
repeated in a new location.
[0123] The spectral properties may also be used to develop a
frequency-depth function. A frequency-depth function may be
established using known locations, parameters, and/or calculations
to allow the depth values for similar locations to be determined
once the spectral characteristics of the signals are analyzed and
determined. The frequency-depth relationship for seismic signals
may depend on the Earth's resistivity, formation properties, types
of components present, and, more generally, the electrical
properties for a particular area and corrections may need to be
applied to the spectral properties from location to location. In
some instances, the frequency-depth correlation variation from area
to area may not be so great that for many purposes an approximate
or a more or less typical frequency-depth function may be used. For
example, a frequency-depth function may be derived from empirical
data taking into consideration various locations that have been
reported in the literature. This function may be considered more or
less representative of the "typical" relationship between frequency
and depth. Other suitable sources of data may be considered
including, for example, an approximate frequency-depth function
derived from conventional skin effect conductivity analyses. The
resulting typical frequency-depth function may be used to derive
depth information for the corresponding detected electromagnetic
signal of interest based on the analysis described herein. In an
embodiment, the frequency-depth relationship is determined based on
a classification method comprising at least one method selected
from the group consisting of: a neural network, a decision tree, a
Bayes-based classifier, a fuzzy logic-based classifier, and a
conventional statistical classifier.
[0124] In still another embodiment, the various properties of the
subterranean formation including the frequency-depth function may
be determined by developing a geological model of the subterranean
formation. Various modeling programs may be used to develop the
model of the subterranean formation and can provide predicted
outputs based on the model. The predicted outputs can then be
compared with the detected signals (e.g., a detected seismic
signal) to determine if the model is accurate. When a discrepancy
is detected, the geological model can be altered and the process
repeated. Such a process may result in a match between the
geological model and the detected signal, thereby providing one or
more properties of the subterranean formation.
[0125] The processing described herein may be used to determine the
existence of a fluid in a subterranean formation as well as other
properties of the subterranean formation. In an embodiment, the
processing of the seismic signal may provide an indication of at
least one property of a subterranean formation including, but not
limited to, an existence of the subsurface earth formation
containing at least one fluid, a depth of the subsurface earth
formation, a porosity, a fluid permeability, a composition of at
least one fluid within the subsurface earth formation, a spatial
extent of the subsurface earth formation, an orientation of the
boundaries of the subsurface earth formation, a resistivity, and
any combination thereof. Further, the processing may detect one or
more fluids within the subterranean formation, where detectable
fluids may include, but are not limited to, an aqueous fluid, a
hydrocarbon, a petroleum, and any combination thereof.
[0126] While the measurement of a signal at a single location
followed by an analysis of that signal has been described herein,
the detection and analysis steps may be repeated any number of
times. For example, multiple measurements may be made at a single
location over several time periods. The results may be
statistically analyzed to provide an improved accuracy correlation
and/or survey. In addition, one or more samples may be taken at
varying locations sequentially in time or concurrently in time
using one or multiple sensors. For example, multiple measurements
may be made at varying locations around a site of interest. Various
grid patterns and/or random sample locations may be chosen to
generate a plurality of measurements across an area. For example,
the grid and/or array of detectors described above may be used to
generate a plurality of detected signals for use with the
processing techniques described herein. The multiple measurements
may be performed sequentially or concurrently (e.g., using multiple
sensors) at a single location, and/or the measurements may be
performed sequentially and/or concurrently in the various locations
around a site of interest when a plurality of locations are used to
measure the signal of interest. The resulting hydrocarbon
indications and resulting depth measurements may be used to
generate a two dimensional, a three dimensional, and/or a four
dimensional model (e.g., where the fourth dimension is time) of a
subterranean formation and the one or more fluids contained
therein.
Passive, Sequential Surveying and Processing
[0127] In an embodiment, passive surveying may be carried out by
sequentially detecting and/or separately processing an
electromagnetic signal and a seismic signal. For example, the
detection of both an electromagnetic signal and a seismic signal
may occur at different times and/or locations. The two signals may
be cross-correlated to some degree to determine if one or more
fluids are present in a subterranean formation of interest. In this
embodiment, the seismic signal may be detected using any of the
seismic sensor configurations described above, including the use of
a plurality of seismic sensors configured in an array. Similarly,
the electromagnetic signal may be detected using any of the
electromagnetic sensor configurations described above, including
the use of a plurality of electromagnetic sensors configured in an
array. The electromagnetic signal may comprise a modulated
electromagnetic signal and/or the electromagnetic signal directly
originating from the electroseismic and seismoelectric conversions.
The electromagnetic signal and/or the seismic signal may comprise
nonlinear components and/or responses.
[0128] The cross-correlation of the electromagnetic signal and the
seismic signal, which may also be referred to in some contexts as
joint processing, may be used to identify the features that are
common in both detected data sets. Electroseismic and
seismoelectric signals originate in the same physical conversion
mechanism at boundaries between dissimilar rocks or at boundaries
between different fluids in rock pore spaces, as described in more
detail above. However, the seismic detectors and the
electromagnetic field detectors may not be equally sensitive to
rapid signal changes or to small signal amplitude differences.
Thus, the processed electromagnetic signal and the seismic signal
may be similar but likely will not be identical. The use of a
cross-correlation may be used to enhance and/or isolate the common
information in both data sets. Several methods are known to those
skilled in the art for comparing the information in the two
measurements with the goal of learning more about subterranean
features of interest.
[0129] In an embodiment, the cross-correlation of the
electromagnetic signal and the seismic signal may be carried out at
a variety of points in the analysis of each signal as described
above with respect to the processing of the electromagnetic signal
alone and the seismic signal alone. In general, the
cross-correlation of the electromagnetic signal and the seismic
signal may be carried out on a data set using the same or similar
processing for each data set to allow for the comparison of similar
data sets. For example, both the electromagnetic signal and the
seismic signal may be optionally pre-processed using one or more
similar filters and then cross-correlated. In other embodiments,
the electromagnetic signal and the seismic signal may be
cross-correlated following the demodulation of the signal or after
any additional processing is performed.
[0130] The sequential detection and/or separate processing an
electromagnetic signal and a seismic signal described herein may be
used to determine the existence of a fluid in a subterranean
formation as well as other properties of the subterranean
formation. In an embodiment, the processing of the electromagnetic
signal and the seismic signal may provide an indication of at least
one property of a subterranean formation including, but not limited
to, an existence of the subsurface earth formation containing at
least one fluid, a depth of the subsurface earth formation, a
porosity, a fluid permeability, a composition of at least one fluid
within the subsurface earth formation, a spatial extent of the
subsurface earth formation, an orientation of the boundaries of the
subsurface earth formation, a resistivity, and any combination
thereof. Further, the sequential detection and/or separate
processing may detect one or more fluids within the subterranean
formation, where detectable fluids may include, but are not limited
to, an aqueous fluid, a hydrocarbon, a petroleum, and any
combination thereof.
Passive, Simultaneous Surveying and Processing
[0131] In an embodiment, passive surveying may be carried out by
detecting both an electromagnetic signal and a seismic signal at
overlapping time intervals to allow for cross-correlation of the
two detected signals. The resulting signals may then be processed
to determine if one or more fluids are present in a subterranean
formation of interest. In this embodiment, an electromagnetic and
seismic signal may be detected using any of the sensor
configurations described above, including the use of a plurality of
sensors configured in an array.
[0132] In an embodiment, processing of the detected signals may
comprise cross-correlating the detected electromagnetic field with
the detected seismic signal and isolating at least a portion of the
detected seismic signal arising from the electroseismic conversion.
The ability to isolate the detected seismic signal arising from the
electroseismic conversion may depend on the various components
(e.g., desired signal versus noise components) of the detected
electromagnetic field and/or the detected seismic signal. For
example, the amplitude of the earth's electromagnetic field in the
detected electromagnetic field and the seismic waves resulting from
the electroseismic conversion within the subsurface earth formation
in the detected seismic signal may be smaller than the various
noise levels at the electromagnetic field detectors and/or seismic
sensors. Various processing techniques may be used when an increase
in the signal-to-noise ratio and/or removal of at least a portion
of the noise in the detected electromagnetic field and the detected
seismic signals is needed. In an embodiment, at least one
processing technique may be used to eliminate at least a portion of
a coherent noise and/or a random noise in the detected seismic
signal and/or the detected electromagnetic field. Each of these
processing techniques will be described in more detail below.
[0133] Coherent noise refers to cyclic electromagnetic and/or
seismic signals that have an approximately constant frequency over
a predetermined measurement period. Many coherent, electromagnetic
noise sources can be found in a typical measurement setting and can
be accounted for through various processing techniques. For
example, the power-line frequency of 60 Hertz (Hz) can generate a
high amplitude electromagnetic signal that can propagate into the
earth, where the resulting amplitude at the one or more
electromagnetic field detectors may be hundreds or thousands of
times larger than the desired background electromagnetic field
within the earth. Similarly, unbalanced power-lines can generate
180 Hz noise and motors can generate 400 Hz noise. As a further
example, cathodic protection circuits can produce poorly-rectified
alternating current (AC) signals at several frequencies that result
in electromagnetic noise at the one or more electromagnetic field
detectors. In order to improve the signal-to-noise ratio, at least
a portion of the noise may be removed from the detected
electromagnetic field.
[0134] In an embodiment, at least a portion of coherent noise may
be removed from the detected electromagnetic field through the use
of data intervals having selected start times and durations. The
intervals may be non-overlapping and the start times may be chosen
so that a portion of the detected electromagnetic field is not
taken into consideration in a data interval. The data points within
the data intervals may then be summed to remove at least a portion
of the coherent noise and/or improve the signal-to-noise ratio of
the detected electromagnetic field through cancellation of the
overlapping coherent noise signal. In order to facilitate the
selection of the intervals, the detected electromagnetic field and
the detected seismic signal may be recorded and the intervals
selected from the recorded data. In an embodiment, the intervals
may be processed in real time, using for example a transitory
storage medium, to temporarily store the data in the intervals
during removal of the coherent noise. In this embodiment, only the
final signal having an improved signal-to-noise ratio and/or a
reduced noise content may be recorded.
[0135] In an embodiment, a start time may be chosen to correspond
to the zero crossing voltage of a coherent noise source. For
example, the start time for a first interval may be selected to
occur where the 60 Hz voltage crosses zero volts. After collecting
data for a given duration, the start time for the next interval may
be chosen as the zero crossing voltage plus some additional time
period. In an embodiment, the additional time period may correspond
to an irrational number or a whole number fraction of an irrational
number. For example, the irrational number may be it, though other
irrational numbers may also be used. For example, the start time
for the second interval in the example above may be chosen to
correspond to the 60 Hz zero crossing voltage plus a small time
that is not an even fraction of the 60 Hz period, which is 16.666
milliseconds (ms) for the 60 Hz signal. If the second and
subsequent intervals have a predetermined start time corresponding
to the 60 Hz zero crossing plus a cumulative time of .pi./10,000
(i.e., 0.31416 microseconds (.mu.s)) for each additional interval,
then the collection of the data during the data interval will never
repeat the 60 Hz phase of the first interval. In other words, each
start time would begin 0.31416 .mu.s beyond the start time of the
preceding interval relative to the time corresponding to the zero
crossing voltage. By adding a plurality of the resulting data
intervals, the positive amplitudes (e.g., positive voltage
portions) of the coherent signals will cancel with the negative
amplitudes (e.g., negative voltage portions) of the coherent
signals such that any coherent noise sources will not add up in the
data collection process. By this technique, at least a portion of
the coherent noise may be removed from the detected electromagnetic
field.
[0136] In general, coherent noise sources may not have exactly
constant frequency over a predetermined measurement period. These
imperfections may be due to phase changes in the coherent noise
sources. For example, electromagnetic noise generated by power
lines can experience some variations in the power-line voltage. The
method described herein may comprise monitoring the phase of the
coherent noise source to adjust the start times to correspond to
the phase of the coherent noise for each interval. The coherent
noise source may also experience amplitude variations over time,
which may result in a partial cancellation of the coherent noise
upon the summing of the intervals. In an embodiment, a frequency
filter (e.g., a frequency notch filter) may be applied to the
detected electromagnetic field to further enhance the
signal-to-noise ratio and/or reduce a portion of the coherent noise
in the background electromagnetic field.
[0137] As a second coherent noise cancellation method, the start
time may be chosen to correspond to events (e.g., large amplitude
spikes) in the in the earth's electric and/or magnetic fields. If
these events occur at random intervals in time, then effective
coherent noise cancellation may be achieved through summing of the
resulting data intervals. The number of events may vary from
location to location and from time to time. As a result, this
method may not yield as many data collection intervals per day as
frequency based method, which may result in a slower data interval
collection.
[0138] Slower data collection resulting from choosing the
predetermined start time to correspond to an event in the earth's
electric and/or magnetic fields may be balanced by concentrating on
the largest peak events in the earth's electric and/or magnetic
fields. Events comprising low amplitude electric and/or magnetic
fields may result in low-amplitude seismic waves, which may be
difficult to detect in the presence of noise. In order to select
larger events and suppress the low amplitude peaks in the earth's
electric and/or magnetic fields, the detected electromagnetic field
and the detected seismic signal data may be squared while retaining
the sign. Cross-correlation of the squared detected electromagnetic
field and detected seismic signal data may then result in the
selection of the largest events while suppressing the
small-amplitude noise.
[0139] The duration of the data intervals may comprise any period
of time sufficient to include at least one coherent noise cycle.
The frequency of the coherent noise, and thus the period of time of
a coherent noise cycle, may be estimated based on localized sources
of electromagnetic noise or measured using an electromagnetic field
detector. In an embodiment, the predetermined duration comprises a
time of at least a plurality of coherent noise cycles. In an
embodiment, the duration comprises a period of time of between
about 10 seconds and about 100 minutes, alternatively between about
30 seconds and about 10 minutes.
[0140] In an embodiment, each interval in the plurality of
intervals does not overlap with any other interval and each
interval may have a period of time between successive intervals in
which the data are not considered. The period of time between
intervals may be the same between each interval or may vary between
the intervals. In an embodiment, the period of time between the
intervals may comprise an uneven fraction of a coherent noise cycle
or an uneven fraction of a coherent noise cycle plus an amount of
time corresponding to a whole number of coherent noise cycles. In
an embodiment, the period of time between intervals may comprise a
fraction of .pi.. For example, each interval may have a period of
time between intervals of a time of .pi./10,000 (i.e., 0.31416
.mu.s), as described in the example above. As an alternative
example, one or more full cycles of the coherent noise may be
allowed to pass before beginning the next interval at a start time
according to one of the methods disclosed herein. This may allow
for system processing and/or recording latencies to be taken into
account between data intervals.
[0141] The techniques used to remove at least a portion of coherent
noise from the detected electromagnetic field may also be applied
to the detected seismic signal. Various sources of coherent seismic
noise may be present in a typical measurement setting, including
for example, motor noise, industrial equipment, etc. It should be
noted that the start time and duration for each corresponding
interval of both the detected electromagnetic field and the
detected seismic signal are the same so that the signals can be
cross-correlated. In an embodiment, the start time and duration may
be chosen to allow cancellation of at least a portion of the
coherent noise in both the detected electromagnetic field and the
detected seismic signal.
[0142] In an embodiment, a method of removing at least a portion of
a coherent noise signal may use the techniques described above. In
an embodiment, a method comprises generating a detected
electromagnetic field by detecting the earth's electromagnetic
field within the earth, and generating a detected seismic signal by
detecting a seismic wave related to the earth's electromagnetic
field. The detected electromagnetic field and the detected seismic
signal may optionally be recorded, though processing of the
detected signals without first being recorded may also be used. A
first signal may be generated by adding a plurality of intervals of
the detected electromagnetic field, where each of the plurality of
intervals of the detected electromagnetic field begins at a start
time and continues for a duration. A second signal may be generated
by adding a plurality of intervals of the detected seismic signal
corresponding to the plurality of intervals of the detected
electromagnetic field, wherein each of the plurality of intervals
of the detected seismic signal begins at the start time and
continues for the duration. Processing of the first signal and the
second signal may then be performed to determine at least one
property of the subsurface earth formation. The number of the
plurality of intervals may be chosen to produce a predetermined
signal-to-noise ratio and/or remove a predetermined amount of a
coherent noise in one or more of the detected signals at one or
more of the sensors.
[0143] Additional processing techniques may be used to eliminate at
least a portion of a coherent noise and/or a random noise in the
detected electromagnetic field and/or the detected seismic signal.
As discussed above, the electroseismic conversion between
electromagnetic and seismic energy may occur when ions in pore
fluids are displaced relative to grain interfaces within the pores
of a rock in a subsurface earth formation. The displaced ions may
impose a force on the fluid in the pore space and the pore
surfaces. The moving fluids and displaced ions may track each other
and may be expected to have a corresponding, though not necessarily
identical, time dependence. In other words, the electroseismic
conversion may generate a seismic response to a time-dependent
electromagnetic field with a corresponding time dependence.
Accordingly, the resulting seismic wave may have the same
time-dependence as the earth's background electromagnetic field,
only delayed by the seismic travel time. The electromagnetic signal
travel time may be neglected since the electromagnetic propagation
time down to the reservoir is much shorter than the seismic travel
time to the surface. This result may be used to remove at least a
portion of a noise signal that does not possess the expected time
dependence between the detected electromagnetic field and the
detected seismic signal.
[0144] In addition to the expected time dependence, the seismic
wave characteristics may be used to remove at least a portion of a
noise signal in the detected seismic signal. The detected seismic
signal resulting from the electroseismic conversion of the earth's
background electromagnetic field at a horizontal reservoir
interface can be assumed to be a vertically-traveling plane wave,
as described in more detail above. Even inhomogeneous reservoir
interfaces that on average are horizontal can produce vertical
plane waves. The vertical plane wave may arrive at all of the
seismic sensors at the same time, and accordingly, may not have any
horizontal components. This result may be used to remove at least a
portion of a noise signal by accounting for detected seismic signal
components that are not common to all of the seismic sensors and/or
that do not represent a plane wave propagating from a subsurface
earth formation.
[0145] In order to determine at least one property of the
subsurface earth formation, the detected electromagnetic field and
the detected seismic signal may be processed, for example, using a
cross-correlation of the detected signals. Cross-correlation of the
background electromagnetic field with the detected seismic signal
from one or more seismic sensors may produce a peak in amplitude
corresponding to the seismic travel time. When a plurality of
seismic sensors are used to generate one or more detected seismic
signals, the result of the cross-correlation between the background
electromagnetic field and the detected seismic signal should be the
same for each seismic sensor. Local sources of noise (e.g.,
non-uniform noise sources) may corrupt this expectation, and the
differences may be used to remove at least a portion of the noise
signal. For example, when considering a pseudo-random noise source
with a non-repeating character, the cross-correlation of the
detected electromagnetic field and the detected seismic signal may
be convoluted with the autocorrelation of the detected
electromagnetic field.
[0146] In an embodiment, local sources of noise may be rejected
using several methods. First, the detected seismic signals from
each of the seismic sensors may be filtered in the spatial domain
to reject surface waves traveling across the plurality of seismic
sensors. The filter may use at least a portion of the horizontal
component of the detected seismic signal across the plurality of
seismic sensors to remove at least a portion of a noise signal from
the detected seismic signal at each seismic sensor. In an
embodiment, only a portion of the seismic sensors may be configured
to measure a horizontal component of the seismic wave, and these
seismic sensors may be used to generate the horizontal components
used in the spatial filter. In an embodiment, each of the plurality
of seismic sensors may be configured to measure a horizontal
component of the seismic wave, and each of the plurality of seismic
sensors may be used with the filter in the spatial domain to reject
surface waves traveling across the plurality of seismic
sensors.
