NZ609503A - Method for acquiring marine seismic data - Google Patents
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Abstract
609503 A method for acquiring marine seismic data is disclosed. At least one seismic energy source (SA1, SA2) is towed in a body of water at a substantially constant speed. The at least one seismic energy source are actuated at a plurality of spatial positions. A distance between each of the plurality of actuations is randomly different than any other such distance and the difference between successive distances is sufficiently large to enable detection of corresponding differences between a position of the at least one seismic energy source corresponding to each actuation. Seismic energy detected by at least one seismic sensor (2a-2d, 10) is substantially continuously recorded (6) through a plurality of actuations of the at least one seismic energy source. The recording comprises recording a geodetic position of the at least one seismic energy source and the at least one seismic sensor at each actuation.
Description
METHOD FOR ACQUIRING MARINE SEISMIC DATA
BACKGROUND OF THE INVENTION
This disclosure relates generally to the field of marine seismic data
acquisition. More specifically, the disclosure relates to methods for acquiring marine
seismic data in which the positions of seismic energy source(s) used to generate the
seismic data are randomly geodetically distributed in order to improve quality of the data
obtained.
Seismic surveying is known in the art for determining structures of rock
formations below the earth's surface. Seismic surveying generally includes deploying an
array of seismic sensors at the surface of the earth in a selected pattern, and selectively
actuating a seismic energy source positioned near the seismic sensors. The energy source
may be an explosive, a vibrator, or in the case of seismic surveying performed in the
ocean, one or more air guns or water guns.
Seismic energy which emanates from the source travels through the earth
formations until it reaches an acoustic impedance boundary in the formations. Acoustic
impedance boundaries typically occur where the composition and/or mechanical properties
of the earth formation change. Such boundaries are typically referred to as "bed
boundaries". At an acoustic impedance boundary, some of the seismic energy is reflected
back toward the earth's surface, where it may be detected by one or more of the seismic
sensors deployed on or below the surface when onshore, and in the water when offshore.
Other portions of the energy are refracted and continue propagating in a generally
downward direction until another impedance boundary is reached. Seismic signal
processing known in the art has as an objective the determination of the depths, geographic
locations and physical properties of rocks forming a bed boundary below the earth's
surface. The depth and location of the bed boundaries is inferred from the travel time of the
seismic energy to the acoustic impedance boundaries and back to the sensors at the surface.
16/04/14,ag op2320 amended speci & claim pages,1
Seismic surveying (marine seismic surveying) is performed in bodies of
water suck as lakes or the ocean to determine the structure of earth formations below the
water bottom. Marine seismic surveying known in the art includes having a vessel tow one
or more seismic energy sources, and the same or a different vessel tow one or more
"streamers", which are arrays of seismic sensors forming part of or otherwise affixed to a
cable. Typically, a seismic vessel will tow a plurality of such streamers arranged to be
separated by a selected fixed or variable lateral distance from each other, in a pattern
selected to enable relatively complete determination of geologic structures in three
dimensions.
The signals detected by the seismic sensors at the earth's surface include
components of seismic energy reflected at the bed boundaries, as previously explained. In
addition, both coherent noise (noise which has a determinable pattern, such as may be
caused by a ship propeller) and incoherent (random) noise may be present. The presence of
such noise in the signals received by the seismic sensors reduces the signal-to-noise ratio
("SNR") of the seismic signals of interest. An objective of seismologists, therefore, is to
seek methods of eliminating the effects of noise on the signals detected by the sensors
without appreciably reducing the true seismic signal component of the detected signals.
The resolution of the resultant seismic data is typically dependent on the spatial sampling
of the signal and the noise.
Prior art methods which have been used to reduce the effects of noise and
acquire a higher quality seismic representation of a particular subsurface structure include
using multiple actuations of the seismic source (multiple "firings" or "shots") to record a
plurality of sensor measurements from substantially the same subsurface structure, and
then summing or "stacking" such measurements to enhance signal strength while
substantially reducing the effects of random or incoherent noise. In most such techniques
known in the art, the multiple firings are performed such that the source is disposed at
regularly spaced positions, and signal processing of the recorded signals follows
accordingly.
The idea of random spatial sampling rather than regular spatial sampling of
the subsurface has been proposed as a way that can lead to improved resolution of the
16/04/14,ag op2320 amended speci & claim pages,2
subsequent data. These design principles and theoretical justification come from a
relatively new field of mathematics known as “compressive sampling”. See, e.g., Candes,
E., Romberg J., and Tao T., (2006) Stable signal recovery from incomplete and inaccurate
measurements. Communications on Pure and Applied Mathematics 59, 1207-1223. See
also, Donoho, D.L., (2006) Compressed sensing; IEEE Transactions on Information
Theory. 52, 1289-1306.
Typically the approach of random sampling has been suggested as a way to
obtain more information from fewer samples and has been considered in a theoretical sense
for seismic data. See, Herrman, F., (2009) Sub-Nyquist sampling and sparsity: how to get
more information from fewer samples, Proceedings of the 2009 Annual SEG meeting
3410-3415.
