U.S. patent application number 14/202856 was filed with the patent office on 2014-07-10 for combining seismic data from sensors to attenuate noise.
This patent application is currently assigned to WESTERNGECO L.L.C.. The applicant listed for this patent is WESTERNGECO L.L.C.. Invention is credited to PASCAL EDME, JULIAN EDWARD KRAGH, QUINGLIN LIU, EVERHARD MUIJZERT, JOHAN O. A. ROBERTSSON.
Application Number | 20140192620 14/202856 |
Document ID | / |
Family ID | 43823847 |
Filed Date | 2014-07-10 |
United States Patent
Application |
20140192620 |
Kind Code |
A1 |
EDME; PASCAL ; et
al. |
July 10, 2014 |
COMBINING SEISMIC DATA FROM SENSORS TO ATTENUATE NOISE
Abstract
To perform noise attenuation for seismic surveying, a sensor
assembly is deployed on a ground surface, where the sensor assembly
has a seismic sensor to measure seismic waves propagated through a
subterranean structure, and a divergence sensor comprising a
pressure sensor to measure noise. First data is received from the
seismic sensor, and second data is received from the divergence
sensor. The first data and the second data are combined to
attenuate noise in the first data.
Inventors: |
EDME; PASCAL; (CAMBRIDGE,
GB) ; MUIJZERT; EVERHARD; (GIRTON, GB) ;
KRAGH; JULIAN EDWARD; (GREAT BARDFIELD, GB) ;
ROBERTSSON; JOHAN O. A.; (WALD, CH) ; LIU;
QUINGLIN; (BEIJING, CN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
WESTERNGECO L.L.C. |
HOUSTON |
TX |
US |
|
|
Assignee: |
WESTERNGECO L.L.C.
HOUSTON
TX
|
Family ID: |
43823847 |
Appl. No.: |
14/202856 |
Filed: |
March 10, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12573266 |
Oct 5, 2009 |
8712694 |
|
|
14202856 |
|
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Current U.S.
Class: |
367/38 |
Current CPC
Class: |
G01V 1/366 20130101;
G01V 1/364 20130101; G01V 1/30 20130101; G01V 2210/32 20130101 |
Class at
Publication: |
367/38 |
International
Class: |
G01V 1/30 20060101
G01V001/30 |
Claims
1. A method of noise attenuation for land-based seismic surveying,
comprising: deploying a sensor assembly at a ground surface,
wherein the sensor assembly has a seismic sensor to measure seismic
waves reflected from a subterranean structure in response to
seismic waves transmitted due to activation of at least one seismic
source, and a divergence sensor, wherein the divergence sensor
includes a container containing a material and a pressure sensor
immersed in the material, and wherein a portion of the sensor
assembly including the divergence sensor is connected with the
ground so that the divergence sensor senses pressure fluctuations
in the material resulting at least partially from ground roll;
receiving first data representing the reflected seismic waves from
the seismic sensor and second data representing the pressure
fluctuations from the divergence sensor, the first data
representing measurements along one or more axes, and the second
data comprising measurements insensitive to a direction of wave
propagation; and combining the first data and the second data to
attenuate ground-roll noise in the first data.
2. The method of claim 1, wherein the seismic sensor and divergence
sensor are physically spaced apart by a predetermined distance.
3. The method of claim 1, wherein combining the first data and the
second data comprises subtracting the second data from the first
data.
4. The method of claim 3, wherein subtracting the second data from
the first data comprises subtracting a product of the second data
and a filter operator from the first data.
5. The method of claim 1, wherein the material is selected from the
group consisting of a liquid, a gel and a solid.
6. The method of claim 1, wherein the second data from the
divergence sensor provides a better noise model than a component
orthogonal to, or near-orthogonal to, the first data from the
seismic sensor.
7. The method of claim 1, further comprising deploying additional
sensor assemblies at the ground surface, where each of the
additional sensor assemblies has a seismic sensor to measure
seismic waves reflected from the subterranean structure, and a
divergence sensor to measure noise.
8. The method of claim 7, wherein deploying the sensor assemblies
comprises deploying the sensor assemblies in an environment that
includes one or more obstructions that disturbs a regular pattern
of the sensor assemblies, wherein provision of the divergence
sensors enable noise attenuation even without the regular pattern
of the sensor assemblies.
