U.S. patent application number 14/028933 was filed with the patent office on 2014-01-16 for acquisition scheme for vibroseis marine sources.
This patent application is currently assigned to CGG SERVICES SA. The applicant listed for this patent is CGG SERVICES SA. Invention is credited to Laurent RUET.
Application Number | 20140016435 14/028933 |
Document ID | / |
Family ID | 46829797 |
Filed Date | 2014-01-16 |
United States Patent
Application |
20140016435 |
Kind Code |
A1 |
RUET; Laurent |
January 16, 2014 |
ACQUISITION SCHEME FOR VIBROSEIS MARINE SOURCES
Abstract
Control mechanisms, computer software and methods for driving
vibrational source arrays underwater. An incoherent acquisition
scheme drives individual source elements simultaneously and
incoherently while a coherent acquisition scheme drives
high-frequency individual source elements simultaneously and
incoherently and low-frequency individual source elements
simultaneously and coherently. Thus, denser coverage and an
increased energy input is achieved for the source arrays.
Inventors: |
RUET; Laurent; (Massy,
FR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
CGG SERVICES SA |
Massy Cedex |
|
FR |
|
|
Assignee: |
CGG SERVICES SA
Massy Cedex
FR
|
Family ID: |
46829797 |
Appl. No.: |
14/028933 |
Filed: |
September 17, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13687059 |
Nov 28, 2012 |
8565041 |
|
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14028933 |
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13415225 |
Mar 8, 2012 |
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13687059 |
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Current U.S.
Class: |
367/23 |
Current CPC
Class: |
G01V 1/3808 20130101;
G01V 1/3861 20130101; Y02A 90/36 20180101; G01V 1/005 20130101;
G01V 1/38 20130101; Y02A 90/30 20180101 |
Class at
Publication: |
367/23 |
International
Class: |
G01V 1/38 20060101
G01V001/38 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 19, 2011 |
FR |
1159433 |
Claims
1. An incoherent acquisition method for driving vibrational source
arrays under water, the method comprising: towing with a vessel a
first source array and a second source array underwater, wherein
the first source array includes plural first individual source
elements and the second source array includes plural second
individual source elements; and activating simultaneously the first
source array and the second source array so that incoherent coded
driving signals drive the first and second source arrays, wherein
the coded driving signals include corresponding signatures so that
during a processing phase, seismic signals from the first source
array are separated from seismic signals of the second source
array.
2. The method of claim 1, wherein the incoherent coded driving
signals of the first and second source arrays include multiple
frequencies and coded driving signals for the first source array
are not correlated to coded driving signals for the second source
array.
3. The method of claim 1, wherein the plural first individual
source elements and the plural second individual source elements
are configured to emit a frequency lower than about 32 Hz.
4. The method of claim 3, wherein the first source array and the
second source array include plural third individual source elements
that are configured to emit a frequency higher than about 32
Hz.
5. The method of claim 4, further comprising: towing the plural
first and second individual source elements at a depth larger than
a depth of the plural third individual source elements.
6. The method of claim 1, wherein the plural individual first and
second source elements are electro-mechanical sources.
7. The method of claim 6, further comprising: activating an
electro-magnetic actuator system to generate a first seismic wave;
and activating a pneumatic mechanism to generate a second seismic
wave, wherein the electro-magnetic actuator system and the
pneumatic mechanism are part of an individual source element.
8. A control mechanism configured to implement an incoherent
acquisition method for driving vibrational source arrays under
water, the control mechanism comprising: a processor configured to
activate simultaneously a first source array and a second source
array so that incoherent coded driving signals drive the first and
second source arrays, wherein the first source array includes
plural first individual source elements and the second source array
includes plural second individual source elements, and wherein the
coded driving signals include corresponding signatures so that
during a processing phase, seismic signals from the first source
array are separated from seismic signals of the second source
array.
9. The control mechanism of claim 8, wherein the incoherent coded
driving signals of the first and second source arrays include
multiple frequencies and codded driving signals for the first
source array are not correlated to codded driving signals for the
second source array.
10. The control mechanism of claim 8, wherein the plural first
individual source elements and the plural second individual source
elements are configured to emit a frequency lower than about 32 Hz
and the first source array and the second source array include
plural third individual source elements that are configured to emit
a frequency higher than about 32 Hz.