For land seismic acquisition, it is relatively easy to randomize the spatial
positions of shots. Randomizing the positions of sensors is also possible with wireless
systems, though more conventional wired systems would limit the potential. The concept
of using random sampling in a land environment to get the same result through acquisition
of less data has been described in, Milton A., Trickett, S., and Burroughs L., (2011)
Reducing acquisition costs with random sampling and multi-dimensional interpolation,
Proceedings of the 2011 Annual SEG meeting 52-56.
In the marine environment seismic data are typically acquired in straight
lines with a set of sensors towed behind the vessel. There is in effect no capability to vary
the relative positions of the sensors as these are constrained within a streamer towed
behind the vessel. Some natural randomization of sensor positions may occur simply
through the deviation of the streamers from the intended track due to currents, but the
spacing of the sensor positions within the streamer is fixed.
Marine seismic sources are typically fired sequentially and alternately (in
the case of 2 sources and a single vessel), with the objective of firing the sources at
regularly spaced locations along a designated vessel track. There are known deviations
from this practice known in the art involving the number of sources being activated, and
the timing of the source activation. .
16/04/14,ag op2320 amended speci & claim pages,3
A first technique known in the art is that multiple sources are fired
sequentially with small deviations in timing between firings in each sequence. See, e.g.,
U.S. Patent No. 6,906,981 B2 issued to Vaage, entitled, Method and system for acquiring
marine seismic data using multiple sources. The method disclosed in the foregoing patent
still has as an objective acquiring seismic data on a regular spatial sampling basis and
recording the data into discrete records of fixed time duration. By introducing slight
variations in the actuation timing of the secondary source some variation in the position of
the second source is obtained, however such position randomization is relatively small.
The purpose of the technique disclosed in the foregoing patent is to achieve randomization
of source firing timing so that essentially simultaneously operated sources can have their
energy individually identified and separated from the recorded seismic signals in a single
discrete record. Note that for purposes of identifying the source position, the sources are
actuated at essentially the same time, and at regularly spaced apart spatial positions.
Another technique known in the art provides that the track of the vessel not
be straight, but be approximately circular. This essentially creates a pseudo random set of
resultant source positions, but the seismic energy sources are still fired at regular spatial
intervals along the vessel track. This is described in, Moldoveannu, N., (2010) Random
Sampling: A new strategy for marine acquisition, Proceedings of the 2010 Annual SEG
meeting 51-54. Note however, that using the foregoing technique the source positions are
not randomized along the vessel track, but it is simply a result of the fact that the vessel
track is not straight that results in spatial variation of the source position.
What is needed is a technique to randomize seismic energy source position
for marine seismic data acquisition to obtain the benefits thereof.
SUMMARY
One aspect is a method for acquiring marine seismic data including towing a
seismic energy source in a body of water and towing a seismic sensor at a selected distance
from the seismic energy source. The seismic energy source is actuated at a plurality of
positions, a spatial distance between each of the plurality of actuations being randomly
different than any other such spatial distance. Seismic energy detected by the seismic
16/04/14,ag op2320 amended speci & claim pages,4
sensor is substantially continuously recorded through a plurality of actuations of the at
least one seismic energy source. The recording includes recording a geodetic position of
the at least one seismic energy source and the at least one seismic sensor at each actuation.
Other aspects and advantages of the disclosure will be apparent from the
description and claims which follow.
BRIEF DESCRIPTION OF THE DRAWINGS
is a diagram of one embodiment of a marine seismic acquisition
system.
shows an example of seismic energy paths (ray paths) from a source
to a plurality of seismic sensors towed by a vessel as the seismic energy reflects from an
acoustic impedance boundary.
shows positions of seismic energy sources wherein a distance
between successive actuations occurs randomly.
shows positions of seismic energy source sub arrays wherein a
distance between successive actuations occurs randomly.
shows an example of continuous signal recording during multiple
source actuations.
shows an example of seismic energy source arrays each comprising a
plurality of sub arrays.
DETAILED DESCRIPTION
In the description below, the term “seismic source” is used to describe a set
of seismic energy sources such as air guns and water guns which are fired substantially
simultaneously. Such a seismic source, known as an “array” will normally include several
air guns or water guns, but might also consist of only one such gun, or one gun in one or
16/04/14,ag op2320 amended speci & claim pages,5
more of a plurality of “sub-arrays”. A seismic vessel will typically tow one, two or more
seismic source arrays which are actuated (“fired”) at separate times. In some of following
description two sources are used. It should be clearly understood, however, that a method
and system according to various examples can use single sources or more than two sources
on each vessel. Also, it is not necessary that all vessels, operating together, tow the same
number of sources.
shows an example of a marine seismic data acquisition arrangement
which may be used in example implementations. A seismic vessel (SEV) 1 tows the first
seismic sources SA1, SA2, and one or more "streamers" or seismic sensor arrays as shown
at 2a-2d. Each streamer 2a-2d includes a plurality of seismic sensors (typically
hydrophones) disposed thereon at spaced apart locations along each streamer 2a-2d. The
seismic sensors are shown generally at 10. The streamers 2a-2d are disposed along lines
substantially parallel to the survey line 5. For purposes of the present examples, only one
streamer need be towed by the SEV 1, however, having a plurality of streamers as shown
in may improve the efficiency and speed of data acquisition, as is known in the art.
