U.S. patent application number 11/198477 was filed with the patent office on 2006-09-28 for multicomponent marine geophysical data gathering system.
Invention is credited to Audun Sodal, Andre Stenzel, Stig Rune Lennart Tenghamn.
Application Number | 20060215490 11/198477 |
Document ID | / |
Family ID | 27765857 |
Filed Date | 2006-09-28 |
United States Patent
Application |
20060215490 |
Kind Code |
A1 |
Tenghamn; Stig Rune Lennart ;
et al. |
September 28, 2006 |
Multicomponent marine geophysical data gathering system
Abstract
In one embodiment the invention comprises a particle velocity
sensor that includes a housing with a geophone mounted in the
housing. A fluid that substantially surrounds the geophone is
included within the housing. The particle velocity sensor has an
acoustic impedance within the range of about 750,000 Newton seconds
per cubic meter (Ns/m.sup.3) to about 3,000,000 Newton seconds per
cubic meter (Ns/m.sup.3). In another embodiment the invention
comprises method of geophysical exploration in which a seismic
signal is generated in a body of water and detected with a
plurality of co-located particle velocity sensors and pressure
gradient sensors positioned within a seismic cable. The output
signal of either or both of the particle velocity sensors or the
pressure gradient sensors is modified to substantially equalize the
output signals from the particle velocity sensors and the pressure
gradient sensors. The output signals from particle velocity sensors
and pressure gradient sensors are then combined.
Inventors: |
Tenghamn; Stig Rune Lennart;
(Katy, TX) ; Sodal; Audun; (Ranheim, NO) ;
Stenzel; Andre; (Richmond, TX) |
Correspondence
Address: |
E. Eugene Thigpen;Petroleum Geo-Services, Inc.
P.O. Box 42805
Houston
TX
77242-2805
US
|
Family ID: |
27765857 |
Appl. No.: |
11/198477 |
Filed: |
August 5, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10233266 |
Aug 30, 2002 |
|
|
|
11198477 |
Aug 5, 2005 |
|
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|
Current U.S.
Class: |
367/20 |
Current CPC
Class: |
G01V 1/189 20130101;
G01V 1/181 20130101; G01V 1/3808 20130101; G01V 1/185 20130101;
G01V 2210/56 20130101 |
Class at
Publication: |
367/020 |
International
Class: |
G01V 1/38 20060101
G01V001/38 |
Claims
1. A method of geophysical exploration comprising: generating a
seismic signal in a body of water; detecting said seismic signal
with a plurality of co-located particle motion sensor assemblies
and pressure gradient sensors positioned within a seismic cable
deployed in said body of water; modifying the output signal of at
least one of said particle motion sensor assemblies or said
pressure gradient sensors to substantially equalize the output
signals from said particle motion sensor assemblies and said
pressure gradient sensors within at least a selected frequency
range; and combining the modified output signals from co-located
particle motion sensor assemblies and pressure gradient sensors
within at least said selected frequency range.
2. The method of claim 1 wherein the amplitude and phase of the
output signals from said particle motion sensor assemblies and said
pressure gradient sensors are substantially matched within said at
least a selected frequency range
3. The method of claim 1 wherein modifying the output signals from
either said particle motion sensor assemblies or said pressure
gradient sensors is performed independently of the acoustic
impedance of material through which said seismic signal
travels.
4. The method of claim 1 wherein the output signals of said
pressure gradient sensors and said particle motion sensor
assemblies are substantially equalized during processing and
combined.
5. The method of claim 1 wherein the amplitude and phase of the
pressure gradient sensors and the particle motion sensor assemblies
are equalized.
6. The method of claim 1 wherein output signals from said particle
motion sensor assemblies and said pressure gradient sensor are
combined to reduce spectral notches above frequencies of about 20
Hz.
7. The method of claim 1 wherein said particle motion sensor
assemblies and pressure gradient sensors are positioned in the
interior of a seismic cable having an inside diameter of about 55
millimeters.
8. The method of claim 1 wherein said particle motion sensor
assemblies and pressure gradient sensors are positioned in the
interior of a seismic cable having an inside diameter of about 66
millimeters.
9. The method of claim 1 wherein said seismic cable is deployed at
a depth of less that six meters.
10. The method of claim 1 wherein said seismic cable is deployed at
a depth of greater than nine meters.
11. The method of claim 1 wherein said particle motion sensor
assemblies have an acoustic impedance with the range of about
750,000 Newton seconds per cubic meter to about 3,000,000 Newton
seconds per cubic meter.
12. The method of claim 1 wherein said particle motion sensor
assemblies have an acoustic impedance substantially equal to the
acoustic impedance of the water in said body of water in which said
cable is deployed.
13. The method of claim 1 wherein said seismic cable is a
liquid-filled cable.
14. The method of claim 1 wherein said seismic cable is a
gel-filled cable.
15. The method of claim 1 wherein said seismic cable is a solid
cable.
16. The method of claim 1 wherein said cable is towed through said
body of water.
17. The method of claim 1 wherein said cable is maintained at a
substantially stationary position.
18. The method of claim 1 wherein at least a portion of the
particle motion sensor assemblies are electrically interconnected
in groups to generate a particle velocity output signals.