9. The method of claim 8, wherein deploying the sensor assemblies
comprises providing sensor assemblies that are spaced apart from
each other by a distance larger than half a shortest wavelength of
noise recorded by the divergence sensors.
10. A system comprising: a controller having a processor to receive
data collected by sensor assemblies deployed at a ground surface,
where each of the sensor assemblies has a seismic sensor to measure
seismic waves reflected from a subterranean structure, and a
divergence sensor, wherein the divergence sensor includes a
container containing a material and a pressure sensor immersed in
the material, and the divergence sensor is insensitive to a
direction of wave propagation, and wherein the sensor assembly
including the divergence sensor is connected with the ground so
that the divergence sensor senses pressure fluctuations in the
material resulting at least partially from ground roll; wherein the
processor is configured to combine first data representing the
reflected seismic waves from the seismic sensors with second data
representing the pressure fluctuations from the divergence sensors
to attenuate ground-roll noise in the first data from the seismic
sensors, the first data representing measurements along one or more
axes, and the second data comprising measurements insensitive to a
direction of wave propagation.
11. The system of claim 10, wherein the material is selected from
the group of a liquid, a gel and a solid.
12. The system of claim 10, wherein, in each of the sensor
assemblies, the seismic sensor is provided above and external to
the container of the divergence sensor.
14. The system of claim 10, wherein the controller is configured to
communicate over a cable with the sensor assemblies.
15. The system of claim 10, wherein the controller is configured to
communicate wirelessly with the sensor assemblies.
16. The system of claim 10, wherein combining the first data and
the second data comprises subtracting the second data from the
first data.
17. The system of claim 16, wherein subtracting the second data
from the first data comprises subtracting a product of the second
data and a filter operator from the first data.
18. An article comprising at least one non-transitory
computer-readable storage medium containing instructions that upon
execution cause a system having a processor to: receive data
collected by sensor assemblies arranged at a ground surface, where
each of the sensor assemblies has a seismic sensor to measure
seismic waves reflected from a subterranean structure, and a
divergence sensor, wherein the divergence sensor includes a
container containing a material and a pressure sensor immersed in
the material, and the divergence sensor is insensitive to a
direction of wave propagation, and wherein a portion of the sensor
assembly including the divergence sensor is connected with the
ground so that the divergence sensor senses pressure fluctuations
resulting at least partially from ground roll; and combine first
data representing the reflected seismic waves from the seismic
sensors with second data representing the pressure fluctuations
from the divergence sensors to attenuate ground-roll noise in the
first data from the seismic sensors, the first data representing
measurements along one or more axes, and the second data comprising
measurements insensitive to a direction of wave propagation.
19. The article of claim 18, wherein combining the first data and
the second data comprises subtracting the second data from the
first data.
20. The method of claim 1, wherein deploying the sensor assembly
comprises deploying the sensor assembly having a housing containing
the seismic sensor and the divergence sensor.
21. The system of claim 10, wherein at least one of the sensor
assemblies has a housing containing the corresponding seismic
sensor and the corresponding divergence sensor.
22. The article of claim 19, wherein at least one of the sensor
assemblies has a housing containing the corresponding seismic
sensor and the corresponding divergence sensor.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to U.S. patent application
Ser. No. 12/573,266 filed Oct. 5, 2009, which is incorporated
herein by reference in its entirety.
BACKGROUND
[0002] Seismic surveying is used for identifying subterranean
elements, such as hydrocarbon reservoirs, freshwater aquifers, gas
injection zones and so forth. In seismic surveying, seismic sources
are placed at various locations on a land surface or sea floor,
with the seismic sources activated to generate seismic waves
directed into a subterranean structure.
[0003] The seismic waves generated by a seismic source travel into
the subterranean structure, with a portion of the seismic waves
reflected back to the surface for receipt by seismic receivers
(e.g., geophones, accelerometers, etc.). These seismic receivers
produce signals that represent detected seismic waves. Signals from
the seismic receivers are processed to yield information about the
content and characteristic of the subterranean structure.
[0004] A typical land-based seismic survey arrangement includes
deploying an array of seismic receivers on the ground with the
seismic receivers provided in an approximate grid formation. The
seismic receivers can be multi-component geophones that enable the
measurement of an incoming wavefield in three orthogonal directions
(vertical z, horizontal inline x, and horizontal crossline y).