[0147] Second, a horizontal component of the detected seismic
signal may be used as a predictive filter to remove local noise
from the vertical component of each seismic sensor. The filter may
utilize one horizontal component or two horizontal components as
detected by the seismic sensor. This predictive filter may be
applied to each seismic sensor of the plurality of seismic sensors
that is configured to measure a horizontal component of the seismic
wave.
[0148] This noise cancellation method may also be used with the
detected electromagnetic signal. In an embodiment, a horizontal
component of the detected electromagnetic signal may be used with a
predictive filter to remove noise from the vertical component of
the detected electromagnetic field. The predictive filter may
utilize one horizontal component or two horizontal components as
detected by the one or more electromagnetic field detectors. This
predictive filter may be applied to each electromagnetic field
detector of the plurality of electromagnetic field detectors.
[0149] Third, local noise waves may propagate across the plurality
of seismic sensors in expected spreading patterns, which may be
analogous to water waves on a pond. The propagating noise waves may
be suppressed by determining the direction of travel and speed, and
applying a spatial filter that makes use of the spreading symmetry
of the noise wave. The spatial filter may remove the local noise
from the detected seismic signals at each sensor having affected by
the noise. In an embodiment, a predictive filter may be employed to
predict the arrival and amplitude of the local noise wave at a
seismic sensor and remove the local noise wave during the
generation of the detected seismic signal. In an embodiment, only a
portion of the seismic sensors may be configured to measure a
horizontal component of the seismic wave, and these seismic sensors
may be used to determine the spreading geometry of the local noise
wave. The spatial filter may then be applied to each of the
plurality of seismic sensors, including those that are not
configured to measure a horizontal component of the seismic wave.
In an embodiment, each of the plurality of seismic sensors may be
configured to measure a horizontal component of the seismic wave,
and each of the plurality of seismic sensors may be used to predict
and verify the spreading geometry of the local noise wave used with
the spatial filter. In an embodiment, one or more additional
seismic sensors at a distance away from the plurality seismic
sensors may be used to measure a seismic wave, which may include
the local noise wave. The ability to measure the local noise wave
at a distance from the plurality of seismic sensors may provide
better prediction of the local noise wave and an improvement of the
reduction of the local noise wave in the detected seismic
signal.
[0150] Fourth, the detected seismic signals may be
cross-correlated, and the cross-correlated data from all of the
seismic sensors may be summed. The resulting summed data may be
used as a predictive filter to enhance spatial continuity across
the plurality of seismic sensors. The summing of the data may
result in an increase in the amplitude of the seismic waves
arriving at the same time, for example from a plane wave. Any
seismic components resulting from a noise source traveling across
the plurality of seismic sensors and/or any components that are not
traveling as a plane wave will tend not to add. As a result, the
summed data may be used to identify the seismic waves resulting
from an electroseismic conversion and identify the other seismic
components as local noise sources.
[0151] Fifth, the assumption that the seismic wave resulting from
the electroseismic conversion can be represented as a plane wave
may be used to remove at least a portion of a noise signal from the
detected seismic signal from one or more of the seismic sensors. In
an embodiment, a dip filter can be used to reject detected seismic
signals arriving at a non-normal angle to the plurality of seismic
sensors. In an embodiment, the dip filter may be applied after
cross-correlating the detected seismic signals from two or more of
the seismic sensors.
[0152] In an embodiment, the application of one or more of the
methods disclosed here may result in the suppression of at least a
portion of a noise source in a detected field and/or signal. When
at least a portion of the noise source is removed, the
cross-correlation between two or more detected seismic signals may
more closely approximate an autocorrelation of each detected
seismic signal from each seismic sensor. Differences between the
cross-correlation and the autocorrelation may be due, at least in
part, to differences in the detected seismic signal amplitudes at
each seismic sensor due to ground inhomogeneities and/or seismic
scattering. The processing of the detected electromagnetic field
and the detected seismic signal with a reduction in at least one
noise signal may then be used to determine at least one property of
the subsurface earth formation.
[0153] In addition to the detected electromagnetic field and the
detected seismic signal, other detected signals may also be used to
determine at least one property of the subsurface earth formation.
For example, nonlinear electroseismic conversions may produce
signals useful during processing. In an embodiment, nonlinear,
harmonic signals having frequency components at higher frequency
harmonics of the source's fundamental frequency (i.e., those
frequencies present in the earth's background electromagnetic
field) may be detected as a result of distortions of the background
electromagnetic field interacting with the subsurface earth
formation containing at least one fluid. The harmonic signals may
be processed alone or in conjunction with the fundamental
frequencies of the detected seismic signal and/or the detected
electromagnetic field to determine one or more properties of the
subsurface earth formation. In an embodiment, the harmonic signals
may be present, detected, and/or isolated in both the detected
electromagnetic field and the detected seismic signal.
[0154] One or more harmonic signals may be detected and/or isolated
in the detected seismic signal using a variety of methods. In an
embodiment, the detected seismic signal may be cross-correlated
with the detected electromagnetic field. A frequency analysis of
the data resulting from the cross-correlation may be used to
identify frequencies in the detected seismic signal that are higher
than those present in the detected electromagnetic field. The
frequencies present in the detected electromagnetic field may then
be used to remove (e.g., using a filter) at least a portion of the
corresponding frequencies (e.g., the fundamental frequencies) from
the detected seismic signal and detect and/or isolate one or more
of the harmonic signals. In an embodiment, the harmonic signal may
comprise a coherent harmonic signal.
[0155] In an embodiment, the harmonic signals may be detected
and/or isolated in the detected seismic signal by partially
rectifying the detected seismic signal and/or the harmonic signals
detected and/or isolated from the detected seismic signal. The
harmonic signals may resemble a partially-rectified sine wave,
which may be asymmetrical about zero amplitude. In an embodiment,
the positive amplitudes may be larger than the negative amplitudes.
The resulting asymmetry may be utilized by arbitrarily reducing the
positive portions of the source waveform before cross-correlation.
In an embodiment, the negative amplitudes may be larger than the
positive amplitudes. The resulting asymmetry may be utilized by
arbitrarily reducing the negative portions of the source waveform
before cross-correlation. Signal measurement and processing may be
used to determine which portion of the amplitude (i.e., the
positive amplitude portion or the negative amplitude portion), if
either, is larger.
[0156] The detected seismic signal may be filtered prior to
cross-correlating the detected harmonic signals in the detected
seismic signal with the detected electromagnetic field and/or one
or more harmonic signals in the detected electromagnetic field,
which are described in more detail below. An autocorrelation of the
detected electromagnetic field may have lower frequency components
than the autocorrelation of the detected seismic signal from one or
more of the seismic sensors. In an embodiment, the detected seismic
signal may be band-pass filtered to remove frequencies below the
fundamental frequencies present in the detected electromagnetic
field, which may be used to identify the harmonic signals. The
filter may be applied before processing the detected seismic signal
and the detected electromagnetic field.
[0157] In an embodiment, the detected harmonic signals may be
processed with the detected electromagnetic field to determine at
least one property of the subsurface earth formation. In an
embodiment, the processing of the detected harmonic signals with
the detected electromagnetic field may comprise cross-correlating
the detected harmonic signals with the detected electromagnetic
field.
[0158] One or more nonlinear signals may be detected and/or
isolated in the detected electromagnetic signal using a variety of
methods. The nonlinear signals in the detected electromagnetic
field, which may include harmonic signals, may result from the
conversion of the electromagnetic energy in the earth's background
electromagnetic field to seismic energy, as described in more
detail above. This point of conversion may also result in a
frequency shift or time delay in the electromagnetic energy in the
earth's background electromagnetic field, generating nonlinear
signals. At least a portion of the resulting nonlinear signals may
be detected by the electromagnetic field detectors and used to
determine at least one property of the subsurface earth
formation.
[0159] Without intending to be limited by theory, it is believed
that the point of electroseismic conversion may also produce
nonlinear electromagnetic conversions that may be detected and/or
isolated in the detected electromagnetic signal. As discussed
above, the electroseismic energy conversion may occur at the
boundary between two types of rock. For example, the electroseismic
energy conversion may occur at the boundary between reservoir rock
and the sealing and/or confining rock. Alternatively,
electroseismic energy conversion may occur at an interface between
pore fluids, for example, between oil and water. At the rock and/or
fluid interfaces there may be a gradient in the chemical potential.
For example, at the boundary between a silicate rock and a
carbonate rock, a chemical reaction may occur in the comingled pore
fluids. For example, the silicate may dissolve the carbonate, and
the silicate ions in solution may react with the carbonate ions in
solution. The overall reaction may be driven by a gradient in the
chemical potential at the interface. The reaction product between
positive and negative ions in solution is electrically neutral and
may precipitate out of solution. When a precipitate is formed, the
resulting deposition of the precipitate strengthens the rock,
increases it hardness, and increases the electrical resistivity of
the interface. During the reactions in pore spaces, concentration
gradients of charged ions may be created within the pore
fluids.
[0160] These concentration gradients may produce an
electrochemical-potential gradient which may manifest itself as a
macroscopic electrical potential gradient. The internal electrical
potential gradients at the interfaces may create internal stresses,
and the interaction of the earth's background electromagnetic field
with the electrochemical-potential gradient may change these
internal stresses. Due to the natural modulations in the earth's
background electromagnetic field, the internal stresses may be
modulated, accounting for the nonlinear electroseismic conversions
that may be measured and used with the systems and method disclosed
herein.
[0161] In an embodiment, the interface where electroseismic
conversions occur can be modeled as a charged capacitor that
comprises a planar region of high resistance and an existing,
internal electromagnetic field. The interface can then be
understood as having a resistor-capacitor (RC) time constant. The
RC time constant may vary over a considerable range of values
depending on the resistance of the rock interface and the internal
electric field. The RC time constant may have the effect of
smoothing out a portion of the background electromagnetic field,
which may be detected by one or more of the electromagnetic field
detectors. In an embodiment, the extent of the resulting smoothing
of the background electromagnetic field may be used during
processing to determine at least one property of the subsurface
earth formation.
[0162] The background electromagnetic field may be modified
depending on the orientation of the background electromagnetic
field with respect to the interface. When the background
electromagnetic field is parallel to the internal electric field at
the interface, the internal field and internal stresses may not be
modified significantly. In this orientation, the interface behaves
as a simple resistor of high value with mobile fluids in the pore
space, and the RC time constant may not significantly affect the
background electromagnetic field. However, some of the electrical
field energy may be converted into seismic energy in the
electroseismic response.
[0163] When the background electromagnetic field is anti-parallel
with respect to the internal field at the interface, the internal
chemical reactions may be temporarily halted, the stresses and
effective resistance may be reduced, and the net electric field may
decrease. In this orientation, the applied field (e.g., the earth's
background electromagnetic field) may be at least partially
rectified to a reduced value and the change in internal stresses
may produce a seismic response. In terms of the overall subsurface
earth formation, the earth's background electromagnetic field may
be at least partially rectified at the boundaries between rock
masses. As a result, the earth's background electromagnetic field
that is interacting with a charged dipole layer where an
electroseismic conversion occurs may be altered, and the
alterations may be detected by one or more background
electromagnetic field detectors. By detecting and processing the
resulting electromagnetic field signals, at least one property of
the subsurface earth formation may be determined.
[0164] In an embodiment, the partial rectification of the
background electromagnetic field may be used to determine an
orientation, resistivity, or both of at least one interface in the
subsurface earth formation. The apparent subsurface resistivity may
depend on the background electromagnetic field's polarization. In
one polarity of the background electromagnetic field, the
conversion surface looks like a simple resistor. In the opposite
polarity it appears to be a capacitor with a long RC time constant.
This time constant may at least partially smooth out one polarity
of the source signal, resulting in one polarity having an
observable induced polarization while the opposite polarity may
not. The degree of induced polarization may act as an indicator of
the resistivity of the interface, and the determination of the
polarity being affected may act as an indicator of the orientation
of the rock interface.
[0165] The properties of the background electromagnetic field may
be spatially dependent, allowing for a determination of the lateral
extent of the subsurface earth formation structure. The extent of
the lateral variation in the induced polarization and generation of
nonlinear signals may be smoothed out due of the long wavelengths
present in the earth's background electromagnetic field. As a
result, the detected electromagnetic field may have a limited
resolution with respect to the edges of the reservoir.
[0166] In an embodiment, low frequency measurements (e.g., below 1
Hz) of the earth's background electromagnetic field may be useful
in measuring the polarity dependence of the induced polarization.
In the measurements of the seismic signals resulting from the
electroseismic conversions, the seismic wavelengths may be useful
for spatial delineation and the seismic velocity may be useful for
depth determination. In these measurements, frequency and time
information may be important characterizations. In an embodiment,
the frequency and time information may be determined by integrating
the amplitudes of different polarities in the detected
electromagnetic field and the detected seismic signal from one or
more seismic sensors.
[0167] The nonlinear signals in the detected electromagnetic field
resulting from the conversions at the subsurface earth formation
interfaces may be detected using a variety of methods. In an
embodiment, the positive and negative polarities of the earth's
background electromagnetic field may have different amplitudes and
different frequency spectra after being affected by the interface.
These differences may be used in determining the nonlinear
components of the detected electromagnetic field. The resulting
linear electroseismic response may be detected from the detected
seismic signal at one or more seismic sensors. Through a
cross-correlation, the resulting linear components of the detected
electromagnetic field may be determined and isolated. Using the
linear components as a filter, the non-linear components may be
isolated from the detected electromagnetic field. The filtered
electromagnetic field may be further processed to identify the
nonlinear components or reduce any noise signals present in the
remaining detected electromagnetic field after being filtered. For
example, additional filters may be applied and/or autocorrelations
performed.
[0168] In an embodiment, the detected electromagnetic field may be
compared to the earth's background electromagnetic field measured
at a distant location. The detected electromagnetic field may have
harmonic frequencies and low frequencies that are not present in a
signal measured at a distant point. In this embodiment, a detected
electromagnetic field at a distant electromagnetic field detector
may be used to filter the detected electromagnetic field above the
subsurface earth formation of interest. The remaining signal
present after applying the filter may contain the various harmonic,
nonlinear, and/or low frequencies of interest. These signals may be
further processed or filtered, for example to remove one or more
noise signals.
[0169] In an embodiment, any harmonic, nonlinear, and/or low
frequencies present in the detected electromagnetic field above the
subsurface earth formation of interest may be detected by comparing
the detected electromagnetic field measured in the earth to those
measured in the atmosphere. If the earth's background
electromagnetic field modulation creates a seismic response, then
the surface where energy conversion occurs may behave as a source
of electromagnetic radiation since there is a finite region of
modulated electromagnetic field and charge separation. The earth's
background electromagnetic field within the earth may itself take
on a character reflecting the nonlinear conversion. The resulting
electromagnetic radiation may manifest itself as a change in
boundary conditions at the earth's surface. Specifically, the
resulting electromagnetic radiation may create a vertical electric
field that may not be continuous across the earth/atmosphere
boundary. The use of a detected electromagnetic field above the
surface of the earth may be used to filter the detected
electromagnetic field within the earth. The remaining signal
present after applying the filter may contain the various harmonic,
nonlinear, and/or low frequencies of interest. These signals may be
further processed or filtered, for example to remove one or more
noise signals.
[0170] The processing described herein may be used to determine the
existence of a fluid in a subterranean fluid as well as other
properties of the subterranean formation. In an embodiment, the
processing of the electric/electromagnetic signal and the seismic
signal may provide an indication of at least one property of a
subterranean formation including, but not limited to, an existence
of the subsurface earth formation containing at least one fluid, a
depth of the subsurface earth formation, a porosity, a fluid
permeability, a composition of at least one fluid within the
subsurface earth formation, a spatial extent of the subsurface
earth formation, an orientation of the boundaries of the subsurface
earth formation, a resistivity, and any combination thereof.
Further, the processing may detect one or more fluids within the
subterranean formation, where detectable fluids may include, but
are not limited to, an aqueous fluid, a hydrocarbon, a petroleum,
and any combination thereof.
[0171] As noted above, the detection and analysis steps described
herein may be repeated any number of times. For example, multiple
measurements may be made at a single location over several time
periods. The results may be statistically analyzed to provide an
improved accuracy correlation and/or survey. In addition, one or
more samples may be taken at varying locations sequentially in time
or concurrently in time using one or multiple sensors. For example,
multiple measurements may be made at varying locations around a
site of interest. Various grid patterns and/or random sample
locations may be chosen to generate a plurality of measurements
across an area. For example, the grid and/or array of detectors
(e.g., using about 10 to about 10,000 seismic sensors and/or
electromagnetic field detectors) described above may be used to
generate a plurality of detected signals for use with the
processing techniques described herein. The multiple measurements
may be performed sequentially or concurrently (e.g., using multiple
sensors) at a single location, and/or the measurements may be
performed sequentially and/or concurrently in the various locations
around a site of interest when a plurality of locations are used to
measure the signal of interest. The resulting hydrocarbon
indications and resulting depth measurements may be used to
generate a two dimensional and/or three dimensional model of a
subterranean formation and the one or more fluids contained
therein.
Additional Surveying Steps
[0172] In an embodiment, more information may be obtained about the
subterranean formation by conducting one or more further surveys
after any of the passive surveying techniques described herein have
been carried out. For example, a seismological survey, a
controlled-source electromagnetics survey, a controlled-source
electroseismic survey, a controlled-source seismoelectric survey a
gravity survey, and/or a magnetic survey may be conducted based on
an indication of a fluid (e.g., a hydrocarbon) present in the
subterranean formation of interest. In an embodiment, an additional
passive survey may be performed as described herein. The additional
passive survey may provide for more data over a greater number of
sensors and/or detectors to obtain higher quality information about
the subterranean formation. Thus, the system and methods described
herein may be used in combination with other surveying techniques
to provide information about a subterranean formation of interest.
In an embodiment, a wellbore may be commenced and/or drilled into
the subterranean formation when an envelope is detected in order to
recover one or more hydrocarbons contained therein.
[0173] These systems and methods may have several advantages over
previous prospecting techniques. The detection of an electroseismic
conversion allows for the detection of an electromagnetic signal
and a generated seismic response, alone or in combination. As a
result, the passive surveying system allows for surveying without
an active electromagnetic energy source, which may improve site
safety and reduce any environmental impacts. The reduction in the
amount of equipment and power, along with the corresponding reduced
footprint at the measurement site, represents an advantage over
other surveying systems and methods. From an environmental and
health perspective, the reduction in transportation, site
preparation, and high energy sources may improve the overall health
and safety of the workers operating the equipment. In addition, the
earth's naturally occurring electromagnetic field comprises a broad
spectrum of frequencies, from sub-hertz frequencies to tens of
thousands of hertz frequencies, along with a broad coverage over
the surface of the earth. This broad spectrum allows for a broad
range of penetration depths from tens of meters to tens of
kilometers.
Computer Based Implementations
[0174] The various methods disclosed herein for removing coherent
and random noise sources and/or isolating and processing nonlinear
signals may use real-time processing or may use data that has been
recorded (e.g., post processing). In an embodiment, one or more of
the methods disclosed herein may be implemented on any computer
with sufficient processing power, memory resources, and network
throughput capability to handle the necessary workload placed upon
it. FIG. 9 illustrates a typical, computer system suitable for
implementing one or more embodiments disclosed herein. The computer
system 980 includes a processor 982 (which may be referred to as a
central processor unit or CPU) that is in communication with memory
devices including secondary storage 984, read only memory (ROM)
986, random access memory (RAM) 988, input/output (I/O) devices
990, and network connectivity devices 992. The processor may be
implemented as one or more CPU chips.