11. An incoherent acquisition method for driving vibrational source
arrays under water, the method comprising: towing with a vessel
first and second source arrays, the first source array having first
high-frequency and first low-frequency source elements and the
second source array having second high-frequency and second
low-frequency source elements; and activating simultaneously the
first high-frequency, first low-frequency, second high-frequency
and second low-frequency source elements with incoherent coded
driving signals.
12. The method of claim 11, wherein the coded driving signals
include corresponding signatures so that during a processing phase,
seismic signals from the first source array are separated from
seismic signals from the second source array.
13. The method of claim 11, wherein the incoherent coded driving
signals of the first and second source arrays include multiple
frequencies.
14. The method of claim 11, wherein coded driving signals for the
first source array are not correlated to coded driving signals for
the second source array.
15. The method of claim 14, further comprising: towing the first
and second high-frequency source elements at a depth larger than a
depth of the first and second low-frequency source elements.
16. The method of claim 11, wherein the first high-frequency and
first low-frequency source elements and the second high-frequency
and second low-frequency source elements are electro-mechanical
sources having pistons that simultaneously move in opposite
directions to generate seismic waves.
17. The method of claim 16, further comprising: activating an
electro-magnetic actuator system to generate a first seismic wave;
and activating a pneumatic mechanism to generate a second seismic
wave, wherein the electro-magnetic actuator system and the
pneumatic mechanism are part of an individual source element.
18. A control mechanism configured to implement an incoherent
acquisition method for driving vibrational source arrays under
water, the control mechanism comprising: a processor configured to
activate simultaneously a first source array and a second source
array so that incoherent coded driving signals drive the first and
second source arrays, wherein the first source array has first
high-frequency and first low-frequency source elements and the
second source array has second high-frequency and second
low-frequency source elements.
19. The control mechanism of claim 18, wherein the coded driving
signals include corresponding signatures so that during a
processing phase, seismic signals from the first source array are
separated from seismic signals from the second source array.
20. The control mechanism of claim 18, wherein coded driving
signals for the first source array are not correlated to coded
driving signals for the second source array.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation of U.S. patent
application Ser. No. 13/687,059 filed on Nov. 28, 2012, which is a
continuation of U.S. patent application Ser. No. 13/415,225 filed
on Mar. 8, 2012 and claims the benefit of priority of French
Application No. 1159433 filed on Oct. 19, 2011, the entire content
of which is incorporated herein by reference.
BACKGROUND
[0002] 1. Technical Field
[0003] Embodiments of the subject matter disclosed herein generally
relate to methods and systems and, more particularly, to mechanisms
and techniques for generating an acquisition scheme for vibroseis
marine sources.
[0004] 2. Discussion of the Background
[0005] Reflection seismology is a method of geophysical exploration
to determine the properties of a portion of a subsurface layer in
the earth, which information is especially helpful in the oil and
gas industry. Marine reflection seismology is based on the use of a
controlled source that sends energy waves into the earth. By
measuring the time it takes for the reflections to come back to
plural receivers, it is possible to estimate the depth and/or
composition of the features causing such reflections. These
features may be associated with subterranean hydrocarbon
deposits.
[0006] For marine applications, seismic sources are essentially
impulsive (e.g., compressed air is suddenly allowed to expand). One
of the most used sources are airguns. The airguns produce a high
amount of acoustics energy over a short time. Such a source is
towed by a vessel either at the water surface or at a certain
depth. The acoustic waves from the airguns propagate in all
directions. A typical frequency range of the acoustic waves emitted
by the impulsive sources is between 6 and 300 Hz. However, the
frequency content of the impulsive sources is not fully
controllable and different sources are selected depending on the
needs of a particular survey. In addition, the use of impulsive
sources can pose certain safety and environmental concerns.
[0007] Thus, another class of sources that may be used are
vibratory sources. Vibratory sources, including hydraulically
powered sources and sources employing piezoelectric or
magnetostrictive material, have been used in marine operations.
However, there is no large scale use of such sources as they have
limited power and are not reliable due to the number of moving
parts required to generate the seismic waves. A positive aspect of
the vibratory sources is that they can generate signals over
various frequency bands, commonly referred to as "frequency
sweeps". The frequency band of such sources may be better
controlled compared to impulsive sources. However, the known
vibratory sources do not have a high vertical resolution as the
typical frequency range of a marine seismic source represents
approximately four octaves. A few examples of such sources are now
discussed.