19. The method of claim 18 wherein at least a portion of the
particle motion sensor assemblies are electrically interconnected
in series in groups of at least three sensors.
20. The method of claim 18 wherein at least a portion of the
particle motion sensor assemblies are electrically interconnected
in parallel.
21. The method of claim 1 wherein particle motion sensor assemblies
include sensors mounted in said cable in an orientation to detect
signals in the vertical direction, the cross line direction and
in-line direction.
22. A method of geophysical exploration comprising: deploying a
seismic cable in a body of water, said seismic cable having a
plurality of particle motion sensor assemblies included within said
cable, said particle motion sensor assemblies having an acoustic
impedance within the range of about 750,000 Newton seconds per
cubic meter to about 3,000,000 Newton seconds per cubic meter; and
utilizing said seismic cable for detecting seismic data
signals.
23. The method of claim 22 wherein said particle motion sensor
assemblies have an acoustic impedance equal to about the impedance
of the water in said body of water in which said seismic cable is
deployed.
24. A method of geophysical exploration comprising: deploying a
seismic cable in a body of water, said seismic cable having a
plurality of motion sensor assemblies included within said cable,
said particle motion sensor assemblies having a density less than 2
grams per cubic centimeter; and utilizing said seismic cable for
detecting seismic data signals.
25. The method of claim 24 wherein the density of said particle
motion sensor assemblies is substantially equal to the density of
the water in said body of water in which said seismic cable is
deployed.
26. A method of geophysical exploration comprising: deploying a
seismic cable in a body of water, said seismic cable having a
plurality of particle motion sensor assemblies included within said
cable; wherein said particle motion sensor assemblies comprise a
housing, a gimbal-mounted particle motion sensor mounted in said
housing, a fluid within said housing substantially surrounding said
particle motion sensor, said fluid having a viscosity selected to
restrain noise-generating movement of said particle motion sensor
and to allow said particle motion sensor to maintain a selected
orientation as said housing is rotate; and utilizing said seismic
cable for detecting seismic data signals.
27. The method of claim 26 wherein said fluid has a viscosity of
greater than about 500 centistokes and less than about 5000
centistokes.
28. A method of processing marine seismic data to reduce spectral
notches resulting from surface ghost reflections, comprising:
determining the amplitude and phase variation with frequency of the
output of a particle motion sensor assembly of a co-located sensor
pair comprising a particle motion sensor assembly and a pressure
gradient sensor, independently of any variation in amplitude or
phase with frequency of said particle motion sensor assembly
resulting from impedance mismatch between said particle motion
sensor assembly and a medium from which a seismic wave is coupled
to said particle motion sensor assembly; modifying the output
signal of at least one of said motion sensor assemblies or pressure
gradient sensors to compensate for said determined amplitude and
phase variation, thereby generating modified output signals; and
summing said modified output signals from said pressure gradient
sensor and said motion sensor assembly.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional application of, and claims
priority from, U.S. Nonprovisional patent application Ser. No.
10/233266, filed on Aug. 30, 2002, the entirety of which is
incorporated herein by reference.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not applicable.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] The present invention is related to marine geophysical
exploration. More specifically, the invention is related to sensors
for detecting seismic signals and to marine seismic data
gathering.
[0005] 2. Description of Relevant Art
[0006] In seismic exploration, geophysical data are obtained by
applying acoustic energy to the earth at the surface and detecting
seismic energy reflected from interfaces between different layers
in subsurface formations. The seismic wave is reflected when there
is a difference in impedance between the layer above the interface
and the layer below the interface.
[0007] In marine seismic exploration, a seismic shock generator,
such as an airgun, for example, is commonly used to generate an
acoustic pulse. The resulting seismic wave is reflected back from
subsurface interfaces and detected by sensors deployed in the water
or on the water bottom.
[0008] In a typical marine seismic operation, a streamer cable is
towed behind an exploration vessel at a water depth between about
six to about nine meters. Hydrophones are included in the streamer
cable for detecting seismic signals. A hydrophone is a submersible
pressure gradient sensor that converts pressure waves into
electrical signals that are typically recorded for signal
processing, and evaluated to estimate characteristics of the
earth's subsurface.
[0009] After the reflected wave reaches the streamer cable, the
wave continues to propagate to the water/air interface at the water
surface, from which the wave is reflected downwardly, and is again
detected by the hydrophones in the streamer cable. The reflection
coefficient at the surface is nearly unity in magnitude and
negative in sign. The seismic wave will be phase-shifted 180
degrees. The downwardly traveling wave is commonly referred to as
the "ghost" signal, and the presence of this ghost reflection
creates a spectral notch in the detected signal. Because of the
spectral notch, some frequencies in the detected signal are
amplified and some frequencies are attenuated.
[0010] Because of the ghost reflection, the water surface acts like
a filter, making it difficult to record data outside a selected
bandwidth without excessive attenuation or notches in the recorded
data.
[0011] Maximum attenuation will occur at frequencies for which the
distance between the detecting hydrophone and the water surface is
equal to one-half wavelength. Maximum amplification will occur at
frequencies for which the distance between the detecting hydrophone
and the water surface is one-quarter wavelength. The wavelength of
the acoustic wave is equal to the velocity divided by the
frequency, and the velocity of an acoustic wave in water is about
1500 meters per second. Accordingly the location in the frequency
spectrum of the resulting spectral notch is readily determinable.