[0005] For land-based seismic surveying, various types of unwanted
wavefields may be present, including ground-roll noise, such as
Rayleigh or Love surface waves. The unwanted wavefields can
contaminate seismic data acquired by seismic receivers. Although
various conventional techniques exist to remove unwanted wavefields
from seismic data, such techniques are relatively complex and may
be costly.
SUMMARY
[0006] In general, according to an embodiment, a method of noise
attenuation for seismic surveying includes deploying a sensor
assembly on a land surface, where the sensor assembly has a seismic
sensor to measure seismic waves propagated through a subterranean
structure, and a divergence sensor comprising a pressure sensor to
measure noise. First data received from the seismic sensor and
second data received from the divergence sensor are combined to
attenuate noise in the first data.
[0007] Other or alternative features will become apparent from the
following description, from the drawings, and from the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is a schematic diagram of an example arrangement of
sensor assemblies that can be deployed to perform a land-based
seismic survey, according to an embodiment;
[0009] FIG. 2 illustrates a sensor assembly according to an
embodiment that can be employed in the arrangement of FIG. 1;
[0010] FIGS. 3A-3B are graphs illustrating propagation of
wavefields that are detectable by a sensor assembly according to an
embodiment;
[0011] FIGS. 4A-4C are graphs illustrating data in the time-offset
domain as acquired by sensor assemblies according to some
embodiments; and
[0012] FIG. 5 is a flow diagram of a process of performing seismic
surveying, according to an embodiment.
DETAILED DESCRIPTION
[0013] In the following description, numerous details are set forth
to provide an understanding of the present invention. However, it
will be understood by those skilled in the art that the present
invention may be practiced without these details and that numerous
variations or modifications from the described embodiments are
possible.
[0014] In accordance with some embodiments, to attenuate noise in
seismic data in a land-based survey arrangement, sensor assemblies
each having at least one seismic sensor and at least one divergence
sensor (for measuring noise) are employed. In some embodiments, the
divergence sensor is formed using a container filled with a
material in which a pressure sensor (e.g., a hydrophone) is
provided. The pressure sensor in such an arrangement is able to
record mainly noise, such that the data from the pressure sensor in
the sensor assemblies can be used to develop a noise reference
model for cleansing seismic data acquired by the seismic sensors.
The material in which the pressure sensor is immersed can be a
liquid, a gel, or a solid such as sand or plastic.
[0015] One type of noise is ground-roll noise. Ground-roll noise
refers to seismic waves produced by seismic sources that travel
generally horizontally along a ground surface towards seismic
receivers. These horizontally traveling seismic waves, such as
Rayleigh waves or Love waves, are undesirable components that can
contaminate seismic data. Generally, "noise" refers to any signal
component that is unwanted from seismic data (such as data
representing reflected signals from subterranean elements). Other
types of noise include flexural waves present in data acquired over
frozen surfaces such as a body of water or permafrost; and airborne
noise caused by the environment such as due to wind, rain, or human
activity such as traffic, air blasts, flare noise or other
industrial processes.
[0016] FIG. 1 is a schematic diagram of an arrangement of sensor
assemblies 100 that are used for land-based seismic surveying. The
sensor assemblies 100 are deployed on a ground surface 108 (in a
row or in an array). A sensor assembly 100 being "on" a ground
surface means that the sensor assembly 100 is either provided on
and over the ground surface, or buried (fully or partially)
underneath the ground surface such that the sensor assembly 100 is
with 10 meters of the ground surface. The ground surface 108 is
above a subterranean structure 102 that contains at least one
subterranean element 106 of interest (e.g., hydrocarbon reservoir,
freshwater aquifer, gas injection zone, etc.). One or more seismic
sources 104, which can be vibrators, air guns, explosive devices,
and so forth, are deployed in a survey field in which the sensor
assemblies 100 are located.
[0017] Activation of the seismic sources 104 causes seismic waves
to be propagated into the subterranean structure 102.
Alternatively, instead of using controlled seismic sources as noted
above to provide controlled source or active surveys, some
embodiments can also be used in the context of passive surveys.