[0175] It is understood that by programming and/or loading
executable instructions onto the computer system 980, at least one
of the CPU 982, the RAM 988, and the ROM 986 are changed,
transforming the computer system 980 in part into a particular
machine or apparatus having the novel functionality taught by the
present disclosure. It is fundamental to the electrical engineering
and software engineering arts that functionality that can be
implemented by loading executable software into a computer can be
converted to a hardware implementation by well known design rules.
Decisions between implementing a concept in software versus
hardware typically hinge on considerations of stability of the
design and numbers of units to be produced rather than any issues
involved in translating from the software domain to the hardware
domain. Generally, a design that is still subject to frequent
change may be preferred to be implemented in software, because
re-spinning a hardware implementation is more expensive than
re-spinning a software design. Generally, a design that is stable
that will be produced in large volume may be preferred to be
implemented in hardware, for example in an application specific
integrated circuit (ASIC), because for large production runs the
hardware implementation may be less expensive than the software
implementation. Often a design may be developed and tested in a
software form and later transformed, by well known design rules, to
an equivalent hardware implementation in an application specific
integrated circuit that hardwires the instructions of the software.
In the same manner as a machine controlled by a new ASIC is a
particular machine or apparatus, likewise a computer that has been
programmed and/or loaded with executable instructions may be viewed
as a particular machine or apparatus.
[0176] The secondary storage 984 is typically comprised of one or
more disk drives or tape drives and is used for non-volatile
storage of data and as an over-flow data storage device if RAM 988
is not large enough to hold all working data. Secondary storage 984
may be used to store programs which are loaded into RAM 988 when
such programs are selected for execution. The ROM 986 is used to
store instructions and perhaps data which are read during program
execution. ROM 986 is a non-volatile memory device which typically
has a small memory capacity relative to the larger memory capacity
of secondary storage 984. The RAM 988 is used to store volatile
data and perhaps to store instructions. Access to both ROM 986 and
RAM 988 is typically faster than to secondary storage 984. The
secondary storage 984, the RAM 988, and/or the ROM 986 may be
referred to in some contexts as computer readable storage media
and/or non-transitory computer readable media.
[0177] I/O devices 990 may include printers, video monitors, liquid
crystal displays (LCDs), touch screen displays, keyboards, keypads,
switches, dials, mice, track balls, voice recognizers, card
readers, paper tape readers, or other well-known input devices.
[0178] The network connectivity devices 992 may take the form of
modems, modem banks, Ethernet cards, universal serial bus (USB)
interface cards, serial interfaces, token ring cards, fiber
distributed data interface (FDDI) cards, wireless local area
network (WLAN) cards, radio transceiver cards such as code division
multiple access (CDMA), global system for mobile communications
(GSM), long-term evolution (LTE), worldwide interoperability for
microwave access (WiMAX), and/or other air interface protocol radio
transceiver cards, and other well-known network devices. These
network connectivity devices 992 may enable the processor 982 to
communicate with the Internet or one or more intranets. With such a
network connection, it is contemplated that the processor 982 might
receive information from the network, or might output information
to the network in the course of performing the above-described
method steps. Such information, which is often represented as a
sequence of instructions to be executed using processor 982, may be
received from and outputted to the network, for example, in the
form of a computer data signal embodied in a carrier wave.
[0179] Such information, which may include data or instructions to
be executed using processor 982 for example, may be received from
and outputted to the network, for example, in the form of a
computer data baseband signal or signal embodied in a carrier wave.
The baseband signal or signal embodied in the carrier wave
generated by the network connectivity devices 992 may propagate in
or on the surface of electrical conductors, in coaxial cables, in
waveguides, in an optical conduit, for example an optical fiber, or
in the air or free space. The information contained in the baseband
signal or signal embedded in the carrier wave may be ordered
according to different sequences, as may be desirable for either
processing or generating the information or transmitting or
receiving the information. The baseband signal or signal embedded
in the carrier wave, or other types of signals currently used or
hereafter developed, may be generated according to several methods
well known to one skilled in the art. The baseband signal and/or
signal embedded in the carrier wave may be referred to in some
contexts as a transitory signal.
[0180] The processor 982 executes instructions, codes, computer
programs, scripts which it accesses from hard disk, floppy disk,
optical disk (these various disk based systems may all be
considered secondary storage 984), ROM 986, RAM 988, or the network
connectivity devices 992. While only one processor 982 is shown,
multiple processors may be present. Thus, while instructions may be
discussed as executed by a processor, the instructions may be
executed simultaneously, serially, or otherwise executed by one or
multiple processors. Instructions, codes, computer programs,
scripts, and/or data that may be accessed from the secondary
storage 984, for example, hard drives, floppy disks, optical disks,
and/or other device, the ROM 986, and/or the RAM 988 may be
referred to in some contexts as non-transitory instructions and/or
non-transitory information.
[0181] In an embodiment, the computer system 980 may comprise two
or more computers in communication with each other that collaborate
to perform a task. For example, but not by way of limitation, an
application may be partitioned in such a way as to permit
concurrent and/or parallel processing of the instructions of the
application. Alternatively, the data processed by the application
may be partitioned in such a way as to permit concurrent and/or
parallel processing of different portions of a data set by the two
or more computers. In an embodiment, virtualization software may be
employed by the computer system 980 to provide the functionality of
a number of servers that is not directly bound to the number of
computers in the computer system 980. For example, virtualization
software may provide twenty virtual servers on four physical
computers. In an embodiment, the functionality disclosed above may
be provided by executing the application and/or applications in a
cloud computing environment. Cloud computing may comprise providing
computing services via a network connection using dynamically
scalable computing resources. Cloud computing may be supported, at
least in part, by virtualization software. A cloud computing
environment may be established by an enterprise and/or may be hired
on an as-needed basis from a third party provider. Some cloud
computing environments may comprise cloud computing resources owned
and operated by the enterprise as well as cloud computing resources
hired and/or leased from a third party provider.
[0182] In an embodiment, some or all of the functionality disclosed
above may be provided as a computer program product. The computer
program product may comprise one or more computer readable storage
medium having computer usable program code embodied therein to
implement the functionality disclosed above. The computer program
product may comprise data structures, executable instructions, and
other computer usable program code. The computer program product
may be embodied in removable computer storage media and/or
non-removable computer storage media. The removable computer
readable storage medium may comprise, without limitation, a paper
tape, a magnetic tape, magnetic disk, an optical disk, a solid
state memory chip, for example analog magnetic tape, compact disk
read only memory (CD-ROM) disks, floppy disks, jump drives, digital
cards, multimedia cards, and others. The computer program product
may be suitable for loading, by the computer system 980, at least
portions of the contents of the computer program product to the
secondary storage 984, to the ROM 986, to the RAM 988, and/or to
other non-volatile memory and volatile memory of the computer
system 980. The processor 982 may process the executable
instructions and/or data structures in part by directly accessing
the computer program product, for example by reading from a CD-ROM
disk inserted into a disk drive peripheral of the computer system
980. Alternatively, the processor 982 may process the executable
instructions and/or data structures by remotely accessing the
computer program product, for example by downloading the executable
instructions and/or data structures from a remote server through
the network connectivity devices 992. The computer program product
may comprise instructions that promote the loading and/or copying
of data, data structures, files, and/or executable instructions to
the secondary storage 984, to the ROM 986, to the RAM 988, and/or
to other non-volatile memory and volatile memory of the computer
system 980.
[0183] In some contexts, a baseband signal and/or a signal embodied
in a carrier wave may be referred to as a transitory signal. In
some contexts, the secondary storage 984, the ROM 986, and the RAM
988 may be referred to as a non-transitory computer readable medium
or a computer readable storage media. A dynamic RAM embodiment of
the RAM 988, likewise, may be referred to as a non-transitory
computer readable medium in that while the dynamic RAM receives
electrical power and is operated in accordance with its design, for
example during a period of time during which the computer 980 is
turned on and operational, the dynamic RAM stores information that
is written to it. Similarly, the processor 982 may comprise an
internal RAM, an internal ROM, a cache memory, and/or other
internal non-transitory storage blocks, sections, or components
that may be referred to in some contexts as non-transitory computer
readable media or computer readable storage media.
EXAMPLES
[0184] The disclosure having been generally described, the
following examples are given as particular embodiments of the
disclosure and to demonstrate the practice and advantages thereof.
It is understood that the examples are given by way of illustration
and are not intended to limit the specification or the claims in
any manner.
Example 1
[0185] Referring now to FIG. 10, the results of passive
seismoelectric detection used to detect and process electromagnetic
waves are shown. The processing of the detected signal illustrates
characteristic depths of subterranean formations of interest. The
data of FIG. 10 were collected at Harris County Tex. The
subterranean features of interest are aquifers that are
well-characterized by standard measurement methods and the depths
of the subject aquifers are tabulated in the public domain. FIG. 10
displays the amplitude of seismoelectric signals on the vertical
axis and the depth below the surface on the horizontal axis. The
highest amplitude peaks on the data of FIG. 10 occur at depths
corresponding to the known aquifer depths, which are known to occur
at about 200 feet, about 300 feet, and about 400 feet.
Additionally, the data of FIG. 10 have a peak near 100 feet that is
the expected signal originating from seismic waves traveling
between the aquifers at 200 and 300 feet.
Example 2
[0186] Referring now to FIG. 11, the results of passive
electroseismic testing used to detect and process a seismic signal
are shown. The processing of the seismic signal illustrates
characteristic depths to subterranean formations that were also
detected with the seismoelectric testing of Example 1. The seismic
testing reveals additional subterranean structures not found with
the seismoelectric test. Those skilled in the art will recognize
that seismic waves have shorter wavelengths than electromagnetic
waves. The shorter wavelengths of seismic waves enable seismic
detectors to resolve additional structures that are not visible to
electromagnetic waves. FIG. 11 has peaks indicating structures near
100 feet, 200 feet, 300 feet, and 400 feet. A peak at 24 feet is
expected as a seismic wave originating at the near-surface water
table. Double peaks near 100 feet, 200 feet, and 400 feet are
expected to arise from electroseismic conversions at both the top
and bottom of aquifers.
Example 3
[0187] Referring now to FIG. 12, cross-correlation (i.e., joint
processing) of the results of Example 1 (shown in FIG. 10) and
Example 2 (shown in FIG. 11) shows the features that are common in
the FIG. 10 and FIG. 11 data sets. Electroseismic and
seismoelectric signals originate in the same physical conversion
mechanism at boundaries between dissimilar rocks or at boundaries
between different fluids in rock pore spaces. The seismic detectors
and the electromagnetic field detectors are not equally sensitive
to rapid signal changes or to small signal amplitude differences.
Thus, the processed data shown in FIGS. 10 and 11 are similar but
not identical. FIG. 12 enhances the common information in both data
sets. There exist several methods known to those skilled in the art
for comparing the information in the two measurements with the goal
of learning more about subterranean features of interest.
Example 4
[0188] Referring now to FIG. 13, the results of passive detection
used to detect and process electromagnetic fields are shown. The
processing of the detected signal illustrates characteristic depths
of subterranean formations of interest. The data of FIG. 13 were
collected in Texas. The subterranean features of interest are
hydrocarbon reservoirs and strong seismic reflectors that have been
previously characterized by standard seismic surveying techniques.
FIG. 13 displays the amplitude of seismoelectric signals on the
vertical axis and the depth on the horizontal axis. The highest
amplitude peaks on the data of FIG. 13 occur at time of about 0.2
seconds, 0.365 seconds, 0.608 seconds, and 1.028 seconds. These
seismic travel times correspond to strong reflectors measured in
seismology and the strong reflector at 1.028 seconds, corresponding
to a depth of approximately 10,000 feet, is the depth of a recently
discovered reservoir. Other peaks in FIG. 13 also correspond to
known depths of strong seismic reflectors. The results of FIG. 13
illustrate that the application of the techniques described herein
can be used to identify the depth of seismic reflectors and
hydrocarbons in a subsurface formation.
System and Method Embodiments
[0189] Having described the systems and methods for passive
surveying, various embodiments will now be described.
[0190] 1. In an embodiment, a method of passive surveying
comprises: generating one or more detected signals by passively
detecting a signal generated within a subsurface earth formation
due to a seismoelectric response or an electroseismic response in
at least one porous subsurface earth formation containing at least
one fluid; and processing the one or more detected signals to
determine at least one property of the subsurface earth
formation.
[0191] 2. The method of embodiment 1, wherein generating the one or
more detected signals comprises: generating a detected
electromagnetic field by detecting earth's electromagnetic field
using an electromagnetic field detector; and wherein processing the
one or more detected signals comprises: demodulating a portion of
the signal to identify an envelope of the signal, wherein the
envelope is indicative of the presence of the hydrocarbons; and
analyzing the envelope to determine a value correlated to a depth
of the hydrocarbons in the subterranean formation.
[0192] 3. The method of embodiment 2, wherein demodulating the
portion of the signal comprises extracting a frequency modulation,
a phase modulation, an amplitude modulation, or any combination
thereof from the signal.
[0193] 4. The method of embodiment 3, wherein the envelope
comprises at least one of the frequency modulation, the phase
modulation, or the amplitude modulation.
[0194] 5. The method of any of embodiments 2 to 4, wherein the
sensor is disposed on or above the surface of the earth.
[0195] 6. The method of any of embodiments 2 to 5, wherein the
sensor is moving during the detection of the electromagnetic
field.
[0196] 7. The method of any of embodiments 2 to 6, wherein the
sensor is disposed in a moving vehicle.
[0197] 8. The method of any of embodiments 2 to 7, further
comprising recording the signal using a recording apparatus.
[0198] 9. The method of embodiment 8, wherein the signal is
recorded for at least about 5 seconds.
[0199] 10. The method of embodiment 8, wherein the signal is
recorded for at least about 0.1 seconds.
[0200] 11. The method of any of embodiments 8 to 10, wherein the
recording apparatus comprises a digital or analog recording
device.
[0201] 12. The method of any of embodiments 2 to 11, wherein
analyzing the envelope comprises determining one or more spectral
properties of the envelope.
[0202] 13. The method of any of embodiments 2 to 12, wherein the
value comprises a frequency distribution of the envelope and the
value is correlated to the depth of the hydrocarbons using a
frequency-depth function.
[0203] 14. The method of embodiment 13, wherein the frequency-depth
function is derived using data from a known location.
[0204] 15. The method of any of embodiments 2 to 14, further
comprising repeating the detecting, demodulating, and analyzing a
plurality of times.
[0205] 16. The method of embodiment 15, wherein the repeating is
performed at a single location.
[0206] 17. The method of embodiment 15, wherein the repeating is
performed at different locations.
[0207] 18. The method of embodiment 17, wherein the repeating is
performed sequentially or concurrently at each of the different
locations.
[0208] 19. The method of embodiment 17 or 18, wherein the different
locations correspond to a plurality of grid positions.
[0209] 20. The method of embodiment 19, further comprising
generating a multi-dimension model of a subterranean formation
using a plurality of values, wherein at least one value of the
plurality of values is determined at each corresponding grid
position of the plurality of grid positions.
[0210] 21. The method of any of embodiments 2 to 20, further
comprising: performing a further survey of the subterranean
formation when an envelope is identified.
[0211] 22. The method of embodiment 21, wherein the performing of
the further survey comprises a seismological survey, a
controlled-source electromagnetics survey, a controlled-source
electroseismic survey, a gravity survey, a magnetic survey, or a
passive survey.
[0212] 23. The method of any of embodiments 2 to 22, wherein
detecting the earth's electromagnetic field comprises measuring a
vertical electric component of the earth's electromagnetic
field.
[0213] 24. The method of embodiment 23, further comprising
generating a detected magnetic field by detecting earth's magnetic
field using a magnetic field detector, and processing the detected
magnetic field with the vertical electric component and the
detected seismic signal.
[0214] 25. The method of any of embodiments 2 to 24, wherein
detecting the earth's electromagnetic field comprises measuring the
earth's electromagnetic field in at least one horizontal
direction.
[0215] 26. The method of any of embodiments 2 to 25, wherein the
earth's electromagnetic field comprises a time-varying
electromagnetic field.
[0216] 27. The method of any of embodiments 2 to 26, wherein the
earth's electromagnetic field is detected using a pair of porous
pot electrodes.
[0217] 28. The method of embodiment 27, wherein the pair of porous
pot electrodes comprises at least one component selected from the
group consisting of: copper sulfate, silver chloride, cadmium
chloride, mercury chloride, and lead chloride.
[0218] 29. The method of any of embodiments 2 to 28, wherein the
earth's electromagnetic field is detected using a plurality of
pairs of porous pot electrodes.
[0219] 30. The method of any of embodiments 2 to 29, wherein the
earth's electromagnetic field is detected using a pair of
conductive electrodes.
[0220] 31. The method of embodiment 30, wherein the pair of metal
electrodes comprises at least one conductive material selected from
the group consisting of: copper, stainless steel, aluminum, gold,
galvanized metal, iron, lead, brass, graphite, and steel.
[0221] 32. The method of any of embodiments 2 to 31, wherein the
earth's electromagnetic field is detected using a conductive
electrode coupled to a porous pot electrode.
[0222] 33. The method of any of embodiments 2 to 32, wherein the
detected electromagnetic field is generated using one or more
antennas disposed on or above a surface of the earth.
[0223] 34. The method of embodiment 33, wherein the one or more
antennas comprises at least one antenna selected from the group
consisting of: a parallel-plate capacitor antenna comprising two or
more parallel conducting plates, a single-plate capacitor antenna
comprising one electrode electrically coupled to the earth, a
monopole antenna comprising a conducting element, a dipole antenna
comprising two conducting elements, a multi-pole antenna comprising
a plurality of conducting elements, a directional antenna
comprising conducting elements arranged to augment a signal
amplitude in a particular direction, a coil antenna comprising one
or more coils of wire, and any combination thereof.
[0224] 35. The method of embodiment 33, wherein the one or more
antennas comprise a concentric electric dipole.
[0225] 36. The method of any of embodiments 2 to 35, wherein the at
least one property of the subsurface earth formation comprises at
least one property selected from the group consisting of: an
existence of the subsurface earth formation containing at least one
fluid, a depth of the subsurface earth formation, a porosity, a
fluid permeability, a composition of at least one fluid within the
subsurface earth formation, a spatial extent of the subsurface
earth formation, an orientation of the boundaries of the subsurface
earth formation, a resistivity, and any combination thereof.
[0226] 37. The method of any of embodiments 2 to 36, wherein the at
least one fluid comprises at least one component selected from the
group consisting of: an aqueous fluid, a hydrocarbon, a petroleum,
carbon dioxide, and any combination thereof.
[0227] 38. The method of embodiment 37 where the aqueous fluid
comprises at least one of potable water, fresh water, or brine.
[0228] 39 The method of any of embodiments 2 to 38, wherein the
detected electromagnetic field results, at least in part, from a
natural event comprising at least one of: a electromagnetic
fluctuation in the ionosphere, and a naturally occurring electric
discharge in the atmosphere.
[0229] 40. The method of any of embodiments 2 to 39, further
comprising a plurality of electromagnetic field detectors.
[0230] 41. The method of embodiment 40, wherein the plurality of
electromagnetic field detectors are arranged in an array.
[0231] 42. The method of any of embodiments 2 to 41, wherein
processing the one or more detected signals further comprises:
filtering the detected electromagnetic field.
[0232] 43. The method of embodiment 42, wherein the filtering and
processing occur in real time.
[0233] 44. The method of any of embodiments 42 to 43, wherein
filtering comprises filtering a direct current (DC) portion of the
detected electromagnetic field prior to performing the
processing.
[0234] 45. The method of any of embodiments 42 to 44, wherein
filtering comprises decimating a data set representing the detected
electromagnetic field prior to performing the processing.