For example, for a streamer water depth of 7 meters, as illustrated
by curve 54 in FIG. 1, maximum attenuation will occur at a
frequency of about 107 Hz. and maximum amplification will occur at
a frequency of about 54 Hz.
[0012] It has not been practical to tow cables deeper than about 9
meters because the location of the spectral notch in the frequency
spectrum of the signal detected by a hydrophone substantially
diminishes the utility of the recorded data. It has also not been
practical to tow cables at a depth shallower than about 6 meters,
because the ghost signal reflected from the water surface
substantially attenuates the signal detected by a hydrophone within
the frequency band of interest.
[0013] It is also common to perform marine seismic operations in
which sensors are deployed on the water bottom. Such operations are
typically referred to as "ocean bottom seismic" operations. In
ocean bottom seismic operations, both hydrophones and geophones are
employed for recording the seismic data, with the geophone normally
being placed in direct contact with the ocean bottom. To improve
the contact between the geophone and the ocean floor, the geophone
assembly is typically made to be quite heavy, with a typical
density of between 3 and 7 grams per cubic centimeter.
[0014] A geophone detects a particle velocity signal, whereas the
hydrophone detects a pressure gradient signal. The geophone has
directional sensitivity, whereas the hydrophone does not.
Accordingly, the upgoing wavefield signals detected by the geophone
and the hydrophone will be in phase, but the downgoing wavefield
signals detected by the geophone and the hydrophone will be 180
degrees out of phase. Various techniques have been proposed for
using this phase difference to reduce the spectral notch caused by
the ghost reflection.
[0015] U.S. Pat. No. 4,486,865 to Ruehle, for example, teaches a
system said to suppress ghost reflections by combining the outputs
of pressure and velocity detectors. The detectors are paired, one
pressure detector and one velocity detector in each pair. A filter
is said to change the frequency content of at least one of the
detectors so that the ghost reflections cancel when the outputs are
combined.
[0016] U.S. Pat. No. 5,621,700 to Moldovenu also teaches using at
least one sensor pair comprising a pressure sensor and a velocity
sensor in an ocean bottom cable in a method for attenuating ghosts
and water layer reverberations.
[0017] U.S. Pat. No. 4,935,903 to Sanders et al. teaches a marine
seismic reflection prospecting system that detects seismic waves
traveling in water by pressure sensor-particle velocity sensor
pairs (e.g., hydrophone-geophone pairs) or alternatively
vertically-spaced pressure sensors. Instead of filtering to
eliminate ghost reflection data, the system calls for enhancing
primary reflection data for use in pre-stack processing by adding
the ghost data.
[0018] U.S. Pat. No. 4,979,150 provides a method for marine seismic
prospecting said to attenuate coherent noise resulting from water
column reverberation by applying a scale factor to the output of a
pressure transducer and a particle velocity transducer positioned
substantially adjacent one another in the water. In this method,
the transducers may be positioned either on the ocean bottom or at
a location in the water above the bottom, although the ocean bottom
is said to be preferred.
[0019] Four component system have also been utilized on the sea
floor. A four component system utilizes a hydrophone for detecting
a pressure signal, together with a three-component geophone for
detecting particle velocity signals in three orthogonal directions:
vertical, in-line and cross line. The vertical geophone output
signal is used in conjunction with the hydrophone signal to
compensate for the surface reflection. The three orthogonally
positioned geophones are used for detecting shear waves, including
the propagation direction of the shear waves.
[0020] The utility of simultaneously recording pressure and
vertical particle motion in marine seismic operations has long been
recognized. However, a geophone (or accelerometer) for measuring
vertical particle motion must be maintained in a proper orientation
in order to accurately detect the signal. Maintaining such
orientation is non-trivial in a marine streamer and significantly
more problematic than maintaining such orientation on the ocean
bottom. Exploration streamers towed behind marine vessels are
typically over one mile in length. Modem marine seismic streamers
may use more than 10,000 transducers. To maintain a particle
velocity sensor (a geophone or accelerometer) in proper orientation
to detect vertical motion, the prior art has proposed various
solutions. The use of gimbals has been proposed repeatedly. One
example is a "gimbal lock system for seismic sensors" described in
U.S. Pat. No. 6,061,302 to Brink et al. Another example is a "dual
gimbal geophone" described in U.S. Pat. No. 5,475,652 to McNeel et
al. Still another example is a "self-orienting directionally
sensitive geophone" described in U.S. Pat. No. 4,618,949 to Lister.
Nevertheless, no streamers containing both hydrophone and geophones
are in commercial use.
[0021] In addition to the problem of maintaining orientation,
severe noise from streamer cables has been considered prohibitive
to use of particle velocity sensors in streamers. Because the
voltage output signal from particle velocity sensors is normally
not as strong as the output signal from hydrophones, the noise
level in streamer cables has been a detriment to the use of
particle velocity sensors.