Passive surveys use the sensor assemblies 100 to perform one or
more of the following: (micro)earthquake monitoring; hydro-frac
monitoring where microearthquakes are observed due to rock failure
caused by fluids that are actively injected into the subsurface,
such as a hydrocarbon reservoir; and so forth. Seismic waves
reflected from the subterranean structure 102 (and from the
subterranean element 106 of interest) are propagated upwardly
towards the sensor assemblies 100. Seismic sensors 112 (e.g.,
geophones, accelerometers, etc.) in the corresponding sensor
assemblies 100 measure the seismic waves reflected from the
subterranean structure 102. Moreover, the sensor assemblies 100
further include divergence sensors 114 that are designed to measure
noise, such as ground-roll noise or other types of noise. The data
from the divergence sensors 114 can be employed to develop a noise
reference model to attenuate noise in the measured seismic
signals.
[0018] In one embodiment, the sensor assemblies 100 are
interconnected by an electrical cable 110 to a controller 116.
Alternatively, instead of connecting the sensor assemblies 100 by
the electrical cable 110, the sensor assemblies 100 can communicate
wirelessly with the controller 116. In some implementations,
intermediate routers or concentrators may be provided at
intermediate points of the network of sensor assemblies 100 to
enable communication between the sensor assemblies 100 and the
controller 116.
[0019] The controller 116 shown in FIG. 1 further includes
processing software 120 that is executable on a processor 122. The
processor 122 is connected to storage media 124 (e.g., one or more
disk-based storage devices and/or one or more memory devices). In
the example of FIG. 1, the storage media 124 is used to store
seismic sensor data 126 communicated from the seismic sensors 112
of the sensor assemblies 100 to the controller 116, and to store
divergence data 128 communicated from the divergence sensors 114 of
the sensor assemblies 100.
[0020] In operation, the software 120 is used to process the
seismic sensor data 126 and the hydrophone data 128. The hydrophone
data 128 is combined with the seismic sensor data 126, using
techniques discussed further below, to attenuate noise in the
seismic sensor data 126 (to produce a cleansed version of the
seismic sensor data). The software 120 can then produce an output
to characterize the subterranean structure 102 based on the
cleansed seismic sensor data 126.
[0021] A sensor assembly 100 according to some embodiments is
depicted in greater detail in FIG. 2. The seismic sensor 112 in the
sensor assembly can be a geophone for measuring particle velocity
induced by seismic waves in the subterranean structure 102, or
alternatively, the seismic sensor 112 can be an accelerometer for
measuring acceleration induced by seismic waves propagated through
the subterranean structure 102.
[0022] In some embodiments, the seismic sensor 112 is a vertical
component seismic sensor for measuring seismic waves in the
vertical direction (represented by axis z in FIG. 1). In
alternative embodiments, the sensor assembly 100 can additionally
or alternatively include seismic sensors for detecting seismic
waves in generally horizontal directions, such as the x or y
directions that are generally parallel to the ground surface
108.
[0023] The divergence sensor 114 that is also part of the sensor
assembly 100 (within a housing 101 of the sensor assembly 100) is
used for measuring an input (e.g., noise) different from the
seismic waves propagated through the subterranean structure 102
that are measured by the seismic sensor 112. In an alternative
embodiment, the divergence sensor 114 of the sensor assembly 100
can be physically spaced apart from the seismic sensor 112 by some
predetermined distance.
[0024] The divergence sensor 114 has a closed container 200 that is
sealed. The container 200 contains a volume of liquid 202 (or other
material such as a gel or a solid such as sand or plastic) inside
the container 200. Moreover, the container 200 contains a
hydrophone 204 (or other type of pressure sensor) that is immersed
in the liquid 202 (or other material). The pressure sensor being
immersed in the material means that the pressure sensor is
surrounded by or otherwise attached to or in contact with the
material. In the ensuing discussion, reference is made to the
hydrophone 204 that is immersed in the liquid 202--note that in
alternative embodiments, other types of pressure sensors can be
immersed in other types of material. The hydrophone 204, which is
neutrally buoyantly immersed in the liquid 202, is mechanically
decoupled from the walls of the container 200. As a result, the
hydrophone 204 is sensitive to just acoustic waves that are induced
into the liquid 202 through the walls of the container 200. To
maintain a fixed position, the hydrophone 204 is attached by a
coupling mechanism 206 that dampens propagation of acoustic waves
through the coupling mechanism 206.