[0008] The vibratory sources need to be spatially arranged, when
towed, so that they reasonably cover the subsurface desired to be
investigated and also provide a high energy output so that the
receivers are able to record the reflected seismic waves. Various
arrangements are known in the art for impulsive sources that may
also be used for the vibratory sources. For example, FIG. 1 shows a
system 10 in which a source array 20 is towed underwater with
plural streamers 30 (four in this case). The figure illustrates a
cross-sectional view of this system, i.e., in a plane perpendicular
to the streamers. The seismic waves 22a-d emitted by the source are
reflected from a surface 40 and recorded by receivers of the
streamers 30. A distance "a" between two successive reflections is
called a bin size. Because this bin size is measured along a
cross-line, "a" represents the cross-line bin size. The cross-line
is defined as a line substantially perpendicular to the streamers,
different from an axis Z that describes the depth of the streamers
underwater. An inline is a line that extends substantially along
the streamers and is perpendicular on the cross-line. For example,
the Cartesian system shown in FIG. 1 has the X axis parallel to the
inline, the Y axis parallel to the cross-line and the Z axis
describes the depth of the streamers.
[0009] With this arrangement, the cross-line bin size is half the
cross-line distance 42 between two consecutive streamers. It is
noted that the streamers are typically placed 100 m from each
other. The inline bin size may be much smaller as it depends mainly
on the separation between the receivers in the streamer itself,
which may be around 12 to 15 m. Thus, it is desired to decrease the
cross-line bin size. With a cross-line bin size in the order of 50
m, aliasing effects may be produced, especially for the highest
frequencies as the maximum bin size is inversely proportional to
the frequency.
[0010] A common technique for reducing the cross-line bin size is
the flip-flop acquisition scheme. In this mode, the vessel tows two
sources 20 and 20' as shown in FIG. 2. This arrangement 50 is
configured to shoot one source 20, listen for a predetermined time
for the reflections of the first emitted wave, and then to shoot
the other source 20' and listen for the reflections of the second
emitted wave. Then, the process is repeated. This scheme doubles
the coverage and reduces the cross-line bin size to a distance "b",
which is smaller than "a".
[0011] However, due to the particulars of the vibro-acoustic
sources, there are additional acquisition schemes, not applicable
to impulsive sources, that can be used to increase the performances
of the acquisition as discussed next.
SUMMARY
[0012] According to one exemplary embodiment, there is an
incoherent acquisition method for driving vibrational source arrays
under water. The method includes a step of towing with a vessel a
first source array and a second source array underwater, wherein
the first source array includes plural first individual source
elements and the second source array includes plural first
individual source elements; and a step of activating simultaneously
the first source array and the second source array so that
incoherent coded driving signals drive the first and second source
arrays.
[0013] According to still another exemplary embodiment, there is a
control mechanism configured to implement an incoherent acquisition
method for driving vibrational source arrays under water. The
control mechanism includes a processor configured to activate
simultaneously a first source array and a second source array so
that incoherent coded driving signals drive the first and second
source arrays. The first source array includes plural first
individual source elements and the second source array includes
plural first individual source elements.
[0014] According to yet another exemplary embodiment, there is a
coherent acquisition method for driving vibrational source arrays
under water. The method includes a step of towing with a vessel
high-frequency first and second source arrays and a low-frequency
source array underwater, wherein the high-frequency first and
second source arrays include plural high-frequency individual
source elements and the low-frequency source array includes plural
low-frequency individual source elements; a step of activating
simultaneously the high-frequency first source array and the
high-frequency second source array so that incoherent coded driving
signals drive the high-frequency first and second source arrays;
and a step of activating simultaneously the plural low-frequency
individual source elements of the low-frequency source array so
that coherent coded driving signals drive the low-frequency
individual source elements.
[0015] According to still another exemplary embodiment, there is a
control mechanism configured to implement a coherent acquisition
method for driving vibrational source arrays under water. The
control mechanism includes a processor configured to, activate
simultaneously a high-frequency first source array and a
high-frequency second source array so that incoherent coded driving
signals drive the high-frequency first and second source arrays;
and activate simultaneously plural low-frequency individual source
elements of a low-frequency source array so that coherent coded
driving signals drive the low-frequency individual source elements.