[0235] 46. The method of any of embodiments 42 to 45, wherein
filtering comprises using a noise filter.
[0236] 47. The method of embodiment 46, wherein the noise filter
comprises a high pass filter, a low pass filter, a wide band
frequency filter, a narrow band frequency filter, or any
combination thereof.
[0237] 48. The method of any of embodiments 42 to 47, wherein
filtering comprises using one or more band-pass filters.
[0238] 49. The method of embodiment 48, wherein at least one of the
band-pass filters of the one or more band-pass filters comprises a
linear phase filter, a finite impulse response filter, a forward
infinite impulse response filter, a reverse infinite impulse
response filter, or any combination thereof.
[0239] 50. The method of any of embodiments 2 to 49, wherein
generating the one or more detected signals further comprises:
generating at least one nonlinear electromagnetic field response by
isolating the at least one nonlinear electromagnetic field response
from the detected electromagnetic field; and wherein processing the
one or more detected signals further comprises: processing the at
least one nonlinear electromagnetic field response to determine at
least one property of the subsurface earth formation.
[0240] 51. In an embodiment, a method for identifying deposits in a
subterranean formation comprises: obtaining a signal corresponding
to a vertical electric field in the earth from an area of interest,
wherein the signal comprises a modulating signal generated within
the subterranean formation; filtering the signal using a set of
predetermined frequency bandwidths to generate a plurality of
filtered signals; identifying a plurality of envelopes, wherein
each envelope of the plurality of envelopes corresponds to each
filtered signal of the plurality of filtered signals; and analyzing
the plurality of envelopes to determine one or more values
correlated to a depth of the deposits in the subterranean
formation.
[0241] 52. The method of embodiment 51, wherein the modulating
signal is generated based on a combination of electroseismic and
seismoelectric effects due to interactions between the deposits in
the subterranean formation and interactions with a background
electromagnetic field of the earth.
[0242] 53. The method of embodiment 51 or 52, wherein the
modulating signal comprises at least one of a frequency modulation,
a phase modulation, or an amplitude modulation.
[0243] 54. The method of embodiment 53, wherein the envelope
comprises at least one of the frequency modulation, the phase
modulation, or the amplitude modulation.
[0244] 55. The method of any of embodiments 51 to 54, further
comprising recording the signal using a recording apparatus.
[0245] 56. The method of any of embodiments 51 to 55, wherein the
filtering, identifying, and analyzing occur in real time.
[0246] 57. The method of any of embodiments 51 to 56, wherein a
positive identification of a plurality of envelopes indicates the
presence of a fluid within a pore in the subterranean
formation.
[0247] 58. The method of any of embodiments 51 to 57, wherein a
positive identification of a plurality of envelopes indicates the
presence of a fluid of a certain type within a pore in the
subterranean formation.
[0248] 59. The method of any of embodiments 51 to 58, further
comprising filtering a direct current (DC) portion of the signal
from the signal prior to identifying a plurality of envelopes.
[0249] 60. The method of any of embodiments 51 to 59, further
comprising decimating a data set representing the signal prior to
identifying the plurality of envelopes.
[0250] 61. The method of any of embodiments 51 to 60, further
comprising filtering the signal with a noise filter.
[0251] 62. The method of embodiment 61, wherein the noise filter
comprises a high pass filter, a low pass filter, a wide band
frequency filter, a narrow band frequency filter, or any
combination thereof.
[0252] 63. The method of any of embodiments 51 to 62, wherein a
plurality of band-pass filters is used to generate the plurality of
filtered signals.
[0253] 64. The method of embodiment 63, wherein at least one of the
band-pass filters of the plurality of band-pass filters comprises a
linear phase filter, a finite impulse response filter, a forward
infinite impulse response filter, a reverse infinite impulse
response filter, or any combination thereof.
[0254] 65. The method of any of embodiments 51 to 64, wherein
identifying a plurality of envelopes comprises using a Hilbert
transform method.
[0255] 66. The method of any of embodiments 51 to 65, wherein
analyzing the plurality of envelopes comprises determining one or
more spectral properties of at least one of the plurality of
envelopes.
[0256] 67. The method of embodiment 66, wherein determining one or
more spectral properties comprises calculating a power spectral
density for the at least one of the plurality of envelopes.
[0257] 68. The method of embodiment 66 or 67, wherein determining
one or more spectral properties comprises calculating a frequency
profile using a fast Fourier transform of the at least one of the
plurality of envelopes.
[0258] 69. The method of any of embodiments 66 to 68, wherein
determining one or more spectral properties comprises using at
least one of a lock-in amplifier or a spectrum analyzer.
[0259] 70. The method of any of embodiments 66 to 69, further
comprising correlating a ratio of at least one spectral property of
at least one frequency band from one or more envelopes of the
plurality of envelopes to at least one corresponding spectral
property of at least a second frequency band from the one or more
envelopes of the plurality of envelopes.
[0260] 71. The method of any of embodiments 51 to 70, further
comprising correlating the value to a depth of the deposits in the
subterranean formation.
[0261] 72. The method of embodiment 71, wherein correlating
comprises determining a frequency-depth relationship.
[0262] 73. The method of embodiment 72, wherein the frequency-depth
relationship is determined based on a classification method
comprising at least one method selected from the group consisting
of: a neural network, a decision tree, a Bayes-based classifier, a
fuzzy logic-based classifier, and a conventional statistical
classifier.
[0263] 74. The method of embodiment 72 or 73, wherein the
frequency-depth relationship is determined based on a regression
analysis of a set of known empirical data.
[0264] 75. The method of any of embodiments 66 to 74, further
comprising analyzing the one or more spectral properties to
determine a relationship between the one or more spectral
properties and a presence of a fluid in a pore in the subterranean
formation based on a classification method comprising at least one
method selected from the group consisting of: a neural network, a
decision tree, a Bayes-based classifier, a fuzzy logic-based
classifier, and a conventional statistical classifier.
[0265] 76. The method of any of embodiments 66 to 75, further
comprising analyzing the one or more spectral properties to
determine a relationship between the one or more spectral
properties and a presence of a fluid in a pore in the subterranean
formation based on a regression analysis of a set of known
empirical data.
[0266] 77. The method of any of embodiments 51 to 76, further
comprising: developing a geological model of the subterranean
formation; obtaining one or more predicted envelopes from the
geological model; comparing the one or more predicted envelopes to
the plurality of envelopes; and determining one or more properties
of the subterranean formation based on the comparing.
[0267] 78. The method of any of embodiments 51 to 77, further
comprising: performing a further survey of the subterranean
formation when an envelope is identified.
[0268] 79. In an embodiment, a method comprises: providing a sensor
disposed on or above a surface of the earth; generating a reference
signal; introducing the reference signal into the earth, wherein
the reference signal is modulated by a modulating signal generated
within the subterranean formation to generate a modulated reference
signal; detecting the modulated reference signal; and processing
the modulated reference signal and the reference signal to isolate
the modulating signal.
[0269] 80. The method of embodiment 79 wherein the modulating
signal comprises at least one of a frequency modulation, a phase
modulation, or an amplitude modulation.
[0270] 81. The method of embodiment 79 or 80, wherein the
modulating signal is generated based on at least one of an
electroseismic effect or a seismoelectric effect due to an
interaction between a fluid in a subterranean formation and a
background electromagnetic field of the earth.
[0271] 82. The method of any of embodiments 79 to 81, further
comprising recording the modulated reference signal using a
recording apparatus.
[0272] 83. The method of embodiment 82, wherein the modulated
reference signal is recorded for at least about 5 seconds.
[0273] 84. The method of embodiment 82, wherein the modulated
reference signal is recorded for at least about 0.1 seconds.
[0274] 85. The method of any of embodiments 79 to 84, wherein
processing the modulated reference signal comprises demodulating
the modulated reference signal.
[0275] 86. The method of embodiment 85, wherein the demodulating
comprises using a Hilbert transform method.
[0276] 87. The method of any of embodiments 79 to 86, wherein
processing the modulated reference signal and the reference signal
comprises using a lock-in amplifier receiving the modulated
reference signal and the reference signal to isolate the modulating
signal.
[0277] 88. The method of any of embodiments 79 to 88, further
comprising: performing a further survey of the subterranean
formation when an envelope is identified.
[0278] 89. In an embodiment, a system for identifying hydrocarbons
in a subterranean formation comprises: one or more sensors coupled
to a processor that detect one or more signals generated within a
subsurface earth formation due to a seismoelectric response or an
electroseismic response in at least one porous subsurface earth
formation containing at least one fluid; and an analysis tool, that
when executed on the processor, configures the processor to:
receive the one or more signals from the one or more sensors; and
process at least a portion of the one or more signals to determine
at least one property of the subsurface earth formation.
[0279] 90. The system of embodiment 89, wherein the one or more
sensors comprise: one or more electromagnetic field detectors that
measures the earth's electromagnetic field and produces a signal
indicative of the detected electromagnetic field; and wherein the
analysis tool configures the processor to: receive the signal from
the one or more sensors; demodulate a portion of the signal to
identify an envelope of the signal; analyze the envelope to
determine one or more properties indicative of the presence of the
hydrocarbons; and analyze the envelope to determine the depth of
the hydrocarbons in the subterranean formation.
[0280] 91. The system of embodiment 90, wherein the envelope of the
signal comprises at least one of a frequency modulation, a phase
modulation, or an amplitude modulation.
[0281] 92. The system of embodiment 90 or 91, further comprising a
recording apparatus coupled to the sensor and the analysis tool,
wherein the recording apparatus is configured to record the
signal.
[0282] 93. The system of any of embodiments 90 to 92, wherein the
analysis tool further configures the processor to: remove a direct
current (DC) portion of the signal.
[0283] 94. The system of any of embodiments 90 to 93, wherein the
analysis tool further configures the processor to: decimate a data
set representing the signal.
[0284] 95. The system of any of embodiments 90 to 94, wherein the
analysis tool further configures the processor to: filter a noise
component from the signal.
[0285] 96. The system of any of embodiments 90 to 95, wherein the
analysis tool further configures the processor to: filter the
signal using a predetermined frequency bandwidth.
[0286] 97. The system of any of embodiments 90 to 96, wherein the
analysis tool configures the processor to demodulate a portion of
the signal using a Hilbert transform.
[0287] 98. The system of any of embodiments 90 to 97, wherein the
analysis tool configures the processor to analyze the envelope to
determine a spectral property of the envelope.
[0288] 99. The system of embodiment 98, wherein the spectral
property comprises a power spectral density.
[0289] 100. The system of embodiment 98, wherein the spectral
property comprises a frequency distribution determined by a fast
Fourier transform.
[0290] 101. The system of any of embodiments 90 to 100, wherein the
analysis tool further configures the processor to: correlate the
envelope to a depth of the hydrocarbons in the subterranean
formation.
[0291] 102. The system of embodiment 101, wherein the analysis tool
configures the processor to: correlate the envelope to a depth
using a frequency-depth function.
[0292] 103. The system of any of embodiments 90 to 102, wherein the
electromagnetic field detector comprises a plurality of pairs of
porous pot electrodes, wherein each pair of porous pot electrodes
are electrically coupled.
[0293] 104. The system of embodiment 103, wherein the pairs of
porous pot electrodes comprise at least one component selected from
the group consisting of: copper sulfate, silver chloride, cadmium
chloride, mercury chloride, and lead chloride.
[0294] 105. The system of any of embodiments 90 to 104, wherein the
electromagnetic field detector comprises a plurality of pairs of
conductive electrodes, wherein each pair of conductive electrodes
are electrically coupled.
[0295] 106. The system of any of embodiments 90 to 105, wherein the
plurality of conductive electrodes comprises at least one
conductive material selected from the group consisting of: copper,
stainless steel, aluminum, gold, galvanized metal, iron, lead,
brass, graphite, steel, alloys thereof, and any combination
thereof.
[0296] 107. The system of any of embodiments 90 to 106, wherein the
electromagnetic field detector comprises a conductive electrode
coupled to a porous pot electrode.
[0297] 108. The system of any of embodiments 90 to 107, wherein the
at least one fluid comprises at least one component selected from
the group consisting of: an aqueous fluid, a hydrocarbon, a
petroleum, carbon dioxide, and any combination thereof.
[0298] 109. The system of embodiment 108, wherein the aqueous fluid
comprises at least one of potable water, fresh water, or brine.
[0299] 110. The system of any of embodiments 90 to 109, wherein the
electromagnetic field detector comprises an antenna disposed on or
above the surface of the earth.
[0300] 111. The system of embodiment 110, wherein the antenna
comprises at least one antenna selected from the group consisting
of: a parallel-plate capacitor antenna comprising two or more
parallel conducting plates, a single-plate capacitor antenna
comprising one electrode electrically coupled to the earth, a
monopole antenna comprising a conducting element, a dipole antenna
comprising two conducting elements, a multi-pole antenna comprising
a plurality of conducting elements, a directional antenna
comprising conducting elements arranged to augment a signal
amplitude in a particular direction, a coil antenna comprising one
or more coils of wire, and any combination thereof.
[0301] 112. The system of embodiment 110, wherein the antenna
comprises a concentric electric dipole.
[0302] 113. In an embodiment, a system for detecting a signal
comprises: a signal generator configured to generate a reference
signal and transmit the reference signal into the earth, wherein
the reference signal is modulated by a modulating signal generated
within a subterranean formation and generates a modulated reference
signal; a sensor disposed on or above a surface of the earth and
coupled to a processor that detects the modulated reference signal;
and a filter coupled to the signal generator and the sensor,
wherein the filter is configured to receive the reference signal
from the signal generator and the modulated reference signal from
the sensor, and process the modulated reference signal and the
reference signal to isolate the modulating signal.
[0303] 114. The system of embodiment 113, wherein the sensor
comprises one or more capacitive plates arranged parallel to a
surface of the earth.
[0304] 115. The system of embodiment 113 or 114, wherein the filter
comprises a lock-in amplifier.
[0305] 116. The system of any of embodiments 113 to 115, wherein
the modulating signal comprises at least one of a frequency
modulation, a phase modulation, or an amplitude modulation.
[0306] 117. In an embodiment, a system for identifying hydrocarbons
in a subterranean formation comprises: a memory comprising a
non-transitory computer readable media; a processor; and an
analysis tool, that when executed on the processor, configures the
processor to: receive one or more signals from one or more sensors,
wherein the one or more sensors detect one or more signals
generated within a subsurface earth formation due to a
seismoelectric response or an electroseismic response in at least
one porous subsurface earth formation containing at least one
fluid; and process at least a portion of the one or more signals to
determine at least one property of the subsurface earth
formation.
[0307] 118. The system of embodiment 117, wherein the one or more
sensors comprise one or more electromagnetic field detectors that
measures the earth's electromagnetic field and produces a signal
indicative of the detected electromagnetic field; and wherein the
analysis tool further configures the processor to: receive the
signal from the one or more electromagnetic field detectors;
demodulate a portion of the signal to identify an envelope of the
signal; analyze the envelope to determine one or more properties
indicative of the presence of the hydrocarbons; and analyze the
envelope to determine the depth of the hydrocarbons in the
subterranean formation.
[0308] 119. A method of passive surveying comprising: generating
one or more detected signals by passively detecting a signal
generated within a subsurface earth formation due to a
seismoelectric response or an electroseismic response in at least
one porous subsurface earth formation containing at least one
fluid; and processing the one or more detected signals to determine
at least one property of the subsurface earth formation.
[0309] 120. The method of embodiment 119 wherein generating the one
or more detected signals comprises: generating a detected
electromagnetic field by detecting earth's electromagnetic field
using an electromagnetic field detector; and wherein processing the
one or more detected signals comprises: processing the detected
electromagnetic field to determine the at least one property of the
subsurface earth formation.
[0310] 121. The method of embodiment 120, wherein detecting the
earth's electromagnetic field comprises measuring a vertical
electric component of the earth's electromagnetic field.
[0311] 122. The method of embodiment 121, further comprising
generating a detected magnetic field by detecting earth's magnetic
field using a magnetic field detector, and processing the detected
magnetic field with the vertical electric component.
[0312] 123. The method of any of embodiments 120 to 122, wherein
detecting the earth's electromagnetic field comprises measuring the
earth's electromagnetic field in at least one horizontal
direction.
[0313] 124. The method of any of embodiments 120 to 123, wherein
the earth's electromagnetic field comprises a time-varying
electromagnetic field.
[0314] 125. The method of any of embodiments 120 to 124, wherein
the earth's electromagnetic field is detected using a pair of
porous pot electrodes.
[0315] 126. The method of embodiment 125, wherein the pair of
porous pot electrodes comprises at least one component selected
from the group consisting of: copper sulfate, silver chloride,
cadmium chloride, mercury chloride, and lead chloride.
[0316] 127. The method of any of embodiments 120 to 126, wherein
the earth's electromagnetic field is detected using a plurality of
pairs of porous pot electrodes.
[0317] 128. The method of any of embodiments 120 to 127, wherein
the earth's electromagnetic field is detected using a pair of
conductive electrodes.
[0318] 129. The method of embodiment 128, wherein the pair of metal
electrodes comprises at least one conductive material selected from
the group consisting of: copper, stainless steel, aluminum, gold,
galvanized metal, iron, lead, brass, graphite, and steel.
[0319] 130. The method of any of embodiments 120 to 129, wherein
the earth's electromagnetic field is detected using a conductive
electrode coupled to a porous pot electrode.
[0320] 131. The method of any of embodiments 120 to 130, wherein
the electromagnetic field detector is disposed on or above the
surface of the earth.
[0321] 132. The method of any of embodiments 120 to 131, wherein
the electromagnetic field detector is moving during the generation
of the detected electromagnetic field.
[0322] 133. The method of any of embodiments 120 to 132, wherein
the electromagnetic field detector is disposed in a moving
vehicle.
[0323] 134. The method of any of embodiments 120 to 133, wherein
the detected electromagnetic field is generated using one or more
antennas disposed on or above a surface of the earth.
[0324] 135. The method of embodiment 134, wherein the one or more
antennas comprises at least one antenna selected from the group
consisting of: a parallel-plate capacitor antenna comprising two or
more parallel conducting plates, a single-plate capacitor antenna
comprising one electrode electrically coupled to the earth, a
monopole antenna comprising a conducting element, a dipole antenna
comprising two conducting elements, a multi-pole antenna comprising
a plurality of conducting elements, a directional antenna
comprising conducting elements arranged to augment a signal
amplitude in a particular direction, a coil antenna comprising one
or more coils of wire, and any combination thereof.
[0325] 136. The method of embodiment 134, wherein the one or more
antennas comprise a concentric electric dipole.
[0326] 137. The method of any of embodiments 120 to 136, wherein
the at least one property of the subsurface earth formation
comprises at least one property selected from the group consisting
of: an existence of the subsurface earth formation containing at
least one fluid, a depth of the subsurface earth formation, a
porosity, a fluid permeability, a composition of at least one fluid
within the subsurface earth formation, a spatial extent of the
subsurface earth formation, an orientation of the boundaries of the
subsurface earth formation, a resistivity, and any combination
thereof.
[0327] 138. The method of any of embodiments 120 to 137, further
comprising recording the detected electromagnetic field.
[0328] 139. The method of embodiment 138, wherein the recording
occurs for at least about 5 seconds.
[0329] 140. The method of embodiment 138, wherein the modulated
reference signal is recorded for at least about 0.1 seconds.
[0330] 141. The method of any of embodiments 120 to 140, wherein
the at least one fluid comprises at least one component selected
from the group consisting of: an aqueous fluid, a hydrocarbon, a
petroleum, carbon dioxide, and any combination thereof.
[0331] 142. The method of embodiment 141 where the aqueous fluid
comprises at least one of potable water, fresh water, or brine.