[0025] Examples of the liquid 202 include the following: kerosene,
mineral oil, vegetable oil, silicone oil and water. In other
embodiments, other types of liquids can be employed. A liquid with
a higher viscosity can be used to change the sensitivity to
different types of waves, including P (compression) waves, S
(shear) waves, Rayleigh waves and Love waves. Moreover, the amount
of liquid 202 provided in the container 200 of the divergence
sensor 114 determines the sensitivity of the hydrophone 204. A
container 200 that is only partially filled with liquid records a
weaker signal. In some embodiments, the container 200 can be
partially filled with liquid to provide an expansion volume within
the container 200. Expansion of the liquid 202, such as due to a
temperature rise of the liquid 202, can be accommodated in the
expansion volume (which can be filled with a gas).
[0026] As further shown in FIG. 2, the sensor assembly 100 also
includes electronic circuitry 208 that is electrically coupled to
both the seismic sensor 112 and the divergence sensor 114. The
electronic circuitry 208 can include storage elements, processing
elements, and communications elements for communicating data
acquired by the seismic sensor 112 and divergence sensor 114 over
the electrical cable 110 to the controller 116 (FIG. 1).
[0027] As depicted in FIG. 2, the seismic sensor 112 is positioned
above and external to the container 200 of the divergence sensor
114. Alternatively, the seismic sensor 112 can have some other
arrangement with respect to the divergence sensor 114. At least a
portion of the divergence sensor 114 is below the ground surface
108, such that the hydrophone 204 is at or below the ground surface
108, but not above the ground surface 108. When planted, the
divergence sensor 114 of the sensor assembly 100 is firmly in
contact with the earth medium underneath the ground surface 108,
which improves data quality of signals acquired by the hydrophone
204 in the divergence sensor 114.
[0028] In embodiments that employ the cable 110, power is provided
from a remote power supply (such as a power supply located at the
controller 116) through the cable 110 to the sensor assemblies 100.
In embodiments that employ wireless communications and that do not
use the cable 110, the sensor assembly 100 can be provided with
batteries to provide local power.
[0029] In land-based seismic surveying, particle displacement (or
velocity or acceleration) is measured by seismic sensors just below
the free surface (ground surface 108). As a result, the observed
signal components contain not only the impinging-upcoming seismic
waves but also the additional contribution of downwardly
reflected/converted waves at the solid-air interface (ground
surface 108). FIG. 3A shows an example of an incoming compression
or P wave, and FIG. 3B shows an example for an incoming shear or S
wave. The P and S waves are incoming from the subterranean
structure 102 (FIG. 1). A P wave extends in the direction of
propagation of the seismic wave, whereas an S wave extends in a
direction generally perpendicular to the direction of propagation
of the seismic wave.
[0030] In each of FIGS. 3A and 3B, the z direction represents the
vertical direction, while the x direction represents the inline
horizontal direction. The air-solid interface corresponds to the
ground surface 108 shown in FIG. 1. An upcoming P wave is
represented as P.sub.up (FIG. 3A), while an upcoming S wave is
represented as S.sub.up (FIG. 3B). A seismic sensor 112 records not
only the upcoming P or S waves, but also the additional
contribution of the downwardly reflected/converted waves at the
solid-air interface, including the R.sub.PP and R.sub.PS waves
(FIG. 3A), which are reflected in response to the P.sub.up wave.
Similarly, the reflected/converted waves that are reflected from
the S.sub.up wave is represented as R.sub.SP and R.sub.SS in FIG.
3B.
[0031] The R.sub.PP wave is a P wave reflected from the P.sub.up
wave, while the R.sub.PS wave is a reflected S wave from the Pup
wave. Similarly, R.sub.SP is the reflected P wave from the S.sub.up
wave, and R.sub.SS is a reflected S wave from the S.sub.up
wave.
[0032] In contrast to the seismic sensor 112, the hydrophone 204
inside the container 200 filled with liquid 202 in the sensor 114
shown in FIG. 2 is insensitive to the direction and angle of
propagation of waves. As a result, this leads to a destructive
summation of events at a near vertical incidence angle (small
slowness), and thus, the measurement of the hydrophone 204
(represented as U.sub.H) records mainly surface noise (which has
large slowness). Slowness is proportional to the inverse of
apparent velocity--small slowness results from high apparent
velocity, while large slowness results from small apparent
velocity. Thus, U.sub.H (which is the data from the hydrophone 204
in the divergence sensor 114 of FIG. 2) provides a better
representation of noise than U.sub.x (the x component of seismic
data measured by a seismic sensor.) As a result, U.sub.H can be
used for noise removal on U.sub.z, based on adaptive subtraction
and/or polarization. U.sub.z refers to the measured seismic wave in
the z direction. In the ensuing discussion, U.sub.x, U.sub.z, and
U.sub.H are assumed to measure particle displacement or velocity or
acceleration or pressure just below the free surface.