[0022] In ocean bottom cables, the sensors are located on the sea
floor and therefore are less exposed to, noise generated by
vibrations in the cable. Geophones are typically gimbaled to ensure
a correct direction and are made of heavy brass or similar material
to ensure good contact with the sea floor. The geophone housing is
typically filled with fluid to improve the coupling between the
sensor and the seafloor. However, because of the variation in
properties of the seafloor from location to location, impedance
mismatch between the seafloor and the sensor and sensor housing can
cause problems. Such mismatch in impedance can cause various types
of distortion in both the hydrophone signal and the geophone
signal. Also, the boundary effects for the hydrophone and the
geophone due to their closeness to the sea floor can change the
response for the hydrophone and the geophone, giving rise to a need
to correct the amplitude values in processing to be able to use the
signal for elimination of the surface "ghost" reflection.
[0023] Accordingly a need continues to exist for an improved system
for gathering marine seismic data.
SUMMARY OF THE INVENTION
[0024] In one embodiment the invention comprises a particle
velocity sensor that includes a housing with a geophone mounted in
the housing. A fluid that substantially surrounds the geophone is
included within the housing. The particle velocity sensor has an
acoustic impedance within the range of about 750,000 Newton seconds
per cubic meter (Ns/m.sup.3) to about 3,000,000 Newton seconds per
cubic meter (Ns/m.sup.3).
[0025] In another embodiment the invention comprises method of
geophysical exploration in which a seismic signal is generated in a
body of water and detected with a plurality of co-located particle
velocity sensors and pressure gradient sensors positioned within a
seismic cable deployed in the body of water. The output signal of
either or both of the particle velocity sensors or the pressure
gradient sensors is modified to substantially equalize the output
signals from the particle velocity sensors and the pressure
gradient sensors within at least a selected frequency range. The
output signals from co-located particle velocity sensors and
pressure gradient sensors are then combined.
[0026] In yet another embodiment the invention comprises a method
of processing marine seismic data to reduce spectral notches
resulting from surface ghost reflections in which the amplitude and
phase variation with frequency of the output of a particle velocity
sensor of a co-located particle velocity sensor and pressure
gradient sensor pair is determined independently of any variation
in amplitude or phase with frequency of the particle velocity
sensor output resulting from impedance mismatch between the
particle velocity sensor and a medium from which a seismic wave is
coupled to the particle velocity sensor. The output signal of one
or both of the particle velocity sensors or pressure gradient
sensors is modified to compensate for the determined amplitude and
phase variation to generate modified output signals. The modified
output signals are then summed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 shows the frequency spectrum of a seismic signal
detected by a hydrophone at a water depth of 7 meters.
[0028] FIG. 2 illustrates a typical implementation of the
invention, in which a plurality of streamer cables are towed behind
a seismic survey vessel.
[0029] FIG. 3 shows the geophone assembly with the parts exploded
or separated out for illustration.
[0030] FIG. 4 shows a cross section of a geophone assembly.
[0031] FIG. 5 shows particle velocity sensors and pressure gradient
sensors in a seismic streamer cable.
[0032] FIGS. 6A and 6B show a typical phase and amplitude response
for a particle velocity sensor.
[0033] FIG. 7 shows the simulated output responses for a hydrophone
and a geophone at a water depth of 26 meters.
[0034] FIG. 8 provides actual hydrophone and geophone data from a
field test with the cable at about 26 meters.
[0035] FIG. 9 shows a summation of the hydrophone and geophone data
shown in FIG. 8.
[0036] FIG. 10 shows a simulation of streamer data at a one-meter
depth.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0037] FIG. 2 illustrates a typical geophysical exploration
configuration in which a plurality of streamer cables 30 are towed
behind vessel 32. One or more seismic sources 34 are also normally
towed behind the vessel. The seismic source, which typically is an
airgun, but may also be a water gun or other type of source known
to those of ordinary skill in the art, transmits seismic energy or
waves into the earth and the waves are reflected back by reflectors
in the earth and recorded by sensors in the streamers. Paravanes 35
are utilized to maintain the cables 30 in the desired lateral
position. The invention may also be implemented, however, in
seismic cables that are maintained at a substantially stationary
position in a body of water, either floating at a selected depth or
lying on the bottom of the body of water, in which case the source
may be towed behind a vessel to generate shock waves at varying
locations, or the source may also be maintained in a stationary
position. Seismic sensors, in accordance with embodiments of the
present invention are deployed in streamer cables, such as cables
30.
[0038] In a particular implementation, the present invention
comprises a particle velocity sensor in the form of a geophone
assembly. Such a geophone assembly is shown in FIGS. 3 and 4. FIG.
3 shows the geophone assembly 3 with the parts exploded or
separated out for illustration. FIG. 4 shows a cross section of the
geophone assembly 3 of FIG. 3 with the various parts assembled (not
exploded).
[0039] With reference to FIGS. 3 and 4, geophone 10 is mounted in a
housing 20 comprising outer sleeve 12 and end cups 1 and 13.
Geophone 10 is secured in mounting ring (or cradle) 8. Shafts 9
extend from opposite sides of mounting ring 10 into bushings 2.