[0332] 143 The method of any of embodiments 120 to 142, wherein the
detected electromagnetic field results, at least in part, from a
natural event comprising at least one of: an electromagnetic
fluctuation in the ionosphere, and a naturally occurring electric
discharge in the atmosphere.
[0333] 144. The method of any of embodiments 120 to 143, further
comprising a plurality of electromagnetic field detectors.
[0334] 145. The method of embodiment 144, wherein the plurality of
electromagnetic field detectors are arranged in an array.
[0335] 146. The method of any of embodiments 120 to 145, further
comprising performing a correlation analysis of the detected
electromagnetic field in the time domain, the frequency domain, or
both, wherein the correlation analysis provides an indication of a
presence of the at least one fluid.
[0336] 147. The method of embodiment 146, wherein performing the
correlation analysis provides at least one of temporal
characteristics or frequency characteristics.
[0337] 148. The method of embodiment 147, further comprising
deriving a frequency-depth function based on the temporal
characteristics, the frequency characteristics, or both.
[0338] 149. The method of embodiment 148, wherein the
frequency-depth function is derived using data from a known
location.
[0339] 150. The method of any of embodiments 120 to 149, further
comprising repeating the passively detecting, and processing a
plurality of times.
[0340] 151. The method of embodiment 150, wherein the repeating is
performed at a single location.
[0341] 152. The method of embodiment 150 or 151, wherein the
repeating is performed at different locations.
[0342] 153. The method of any of embodiments 150 to 152, wherein
the repeating is performed sequentially or concurrently at each of
the different locations.
[0343] 154. The method of embodiment 153, wherein the different
locations correspond to a plurality of grid positions.
[0344] 155. The method of embodiment 154, further comprising
generating a multi-dimension model of a subterranean formation
using a plurality of detected electromagnetic field signals,
wherein at least one signal of the plurality of signals is
determined at each corresponding grid position of the plurality of
grid positions.
[0345] 156. The method of any of embodiments 120 to 155, further
comprising: performing a further survey of the subterranean
formation when a presence of the at least one fluid is
detected.
[0346] 157. The method of embodiment 156, wherein the further
survey comprises a seismological survey, a controlled-source
electromagnetics survey, a controlled-source electroseismic survey,
a controlled-source seismoelectric survey, a gravity survey, a
magnetic survey, or a passive survey.
[0347] 158. The method of any of embodiments 120 to 157, wherein
processing the one or more detected signals further comprises:
filtering the detected electromagnetic field.
[0348] 159. The method of embodiment 158, wherein the filtering and
processing occur in real time.
[0349] 160. The method of embodiment 158 or 159, wherein filtering
comprises filtering a direct current (DC) portion of the detected
electromagnetic field prior to performing the processing.
[0350] 161. The method of any of embodiments 158 to 160, wherein
filtering comprises decimating a data set representing the detected
electromagnetic field prior to performing the processing.
[0351] 162. The method of any of embodiments 158 to 161, wherein
filtering comprises using a noise filter.
[0352] 163. The method of embodiment 162, wherein the noise filter
comprises a high pass filter, a low pass filter, a wide band
frequency filter, a narrow band frequency filter, or any
combination thereof.
[0353] 164. The method of any of embodiments 158 to 163, wherein
filtering comprises using one or more band-pass filters.
[0354] 165. The method of embodiment 164, wherein at least one of
the band-pass filters of the one or more band-pass filters
comprises a linear phase filter, a finite impulse response filter,
a forward infinite impulse response filter, a reverse infinite
impulse response filter, or any combination thereof.
[0355] 166. The method of any of embodiments 120 to 165, wherein
processing the detected electromagnetic field comprises averaging
an amplitude of the detected electromagnetic field over one or more
frequencies.
[0356] 167. The method of embodiment 166, wherein the plurality of
frequencies comprises one or more fixed frequencies.
[0357] 168. The method of embodiment 166 or 167, wherein the
averaging comprises measuring an amplitude of the detected
electromagnetic field for a length of time that is greater than a
period of oscillation of the detected electromagnetic field at the
one or more frequencies; and averaging the amplitude over the
length of time.
[0358] 169. The method of any of embodiments 120 to 168, wherein
processing the detected electromagnetic field comprises determining
one or more spectral properties of the detected electromagnetic
field.
[0359] 170. The method of embodiment 169, wherein determining one
or more spectral properties comprises using at least one of a
lock-in amplifier or a spectrum analyzer.
[0360] 171. The method of embodiment 169, wherein determining one
or more spectral properties comprises determining a relative
amplitude of each frequency at which an amplitude is
determined.
[0361] 172. The method of any of embodiments 169 to 171, further
comprising analyzing the one or more spectral properties to
determine a relationship between the one or more spectral
properties and the presence of the at least one fluid in the
subterranean formation based on a classification method comprising
at least one method selected from the group consisting of: a neural
network, a decision tree, a Bayes-based classifier, a fuzzy
logic-based classifier, and a conventional statistical
classifier.
[0362] 173. The method of any of embodiments 169 to 172, further
comprising analyzing the one or more spectral properties to
determine a relationship between the one or more spectral
properties and a presence of the at least one fluid in the
subterranean formation based on a regression analysis of a set of
known empirical data.
[0363] 174. The method of any of embodiments 169 to 173, wherein
determining one or more spectral properties comprises calculating
one or more power spectral densities.
[0364] 175. The method of embodiment 174, further comprising
de-trending of the one or more power spectral densities.
[0365] 176. The method of embodiment 174 or 175, further comprising
integrating each of the one or more power spectral densities.
[0366] 177. The method of any of embodiments 174 to 176, further
comprising performing a fast Fourier transform of the one or more
power spectral densities; and determining one or more correlations
between a source signal and the detected electromagnetic field
based on the fast Fourier transform.
[0367] 178. The method of embodiment 177, wherein an existence of
the one or more correlations provides an indication of the presence
of the at least one fluid in the subterranean formation.
[0368] 179. The method of embodiment 177 or 178, wherein the one or
more correlations indicate one or more transit times between the
subterranean formation and the surface of the earth.
[0369] 180. The method of embodiment 179, further comprising
correlating the one or more transit times to a depth of the fluid
in the subterranean formation.
[0370] 181. The method of embodiment 180, wherein the correlating
is based on estimate rock acoustic properties of the earth.
[0371] 182. The method of embodiment 180 or 181, wherein
correlating comprises determining a frequency-depth
relationship.
[0372] 183. The method of embodiment 182, wherein the
frequency-depth relationship is determined based on a
classification method comprising at least one method selected from
the group consisting of: a neural network, a decision tree, a
Bayes-based classifier, a fuzzy logic-based classifier, and a
conventional statistical classifier.
[0373] 184. The method of embodiment 182 or 183, wherein the
frequency-depth relationship is determined based on a regression
analysis of a set of known empirical data.
[0374] 185. The method of any of embodiments 120 to 184, further
comprising: developing a geological model of the subsurface earth
formation; determining at least one predicted result from the
geological model; comparing the at least one predicted result to
the detected electromagnetic field; and determining the at least
one property of the subsurface earth formation based on the
comparing.
[0375] 186. The method of any of embodiments 120 to 185, wherein
generating the one or more detected signals further comprises:
generating at least one nonlinear electromagnetic field response by
isolating the at least one nonlinear electromagnetic field response
from the detected electromagnetic field; and wherein processing the
one or more detected signals further comprises: processing the at
least one nonlinear electromagnetic field response to determine at
least one property of the subsurface earth formation.
[0376] 187. A system for identifying hydrocarbons in a subterranean
formation comprising: one or more sensors coupled to a processor
that detect one or more signals generated within a subsurface earth
formation due to a seismoelectric response or an electroseismic
response in at least one porous subsurface earth formation
containing at least one fluid; and an analysis tool, that when
executed on the processor, configures the processor to: receive the
one or more signals from the one or more sensors; process at least
a portion of the one or more signals to determine at least one
property of the subsurface earth formation.
[0377] 188. The system of embodiment 187, wherein the one or more
sensors comprise: an electromagnetic field detector that measures
the earth's electromagnetic field and produces a signal indicative
of the detected electromagnetic field; and wherein the analysis
tool receives the signal and determines the at least one property
of a subsurface earth formation.
[0378] 189. The system of embodiment 188, wherein the
electromagnetic field detector is disposed within the surface of
the earth.
[0379] 190. The system of embodiment 188 or 189, further comprising
a plurality of electromagnetic field detectors.
[0380] 191. The system of any of embodiments 188 to 190, wherein
the signal comprises a vertical component of the earth's
electromagnetic field.
[0381] 192. The system of any of embodiments 188 to 191, wherein
the signal comprises at least one horizontal component of the
earth's electromagnetic field.
[0382] 193. The system of any of embodiments 188 to 192, wherein
the signal is indicative of a time-varying electromagnetic
field.
[0383] 194. The system of any of embodiments 188 to 193, wherein
the electromagnetic field detector comprises a plurality of pairs
of porous pot electrodes, wherein each pair of porous pot
electrodes are electrically coupled.
[0384] 195. The system of embodiment 194, wherein the pairs of
porous pot electrodes comprise at least one component selected from
the group consisting of: copper sulfate, silver chloride, cadmium
chloride, mercury chloride, and lead chloride.
[0385] 196. The system of any of embodiments 188 to 195, wherein
the electromagnetic field detector comprises a plurality of pairs
of conductive electrodes, wherein each pair of conductive
electrodes are electrically coupled.
[0386] 197. The system of any of embodiments 188 to 196, wherein
the plurality of conductive electrodes comprises at least one
conductive material selected from the group consisting of: copper,
stainless steel, aluminum, gold, galvanized metal, iron, lead,
brass, graphite, steel, alloys thereof, and any combination
thereof.
[0387] 198. The system of any of embodiments 188 to 197, wherein
the electromagnetic field detector comprises a conductive electrode
coupled to a porous pot electrode.
[0388] 199. The system of any of embodiments 188 to 198, wherein
the at least one property of the subsurface earth formation
comprises at least one property selected from the group consisting
of: an existence of the subsurface earth formation containing at
least one fluid, a depth of the subsurface earth formation, a
porosity, a fluid permeability, a composition of at least one fluid
within the subsurface earth formation, a spatial extent of the
subsurface earth formation, an orientation of the boundaries of the
subsurface earth formation, a resistivity, and any combination
thereof.
[0389] 200. The system any of embodiments 188 to 199, further
comprising a recording apparatus configured to record the signal on
a non-transitory media.
[0390] 201. The system of embodiment 200, wherein the recording
apparatus comprises a digital or analog recording device.
[0391] 202. The system of any of embodiments 188 to 201, wherein
the at least one fluid comprises at least one component selected
from the group consisting of: an aqueous fluid, a hydrocarbon, a
petroleum, carbon dioxide, and any combination thereof.
[0392] 203. The system of embodiment 202, wherein the aqueous fluid
comprises at least one of potable water, fresh water, or brine.
[0393] 204. The system of any of embodiments 188 to 203, wherein
the electromagnetic field detector comprises an antenna disposed on
or above the surface of the earth.
[0394] 205. The system of embodiment 204, wherein the antenna
comprises at least one antenna selected from the group consisting
of: a parallel-plate capacitor antenna comprising two or more
parallel conducting plates, a single-plate capacitor antenna
comprising one electrode electrically coupled to the earth, a
monopole antenna comprising a conducting element, a dipole antenna
comprising two conducting elements, a multi-pole antenna comprising
a plurality of conducting elements, a directional antenna
comprising conducting elements arranged to augment a signal
amplitude in a particular direction, a coil antenna comprising one
or more coils of wire, and any combination thereof.
[0395] 206. The system of embodiment 204, wherein the antenna
comprises a concentric electric dipole.
[0396] 207. The system of any of embodiments 188 to 206, wherein
the analysis tool further configures the processor to: remove a
direct current (DC) portion of the signal.
[0397] 208. The system of any of embodiments 188 to 207, wherein
the analysis tool further configures the processor to: decimate a
data set representing the signal.
[0398] 209. The system of any of embodiments 188 to 208, wherein
the analysis tool further configures the processor to: filter a
noise component from the signal.
[0399] 210. The system of any of embodiments 188 to 209, wherein
the analysis tool further configures the processor to: filter the
signal using a predetermined frequency bandwidth.
[0400] 211. In an embodiment, a system for identifying hydrocarbons
in a subterranean formation comprises: a memory comprising a
non-transitory computer readable media; a processor; and an
analysis tool, that when executed on the processor, configures the
processor to: receive one or more signals from one or more sensors,
wherein the one or more sensors detect one or more signals
generated within a subsurface earth formation due to a
seismoelectric response or an electroseismic response in at least
one porous subsurface earth formation containing at least one
fluid; and process at least a portion of the one or more signals to
determine at least one property of the subsurface earth
formation.
[0401] 212. The system of embodiment 211, wherein the one or more
sensors comprise an electromagnetic field detector that measures
the earth's electromagnetic field and produces a signal indicative
of the detected electromagnetic field; and wherein the analysis
tool configures the processor to receive the signal and determine
the at least one property of a subsurface earth formation.
[0402] 213. A method of passive surveying comprising: generating
one or more detected signals by passively detecting a signal
generated within a subsurface earth formation due to a
seismoelectric response or an electroseismic response in at least
one porous subsurface earth formation containing at least one
fluid; and processing the one or more detected signals to determine
at least one property of the subsurface earth formation.
[0403] 214. The method of embodiment 213, wherein generating the
one or more detected signals comprises: generating a detected
seismic signal by passively detecting a seismic wave generated
within the subsurface earth formation due to the seismoelectric
response or the electroseismic response in the at least one porous
subsurface earth formation containing the at least one fluid using
a seismic sensor, and wherein processing the one or more detected
signals comprises processing the detected seismic signal to
determine the at least one property of the subsurface earth
formation.
[0404] 215. The method of embodiment 214, wherein the seismic
sensor comprises at least one sensor selected from the group
consisting of: a hydrophone, a single-component geophone, a
two-component geophone, a three-component geophone, a single-axis
accelerometer, a two-axis accelerometer, and a three-axis
accelerometer.
[0405] 216. The method of embodiment 214 or 215, wherein the at
least one property of the subsurface earth formation comprises at
least one property selected from the group consisting of: an
existence of the subsurface earth formation containing at least one
fluid, a depth of the subsurface earth formation, a porosity, a
fluid permeability, a composition of at least one fluid within the
subsurface earth formation, a spatial extent of the subsurface
earth formation, an orientation of the boundaries of the subsurface
earth formation, a resistivity, and any combination thereof.
[0406] 217. The method of any of embodiments 214 to 216, further
comprising recording the detected seismic signal.
[0407] 218. The method of any of embodiments 214 to 217, wherein
the at least one fluid comprises at least one component selected
from the group consisting of: an aqueous fluid, a hydrocarbon, a
petroleum, carbon dioxide, and any combination thereof.
[0408] 219. The method of embodiment 218, wherein the aqueous fluid
comprises at least one of potable water, fresh water, or brine.
[0409] 220. The method of any of embodiments 214 to 219, wherein
the seismic sensor is disposed on the surface of the earth, within
a wellbore, or both.
[0410] 221. The method of any of embodiments 214 to 220, further
comprising filtering the detected seismic signal to generate a
filtered signal.
[0411] 222. The method of embodiment 221, wherein the filtering and
processing occur in real time.
[0412] 223. The method of embodiment 221 or 222, wherein filtering
comprises decimating a data set representing the detected seismic
signal prior to performing the processing.
[0413] 224. The method of any of embodiments 221 to 223, wherein
filtering the detected seismic signal comprises using a noise
filter.
[0414] 225. The method of embodiment 224, wherein the noise filter
comprises a high pass filter, a low pass filter, a wide band
frequency filter, a narrow band frequency filter, or any
combination thereof.
[0415] 226. The method of any of embodiments 221 to 225, wherein
filtering the detected seismic signal comprises using a plurality
of band-pass filters to generate the filtered signal.
[0416] 227. The method of embodiment 226, wherein at least one of
the band-pass filters of the plurality of band-pass filters
comprises a linear phase filter, a finite impulse response filter,
a forward infinite impulse response filter, a reverse infinite
impulse response filter, or any combination thereof.
[0417] 228. The method of any of embodiments 214 to 227, further
comprising: generating the detected seismic signal using a
plurality of seismic sensors.
[0418] 229. The method of embodiment 228, wherein processing the
detected seismic signal comprises: filtering at least a portion of
a seismic wave traveling in a direction perpendicular to vertical
by applying a spatial filter to the detected seismic signal from
the plurality of seismic sensors.
[0419] 230. The method of embodiment 229, wherein the spatial
filter is based, at least in part, on the spreading symmetry of the
seismic wave traveling in the direction perpendicular to
vertical.
[0420] 231. The method of any of embodiments 228 to 230, wherein
processing the detected seismic signal comprises: removing a
seismic noise from a vertical component of the detected seismic
signal by applying a predictive filter to the detected seismic
signal based on using a horizontal component of the detected
seismic signal from the plurality of seismic sensors.
[0421] 232. The method of any of embodiments 228 to 231, wherein
processing the detected seismic signal comprises: applying a dip
filter to the detected seismic signal based on rejecting at least a
portion of the detected seismic signals from the plurality of
seismic sensors that arrive at a non-normal angle to a surface,
wherein the surface is defined by the plurality of seismic
sensors.
[0422] 233. The method of any of embodiments 214 to 232, wherein
processing the detected seismic signal comprises averaging an
amplitude of the detected seismic signal over one or more
frequencies.
[0423] 234. The method of embodiment 233, wherein the plurality of
frequencies comprises one or more fixed frequencies.
[0424] 235. The method of embodiment 233 or 234, wherein the
averaging comprises measuring the detected seismic signal amplitude
for a length of time that is greater than a period of oscillation
of the signal at the one or more frequencies; and averaging the
detected seismic signal amplitude over the length of time.
[0425] 236. The method of any of embodiments 214 to 235, wherein
processing the detected seismic signal comprises determining one or
more spectral properties of the detected seismic signal.
[0426] 237. The method of embodiment 236, wherein determining one
or more spectral properties comprises using at least one of a
lock-in amplifier or a spectrum analyzer.
[0427] 238. The method of embodiment 236 or 237, further comprising
analyzing the one or more spectral properties to determine a
relationship between the one or more spectral properties and the
presence of the fluid in a pore in the subterranean formation based
on a classification method comprising at least one method selected
from the group consisting of: a neural network, a decision tree, a
Bayes-based classifier, a fuzzy logic-based classifier, and a
conventional statistical classifier.
[0428] 239. The method of any of embodiments 236 to 238, further
comprising analyzing the one or more spectral properties to
determine a relationship between the one or more spectral
properties and a presence of the fluid in a pore in the
subterranean formation based on a regression analysis of a set of
known empirical data.
[0429] 240. The method of any of embodiments 236 to 239, wherein
determining one or more spectral properties comprises determining a
relative amplitude of each frequency at which an amplitude is
determined.
[0430] 241. The method of any of embodiments 236 to 240, wherein
determining one or more spectral properties comprises calculating a
power spectral density.
[0431] 242. The method of embodiment 241, further comprising
de-trending of the power spectral density.
[0432] 243. The method of embodiment 241 or 242, further comprising
integrating the power spectral density.
[0433] 244. The method of any of embodiments 241 to 243, further
comprising performing a fast Fourier transform of the power
spectral density; and determining one or more correlations between
a source signal and the detected seismic signal based on the fast
Fourier transform.
[0434] 245. The method of embodiment 244, wherein an existence of
the one or more correlations provides an indication of the presence
of the fluid in the subterranean formation.
[0435] 246. The method of embodiment 244 or 245, wherein the one or
more correlations indicate one or more transit times between the
subterranean formation and the surface of the earth.