[0033] U.sub.z and U.sub.x are represented according to Eqs. 1 and
2 below:
U.sub.z-(-q.sub..alpha..alpha.+R.sub.PPq.sub..alpha..alpha.-R.sub.PSp.be-
ta.)P.sub.up+(p.beta.-R.sub.SSp.beta.+R.sub.SPq.sub..alpha..alpha.)S.sub.u-
p ,(Eq. 1)
U.sub.x=(p.alpha.+R.sub.PPp.alpha.+R.sub.PSq.sub..beta..beta.)P.sub.up+(-
q.sub..beta..beta.+R.sub.SSq.sub..beta..beta.+R.sub.SPp.alpha.)S.sub.up
,(Eq. 2)
where P.sub.up and S.sub.up are the incident P and S waves
(respectively, as shown in FIGS. 3A-3B), .alpha. and .beta. are the
near-surface P- and S-wave velocities, and
p=sini/.alpha.=sinj/.beta. is horizontal slowness. The vertical
slownesses for P- and S-waves are
q.sub.a=(.alpha..sup.-2-p.sup.2).sup.0.5 and
q.sub..beta.=(.beta..sup.2-p.sup.2).sup.0.5, respectively. The
R.sub.ij terms are the reflection/conversion coefficients for an
incident i wave backward reflected/converted into a j wave at the
solid-air interface just above the sensors. The left hand side of
each of Eqs. 1 and 2 relates the free-surface effect considering an
incident P wave as illustrated in FIG. 3A, while the right hand
side of each of Eqs. 1 and 2 relates the free-surface effect
considering an incident S wave as illustrated in FIG. 3B. Note that
seismic sensor (geophone or accelerometer) signal components are
vectorial measurements--for example, if the incoming P wave case is
considered on the z component, the + and - in the left hand side of
each of Eqs. 1 and 2 relates the direction of propagation, i.e.
upgoing (-) or downgoing (+), while the terms q.sub..alpha..alpha.
(=cosi) or p.beta. (=sinj) relate to the propagation angles, where
i and j are propagation angles (with respect to the vertical) of P
and S waves, respectively.
[0034] In contrast to the seismic sensors 112 (FIG. 2) that record
both P and S waves in a vectorial manner, a hydrophone (204 in FIG.
2) in a liquid cell will only record the pressure fluctuation due
to the P wavefield since S wave propagation is not supported by
liquid. In addition, such a sensor (divergence sensor 114) is
assumed to be insensitive to the direction of propagation, since
pressure is a scalar quantity. Therefore, compared to the seismic
sensor data represented by Eqs. 1 and 2, the angle-direction
related terms vanish for the case of a full isotropic hydrophone
sensor, and the hydrophone data U.sub.H can be written as:
U.sub.H=C.sub.1(P.sub.UP+R.sub.PPP.sub.up+R.sub.SPS.sub.up), (Eq.
3)
where C.sub.1 is a constant that includes (1) a calibration factor
to compensate for the difference in sensitivity between the
hydrophone 204 (that measures pressure) and the seismic sensors 112
(that measure displacement or velocity or acceleration), and (2) a
transmission factor from the ground into the container of the
divergence sensor 114. The hydrophone 204 does not directly record
S waves, but note that S wavefield related energy will be present
in the hydrophone data due to the S-to-P conversion at the free
surface (last term of Eq. 3). Finally, by including into Eq. 3 the
P.sub.up and S.sub.up expressions as a function of U.sub.z and
U.sub.x as well as the expressions for the reflection coefficients,
the following is obtained:
U.sub.H=C.sub.2pU.sub.x ,(Eq. 4)
where C.sub.2 depends on C.sub.1 and on the near-surface properties
in the vicinity of the receiver.