Bearings 4, which are positioned between shafts 9 and bushings 2
enable rotational motion of shafts 9 with respect to bushings 2,
thereby providing a gimbaled mounting. End caps 1 and 13 are
secured together by means of bolts 16 and threaded inserts 18.
Spring 6 provides electrical contact between shafts 9, which are
electrically conductive and are electrically connected to output
terminals (not shown) of the geophone, and bushings 2, which are
also electrically conductive, and which are electrically connected
to the streamer cable wiring. Thrust washers 7 provide pre-load for
bearings 4 to eliminate undesired bearing slack. O-rings 15 provide
a seal between outer sleeve 12 and end caps 1 and 13, and O-rings
14 provide a seal between bushings 2 and end caps 1 and 13. Plug 17
is utilized for plugging the conduit through which fluid is
inserted into the interior of the geophone housing comprising the
two end caps 1 and 13, and the outer sleeve 12. The configuration
of the geophone assembly illustrated in FIGS. 3 and 4 is a
particular implementation of an embodiment of the invention is not
intended to be limiting. The geophone assembly 3 is secured to a
seismic cable strain member for positive location.
[0040] The housing 20, comprising end caps 1 and 13 and outer
sleeve 12, contains a fluid, preferably an oil, which substantially
surrounds the geophone. The fluid provides coupling between the
geophone and the geophone housing of the geophone assembly. The
fluid should preferably surround the geophone, but preferably will
not entirely fill the housing so as to allow room for fluid
expansion and contraction with changes in temperature and pressure.
The fluid has a viscosity that provides sufficient damping of
geophone movement to reduce noise, while enabling sufficient
movement of the geophone 10 on the bearings to maintain the
transducer in the desired orientation. That is, the viscosity of
the fluid should be high enough to restrain the geophone from
unwanted movements but low enough to prevent the geophone from
following rotational movement of the housing and a streamer in
which the geophone assembly may be mounted. A preferred viscosity
for such fluid is in the range of about 500 to about 5000
centistokes.
[0041] A positioner, such as a weight 11, may be mounted on the
lower side of the geophone 10 to assist in maintaining the sensor
10 in the desired orientation. Positioner 11 may be formed
substantially from lead, although other materials having a density
greater than the density of the geophone may be utilized.
Alternatively, or additionally, a positioner (not shown) having a
density lower than the density of the fluid that substantially
surrounds the geophone 10, may be installed on the upper side of
geophone 10 to assist in maintaining the geophone 10 in the desired
orientation. Locating the center of gravity of the geophone below
the rotational axis of the gimbal on which the sensor is mounted
will also assist in maintaining the geophone in the desired
orientation.
[0042] The particle velocity sensor in accordance with this
invention is sufficiently small to fit in the interior of a
cylindrical streamer cable. Typical internal diameters of such
cylindrical streamer cables are either 55 millimeters or 66
millimeters. The space within the streamer surrounding the seismic
sensors and other apparatus (not shown) positioned within the
streamer is typically filled with a liquid, such as an oil, which
provides substantially neutral buoyancy to the cable. The space may
also be filled with a gel or semi-solid material, and the streamer
may also be a solid streamer.
[0043] In a preferred embodiment of the invention, the density of
the overall geophone assembly (including the fluid and all other
elements thereof) is selected to improve coupling between the
geophone assembly and its surroundings. In general, optimum
coupling is obtained when the acoustic impedance of the geophone
assembly is about the same as the acoustic impedance of its
surroundings, which may be achieved by making the density of the
geophone assembly about the same as the density of its
surroundings, and the acoustic velocity of the geophone assembly
about the same as the acoustic velocity of its surroundings.
[0044] When an acoustic wave traveling in one medium encounters the
boundary of a second medium, reflected and transmitted waves are
generated. Further, when the boundary area of the second medium is
much smaller than the wavelength of the acoustic wave, diffraction
results rather than reflection. For plane waves the characteristic
acoustic impedance of a medium is equal to density times velocity,
i.e., z=.+-..rho..sub.0c (Eq. 1)
[0045] in which, z=acoustic impedance [0046] .rho..sub.0=density,
and [0047] c=velocity. Let the incident and reflected wave travel
in a fluid of characteristic acoustic impedance,
r.sub.1=.rho..sub.1c.sub.1, where .rho..sub.1 is equilibrium
density of the fluid and c.sub.1 is the phase speed in the fluid.
Let the transmitted wave travel in a fluid of characteristic
acoustic impedance r.sub.2=.rho..sub.2c.sub.2 . If the complex
pressure amplitude of the incident wave is P.sub.1, that of the
reflected wave P.sub.R, and that of the transmitted wave P.sub.T,
then the pressure reflection coefficient R may be defined as: R = r
2 - r 1 r 2 + r 1 , ( Eq . .times. 2 ) ##EQU1## and since 1+R=T,
the pressure transmission coefficient T can be written as: T = 2
.times. r 2 r 2 + r 1 ( Eq . .times. 3 ) ##EQU2## It follows from
the foregoing explanation that improved reception will be achieved
if the particle velocity sensor is made in such a way that the
density and speed in the sensor assembly, including its housing and
other components, is similar to that of the surrounding fluid. If
they are equal, the reflection coefficient will be R=0 and the
transmission coefficient will be T=1.