The first and second source arrays include plural high-frequency
individual source elements.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The accompanying drawings, which are incorporated in and
constitute a part of the specification, illustrate one or more
embodiments and, together with the description, explain these
embodiments. In the drawings:
[0017] FIG. 1 is a schematic diagram of a traditional acquisition
scheme;
[0018] FIG. 2 is a schematic diagram of a flip-flop acquisition
scheme;
[0019] FIG. 3 is a schematic diagram of vibro-acoustic source
element;
[0020] FIGS. 4a to 4d are schematic diagrams of an incoherent
acquisition scheme according to an exemplary embodiment;
[0021] FIGS. 5a and 5b are schematic diagrams of a coherent
acquisition scheme according to an exemplary embodiment;
[0022] FIG. 6 is a schematic illustration of a bin size when
coherently driving low-frequency individual source elements and
incoherently driving high-frequency individual source elements;
[0023] FIGS. 7a and 7b illustrate another coherent acquisition
scheme according to an exemplary embodiment;
[0024] FIGS. 8a and 8b illustrate various arrangements of
individual source elements in a source array;
[0025] FIG. 9 is a flow chart of an incoherent acquisition scheme
according to an exemplary embodiment;
[0026] FIG. 10 is a flow chart of a coherent acquisition scheme
according to an exemplary embodiment; and
[0027] FIG. 11 is a schematic diagram of a controller according to
an exemplary embodiment.
DETAILED DESCRIPTION
[0028] The following description of the exemplary embodiments
refers to the accompanying drawings. The same reference numbers in
different drawings identify the same or similar elements. The
following detailed description does not limit the invention.
Instead, the scope of the invention is defined by the appended
claims. The following embodiments are discussed, for simplicity,
with regard to the terminology and structure of a vibroseis
acoustic source array. However, the embodiments to be discussed
next are not limited to this structure, but may be applied to other
arrays or sources that generate a seismic wave having a controlled
frequency range.
[0029] Reference throughout the specification to "one embodiment"
or "an embodiment" means that a particular feature, structure, or
characteristic described in connection with an embodiment is
included in at least one embodiment of the subject matter
disclosed. Thus, the appearance of the phrases "in one embodiment"
or "in an embodiment" in various places throughout the
specification is not necessarily referring to the same embodiment.
Further, the particular features, structures or characteristics may
be combined in any suitable manner in one or more embodiments.
[0030] According to an exemplary embodiment, there are at least two
source arrays, each array having two or more individual source
elements. The source arrays are operated (i) simultaneously and
incoherently with coded driving signals or (ii) simultaneously and
coherently. By operating the source arrays simultaneously and
incoherently, a total energy output is doubled relative to a
conventional source array using a flip-flop acquisition scheme. By
operating the source arrays simultaneously and coherently, a total
energy output quadruples relative to a conventional source array
using a flip-flop acquisition scheme. In one application, each
source array is made up of two sub-arrays. A first sub-array may
include individual source elements optimized for a first frequency
range (e.g., low-frequency range, between 2 and 32 Hz) and a second
sub-array may include individual source elements optimized for a
second frequency range (e.g., high-frequency range, between 32 and
128 Hz). A larger number of sub-arrays or different frequencies are
also possible.
[0031] Before discussing a novel acquisition scheme, an example of
a source element is now discussed. It is noted that this one
possible source element and the novel acquisition scheme may be
applied to different source elements (e.g., any vibro-acoustic
source element). According to an exemplary embodiment, an
individual source element is illustrated in FIG. 3. FIG. 3 shows
the individual source element 100 of a seismic source array
including an enclosure 120 that together with pistons 130 and 132
enclose an electro-magnetic actuator system 140 and separate it
from the ambient 150, which might be water. The enclosure 120 has
first and second openings 122 and 124 that are configured to be
closed by the pistons 130 and 132. The electro-magnetic actuator
system 140 is configured to simultaneously drive the pistons 130
and 132 in opposite directions for generating the seismic waves. In
one application, the pistons 130 and 132 are rigid. The
electro-magnetic actuator system 140 may include two or more
individual electro-magnetic actuators 142 and 144. Irrespective of
how many individual electro-magnetic actuators are used in a
individual source element 100, the actuators are provided in pairs
and the pairs are configured to act simultaneously in opposite
directions on corresponding pistons in order to prevent a "rocking"
motion of the individual source element 100.
[0032] The size and configuration of the electro-magnetic actuators
depend on the acoustic output of the individual source element.