[0035] In the slowness domain, U.sub.H is just a p-dependent (p
represents slowness) scaled version of U.sub.x. This means that, at
small slowness (small p), or equivalently at small incident angles,
wave amplitudes on the U.sub.H component are extremely small, even
compared to U.sub.x. This is true for S waves as well as for P
waves. This natural P wave attenuation on U.sub.H is due to the
destructive summation between the upgoing and downgoing reflected
events, R.sub.PP being equal to -1 at vertical incidence (p=0). In
summary, the hydrophone 204 acts as a natural velocity filter, by
attenuating small slownesses (high apparent velocity like
reflections) much more than larger slownesses (slow apparent waves,
typically ground-roll noise).
[0036] By rewriting Eq. 4 (in the slowness domain) into the
conventional time-offset domain (with p=.delta.x/.delta.t), the
following is obtained:
.delta. U H ( t , x ) .delta. t = C 2 .delta. U x ( t , x ) .delta.
x . ( Eq . 5 ) ##EQU00001##
[0037] The above inline (2D) case can be extended to the
3-dimensional case as:
.delta. U H .delta. t .varies. ( .delta. U x .delta. x + .delta. U
y .delta. y ) . ( Eq . 6 ) ##EQU00002##
[0038] The time derivative of the hydrophone signal component
U.sub.H, represented by Eq. 6, is proportional to the divergence of
the wavefield (just below the free-surface).
[0039] FIGS. 4A-4C show simulated data in the time-offset domain,
in which offset refers to the distance between each sensor assembly
100 and a seismic source. FIG. 4A shows the time-offset simulated
data for U.sub.z, FIG. 4B shows the time-offset simulated data for
U.sub.x, and FIG. 4C shows the time-offset simulated data for
U.sub.H. A generally cone-shaped pattern 402 in each of the FIGS.
4B-4C represents noise. Note that the noise appears in each of
U.sub.z, U.sub.x, and U.sub.H. However, the actual seismic data
(represented by the curved structures in FIGS. 4A-4C) is attenuated
in U.sub.H especially at small offsets. Therefore, it is apparent
that the hydrophone 204, which outputs U.sub.H, provides natural
velocity filtering.
[0040] Body waves, and especially reflected P waves at small offset
have even smaller amplitude on U.sub.H than on U.sub.x, in contrast
to the ground-roll cone 402. Therefore U.sub.H provides a better
noise reference model than U.sub.x for ground-roll attenuation on
U.sub.z. In addition, U.sub.H is a better noise model than U.sub.x
(or more generally an orthogonal or near-orthogonal component to
the recorded seismic data by the seismic sensor) because U.sub.H
records also offline scattered events (present on U.sub.z and
U.sub.y, in contrast to U.sub.x).
[0041] Global ground-roll noise correlation between U.sub.H and
U.sub.z should be better than that between U.sub.x and U.sub.z
because S waves and Love waves are weaker on U.sub.H than on
U.sub.x. As a result, U.sub.H can be used to provide better noise
attenuation on U.sub.z based on adaptive subtraction or
polarization filtering. Secondly signal preservation should be
improved by taking the U.sub.H instead of U.sub.x, because U.sub.H
contains less body wave energy (especially at small slowness-offset
where ground-roll noise is dominant).
[0042] Weighted HZ summation (summation of the U.sub.H and U.sub.z
signal components) enables the removal of noise from U.sub.z. Basic
adaptive subtraction can be written as:
U.sub.z.sup.clean(t,x)=U.sub.z(t,x)-F(t,x)U.sub.H(t,x), (Eq. 7)
where F can be a scalar or a frequency dependent operator estimated
for example by matching U.sub.H with U.sub.z in varying time-offset
windows (and for example in the least square sense). In Eq. 7
above, U.sub.z.sup.clean represents the seismic data with the noise
component removed.
[0043] F(t,x) is an operator, which can be a wavelet of several
point length in the general case, or simply a number (scalar) in
the special case where filter length=1. In this latter case, the
number F is simply obtained by dividing U.sub.z by U.sub.H in a
selected time-offset window: F(x,t)=Z(x,t)/H(x,t). F is frequency
independent, since it is just the scaling factor between U.sub.H
and U.sub.z (again in a specific time-offset window).
[0044] More generally, when considering the general case, F(t,x)
can be called a Wiener filter or a transfer function between
U.sub.H and U.sub.z, and F(t,x) becomes frequency dependent. In
such case, the F(t,x) estimation is based on the use of
cross-correlation and auto-correlation of Z(x,t) and H(x,t).