[0048] By making the acoustic velocity in the particle velocity
sensor substantially equal to the acoustic velocity in the water in
which the sensor is deployed, and by making the density of the
particle velocity sensor similar to the density of the water, a
good impedance match is generated between the water and the
particle velocity sensor. The velocity sensor will have a good
impedance match with the surrounding media and no distortion of
amplitude or phase will occur due to reflection, diffraction or
other anomalies of the traveling wave passing through the sensor
and its housing.
[0049] In a preferred embodiment, the density of the particle
velocity sensor is less than about twice the density of water
(about 2 g/cm.sup.3), and more preferably about the same as the
density of water (about 1 g/cm.sup.3). Accordingly, the density of
the particle velocity sensor should typically be between about 0.5
g/cm.sup.3 and 2 g/cm.sup.3, and more preferably, about 1.0
g/cm.sup.3. It is understood, however, that water density may vary
with salinity, and that it may be useful to vary the density of the
particle velocity sensor, depending on the particular body of water
in which the particle velocity sensor is to be employed. Because
the density of particle velocity sensor in accordance with a
preferred embodiment of the invention is substantially less than
the density of geophone assemblies typically available for use in
ocean bottom seismic operations, different components are selected
from which to assemble the geophone assembly. For example, at least
a portion of the housing may be formed from a moldable elastomeric,
such as isoplast or polypropylene, or a moldable composite
material, such as fiber reinforced epoxy.
[0050] Over and above the need for good acoustic coupling, a
low-weight particle velocity sensor is useful because, in a
preferred embodiment of the invention, the seismic cable in which
the sensors are included needs to be neutrally buoyant. As many as
10,000 particle velocity sensors may be utilized in a single cable.
Accordingly, a particle velocity sensor having a density of less
than 2 grams per cubic centimeter facilitates the mechanical
construction of the seismic cable to achieve neutral buoyancy.
[0051] In a particular implementation of the invention, particle
velocity sensors 3 and pressure gradient sensors 5 are utilized
together in a cylindrical seismic cable 30, as shown in FIG. 5. Use
of both particle velocity sensors and pressure gradient sensors
enables signal degradation resulting from surface ghost reflections
to be substantially eliminated from the recorded seismic data. Such
signal improvement is achieved by combining the output signals from
a particle velocity sensor (or an array of particle velocity
sensors) with the output signal from a pressure gradient sensor (or
an array of pressure gradient sensors) positioned at substantially
the same location. Particle velocity sensors and pressure gradient
sensors positioned at substantially the same location may be
referred to hereinafter as "co-located" sensors.
[0052] The phase and amplitude response for a pressure gradient
sensor are substantially constant in the seismic frequency band of
interest (from about 2 Hz. to about 300 Hz.). For example, for the
T-2BX hydrophone marketed by Teledyne Instruments, Inc. of 5825
Chimney Rock Road, Houston, Tex. 77081, the variation in amplitude
over a frequency range of 2-300 Hz. has been measured at less than
1 db, and the variation in phase at less than 0.1 degree. FIGS. 6A
and 6B show a typical amplitude and phase response for a particle
velocity sensor. In FIG. 6A, curve 56 represents amplitude
variation, and in FIG. 6B, curve 58 represents the phase variation.
In contrast to the amplitude and phase response of the hydrophone,
it is evident that there are substantial variations in both the
amplitude and the phase response for a particle velocity sensor in
the seismic frequency range of interest Further, in prior art
systems, in which the impedance of the particle velocity sensor was
not substantially matched to the impedance of the substance (either
the water or the water bottom) from which the seismic wave is
coupled to the particle velocity sensor, additional variations in
amplitude and phase occur in the seismic frequency range because of
the impedance mismatch.
[0053] In accordance with a particular embodiment of the present
invention, where the impedance match between the water and the
particle velocity sensor is more nearly equal, such additional
variations in amplitude and phase are minimized, and, accordingly,
the particle vejocity sensor output and the pressure gradient
sensor output can be matched by utilizing an appropriate filter, of
a type known to those of ordinary skill in the art, without
requiring additional matching for variations caused by impedance
mismatch.
[0054] In one implementation of the invention, the pressure
gradient sensor is a hydrophone and the particle velocity sensor is
a geophone. The ratio of acoustic pressure in a medium to the
associated particle velocity speed is the specific acoustic
impedance (.rho..sub.0c=p/u). For a hydrophone, having a good
impedance match to the medium surrounding the hydrophone, and
having (for example) a pressure sensitivity of 20 volts per bar,
i.e., H=20V/bar, (Eq. 4) which relationship may be expressed as
H=20V/10.sup.5N/m.sup.2, (Eq. 5) and a geophone or a group of
geophones, having a good impedance match to the medium surrounding
the geophone, and having (for example) a voltage sensitivity of:
G=20V/m/s, (Eq. 6) the scale factor (K), expressing the
relationship between velocity output signal of the geophone and the
pressure output signal of the hydrophone will be: K = H .rho. 0
.times. c G = 20 10 - 5 .function. [ V / N / m 2 ] 1.5 10 6
.function. [ Ns / m 3 ] 20 .function. [ V / m / s ] = 15 ( Eq .