FIG. 3 shows that the two actuators 142 and 144 are separated by a
wall 146, which does not have to be at the middle of the actuator
system 140. Further, in one embodiment, the two actuators 142 and
144 are formed as a single unit and there is no interface between
the two actuators. In still another application, the two actuators
142 and 144. In yet another application, the actuator system 140 is
attached to the enclosure 120 by an attachment 148. The attachment
148 may be a strut-type structure. In one application, the
attachment 148 may be a wall that splits the enclosure 120 in a
first chamber 120a and a second chamber 120b. If the attachment 148
is a wall, the actuators 142 and 144 may be attached to the wall
148 or may be attached to the enclosure 120 by other means in such
a way that the actuators 142 and 144 do not contact the wall
148.
[0033] In order to provide the pistons 130 and 132 with the ability
to move relative to the enclosure 120 in order to generate the
seismic waves, a sealing mechanism 160 is provided between the
pistons and the enclosure. The sealing mechanism 160 may be
configured to slide back and forth with the pistons. The sealing
mechanism 160 may be made of an elastomeric material, or may be a
metallic flexible structure. In another application, the sealing
mechanism 160 may be a gas or liquid seal. A gas seal (air bearing
seal) is configured to inject a gas at the interface between the
enclosure and the pistons to prevent the ambient water from
entering the enclosure. A liquid seal may use, e.g., a
ferromagnetic fluid, at the interface between the enclosure and the
pistons to prevent the ambient water from entering the enclosure.
Other seals may be used as will be recognized by those skilled in
the art.
[0034] The embodiment shown in FIG. 3 may also include a pneumatic
regulation mechanism 170. The pneumatic regulation mechanism 170
may be used to balance the external pressure of the ambient 150
with a pressure of the medium enclosed by the enclosure 120 to
reduce a work load of the actuator system 140. It is noted that if
a pressure of the ambient at point 172 (in front of the piston 130)
is substantially equal to a pressure of the enclosed medium 173 of
the enclosure 120 at point 174, the work load of the actuator
system 140 may be used entirely to activate the piston to generate
the acoustic wave instead of a portion thereof used to overcome the
ambient pressure at point 172. The enclosed medium 173 of the
enclosure 120 may be air or other gases or mixtures of gases.
[0035] The pneumatic mechanism 170 may be fluidly connected to a
pressure source (not shown) on the vessel towing the individual
source element 100. The pneumatic mechanism 170 may also be
configured to provide an additional force on the pistons 130 and
132, e.g., at lower frequencies, to increase an acoustic output of
the individual source element and also to extend a frequency
spectrum of the individual source element.
[0036] The embodiment illustrated in FIG. 3 may use a single shaft
(180 and 182) per piston to transmit the actuation motion from the
actuation system 140 to the pistons 130 and 132. However, more than
one shaft per piston may be used depending on the requirements of
the individual source element. To provide a smooth motion of the
shaft 180 relative to the enclosure 120 (e.g., to prevent a
wobbling motion of the shaft), a guiding system 190 may be
provided.
[0037] In one application, heat is generated by the actuation
system 140. This heat may affect the motion of the shafts and/or
the functioning of the actuator system. For this reason, a cooling
system 194 may be provided at the individual source element. The
cooling system 194, as will be discussed later, may be configured
to transfer heat from the actuator system 140 to the ambient
150.
[0038] The pistons 130 and 132 are desired to generate an output
having a predetermined frequency spectrum. To control this output,
a local control system 200 may be provided, inside, outside or both
relative to the enclosure 120. The local control system 200 may be
configured to act in real-time to correct the output of the
individual source element 100. As such, the local control system
200 may include one or more processors and sensors that monitor the
status of the individual source element 100 and provide commands
for the actuator system 140 and/or the pneumatic mechanism 170.
[0039] The source arrays discussed above may be made up entirely of
the individual source element illustrated in FIG. 3. However, the
source arrays may be made up of different vibroseis source elements
or a combination of those shown in FIG. 3 and those known in the
art.
[0040] According to an exemplary embodiment, an incoherent
acquisition scheme is now discussed. This acquisition scheme is
exemplified with reference to FIGS. 4a and 4b, which show, from
side and back, an acquisition system 300 including a vessel 310 and
two source arrays 320a and 320b. Each source array 320a and 320b
may include a first sub-array 340a and 340b, respectively, and a
second sub-array 360a and 360b, respectively. However, it is noted
that it is possible to have a source array 320a that includes only
the sub-array 340a or only the sub-array 360b and the same is true
for the source array 320b.