[0436] 247. The method of embodiment 246, further comprising
correlating the one or more transit times to a depth of the fluid
in the subterranean formation.
[0437] 248. The method of embodiment 247, wherein the correlating
is based on estimate rock acoustic properties of the earth.
[0438] 249. The method of embodiment 247 or 248, wherein
correlating comprises determining a frequency-depth
relationship.
[0439] 250. The method of embodiment 249, wherein the
frequency-depth relationship is determined based on a
classification method comprising at least one method selected from
the group consisting of: a neural network, a decision tree, a
Bayes-based classifier, a fuzzy logic-based classifier, and a
conventional statistical classifier.
[0440] 251. The method of embodiment 249 or 250, wherein the
frequency-depth relationship is determined based on a regression
analysis of a set of known empirical data.
[0441] 252. The method of any of embodiments 214 to 251, further
comprising performing a correlation analysis of the detected
seismic signal in the time domain, the frequency domain, or both,
wherein the correlation analysis provides an indication of a
presence of the at least one fluid.
[0442] 253. The method of embodiment 252, wherein performing the
correlation analysis provides at least one of temporal
characteristics or frequency characteristics.
[0443] 254. The method of embodiment 253, further comprising
deriving a frequency-depth function based on the temporal
characteristics, the frequency characteristics, or both.
[0444] 255. The method of embodiment 254, wherein the
frequency-depth function is derived using data from a known
location.
[0445] 256. The method of any of embodiments 214 to 255, further
comprising repeating the passively detecting, and processing a
plurality of times.
[0446] 257. The method of embodiment 256, wherein the repeating is
performed at a single location.
[0447] 258. The method of embodiment 256, wherein the repeating is
performed at different locations.
[0448] 259. The method of embodiment 258, wherein the repeating is
performed sequentially or concurrently at each of the different
locations.
[0449] 260. The method of embodiment 258 or 259, wherein the
different locations correspond to a plurality of grid
positions.
[0450] 261. The method of embodiment 260, further comprising
generating a multi-dimension model of a subterranean formation
using a plurality of detected seismic signals, wherein at least one
signal of the plurality of detected seismic signals is determined
at each corresponding grid position of the plurality of grid
positions.
[0451] 262. The method of any of embodiments 214 to 261, further
comprising: performing a further survey of the subterranean
formation when a presence of the at least one fluid is
detected.
[0452] 263. The method of embodiment 262, wherein the further
survey comprises a seismological survey, a controlled-source
electromagnetics survey, a controlled-source electroseismic survey,
a controlled-source seismoelectric survey, a gravity survey, a
magnetic survey, or a passive survey.
[0453] 264. The method of any of embodiments 214 to 263, further
comprising: developing a geological model of the subsurface earth
formation; determining at least one predicted result from the
geological model; comparing the at least one predicted result to
the detected seismic signal; and determining the at least one
property of the subsurface earth formation based on the
comparing.
[0454] 265. The method of any of embodiments 214 to 264, wherein
generating the one or more detected signals further comprises:
generating at least one nonlinear seismic signal by isolating the
at least one nonlinear seismic signal from the detected seismic
signal; and wherein processing the one or more detected signals
further comprises: processing the at least one nonlinear seismic
signal to determine at least one property of the subsurface earth
formation.
[0455] 266. In an embodiment, a system for identifying hydrocarbons
in a subterranean formation comprises: one or more sensors coupled
to a processor that detect one or more signals generated within a
subsurface earth formation due to a seismoelectric response or an
electroseismic response in at least one porous subsurface earth
formation containing at least one fluid; and an analysis tool, that
when executed on the processor, configures the processor to:
receive the one or more signals from the one or more sensors;
process at least a portion of the one or more signals to determine
at least one property of the subsurface earth formation.
[0456] 267. The system of embodiment 266, wherein the one or more
sensors comprise a plurality of seismic sensors that detect a
seismic signal related to earth's electromagnetic field and produce
a first signal indicative of the detected seismic signal, and
wherein the analysis tool receives the first signal and processes
the first signal to determine the at least one property of the
subsurface earth formation.
[0457] 268. The system of embodiment 267, wherein the plurality of
seismic sensors is arranged in an array over a portion of the
subsurface earth formation.
[0458] 269. The system of embodiment 267 or 268, wherein the
distance between each of the plurality of seismic sensors is less
than one half of the wavelength of the surface waves at an expected
surface wave frequency.
[0459] 270. The system of any of embodiments 267 to 269, wherein
the plurality of seismic sensors comprises at least one sensor
selected from the group consisting of: a hydrophone, a
single-component geophone, a two-component geophone, a
three-component geophone, a single-axis accelerometer, a two-axis
accelerometer, a three-axis accelerometer, and any combination
thereof.
[0460] 271. The system of any of embodiments 267 to 270, wherein
the at least one property of the subsurface earth formation
comprises at least one property selected from the group consisting
of: an existence of the subsurface earth formation containing at
least one fluid, a depth of the subsurface earth formation, a
porosity, a fluid permeability, a composition of at least one fluid
within the subsurface earth formation, a spatial extent of the
subsurface earth formation, an orientation of the boundaries of the
subsurface earth formation, a resistivity, and any combination
thereof.
[0461] 272. The system of any of embodiments 267 to 271, further
comprising a recording apparatus configured to record the first
signal on a non-transitory media.
[0462] 273. The system of any of embodiments 267 to 272, wherein
the at least one fluid comprises at least one component selected
from the group consisting of: an aqueous fluid, a hydrocarbon, a
petroleum, carbon dioxide, and any combination thereof.
[0463] 274. The method of embodiment 273, wherein the aqueous fluid
comprises at least one of potable water, fresh water, or brine.
[0464] 275. The system of any of embodiments 267 to 274, wherein at
least one of the plurality of seismic sensors is disposed on the
surface of the earth or within a wellbore.
[0465] 276. The system of any of embodiments 267 to 275, wherein
the analysis tool further configures the processor to: decimate a
data set representing the first signal.
[0466] 277. The system of any of embodiments 267 to 276, wherein
the analysis tool further configures the processor to: filter a
noise component from the first signal.
[0467] 278. The system of any of embodiments 267 to 277, wherein
the analysis tool further configures the processor to: filter the
first signal using a predetermined frequency bandwidth.
[0468] 279. In an embodiment, a system for identifying hydrocarbons
in a subterranean formation comprises: a memory comprising a
non-transitory computer readable media; a processor; and an
analysis tool, that when executed on the processor, configures the
processor to: receive one or more signals from one or more sensors,
wherein the one or more sensors detect one or more signals
generated within a subsurface earth formation due to a
seismoelectric response or an electroseismic response in at least
one porous subsurface earth formation containing at least one
fluid; and process at least a portion of the one or more signals to
determine at least one property of the subsurface earth
formation.
[0469] 280. The system of embodiment 279, wherein the one or more
sensors comprise a plurality of seismic sensors that detect a
seismic signal related to earth's electromagnetic field and produce
a first signal indicative of the detected seismic signal, and
wherein the analysis tool configures the processor to receive the
first signal, and process the first signal to determine the at
least one property of the subsurface earth formation.
[0470] 281. In an embodiment, a method of passive surveying
comprises: generating one or more detected signals by passively
detecting a signal generated within a subsurface earth formation
due to a seismoelectric response or an electroseismic response in
at least one porous subsurface earth formation containing at least
one fluid; and processing the one or more detected signals to
determine at least one property of the subsurface earth
formation.
[0471] 282. The method of embodiment 281, wherein generating the
one or more detected signals comprises: generating a detected
electromagnetic field by detecting earth's electromagnetic field
using an electromagnetic field detector; and generating a detected
seismic signal by detecting a seismic wave at least partially
resulting from the electroseismic response in the at least one
porous subsurface earth formation containing the at least one fluid
using a seismic sensor; and wherein processing the one or more
detected signals comprises: processing the detected electromagnetic
field and the detected seismic signal to determine the at least one
property of the subsurface earth formation.
[0472] 283. The method of embodiment 282, wherein the detected
electromagnetic field is generated at a different time than the
detected seismic signal.
[0473] 284. The method of embodiment 282 or 283, wherein the
detected electromagnetic field is generated at a different location
than the detected seismic signal.
[0474] 285. The method of any of embodiments 282 to 284, wherein
detecting the earth's electromagnetic field comprises measuring a
vertical electric component of the earth's electromagnetic
field.
[0475] 286. The method of embodiment 285, further comprising
generating a detected magnetic field by detecting earth's magnetic
field using a magnetic field detector, and processing the detected
magnetic field with the vertical electric component and the
detected seismic signal.
[0476] 287. The method of any of embodiments 282 to 286, wherein
detecting the earth's electromagnetic field comprises measuring the
earth's electromagnetic field in at least one horizontal
direction.
[0477] 288. The method of any of embodiments 282 to 287, wherein
the earth's electromagnetic field comprises a time-varying
electromagnetic field.
[0478] 289. The method of any of embodiments 282 to 288, wherein
the earth's electromagnetic field is detected using a pair of
porous pot electrodes.
[0479] 290. The method of embodiment 289, wherein the pair of
porous pot electrodes comprises at least one component selected
from the group consisting of: copper sulfate, silver chloride,
cadmium chloride, mercury chloride, and lead chloride.
[0480] 291. The method of any of embodiments 282 to 290, wherein
the earth's electromagnetic field is detected using a plurality of
pairs of porous pot electrodes.
[0481] 292. The method of any of embodiments 282 to 291, wherein
the earth's electromagnetic field is detected using a pair of
conductive electrodes.
[0482] 293. The method of embodiment 292, wherein the pair of metal
electrodes comprises at least one conductive material selected from
the group consisting of: copper, stainless steel, aluminum, gold,
galvanized metal, iron, lead, brass, graphite, and steel.
[0483] 294. The method of any of embodiments 282 to 293, wherein
the earth's electromagnetic field is detected using a conductive
electrode coupled to a porous pot electrode.
[0484] 295. The method of any of embodiments 282 to 294, wherein
the electromagnetic field detector is disposed on or above the
surface of the earth.
[0485] 296. The method of any of embodiments 282 to 295, wherein
the electromagnetic field detector is moving during the generation
of the detected electromagnetic field.
[0486] 297. The method of any of embodiments 282 to 296, wherein
the electromagnetic field detector is disposed in a moving
vehicle.
[0487] 298. The method of any of embodiments 282 to 297, wherein
the detected electromagnetic field is generated using one or more
antennas disposed on or above a surface of the earth.
[0488] 299. The method of embodiment 298, wherein the one or more
antennas comprises at least one antenna selected from the group
consisting of: a parallel-plate capacitor antenna comprising two or
more parallel conducting plates, a single-plate capacitor antenna
comprising one electrode electrically coupled to the earth, a
monopole antenna comprising a conducting element, a dipole antenna
comprising two conducting elements, a multi-pole antenna comprising
a plurality of conducting elements, a directional antenna
comprising conducting elements arranged to augment a signal
amplitude in a particular direction, a coil antenna comprising one
or more coils of wire, and any combination thereof.
[0489] 300. The method of embodiment 298, wherein the one or more
antennas comprise a concentric electric dipole.
[0490] 301. The method of any of embodiments 282 to 300, wherein
the seismic sensor comprises at least one sensor selected from the
group consisting of: a hydrophone, a single-component geophone, a
two-component geophone, a three-component geophone, a single-axis
accelerometer, a two-axis accelerometer, and a three-axis
accelerometer.
[0491] 302. The method of any of embodiments 282 to 301, wherein
the at least one property of the subsurface earth formation
comprises at least one property selected from the group consisting
of: an existence of the subsurface earth formation containing at
least one fluid, a depth of the subsurface earth formation, a
porosity, a fluid permeability, a composition of at least one fluid
within the subsurface earth formation, a spatial extent of the
subsurface earth formation, an orientation of the boundaries of the
subsurface earth formation, a resistivity, and any combination
thereof.
[0492] 303. The method of any of embodiments 282 to 302, further
comprising recording at least one of the detected electromagnetic
field or the detected seismic signal.
[0493] 304. The method of embodiment 303, wherein the recording
occurs for at least about 5 seconds.
[0494] 305. The method of embodiment 303, wherein the modulated
reference signal is recorded for at least about 0.1 seconds.
[0495] 306. The method of any of embodiments 282 to 305, wherein
the at least one fluid comprises at least one component selected
from the group consisting of: an aqueous fluid, a hydrocarbon, a
petroleum, carbon dioxide, and any combination thereof.
[0496] 307. The method of embodiment 306, wherein the aqueous fluid
comprises at least one of potable water, fresh water, or brine.
[0497] 308 The method of any of embodiments 282 to 307, wherein the
detected electromagnetic field results, at least in part, from a
natural event comprising at least one of: an electromagnetic
fluctuation in the ionosphere, and a naturally occurring electric
discharge in the atmosphere.
[0498] 309. The method of any of embodiments 282 to 308, wherein
the seismic wave is detected on the surface of the earth.
[0499] 310. The method of any of embodiments 282 to 309, wherein
the seismic wave is detected within a wellbore.
[0500] 311. The method of any of embodiments 282 to 310, further
comprising a plurality of electromagnetic field detectors.
[0501] 312. The method of embodiment 311, wherein the plurality of
electromagnetic field detectors are arranged in an array.
[0502] 313. The method of any of embodiments 282 to 312, further
comprising a plurality of seismic sensors.
[0503] 314. The method of embodiment 313, wherein the plurality of
seismic sensors are arranged in an array over a portion of the
subsurface earth formation.
[0504] 315. The method of embodiment 313 or 314, wherein processing
the detected electromagnetic field and the detected seismic signal
comprises: filtering at least a portion of a seismic wave traveling
in a direction perpendicular to vertical by applying a spatial
filter to the detected seismic signal from the plurality of seismic
sensors.
[0505] 316. The method of embodiment 315, wherein the spatial
filter is based, at least in part, on the spreading symmetry of the
seismic wave traveling in the direction perpendicular to
vertical.
[0506] 317. The method of any of embodiments 313 to 316, wherein
processing the detected electromagnetic field and the detected
seismic signal comprises: removing a seismic noise from a vertical
component of the detected seismic signal by applying a predictive
filter to the detected seismic signal based on using a horizontal
component of the detected seismic signal from the plurality of
seismic sensors.
[0507] 318. The method of any of embodiments 313 to 317, wherein
processing the detected electromagnetic field and the detected
seismic signal comprises: applying a dip filter to the detected
seismic signal based on rejecting at least a portion of the
detected seismic signals from the plurality of seismic sensors that
arrive at a non-normal angle to a surface, wherein the surface is
defined by the plurality of seismic sensors.
[0508] 319. The method of any of embodiments 313 to 318, wherein
processing the detected electromagnetic field and the detected
seismic signal comprises: producing cross-correlated seismic data
by cross-correlating the detected seismic signal from all of the
plurality of seismic sensors; generating summed data by summing the
cross-correlated seismic data; and cross-correlating the summed
data with the detected electromagnetic field.
[0509] 320. The method of any of embodiments 282 to 319, further
comprising performing a correlation analysis of at least one of the
detected electromagnetic field or the detected seismic signal in
the time domain, the frequency domain, or both, wherein the
correlation analysis provides an indication of a presence of the at
least one fluid.
[0510] 321. The method of embodiment 320, wherein performing the
correlation analysis provides at least one of temporal
characteristics or frequency characteristics.
[0511] 322. The method of embodiment 321, further comprising
deriving a frequency-depth function based on the temporal
characteristics, the frequency characteristics, or both.
[0512] 323. The method of embodiment 322, wherein the
frequency-depth function is derived using data from a known
location.
[0513] 324. The method of any of embodiments 282 to 323, further
comprising repeating the passively detecting, and processing a
plurality of times.
[0514] 325. The method of embodiment 324, wherein the repeating is
performed at a single location.
[0515] 326. The method of embodiment 324, wherein the repeating is
performed at different locations.
[0516] 327. The method of embodiment 326, wherein the repeating is
performed sequentially or concurrently at each of the different
locations.
[0517] 328. The method of embodiment 326 or 327, wherein the
different locations correspond to a plurality of grid
positions.
[0518] 329. The method of embodiment 328, further comprising
generating a multi-dimension model of a subterranean formation
using a plurality of signals, wherein at least one signal of the
plurality of signals is determined at each corresponding grid
position of the plurality of grid positions.
[0519] 330. The method of any of embodiments 282 to 329, further
comprising: performing a further survey of the subterranean
formation when a presence of the at least one fluid is
detected.
[0520] 331. The method of embodiment 330, wherein the further
survey comprises a seismological survey, a controlled-source
electromagnetics survey, a controlled-source electroseismic survey,
a controlled-source seismoelectric survey, a gravity survey, a
magnetic survey, or a passive survey.
[0521] 332. The method of any of embodiments 282 to 331, wherein
processing the one or more detected signals further comprises:
filtering at least one of the detected electromagnetic field or the
detected seismic signal.
[0522] 333. The method of embodiment 332, wherein the filtering and
processing occur in real time.
[0523] 334. The method of embodiment 332 or 333, wherein filtering
comprises filtering a direct current (DC) portion of the detected
electromagnetic field prior to performing the processing.
[0524] 335. The method of any of embodiments 332 to 334, wherein
filtering comprises decimating a data set representing the signal
prior to performing the processing.
[0525] 336. The method of any of embodiments 332 to 335, wherein
filtering comprises using a noise filter.
[0526] 337. The method of embodiment 336, wherein the noise filter
comprises a high pass filter, a low pass filter, a wide band
frequency filter, a narrow band frequency filter, or any
combination thereof.
[0527] 338. The method of any of embodiments 332 to 337, wherein
filtering comprises using one or more band-pass filters.
[0528] 339. The method of embodiment 338, wherein at least one of
the band-pass filters of the one or more band-pass filters
comprises a linear phase filter, a finite impulse response filter,
a forward infinite impulse response filter, a reverse infinite
impulse response filter, or any combination thereof.
[0529] 340. The method of any of embodiments 282 to 339, wherein
processing the detected electromagnetic field and the detected
seismic signal comprises averaging an amplitude of at least one of
the detected electromagnetic field or the detected seismic signal
over one or more frequencies.
[0530] 341. The method of embodiment 340, wherein the plurality of
frequencies comprises one or more fixed frequencies.
[0531] 342. The method of embodiment 340 or 341, wherein the
averaging comprises measuring an amplitude of the at least one of
the detected electromagnetic field or the detected seismic signal
for a length of time that is greater than a period of oscillation
of the at least one of the detected electromagnetic field or the
detected seismic signal at the one or more frequencies; and
averaging the amplitude over the length of time.
[0532] 343. The method of any of embodiments 282 to 342, wherein
processing at least one of the detected electromagnetic field or
the detected seismic signal comprises determining one or more
spectral properties of the at least one of the detected
electromagnetic field or the detected seismic signal.
[0533] 344. The method of embodiment 343, wherein determining one
or more spectral properties comprises using at least one of a
lock-in amplifier or a spectrum analyzer.
[0534] 345. The method of embodiment 343 or 344, wherein
determining one or more spectral properties comprises determining a
relative amplitude of each frequency at which an amplitude is
determined.
[0535] 346. The method of any of embodiments 343 to 345, further
comprising analyzing the one or more spectral properties to
determine a relationship between the one or more spectral
properties and the presence of the at least one fluid in the
subterranean formation based on a classification method comprising
at least one method selected from the group consisting of: a neural
network, a decision tree, a Bayes-based classifier, a fuzzy
logic-based classifier, and a conventional statistical
classifier.
[0536] 347. The method of any of embodiments 343 to 346, further
comprising analyzing the one or more spectral properties to
determine a relationship between the one or more spectral
properties and a presence of the at least one fluid in the
subterranean formation based on a regression analysis of a set of
known empirical data.