.times. 7 ) ##EQU3## which indicates that the geophone velocity
output signal needs to be multiplied with a scale factor of K=15
before the pressure and the velocity can be compared. It will be
understood that for hydrophones and geophones having different
sensitivities than in the example discussed above, the scale factor
(K) will be different. Further, because of the variation in the
amplitude (as shown in FIG. 6A) and phase (as shown in FIG. 6B) of
the geophone output as a function of frequency, it is necessary to
compensate for the amplitude and phase response of the geophone
before applying the scale factor.
[0055] The amplitude response (E) and phase response (.phi.) for
the geophone as a function of frequency may be represented by the
following relationships: E = G .function. ( f 2 f n 2 ) .times. ( R
r + R ) ( 1 - f 2 f n 2 ) + 4 .times. b t 2 .times. f 2 f n 2 ( Eq
. .times. 8 ) .PHI. = .alpha. .times. .times. cot .function. ( 2
.times. b 1 .times. f f n 1 - f 2 f n 2 ) ( Eq . .times. 9 )
##EQU4## in which, G=geophone voltage sensitivity; [0056]
f=frequency; [0057] f.sub.n=natural resonance frequency; [0058]
r=winding resistance; [0059] R=load resistance; and [0060]
b.sub.t=total damping. Typical values may be: f.sub.n=10; r=350
ohms; R=.infin.; and b.sub.t=0.6.
[0061] If the amplitude and phase of the geophone output signal is
adjusted to compensate for this variation in phase and amplitude
with frequency, the geophone output signal will have substantially
the same phase and amplitude curve as the hydrophone signal.
Normally the adjustment may be made on the basis of calculations
based on Equations 8 and 9.
[0062] As stated above, in a preferred embodiment of the invention,
particle velocity sensors are constructed to have an acoustic
impedance substantially similar to the acoustic impedance of the
water in the body of water in which the particle velocity sensors
are deployed. Accordingly, problems encountered in prior art
system, in which the impedance of the sensor was not matched to the
acoustic impedance of the medium from which a seismic wave was
coupled to the sensor, are avoided. In prior art systems variations
in amplitude and phase as a function of frequency caused by
impedance mismatch compounded the difficulty of matching the
particle velocity sensor output to the pressure gradient sensor
output. Because of the impedance match achieved in a preferred
embodiment of the present invention, only the variation in
amplitude and phase of the particle velocity sensor itself needs to
be compensated for to enable the particle velocity sensor output to
be combined with the pressure sensor output to attenuate the
spectral notches caused by the ghost reflection.
[0063] In a preferred embodiment of the invention, the phase and
amplitude variations with frequency of the particle velocity sensor
may be calculated based on known (or determinable) characteristics
of the particle velocity sensor, itself. The output signal of the
particle velocity sensor may be modified accordingly to correct for
amplitude and phase variation with frequency using filter
techniques well known to those of ordinary skill in the art. For
co-located pressure gradient sensors and particle velocity sensors,
the signal output of the pressure gradient sensor and the filtered
output of the pressure gradient sensor may then be summed to
attenuate the spectral notches resulting from the ghost reflection.
Although, in a preferred embodiment of the invention, the phase and
amplitude of the particle velocity sensor output is modified to
substantially match the pressure gradient sensor output, those of
ordinary skill in the art would understand that the phase and
amplitude of the pressure gradient sensor output could be modified
to match the particle velocity sensor output signal.
[0064] Because the noise level is generally greater at shallower
water depths, placing the streamer at depths greater than about
nine meters (the greatest depth at which streamer cables are
typically deployed) may reduce noise detected by the sensors, and
the signal to noise ratio of the signals detected by the seismic
sensors is accordingly improved. However, for such greater depths,
notches in a hydrophone spectrum resulting from the surface ghost
reflection are at lower frequencies, and such a hydrophone signal
is normally regarded as undesirable because of the spectral notches
in the frequency range of interest in seismic exploration. In
accordance with an embodiment of the present invention, the output
signal from the particle velocity sensor, which will have notches
in its frequency spectrum at different frequencies from the notches
in the frequency spectrum of the hydrophone, may be combined with
the hydrophone output signal to compensates for the notches and a
substantially ghost free signal can be obtained. FIG. 7 shows
simulated output responses for a hydrophone (curve 42) and a
geophone (curve 44) at a water depth of 26-meters. The graph
indicates that two signals may be combined to compensate for the
spectral notches resulting from the surface reflection. FIG. 8
provides actual data from a field test with the cable at about 26
meters, which confirms the results indicated in the simulation. In
FIG. 8 the geophone output signal is designated by numeral 46 and
the hydrophone output signal is designated by numeral 48. FIG. 9
shows a summation (curve 60) of the hydrophone and geophone data
shown in FIG. 8, and illustrates the attenuation of the spectral
notches
[0065] Because of the potential high noise level in geophone
signals at low frequencies, resulting from mechanical vibrations in
the cable, in a particular implementation of the invention, low
frequency geophone signals are not combined with the hydrophone
signal. In one specific implementation of the invention,
frequencies in the geophone signal lower than about the frequency
of the lowest frequency spectral notch in the hydrophone spectrum
are removed from the geophone signal before the geophone signal is
combined with the hydrophone signal. In another implementation of
the invention, geophone signals of less than about 30 Hz. are not
combined with the hydrophone signal.