[0041] FIGS. 4a and 4b show each source array having two sub-arrays
as the quality of the subsurface's image is better when having two
sub-arrays. For example, the sub-arrays 340a and 340b may include
high-frequency individual source elements and the sub-arrays 360a
and 360b may include low-frequency individual source elements. The
high-frequency individual source elements are towed at a first
depth D1 while the low-frequency individual source elements are
towed at a second depth D2, larger than D1.
[0042] As coded driving signals are applied to the vibrating
individual source elements for emitting the seismic waves (acoustic
waves for example), the individual source elements may be driven
simultaneously and in an incoherent way. A driving signal may
include but is not limited to a random noise, a frequency sweep,
etc. A coded driving signal has a signature that can be recovered
later, i.e., when the seismic wave are recorded, during a
processing stage, the recorded waves may be separated based on the
sources that emitted those waves. Driving the sources incoherently
means that coded driving signals for source array 320a do not
overlap (are not correlated) with coded driving signals for source
array 320b. For these reasons, the recorded seismic waves (after
reflection on the subsurface) can be recovered and separated during
processing, for example, by using signature deconvolution or
cross-correlation with a pilot. This is not possible for the airgun
sources.
[0043] By driving the source arrays 320a and 320b simultaneously
and incoherently with coded driving signals, the total energy
emitted by the two source arrays is doubled (total energy output +3
dB) relative to the case that the sources are using a flip-flop
acquisition scheme. A flip-flop acquisition scheme drives sources
in a given pattern. For example, considering that it is possible to
drive a source in modes A and B, by driving the source ABAB . . .
or ABBABB . . . it is achieved a flip-flop acquisition scheme. It
is noted that a source array may include a predetermined number of
individual source elements, e.g., between 16 and 30. Other numbers
of individual source elements are also possible. The term
"simultaneously" indicates that all individual source elements of
both the source array 320a and the source array 320b are driven at
the same time. However, the term "incoherently" means that the
individual source elements of the source array 320a have a content
different from the individual source elements of the source array
320b. In other words, the individual source elements of the source
array 320a all emit the same content and the individual source
elements of the source array 320b all emit a different content and
thus, any pair of sources, one from the source array 320a and one
from the source array 320b have a different content.
[0044] In another exemplary embodiment, it is possible to drive
simultaneously and incoherently only the sub-arrays 340a and 340b
or only the sub-arrays 360a and 360b. In still another exemplary
embodiment which is illustrated in FIGS. 4c and 4d, it is possible
to have the source arrays 320a and 320b having all the source
elements 360a and 360b, respectively, provided at the same depth D.
Thus, according to this exemplary embodiment, the individual source
elements are not separated based on a frequency content as in FIGS.
4a and 4b. For the exemplary embodiment shown in FIGS. 4c and 4d,
the same novel acquisition scheme as discussed for FIGS. 4a and 4b
is applicable.
[0045] According to another exemplary embodiment a coherent
acquisition scheme is now discussed. This acquisition scheme is
exemplified with reference to FIGS. 5a and 5b, which show, from
side and back, respectively, an acquisition system 400 including a
vessel 410 and three source arrays 440a and 440b and 460. In this
embodiment, each of the source arrays 440a and 440b includes one
sub-array having high-frequency individual source elements and the
source array 460 includes low-frequency individual source elements.
In other words, comparing the embodiment of FIG. 5b with that of
FIG. 4b, the low-frequency individual source elements 360a and 360b
have been merged in a single source arrangement 460. The
high-frequency individual source elements are towed at a first
depth D1 while the low-frequency individual source elements are
towed at a second depth D2, larger than D1.
[0046] As the vibrating individual source elements use coded
driving signals for emitting the seismic waves (acoustic waves for
example), the high-frequency individual source elements may be
driven simultaneously and in an incoherent way while the
low-frequency individual source elements may be driven
simultaneously and in a coherent way. That means that a content of
the signals from source array 440a does not overlap with a content
of the signals from source array 440b. For these reasons, the
recorded seismic waves for the high-frequency spectrum (after
reflection on the subsurface) can be recovered and separated during
processing, for example, by using signature deconvolution or
cross-correlation with a pilot. However, that is not the case now
for the low-frequency spectrum as these individual source elements
are driven by coherent driving signals.