[0537] 348. The method of any of embodiments 343 to 347, wherein
determining one or more spectral properties comprises calculating
one or more power spectral densities.
[0538] 349. The method of embodiment 348, further comprising
de-trending of the one or more power spectral densities.
[0539] 350. The method of embodiment 348 or 349, further comprising
integrating each of the one or more power spectral densities.
[0540] 351. The method of any of embodiments 348 to 350, further
comprising performing a fast Fourier transform of the one or more
power spectral densities; and determining one or more correlations
between a source signal and the at least one of the detected
electromagnetic field or the detected seismic signal based on the
fast Fourier transform.
[0541] 352. The method of embodiment 351, wherein an existence of
the one or more correlations provides an indication of the presence
of the at least one fluid in the subterranean formation.
[0542] 353. The method of embodiment 351 or 352, wherein the one or
more correlations indicate one or more transit times between the
subterranean formation and the surface of the earth.
[0543] 354. The method of embodiment 353, further comprising
correlating the one or more transit times to a depth of the fluid
in the subterranean formation.
[0544] 355. The method of embodiment 354, wherein the correlating
is based on estimate rock acoustic properties of the earth.
[0545] 356. The method of embodiment 354 or 355, wherein
correlating comprises determining a frequency-depth
relationship.
[0546] 357. The method of embodiment 356, wherein the
frequency-depth relationship is determined based on a
classification method comprising at least one method selected from
the group consisting of: a neural network, a decision tree, a
Bayes-based classifier, a fuzzy logic-based classifier, and a
conventional statistical classifier.
[0547] 358. The method of embodiment 356 or 357, wherein the
frequency-depth relationship is determined based on a regression
analysis of a set of known empirical data.
[0548] 359. The method of any of embodiments 282 to 358, further
comprising: developing a geological model of the subsurface earth
formation; determining at least one predicted result from the
geological model; comparing the at least one predicted result to at
least one of the detected electromagnetic field or the detected
seismic signal; and determining the at least one property of the
subsurface earth formation based on the comparing.
[0549] 360. In an embodiment, a method comprises: generating a
detected electromagnetic field by detecting earth's electromagnetic
field using at least one electromagnetic field detector in the
earth; generating a detected seismic signal by detecting a seismic
wave related to the earth's electromagnetic field using a seismic
sensor, wherein the detected electromagnetic field is generated at
a different time, a different location, or both than the detected
seismic signal; generating a detected noise signal by detecting at
least one background noise component; filtering the detected
electromagnetic field, the detected seismic signal, or both using
the detected noise signal to provide a filtered signal, wherein the
filtered signal comprises the detected electromagnetic field, the
detected seismic signal, or both having an increased signal to
noise ratio; and processing the filtered signal and the detected
electromagnetic field, the detected seismic signal, or both to
determine at least one property of the subsurface earth
formation.
[0550] 361. The method of embodiment 360, wherein the background
noise component comprises a background electromagnetic field.
[0551] 362. The method of embodiment 361, wherein the detected
noise signal is generated with an electromagnetic field detector on
or above the surface of the earth.
[0552] 363. The method of embodiment 361 or 362, wherein the
detected noise signal is generated by cross-correlating the signal
from the at least one electromagnetic field detector in the
earth.
[0553] 364. The method of any of embodiments 361 to 363, wherein
the detected noise signal is generated by an electromagnetic field
detector capable of measuring at least one horizontal
electromagnetic field component in the earth.
[0554] 365. The method of any of embodiments 361 to 364, wherein
the detected noise signal is generated with an electromagnetic
field sensor on or above a surface of the earth and distanced from
the at least one electromagnetic field detector in the earth.
[0555] 366. The method of any of embodiments 361 to 365, wherein
the detected noise signal comprises a background seismic signal at
the surface of the earth.
[0556] 367. The method of any of embodiments 361 to 366, wherein
the detected noise signal is generated using a plurality of seismic
sensors.
[0557] 368. In an embodiment, a method comprising: generating a
detected electromagnetic field by detecting earth's electromagnetic
field using at least one electromagnetic field detector; generating
a detected seismic signal by detecting a seismic wave related to
the earth's electromagnetic field using a seismic sensor, wherein
the detected electromagnetic field is generated at a different
time, a different location, or both than the detected seismic
signal; isolating at least one nonlinear electromagnetic field
response from the detected electromagnetic field; and processing
the at least one nonlinear electromagnetic field and the detected
seismic signal to determine at least one property of the subsurface
earth formation.
[0558] 369. The method of embodiment 368, wherein isolating at
least one nonlinear seismic signal comprises: identifying
fundamental frequencies present in the detected electromagnetic
field by cross-correlating the detected electromagnetic field and a
detected distant electromagnetic field generated by a distant
electromagnetic field detector located at least 500 yards from the
at least one electromagnetic field detector; and identifying the at
least one nonlinear electromagnetic field response by filtering the
fundamental frequencies from the detected electromagnetic
field.
[0559] 370. The method of embodiment 368, wherein isolating at
least one nonlinear seismic signal comprises: identifying
fundamental frequencies present in the detected electromagnetic
field by cross-correlating the detected electromagnetic field and a
detected atmospheric electromagnetic field generated by an
atmospheric electromagnetic field detector located above the
surface of the earth; and identifying the at least one nonlinear
electromagnetic field response by filtering the fundamental
frequencies from the detected electromagnetic field.
[0560] 371. In an embodiment, a system for identifying hydrocarbons
in a subterranean formation comprises: one or more sensors coupled
to a processor that detect one or more signals generated within a
subsurface earth formation due to a seismoelectric response or an
electroseismic response in at least one porous subsurface earth
formation containing at least one fluid; and an analysis tool, that
when executed on the processor, configures the processor to:
receive the one or more signals from the one or more sensors;
process at least a portion of the one or more signals to determine
at least one property of the subsurface earth formation.
[0561] 372. The system of embodiment 371, wherein the one or more
sensors comprise: a plurality of seismic sensors that detects a
seismic signal related to earth's electromagnetic field and produce
a first signal indicative of the detected seismic signal; an
electromagnetic field detector that measures the earth's
electromagnetic field and produces a second signal indicative of
the detected electromagnetic field; and wherein the analysis tool
receives the first signal and the second signal and determines the
at least one property of a subsurface earth formation.
[0562] 373. The system of embodiment 372, wherein the first signal
is generated at a different time than the second signal.
[0563] 374. The system of embodiment 373, wherein the first signal
is generated at a different location than the second signal.
[0564] 375. The system of any of embodiments 372 to 374, wherein
the plurality of seismic sensors are arranged in an array over a
portion of the subsurface earth formation.
[0565] 376. The system of any of embodiments 372 to 375, wherein
the electromagnetic field detector is disposed within the surface
of the earth below the plurality of seismic sensors.
[0566] 377. The system of any of embodiments 372 to 376, wherein
the distance between each of the plurality of seismic sensors is
less than one half of the wavelength of the surface waves at an
expected surface wave frequency.
[0567] 378. The system of any of embodiments 372 to 377, further
comprising a plurality of electromagnetic field detectors.
[0568] 379. The system of any of embodiments 372 to 378, wherein
the second signal comprises a vertical component of the earth's
electromagnetic field.
[0569] 380. The system of any of embodiments 372 to 379, wherein
the second signal comprises at least one horizontal component of
the earth's electromagnetic field.
[0570] 381. The system of any of embodiments 372 to 380, wherein
the second signal is indicative of a time-varying electromagnetic
field.
[0571] 382. The system of any of embodiments 372 to 381, wherein
the electromagnetic field detector comprises a plurality of pairs
of porous pot electrodes, wherein each pair of porous pot
electrodes are electrically coupled.
[0572] 383. The system of embodiment 382, wherein the pairs of
porous pot electrodes comprise at least one component selected from
the group consisting of: copper sulfate, silver chloride, cadmium
chloride, mercury chloride, and lead chloride.
[0573] 384. The system of any of embodiments 372 to 383, wherein
the electromagnetic field detector comprises a plurality of pairs
of conductive electrodes, wherein each pair of conductive
electrodes are electrically coupled.
[0574] 385. The system of any of embodiments 372 to 384, wherein
the plurality of conductive electrodes comprises at least one
conductive material selected from the group consisting of: copper,
stainless steel, aluminum, gold, galvanized metal, iron, lead,
brass, graphite, steel, alloys thereof, and any combination
thereof.
[0575] 386. The system of any of embodiments 372 to 385, wherein
the electromagnetic field detector comprises a conductive electrode
coupled to a porous pot electrode.
[0576] 387. The system of any of embodiments 372 to 386, wherein
the plurality of seismic sensors comprises at least one sensor
selected from the group consisting of: a hydrophone, a
single-component geophone, a two-component geophone, a
three-component geophone, a single-axis accelerometer, a two-axis
accelerometer, a three-axis accelerometer, and any combination
thereof.
[0577] 388. The system of any of embodiments 372 to 387, wherein
the at least one property of the subsurface earth formation
comprises at least one property selected from the group consisting
of: an existence of the subsurface earth formation containing at
least one fluid, a depth of the subsurface earth formation, a
porosity, a fluid permeability, a composition of at least one fluid
within the subsurface earth formation, a spatial extent of the
subsurface earth formation, an orientation of the boundaries of the
subsurface earth formation, a resistivity, and any combination
thereof.
[0578] 389. The system of any of embodiments 372 to 388, further
comprising a recording apparatus configured to record the first
signal and the second signal on a non-transitory media.
[0579] 390. The system of embodiment 389, wherein the recording
apparatus comprises a digital or analog recording device.
[0580] 391. The system of any of embodiments 372 to 390, wherein
the at least one fluid comprises at least one component selected
from the group consisting of: an aqueous fluid, a hydrocarbon, a
petroleum, carbon dioxide, and any combination thereof.
[0581] 392. The system of embodiment 391, wherein the aqueous fluid
comprises at least one of potable water, fresh water, or brine.
[0582] 393. The system of any of embodiments 372 to 392, wherein at
least one of the plurality of seismic sensors is disposed on the
surface of the earth or within a wellbore.
[0583] 394. The system of any of embodiments 372 to 393, wherein at
least one of the plurality of seismic sensors is disposed within a
wellbore.
[0584] 395. The system of any of embodiments 372 to 394, wherein
the electromagnetic field detector comprises an antenna disposed on
or above the surface of the earth.
[0585] 396. The system of embodiment 395, wherein the antenna
comprises at least one antenna selected from the group consisting
of: a parallel-plate capacitor antenna comprising two or more
parallel conducting plates, a single-plate capacitor antenna
comprising one electrode electrically coupled to the earth, a
monopole antenna comprising a conducting element, a dipole antenna
comprising two conducting elements, a multi-pole antenna comprising
a plurality of conducting elements, a directional antenna
comprising conducting elements arranged to augment a signal
amplitude in a particular direction, a coil antenna comprising one
or more coils of wire, and any combination thereof.
[0586] 397. The system of embodiment 395, wherein the antenna
comprises a concentric electric dipole.
[0587] 398. The system of any of embodiments 372 to 397, wherein
the analysis tool further configures the processor to: remove a
direct current (DC) portion of the second signal.
[0588] 399. The system of embodiment 272, wherein the analysis tool
further configures the processor to: decimate a data set
representing at least one of the first signal or second signal.
[0589] 400. The system of any of embodiments 372 to 399, wherein
the analysis tool further configures the processor to: filter a
noise component from at least one of the first signal or second
signal.
[0590] 401. The system of any of embodiments 372 to 400, wherein
the analysis tool further configures the processor to: filter at
least one of the first signal or second signal using a
predetermined frequency bandwidth.
[0591] 402. In an embodiment, a system for identifying hydrocarbons
in a subterranean formation comprises: a memory comprising a
non-transitory computer readable media; a processor; and an
analysis tool, that when executed on the processor, configures the
processor to: receive one or more signals from one or more sensors,
wherein the one or more sensors detect one or more signals
generated within a subsurface earth formation due to a
seismoelectric response or an electroseismic response in at least
one porous subsurface earth formation containing at least one
fluid; and process at least a portion of the one or more signals to
determine at least one property of the subsurface earth
formation.
[0592] 403. The system of embodiment 402, wherein the one or more
sensors comprise a plurality of seismic sensors that detects a
seismic signal related to earth's electromagnetic field and produce
a first signal indicative of the detected seismic signal, and an
electromagnetic field detector that measures the earth's
electromagnetic field and produces a second signal indicative of
the detected electromagnetic field; and wherein the analysis tool
further configures the processor to receive the first signal and
the second signal, and determine the at least one property of a
subsurface earth formation.
[0593] 404. In an embodiment, a method of passive surveying
comprises: generating one or more detected signals by passively
detecting a signal generated within a subsurface earth formation
due to a seismoelectric response or an electroseismic response in
at least one porous subsurface earth formation containing at least
one fluid; and processing the one or more detected signals to
determine at least one property of the subsurface earth
formation.
[0594] 405. The method of embodiment 404, wherein generating the
one or more detected signals comprises: generating a detected
electromagnetic field by detecting earth's electromagnetic field
using an electromagnetic field detector; and generating a detected
seismic signal by detecting a seismic wave at least partially
resulting from the electroseismic response in the at least one
porous subsurface earth formation containing the at least one fluid
using a seismic sensor; and wherein processing the one or more
detected signals comprises: processing the detected electromagnetic
field and the detected seismic signal to determine the at least one
property of the subsurface earth formation.
[0595] 406. The method of embodiment 405, wherein detecting the
earth's electromagnetic field comprises measuring a vertical
electric component of the earth's electromagnetic field.
[0596] 407. The method of embodiment 406, further comprising
generating a detected magnetic field by detecting earth's magnetic
field using a magnetic field detector, and processing the detected
magnetic field with the vertical electric component and the
detected seismic signal.
[0597] 408. The method of any of embodiments 405 to 407, wherein
detecting the earth's electromagnetic field comprises measuring the
earth's electromagnetic field in at least one horizontal
direction.
[0598] 409. The method of any of embodiments 405 to 408, wherein
the earth's electromagnetic field comprises a time-varying
electromagnetic field.
[0599] 410. The method of any of embodiments 405 to 409, wherein
the earth's electromagnetic field is detected using a pair of
porous pot electrodes.
[0600] 411. The method of embodiment 410, wherein the pair of
porous pot electrodes comprises at least one component selected
from the group consisting of: copper sulfate, silver chloride,
cadmium chloride, mercury chloride, and lead chloride.
[0601] 412. The method of any of embodiments 405 to 411, wherein
the earth's electromagnetic field is detected using a plurality of
pairs of porous pot electrodes.
[0602] 413. The method of any of embodiments 405 to 412, wherein
the earth's electromagnetic field is detected using a pair of
conductive electrodes.
[0603] 414. The method of embodiment 413, wherein the pair of metal
electrodes comprises at least one conductive material selected from
the group consisting of: copper, stainless steel, aluminum, gold,
galvanized metal, iron, lead, brass, graphite, and steel.
[0604] 415. The method of any of embodiments 405 to 414, wherein
the earth's electromagnetic field is detected using a conductive
electrode coupled to a porous pot electrode.
[0605] 416. The method of any of embodiments 405 to 415, wherein
the electromagnetic field detector is disposed on or above the
surface of the earth.
[0606] 417. The method of any of embodiments 405 to 416, wherein
the electromagnetic field detector is moving during the generation
of the detected electromagnetic field.
[0607] 418. The method of any of embodiments 405 to 417, wherein
the electromagnetic field detector is disposed in a moving
vehicle.
[0608] 419. The method of any of embodiments 405 to 418, wherein
the detected electromagnetic field is generated using one or more
antennas disposed on or above a surface of the earth.
[0609] 420. The method of embodiment 419, wherein the one or more
antennas comprises at least one antenna selected from the group
consisting of: a parallel-plate capacitor antenna comprising two or
more parallel conducting plates, a single-plate capacitor antenna
comprising one electrode electrically coupled to the earth, a
monopole antenna comprising a conducting element, a dipole antenna
comprising two conducting elements, a multi-pole antenna comprising
a plurality of conducting elements, a directional antenna
comprising conducting elements arranged to augment a signal
amplitude in a particular direction, a coil antenna comprising one
or more coils of wire, and any combination thereof.
[0610] 421. The method of embodiment 419, wherein the one or more
antennas comprise a concentric electric dipole.
[0611] 422. The method of any of embodiments 405 to 421, wherein
the seismic sensor comprises at least one sensor selected from the
group consisting of: a hydrophone, a single-component geophone, a
two-component geophone, a three-component geophone, a single-axis
accelerometer, a two-axis accelerometer, and a three-axis
accelerometer.
[0612] 423. The method of any of embodiments 405 to 422, wherein
the at least one property of the subsurface earth formation
comprises at least one property selected from the group consisting
of: an existence of the subsurface earth formation containing at
least one fluid, a depth of the subsurface earth formation, a
porosity, a fluid permeability, a composition of at least one fluid
within the subsurface earth formation, a spatial extent of the
subsurface earth formation, an orientation of the boundaries of the
subsurface earth formation, a resistivity, and any combination
thereof.
[0613] 424. The method of any of embodiments 405 to 423, further
comprising recording at least one of the detected electromagnetic
field or the detected seismic signal.
[0614] 425. The method of embodiment 424, wherein the recording
occurs for at least about 5 seconds.
[0615] 426. The method of embodiment 424, wherein the modulated
reference signal is recorded for at least about 0.1 seconds.
[0616] 427. The method of any of embodiments 405 to 426, wherein
the at least one fluid comprises at least one component selected
from the group consisting of: an aqueous fluid, a hydrocarbon, a
petroleum, carbon dioxide, and any combination thereof.
[0617] 428. The method of embodiment 427, wherein the aqueous fluid
comprises at least one of potable water, fresh water, or brine.
[0618] 429. The method of any of embodiments 405 to 428, wherein
the detected electromagnetic field results, at least in part, from
a natural event comprising at least one of: an electromagnetic
fluctuation in the ionosphere, and a naturally occurring electric
discharge in the atmosphere.
[0619] 430. The method of any of embodiments 405 to 429, wherein
the seismic wave is detected on the surface of the earth.
[0620] 431. The method of any of embodiments 405 to 430, wherein
the seismic wave is detected within a wellbore.
[0621] 432. The method of any of embodiments 405 to 431, wherein
processing the detected electromagnetic field and the detected
seismic signal comprises: cross-correlating the detected
electromagnetic field with the detected seismic signal; and
isolating at least a portion of the detected seismic signal arising
from the conversion of the earth's electromagnetic field to seismic
energy.
[0622] 433. The method of any of embodiments 405 to 432, further
comprising a plurality of electromagnetic field detectors.
[0623] 434. The method of embodiment 433, wherein the plurality of
electromagnetic field detectors are arranged in an array.
[0624] 435. The method of any of embodiments 405 to 434, further
comprising a plurality of seismic sensors.
[0625] 436. The method of embodiment 435, wherein the plurality of
seismic sensors are arranged in an array over a portion of the
subsurface earth formation.
[0626] 437. The method of embodiment 435 or 436, wherein processing
the detected electromagnetic field and the detected seismic signal
comprises: filtering at least a portion of a seismic wave traveling
in a direction perpendicular to vertical by applying a spatial
filter to the detected seismic signal from the plurality of seismic
sensors.
[0627] 438. The method of any of embodiments 435 to 437, wherein
the spatial filter is based, at least in part, on the spreading
symmetry of the seismic wave traveling in the direction
perpendicular to vertical.