Numerous other algorithms can be used to estimate F, such as
solving for F in a L1 norm or L2 norm and least squares. Other
suitable methods to design and apply the filter operator F include
polarization, adaptive, statistical, deterministic, multi-channel
and multi-dimensional filtering techniques.
[0045] Although reference has been made to cleansing the U.sub.z
seismic signals (seismic signal components in the z direction), it
is noted that the divergence sensor output U.sub.H can also be used
to cleanse the seismic signal components U.sub.x or U.sub.y
horizontal directions.
[0046] By employing the noise attenuation technique according to
some embodiments, the spacing between sensor assemblies can be
increased. For example, the spacing between adjacent sensor
assemblies can be provided such that the sensor assemblies are
spaced apart from each other by a distance larger than half a
shortest wavelength of noise recorded by the divergence sensors. As
a result, a less dense array of sensor assemblies has to be
deployed, which reduces equipment costs, and reduces labor costs
associated with deployment of the sensor assemblies in the
field.
[0047] FIG. 2 depicts a divergence sensor 114 with a generally
cuboid container 200. The shape of the container 200 can be changed
to another shape, such as the following shapes: parallelepiped
shape, pyramid shape, quadrilateral frustum shape, dipyramid shape,
ellipsoid shape and spherical shape. Varying the shape of the
container 200 introduces anisotropy, which can yield wavefield
decomposition opportunities (such as to decompose into P and S
waves).
[0048] Instead of immersing the hydrophone 204 (FIG. 2) in a
liquid, the hydrophone 204 can instead be immersed in a more
viscous fluid (e.g., silicone oil), a gel, or even a solid such as
sand or plastic, to allow for S wave detection. This may allow for
decomposition of wavefields, such as P versus S waves, or Rayleigh
versus Love waves.
[0049] FIG. 5 illustrates a general process according to an
embodiment for performing a seismic survey. Sensor assemblies 100
are deployed (at 502) for performing the seismic surveying. One or
more seismic sources (e.g., 104 in FIG. 1) are then activated (at
504). In response to activation of the seismic source(s), seismic
waves are propagated into the subterranean structure 102 (FIG. 1).
Reflected seismic waves are detected by the sensor assemblies
100.
[0050] The seismic data (measured by seismic sensors 112) and
divergence data (measured by the divergence sensors 114) are
received (at 506) by the controller 116. The controller 116 then
combines (at 508) the seismic data and divergence data to cleanse
the seismic data, such as according to Eq. 7. The cleansed seismic
data is then processed (at 510) by the controller 116 to
characterize the subterranean structure 102. Note that the
combination of the seismic measurement and divergence measurement
may be carried out by a central control unit after receiving the
data from the sensor, or locally inside the electronics in the
sensor unit in FIG. 2 or at any intermediate location. The
combination may be carried out in real time, after each shot, after
completion of the survey or later in a data processing center.
[0051] The noise attenuation technique or system according to some
embodiments can be employed in a survey arrangement that has
obstructions, such as buildings or natural obstructions that
prevent placement of sensor assemblies at regular spacings. In
other words, the obstructions disturb the regular pattern of sensor
assemblies. By using the noise attenuation technique according to
some embodiments based on use of divergence sensors, better results
can be obtained.
[0052] Instructions of software described above (including software
120 of FIG. 1) are loaded for execution on a processor (such as
processor 122 in FIG. 1). The processor includes microprocessors,
microcontrollers, processor modules or subsystems (including one or
more microprocessors or microcontrollers), or other control or
computing devices. A "processor" can refer to a single component or
to plural components (e.g., one CPU or multiple CPUs).
[0053] Data and instructions (of the software) are stored in
respective storage devices, which are implemented as one or more
computer-readable or computer-usable storage media. The storage
media include different forms of memory including semiconductor
memory devices such as dynamic or static random access memories
(DRAMs or SRAMs), erasable and programmable read-only memories
(EPROMs), electrically erasable and programmable read-only memories
(EEPROMs) and flash memories; magnetic disks such as fixed, floppy
and removable disks; other magnetic media including tape; and
optical media such as compact disks (CDs) or digital video disks
(DVDs).
[0054] While the invention has been disclosed with respect to a
limited number of embodiments, those skilled in the art, having the
benefit of this disclosure, will appreciate numerous modifications
and variations therefrom. It is intended that the appended claims
cover such modifications and variations as fall within the true
spirit and scope of the invention.
* * * * *