[0047] This specific arrangement for the low and high-frequency
individual source elements is made because the high-frequency
spectrum is desired for accurately determining relative positions
of the various layers and/or interfaces in the subsurface while the
low-frequency spectrum does not affect the clarity of these
features but provide the general background trend. Also, the
maximum bin size to prevent aliasing depends on the frequency and
the high frequencies sources need to be kept separated for this
reason.
[0048] By driving the source arrays 440a and 440b simultaneously
and incoherently with coded driving signals, the energy emitted by
the two source arrays is doubled (total energy output +3 dB)
relative to the case that the sources are using a flip-flop
acquisition scheme. Further, by driving the individual source
elements of the source array 460 simultaneously and coherently, the
energy emitted by the low-frequency individual source elements
quadruple (total energy output +6 dB) at a cost of a bigger bin
size, which is acceptable for the low-frequencies because they can
be interpolated.
[0049] As shown in FIG. 6, two high-frequency source arrays 440a
and 440b and a low-frequency source array 460 are provided
underwater. FIG. 6 also shows streamers 500 and how seismic waves
emitted by the source arrays reflect from the subsurface. A bin
size 400 for the high-frequency source arrays 440a and 440b is
small (but has double energy) and a bin size 402 for the
low-frequency source array 460 is larger (but has quadruple
energy). The data from the low frequencies and high-frequency
recordings can be then interpolated to common points and merged
together.
[0050] In another exemplary embodiment, it is possible to drive
simultaneously and coherently the source arrays 440a and 440b in
addition to the source array 460. In still another exemplary
embodiment illustrated in FIGS. 7a and 7b, it is possible to have
more than two source arrays 720a to 720d for the high-frequency
individual source elements and a single source array 740 for the
low-frequency individual source elements. In another application,
the number of high-frequency individual source elements may be
larger than four. Further, it is possible to have one or more layes
of individual source elements provided between the high-frequency
and the low-frequency source elements. In other words, the method
is applicable not only to individual source elements split as shown
in FIG. 7b but also to source arrays that have the individual
source elements provided at various depths and emitting the same or
different frequencies. Similar to the embodiment shown in FIGS. 5a
and 5b, the source arrays 720a to d may use the incoherent
acquisition scheme while the source array 740 may use the coherent
acquisition scheme.
[0051] The incoherent and coherent acquisition schemes discussed
above may be implemented in a control mechanism illustrated, for
example, in FIG. 11, which is discussed later. The control
mechanism 780 may be provided on the vessel 710 as shown in FIG. 7,
or may be provided as element 200 on the individual source element
as shown in FIG. 3, or may be distributed at the vessel and at the
source arrays. Optionally, the control mechanism may be configured
not only to activate the coherent or incoherent acquisition schemes
but also to control individual source elements, e.g., to control
the activation of an electro-magnetic actuator system (140) of a
low-frequency individual source element to generate a first seismic
wave and/or to activate a pneumatic mechanism (170) of a
low-frequency individual source element to generate a second
seismic wave.
[0052] Any of the source arrays discussed above may include plural
individual source elements. In this respect, FIG. 8a shows a linear
arrangement 800 that includes plural individual source elements 820
and FIG. 8b shows a circular arrangement 900 that includes plural
individual source elements 920. Other arrangements are also
possible. The individual source elements 820 and/or 920 may be the
source element 100 shown in FIG. 3. Other type of individual source
elements may be used. The source arrays 800 or 900 may correspond
to any of the source arrays 320a, 320b, 440a, 440b, and 460.
[0053] The acquisition schemes previously discussed may be
implemented by the following methods. According to an exemplary
embodiment illustrated in FIG. 9, there is an incoherent
acquisition method for driving vibrational source arrays under
water. The method includes a step 900 of towing with a vessel (310)
a first source array (320a) and a second source array (320b)
underwater, where the first source array (320a) includes plural
first individual source elements (360a) and the second source array
(320b) includes plural first individual source elements (360b); and
a step 902 of activating simultaneously the first source array
(320a) and the second source array (320b) so that incoherent coded
driving signals drive the first and second source arrays.