[0628] 439. The method of any of embodiments 435 to 438, wherein
processing the detected electromagnetic field and the detected
seismic signal comprises: removing a seismic noise from a vertical
component of the detected seismic signal by applying a predictive
filter to the detected seismic signal based on using a horizontal
component of the detected seismic signal from the plurality of
seismic sensors.
[0629] 440. The method of any of embodiments 435 to 439, wherein
processing the detected electromagnetic field and the detected
seismic signal comprises: applying a dip filter to the detected
seismic signal based on rejecting at least a portion of the
detected seismic signals from the plurality of seismic sensors that
arrive at a non-normal angle to a surface, wherein the surface is
defined by the plurality of seismic sensors.
[0630] 441. The method of any of embodiments 435 to 440, wherein
processing the detected electromagnetic field and the detected
seismic signal comprises: producing cross-correlated seismic data
by cross-correlating the detected seismic signal from all of the
plurality of seismic sensors; generating summed data by summing the
cross-correlated seismic data; and cross-correlating the summed
data with the detected electromagnetic field.
[0631] 442. The method of any of embodiments 405 to 441, further
comprising performing a correlation analysis of at least one of the
detected electromagnetic field or the detected seismic signal in
the time domain, the frequency domain, or both, wherein the
correlation analysis provides an indication of a presence of the at
least one fluid.
[0632] 443. The method of embodiment 442, wherein performing the
correlation analysis provides at least one of temporal
characteristics or frequency characteristics.
[0633] 444. The method of embodiment 443, further comprising
deriving a frequency-depth function based on the temporal
characteristics, the frequency characteristics, or both.
[0634] 445. The method of embodiment 444, wherein the
frequency-depth function is derived using data from a known
location.
[0635] 446. The method of any of embodiments 405 to 445, further
comprising repeating the passively detecting, and processing a
plurality of times.
[0636] 447. The method of embodiment 446, wherein the repeating is
performed at a single location.
[0637] 448. The method of embodiment 446, wherein the repeating is
performed at different locations.
[0638] 449. The method of embodiment 448, wherein the repeating is
performed sequentially or concurrently at each of the different
locations.
[0639] 450. The method of embodiment 448 or 449, wherein the
different locations correspond to a plurality of grid
positions.
[0640] 451. The method of embodiment 450, further comprising
generating a multi-dimension model of a subterranean formation
using a plurality of signals, wherein at least one signal of the
plurality of signals is determined at each corresponding grid
position of the plurality of grid positions.
[0641] 452. The method of any of embodiments 405 to 451, further
comprising: performing a further survey of the subterranean
formation when a presence of the at least one fluid is
detected.
[0642] 453. The method of embodiment 452, wherein the further
survey comprises a seismological survey, a controlled-source
electromagnetics survey, a controlled-source electroseismic survey,
a controlled-source seismoelectric survey, a gravity survey, a
magnetic survey, or a passive survey.
[0643] 454. The method of any of embodiments 405 to 453, wherein
processing the one or more detected signals further comprises:
filtering at least one of the detected electromagnetic field or the
detected seismic signal.
[0644] 455. The method of embodiment 454, wherein the filtering and
processing occur in real time.
[0645] 456. The method of embodiment 454 or 455, wherein filtering
comprises filtering a direct current (DC) portion of the detected
electromagnetic field prior to performing the processing.
[0646] 457. The method of any of embodiments 454 to 456, wherein
filtering comprises decimating a data set representing the signal
prior to performing the processing.
[0647] 458. The method of any of embodiments 454 to 457, wherein
filtering comprises using a noise filter.
[0648] 459. The method of embodiment 458, wherein the noise filter
comprises a high pass filter, a low pass filter, a wide band
frequency filter, a narrow band frequency filter, or any
combination thereof.
[0649] 460. The method of any of embodiments 454 to 459, wherein
filtering comprises using one or more band-pass filters.
[0650] 461. The method of embodiment 460, wherein at least one of
the band-pass filters of the one or more band-pass filters
comprises a linear phase filter, a finite impulse response filter,
a forward infinite impulse response filter, a reverse infinite
impulse response filter, or any combination thereof.
[0651] 462. In an embodiment, a method comprises: generating a
detected electromagnetic field by detecting earth's electromagnetic
field; generating a detected seismic signal by detecting a seismic
wave related to the earth's electromagnetic field; generating a
first signal by adding a plurality of intervals of the detected
electromagnetic field, wherein each of the plurality of intervals
of the detected electromagnetic field begins at a start time and
continues for a duration; and generating a second signal by adding
a plurality of intervals of the detected seismic signal
corresponding to the plurality of intervals of the detected
electromagnetic field, wherein each of the plurality of intervals
of the detected seismic signals begins at the start time of the
corresponding interval of the detected electromagnetic field and
continues for the duration of the corresponding interval of the
detected electromagnetic field; determining at least one property
of the subsurface earth formation by processing the first signal
and the second signal.
[0652] 463. The method of embodiment 462, wherein the number of the
plurality of intervals of the detected electromagnetic field and
the number of the plurality of intervals of the detected seismic
signal are chosen to produce a predetermined signal to noise
ratio.
[0653] 464. The method of embodiment 462 or 463, wherein at least
one start time corresponds to the zero crossing voltage of a
coherent noise source.
[0654] 465. The method of any of embodiments 462 to 464, wherein at
least one start time corresponds to a natural amplitude spike in
the detected electromagnetic field.
[0655] 466. The method of any of embodiments 462 to 465, wherein
each duration comprises a plurality of coherent noise cycles.
[0656] 467. The method of embodiment 466, wherein each duration
comprises a time of about 30 seconds to about 10 minutes.
[0657] 468. The method of any of embodiments 462 to 467, wherein a
time between the intervals comprises an uneven fraction of a
coherent noise cycle.
[0658] 469. The method of any of embodiments 462 to 468, wherein a
time between each of the plurality of intervals comprises a
fraction of an irrational number.
[0659] 470. The method of any of embodiments 462 to 469, wherein a
time between each of the plurality of intervals comprises a
fraction of .quadrature..
[0660] 471. The method of any of embodiments 462 to 470, further
comprising: applying a frequency filter to the first signal and the
second signal.
[0661] 472. The method of any of embodiments 462 to 471, further
comprising squaring the first signal and the second signal before
processing the first signal and the second signal.
[0662] 473. The method of any of embodiments 462 to 472, further
comprising: recording the detected electromagnetic field and the
detected seismic signal; using the recorded detected
electromagnetic field to generate the first signal; and using the
recorded detected seismic signal to generate the second signal.
[0663] 474. In an embodiment, a method comprises: generating a
detected electromagnetic field by detecting earth's electromagnetic
field using at least one electromagnetic field detector in the
earth; generating a detected seismic signal by detecting a seismic
wave related to the earth's electromagnetic field using a seismic
sensor; generating a detected noise signal by detecting at least
one background noise component; filtering the detected
electromagnetic field, the detected seismic signal, or both using
the detected noise signal to provide a filtered signal, wherein the
filtered signal comprises the detected electromagnetic field, the
detected seismic signal, or both having an increased signal to
noise ratio; and processing the filtered signal and the detected
electromagnetic field, the detected seismic signal, or both to
determine at least one property of the subsurface earth
formation.
[0664] 475. The method of embodiment 474, wherein the background
noise component comprises a background electromagnetic field.
[0665] 476. The method of embodiment 475, wherein the detected
noise signal is generated with an electromagnetic field detector on
or above the surface of the earth.
[0666] 477. The method of embodiment 475 or 476, wherein the
detected noise signal is generated by cross-correlating the signal
from the at least one electromagnetic field detector in the
earth.
[0667] 478. The method of any of embodiments 475 to 477, wherein
the detected noise signal is generated by an electromagnetic field
detector capable of measuring at least one horizontal
electromagnetic field component in the earth.
[0668] 479. The method of any of embodiments 475 to 478, wherein
the detected noise signal is generated with an electromagnetic
field sensor on or above a surface of the earth and distanced from
the at least one electromagnetic field detector in the earth.
[0669] 480. The method of embodiment 474, wherein the detected
noise signal comprises a background seismic signal at the surface
of the earth.
[0670] 481. The method of embodiment 480, wherein the detected
noise signal is generated using a plurality of seismic sensors.
[0671] 482. In an embodiment, a method comprises: generating a
detected electromagnetic field by detecting earth's electromagnetic
field using at least one electromagnetic field detector; generating
a detected seismic signal by detecting a seismic wave related to
the earth's electromagnetic field using a seismic sensor; isolating
at least one nonlinear seismic signal from the detected seismic
signal; and processing the at least one nonlinear seismic signal
and the detected electromagnetic field to determine at least one
property of the subsurface earth formation.
[0672] 483. The method of embodiment 482, wherein isolating at
least one nonlinear seismic signal comprises: identifying
fundamental frequencies present in the detected electromagnetic
field by cross-correlating the detected electromagnetic field and
the detected seismic signal; and identifying the at least one
nonlinear seismic signal by filtering the fundamental frequencies
from the detected seismic signal.
[0673] 484. The method of embodiment 482 or 483, wherein isolating
at least one nonlinear seismic signal comprises at least partially
rectifying the detected seismic signal.
[0674] 485. The method of embodiment 483, wherein filtering the
fundamental frequencies comprises using a band-pass filter.
[0675] 486. In an embodiment, a method comprises: generating a
detected electromagnetic field by detecting earth's electromagnetic
field using at least one electromagnetic field detector; generating
a detected seismic signal by detecting a seismic wave related to
the earth's electromagnetic field using a seismic sensor; isolating
at least one nonlinear electromagnetic field response from the
detected electromagnetic field; and processing the at least one
nonlinear electromagnetic field and the detected seismic signal to
determine at least one property of the subsurface earth
formation.
[0676] 487. The method of embodiment 486, wherein isolating at
least one nonlinear electromagnetic field response comprises:
identifying fundamental frequencies present in the detected
electromagnetic field by cross-correlating the detected
electromagnetic field and the detected seismic signal; and
identifying the at least one nonlinear electromagnetic field
response by filtering the fundamental frequencies from the detected
electromagnetic field.
[0677] 488. The method of embodiment 486 or 487, wherein isolating
at least one nonlinear seismic signal comprises: identifying
fundamental frequencies present in the detected electromagnetic
field by cross-correlating the detected electromagnetic field and a
detected distant electromagnetic field generated by a distant
electromagnetic field detector located at least 500 yards from the
at least one electromagnetic field detector; and identifying the at
least one nonlinear electromagnetic field response by filtering the
fundamental frequencies from the detected electromagnetic
field.
[0678] 489. The method of any of embodiments 486 to 488, wherein
isolating at least one nonlinear seismic signal comprises:
identifying fundamental frequencies present in the detected
electromagnetic field by cross-correlating the detected
electromagnetic field and a detected atmospheric electromagnetic
field generated by an atmospheric electromagnetic field detector
located on or above the surface of the earth; and identifying the
at least one nonlinear electromagnetic field response by filtering
the fundamental frequencies from the detected electromagnetic
field.
[0679] 490. In an embodiment, a system for identifying hydrocarbons
in a subterranean formation comprises: one or more sensors coupled
to a processor that detect one or more signals generated within a
subsurface earth formation due to a seismoelectric response or an
electroseismic response in at least one porous subsurface earth
formation containing at least one fluid; and an analysis tool, that
when executed on the processor, configures the processor to:
receive the one or more signals from the one or more sensors;
process at least a portion of the one or more signals to determine
at least one property of the subsurface earth formation.
[0680] 491. The system of embodiment 490, wherein the one or more
sensors comprise: a plurality of seismic sensors that detects a
seismic signal related to earth's electromagnetic field and produce
a first signal indicative of the detected seismic signal; an
electromagnetic field detector that measures the earth's
electromagnetic field and produces a second signal indicative of
the detected electromagnetic field; and wherein the analysis tool
receives the first signal and the second signal and determines the
at least one property of a subsurface earth formation.
[0681] 492. The system of embodiment 491, wherein the plurality of
seismic sensors are arranged in an array over a portion of the
subsurface earth formation.
[0682] 493. The system of embodiment 491 or 492, wherein the
electromagnetic field detector is disposed within the surface of
the earth below the plurality of seismic sensors.
[0683] 494. The system of any of embodiments 491 to 493, wherein
the distance between each of the plurality of seismic sensors is
less than one half of the wavelength of the surface waves at an
expected surface wave frequency.
[0684] 495. The system of any of embodiments 491 to 494, further
comprising a plurality of electromagnetic field detectors.
[0685] 496. The system of any of embodiments 491 to 495, wherein
the second signal comprises a vertical component of the earth's
electromagnetic field.
[0686] 497. The system of any of embodiments 491 to 496, wherein
the second signal comprises at least one horizontal component of
the earth's electromagnetic field.
[0687] 498. The system of any of embodiments 491 to 497, wherein
the second signal is indicative of a time-varying electromagnetic
field.
[0688] 499. The system of any of embodiments 491 to 498, wherein
the electromagnetic field detector comprises a plurality of pairs
of porous pot electrodes, wherein each pair of porous pot
electrodes are electrically coupled.
[0689] 500. The system of embodiment 499, wherein the pairs of
porous pot electrodes comprise at least one component selected from
the group consisting of: copper sulfate, silver chloride, cadmium
chloride, mercury chloride, and lead chloride.
[0690] 501. The system of any of embodiments 491 to 500, wherein
the electromagnetic field detector comprises a plurality of pairs
of conductive electrodes, wherein each pair of conductive
electrodes are electrically coupled.
[0691] 502. The system of embodiment 501, wherein the plurality of
conductive electrodes comprises at least one conductive material
selected from the group consisting of: copper, stainless steel,
aluminum, gold, galvanized metal, iron, lead, brass, graphite,
steel, alloys thereof, and any combination thereof.
[0692] 503. The system of any of embodiments 491 to 502, wherein
the electromagnetic field detector comprises a conductive electrode
coupled to a porous pot electrode.
[0693] 504. The system of any of embodiments 491 to 503, wherein
the plurality of seismic sensors comprises at least one sensor
selected from the group consisting of: a hydrophone, a
single-component geophone, a two-component geophone, a
three-component geophone, a single-axis accelerometer, a two-axis
accelerometer, a three-axis accelerometer, and any combination
thereof.
[0694] 505. The system of any of embodiments 491 to 504, wherein
the at least one property of the subsurface earth formation
comprises at least one property selected from the group consisting
of: an existence of the subsurface earth formation containing at
least one fluid, a depth of the subsurface earth formation, a
porosity, a fluid permeability, a composition of at least one fluid
within the subsurface earth formation, a spatial extent of the
subsurface earth formation, an orientation of the boundaries of the
subsurface earth formation, a resistivity, and any combination
thereof.
[0695] 506. The system of any of embodiments 491 to 505, further
comprising a recording apparatus configured to record the first
signal and the second signal on a non-transitory media.
[0696] 507. The system of embodiment 506, wherein the recording
apparatus comprises a digital or analog recording device.
[0697] 508. The system of any of embodiments 491 to 507, wherein
the at least one fluid comprises at least one component selected
from the group consisting of: an aqueous fluid, a hydrocarbon, a
petroleum, carbon dioxide, and any combination thereof.
[0698] 509. The system of embodiment 508, wherein the aqueous fluid
comprises at least one of potable water, fresh water, or brine.
[0699] 510. The system of any of embodiments 491 to 509, wherein at
least one of the plurality of seismic sensors is disposed on the
surface of the earth or within a wellbore.
[0700] 511. The system of any of embodiments 491 to 510, wherein at
least one of the plurality of seismic sensors is disposed within a
wellbore.
[0701] 512. The system of any of embodiments 491 to 511, wherein
the electromagnetic field detector comprises an antenna disposed on
or above the surface of the earth.
[0702] 513. The system of embodiment 512, wherein the antenna
comprises at least one antenna selected from the group consisting
of: a parallel-plate capacitor antenna comprising two or more
parallel conducting plates, a single-plate capacitor antenna
comprising one electrode electrically coupled to the earth, a
monopole antenna comprising a conducting element, a dipole antenna
comprising two conducting elements, a multi-pole antenna comprising
a plurality of conducting elements, a directional antenna
comprising conducting elements arranged to augment a signal
amplitude in a particular direction, a coil antenna comprising one
or more coils of wire, and any combination thereof.
[0703] 514. The system of embodiment 512, wherein the antenna
comprises a concentric electric dipole.
[0704] 515. The system of any of embodiments 491 to 514, wherein
the analysis tool further configures the processor to: remove a
direct current (DC) portion of the second signal.
[0705] 516. The system of any of embodiments 491 to 515, wherein
the analysis tool further configures the processor to: decimate a
data set representing at least one of the first signal or second
signal.
[0706] 517. The system of any of embodiments 491 to 516, wherein
the analysis tool further configures the processor to: filter a
noise component from at least one of the first signal or second
signal.
[0707] 518. The system of any of embodiments 491 to 517, wherein
the analysis tool further configures the processor to: filter at
least one of the first signal or second signal using a
predetermined frequency bandwidth.
[0708] 519. In an embodiment, a system for identifying hydrocarbons
in a subterranean formation comprises: a memory comprising a
non-transitory computer readable media; a processor; and an
analysis tool, that when executed on the processor, configures the
processor to: receive one or more signals from one or more sensors,
wherein the one or more sensors detect one or more signals
generated within a subsurface earth formation due to a
seismoelectric response or an electroseismic response in at least
one porous subsurface earth formation containing at least one
fluid; and process at least a portion of the one or more signals to
determine at least one property of the subsurface earth
formation.
[0709] 520. The system of embodiment 519, wherein the one or more
sensors comprise a seismic sensor that detects a seismic signal
related to earth's electromagnetic field and produce a first signal
indicative of the detected seismic signal, and an electromagnetic
field detector that measures the earth's electromagnetic field and
produces a second signal indicative of the detected electromagnetic
field; and wherein the analysis tool configures the processor to
receive the first signal and the second signal and determine the at
least one property of a subsurface earth formation.
[0710] At least one embodiment is disclosed and variations,
combinations, and/or modifications of the embodiment(s) and/or
features of the embodiment(s) made by a person having ordinary
skill in the art are within the scope of the disclosure.
Alternative embodiments that result from combining, integrating,
and/or omitting features of the embodiment(s) are also within the
scope of the disclosure. Where numerical ranges or limitations are
expressly stated, such express ranges or limitations should be
understood to include iterative ranges or limitations of like
magnitude falling within the expressly stated ranges or limitations
(e.g., from about 1 to about 10 includes, 2, 3, 4, etc.; greater
than 0.10 includes 0.11, 0.12, 0.13, etc.). For example, whenever a
numerical range with a lower limit, R.sub.l, and an upper limit,
R.sub.u, is disclosed, any number falling within the range is
specifically disclosed. In particular, the following numbers within
the range are specifically disclosed:
R=R.sub.l+k*(R.sub.u-R.sub.l), wherein k is a variable ranging from
1 percent to 100 percent with a 1 percent increment, i.e., k is 1
percent, 2 percent, 3 percent, 4 percent, 5 percent, . . . , 50
percent, 51 percent, 52 percent, . . . , 95 percent, 96 percent, 97
percent, 98 percent, 99 percent, or 100 percent. Moreover, any
numerical range defined by two R numbers as defined in the above is
also specifically disclosed. Use of the term "optionally" with
respect to any element of a claim means that the element is
required, or alternatively, the element is not required, both
alternatives being within the scope of the claim. Use of broader
terms such as comprises, includes, and having should be understood
to provide support for narrower terms such as consisting of,
consisting essentially of, and comprised substantially of.
Accordingly, the scope of protection is not limited by the
description set out above but is defined by the claims that follow,
that scope including all equivalents of the subject matter of the
claims. Each and every claim is incorporated as further disclosure
into the specification and the claims are embodiment(s) of the
present invention.
* * * * *