[0066] Improved results are also afforded for operations at shallow
depths by the use of particle velocity sensors in seismic cables in
addition to pressure gradient sensors, over operations using solely
pressure gradient sensors. At shallower depths, i.e., less than
about 6 meters, a hydrophone output signal will be attenuated by
the surface ghost in the seismic frequency range of interest.
Because of the phase difference between the upgoing pressure
gradient wavefield and the downgoing pressure gradient wavefield
within the seismic frequency band of interest, the downgoing
wavefield is subtractive with respect to the upgoing wavefield and
the downgoing wavefield effectively attenuates the upgoing
wavefield. For a geophone signal, however, the result is the
opposite, and the surface ghost signal effectively increases the
amplitude of the signal detected by the geophone. The difference in
phase between the upgoing wavefield and the downgoing wavefield is
such that, for shallow depths, the signal detected by the geophone
is additive. Accordingly, substantially improved results are
achieved by use of particle velocity sensors in addition to
pressure gradient sensors at shallow depths over what is achieved
by use of pressure gradient sensors alone. In coastal regions where
the water depth is quite shallow, it may be particularly useful to
be able to deploy the sensors at such shallower depths.
[0067] FIG. 10 shows a simulation of a hydrophone signal (curve 52)
and a geophone signal (curve 50) at one-meter depth. The
attenuation of the hydrophone signal is evident. Combining the
geophone output signal with the hydrophone output signal for data
recorded at the one-meter water depth also compensates for the
influence from the surface reflection.
[0068] Generally, a hydrophone signal will have an amplitude that
is 10 to 20 times greater than the amplitude of a geophone signal.
This relationship will vary depending on the particular sensitivity
of the particular sensors used. Typically a group of hydrophones,
distributed across a selected spatial distance, will be connected
in parallel for noise attenuation, and the hydrophone output signal
that is recorded for use in seismic data processing and analysis is
the combined output from a plurality of individual hydrophones
connected in parallel. Because of the lower signal amplitude of the
geophone output signal, in one implementation of the invention, a
group of geophones, associated with a group of hydrophones
(co-located geophones and hydrophones), will be connected in
series, to increase the amplitude of the output signal as well as
to attenuate noise, and the geophone output signal that is recorded
for use in seismic data processing and analysis will be the
combined output from a plurality of individual geophones connected
in series. However, depending on the needs of a particular survey,
the geophone groups may be connected in parallel or series, or in a
parallel/series combination. Although, in general, the discussion
herein refers to an output signal from various sensors, the output
signal is typically the output signal from a plurality of discrete
sensors interconnected into a sensor array. Further, although the
discussion herein generally refers to a geophone and hydrophone,
particle velocity sensors other than geophones and pressure
gradient sensors other than hydrophones are intended to be within
the scope of the present invention.
[0069] In one embodiment, groups of about 8 pressure gradient
sensors will be used in association with groups of about 2 to about
16 particle velocity sensors (with lower numbers rather than higher
numbers of particle velocity sensors preferred), and each combined
group will be about 12.5 meters apart from another such combined
group. In this embodiment, combined groups of both pressure sensors
and particle velocity sensors will be treated as single
sensors.
[0070] In one embodiment of the invention, three-component particle
velocity sensors are included in the seismic cable. By
"three-component" is meant that, in addition a particle velocity
sensor (typically a geophone) mounted to sense motion in the
vertical direction, two particle velocity sensor are mounted in
orthogonal directions with respect to each other (and to the
vertically mounted geophone) to sense horizontal motion.
Accordingly, a three-component particle velocity sensor will sense
motion in the vertical direction, in an in-line direction and a
cross line direction. Positioning these sensors in these three
directions enables the propagation direction of an incoming signal
to be detected, and also enables the detection of strumming or
other mechanical behavior to the cable.
[0071] Accelerometers could be used as particle motion sensor as an
alternative to use of geophones, although the output signal will
need to be integrated to obtain velocity. An example commercial
accelerometer suitable for use in the present invention is the
VECTOR-SEIS.TM., available from Input Output, Inc. in Houston, Tex.
This particular accelerometer generates a DC output signal which is
indicative of the variation in orientation of the accelerometer
from a selected orientation, accordingly, if sets of 2 (for
situations in which the in-line direction is known) or 3 (if the
in-line direction is not known) of these accelerometers are
utilized, the sensor orientation may be computed and it is not
necessary to gimbal-mount the accelerometers. A single
accelerometer could also be used, but it would need to be
gimbal-mounted. Since the sensor can measure acceleration to DC, it
is possible to determine the true gravity vector by analyzing the
magnitude of G (the gravity vector) each sensor is operable under.
The results of this analysis are stored with the trace data as
direction cosines and describe the tensor rotation required to
recover the signals as if the sensor were deployed at true vertical
orientation.
[0072] The foregoing description of the invention is intended to be
a description of preferred embodiments. Various changes in the
described apparatus and method can be made without departing from
the intended scope of this invention as defined by the appended
claims.
* * * * *