[0054] According to another exemplary embodiment illustrated in
FIG. 10, there is a coherent acquisition method for driving
vibrational source arrays under water. The method includes a step
1000 of towing with a vessel (410) high-frequency first and second
source arrays (440a, 440b) and a low-frequency source array (460)
underwater, where the first and second source arrays (440a, 440b)
include plural high-frequency individual source elements and the
low-frequency source array (460) includes plural low-frequency
individual source elements; a step 1002 of activating
simultaneously the high-frequency first source array (440a) and the
high-frequency second source array (440b) so that incoherent coded
driving signals drive the high-frequency first and second source
arrays; and a step 1004 of activating simultaneously the plural
low-frequency individual source elements of the low-frequency
source array (460) so that coherent coded driving signals drive the
low-frequency individual source elements.
[0055] An example of a representative control system capable of
carrying out operations in accordance with the exemplary
embodiments discussed above is illustrated in FIG. 11. Hardware,
firmware, software or a combination thereof may be used to perform
the various steps and operations described herein. The control
system 1100 of FIG. 11 is an exemplary computing structure that may
be used in connection with such a system.
[0056] The exemplary control system 1100 suitable for performing
the activities described in the exemplary embodiments may include
server 1101. Such a server 1101 may include a central processor
unit (CPU) 1102 coupled to a random access memory (RAM) 1104 and to
a read-only memory (ROM) 1106. The ROM 1106 may also be other types
of storage media to store programs, such as programmable ROM
(PROM), erasable PROM (EPROM), etc. The processor 1102 may
communicate with other internal and external components through
input/output (I/O) circuitry 1108 and bussing 1110, to provide
control signals and the like. For example, the processor 1102 may
communicate with the sensors, electro-magnetic actuator system
and/or the pneumatic mechanism. The processor 1102 carries out a
variety of functions as is known in the art, as dictated by
software and/or firmware instructions.
[0057] The server 1101 may also include one or more data storage
devices, including hard and floppy disk drives 1112, CD-ROM drives
1114, and other hardware capable of reading and/or storing
information such as a DVD, etc. In one embodiment, software for
carrying out the above discussed steps may be stored and
distributed on a CD-ROM 1116, diskette 1118 or other form of media
capable of portably storing information. These storage media may be
inserted into, and read by, devices such as the CD-ROM drive 1114,
the disk drive 1112, etc. The server 1101 may be coupled to a
display 1120, which may be any type of known display or
presentation screen, such as LCD displays, plasma displays, cathode
ray tubes (CRT), etc. A user input interface 1122 is provided,
including one or more user interface mechanisms such as a mouse,
keyboard, microphone, touch pad, touch screen, voice-recognition
system, etc.
[0058] The server 1101 may be coupled to other computing devices,
such as the equipment of a vessel, via a network. The server may be
part of a larger network configuration as in a global area network
(GAN) such as the Internet 1128, which allows ultimate connection
to the various landline and/or mobile client/watcher devices.
[0059] As also will be appreciated by one skilled in the art, the
exemplary embodiments may be embodied in a wireless communication
device, a telecommunication network, as a method or in a computer
program product. Accordingly, the exemplary embodiments may take
the form of an entirely hardware embodiment or an embodiment
combining hardware and software aspects. Further, the exemplary
embodiments may take the form of a computer program product stored
on a computer-readable storage medium having computer-readable
instructions embodied in the medium. Any suitable computer readable
medium may be utilized including hard disks, CD-ROMs, digital
versatile discs (DVD), optical storage devices, or magnetic storage
devices such a floppy disk or magnetic tape. Other non-limiting
examples of computer readable media include flash-type memories or
other known types of memories.
[0060] The disclosed exemplary embodiments provide a source array,
computer software, and method for generating acquisition schemes
for under water vibrational sources. It should be understood that
this description is not intended to limit the invention. On the
contrary, the exemplary embodiments are intended to cover
alternatives, modifications and equivalents, which are included in
the spirit and scope of the invention as defined by the appended
claims. Further, in the detailed description of the exemplary
embodiments, numerous specific details are set forth in order to
provide a comprehensive understanding of the claimed invention.
However, one skilled in the art would understand that various
embodiments may be practiced without such specific details.
[0061] Although the features and elements of the present exemplary
embodiments are described in the embodiments in particular
combinations, each feature or element can be used alone without the
other features and elements of the embodiments or in various
combinations with or without other features and elements disclosed
herein.
[0062] This written description uses examples of the subject matter
disclosed to enable any person skilled in the art to practice the
same, including making and using any devices or systems and
performing any incorporated methods. The patentable scope of the
subject matter is defined by the claims, and may include other
examples that occur to those skilled in the art. Such other
examples are intended to be within the scope of the claims.
* * * * *