The seismic sensors 10 in the streamers 2a-2d are operatively coupled to a recording
system 6 that may be disposed on the SEV 1.
The seismic recording system 6 may also include navigation equipment (not
shown separately) to enable precisely determining the position of the SEV 1 and/or other
vessels and the individual sensors 10 as seismic signals are recorded. The seismic
recording system 6 may also include a source controller which selectively controls
actuation of the one or more sources towed by the SEV 1 and by any additional vessels,
such as shown in at 7 and 8 and further explained below. . Source actuation by the
source controller (not shown separately) will be further explained.
Each of the seismic sources SA1, SA2, in this example, as previously
explained, will typically include an array of air guns, or several sub arrays, as will be
explained further below. Such arrays are used, for among other reasons as is known in the
art, to provide "whiter" seismic energy (including a broader range of frequencies and
having a more nearly constant amplitude for such frequencies). In other embodiments, the
seismic acquisition system may include additional source vessels, shown generally at 7 and
16/04/14,ag op2320 amended speci & claim pages,6
8 in These additional source vessels 7, 8 may each tow one or more additional
seismic sources or arrays thereof, shown generally at SC1 and SC2. The additional source
vessels and sources may be omitted in some examples.
The first SA1, SA2 and second SC1, SC2 seismic energy sources may be
used in marine seismic surveying to increase the coverage area of the seismic data recorded
by the recording system 6. Typically, each of the sources SA1, SA2, SC2, SC2 will be
actuated in a sequence involving consecutive and sequential activations which reduces
interference between sources in the recorded signals. For purposes of the description which
follows of example methods, a "first source" can be either one of the sources towed by the
SEV 1, these being sources SA1 and SA2. A "second source" referred to in the description
can be any other source towed either by SEV1 or any other source vessel e.g., as shown at
7 and 8.
It should also be understood that for purposes of defining the scope of the
disclosure, it is not necessary to have a separate source vessel, or source vessels, to tow the
second source (or any additional sources) as shown in although having such a
separate source vessel provides practical benefits such as increasing the effective
subsurface coverage of the streamers 2a-2d, as is known in the art. For purposes of
defining the scope of this invention, it is only necessary to have one seismic energy source.
However, a second seismic energy source (or source array) may be towed by the same
vessel or a different vessel along (or parallel to) a survey line, such as 5 in at a
selected distance from the first seismic source (or source array), and the following example
uses two sources.
During acquisition of seismic signals, the first sources SA1, SA2 may be
fired in a plurality of firing sequences, the positioning of these source activations will be
further explained, and signals detected by the sensors (not shown) on the streamers 2a-2d
are recorded by the recording system 6.
shows an example of paths 21 ("ray paths") of seismic energy as it
travels from the first source or source arrays (SA1 in , the location along the survey
line (5 in of which is shown at 20, downward through the water 26, to a subsurface
16/04/14,ag op2320 amended speci & claim pages,7
acoustic impedance boundary (bed boundary) 24. Some of the seismic energy is reflected
from the bed boundary 24 and travels upwardly through the water 26 where it is detected
by the sensors on each of the streamers (2a-2d in , the locations of some of which
are shown at 22. The ray paths 21 shown in correspond to the path traveled by the
seismic energy to each tenth sensor in one of the streamers (2a-2d in .
The present example provides a method of acquiring marine seismic data
where the source positions between successive actuations are randomized in distance along
the vessel heading (direction of vessel motion). This randomization is possible in a
direction parallel to the vessel heading (and thus the source heading) and additionally, by
manipulation of certain components of the seismic source array used in some examples,
may be randomized to some extent in the direction perpendicular to the vessel heading.
Randomization of source position may be obtained, for example, by firing the sources at
randomly spaced apart actuation (“firing”) time intervals between successive source
actuations.
Because the source(s) are fired at random spacing between actuations, if the
vessel speed is constant, the seismic sensor signals will be acquired at different time
intervals, and as a result the sensor signals generally cannot be recorded in a conventional
manner as a set of discrete records of the same length indexed to the source actuation time.
In the present example, continuous recording of the sensor signals may be used, and
sequential seismic data records for each source actuation, some of which may overlap in
time, can be extracted from the continuous signal recordings. Any part of or all of the
recordings may or may not contain interfering energy from a subsequent or prior source
actuation, which will depend on the variation in time between source actuations, and
resulting source and sensor positions at the time of source actuation.
illustrates the source locations, e.g., those of SA1 and SA2 in
where the distance between positions of the sources at the time of actuation is varied
randomly along the vessel direction of motion (e.g., 5 in by selecting randomly
changing separations in distance between successive source actuations. The source
positions at each actuation are indicated by the square (for SA1) and diamond (for SA2)
symbols. Preferably the time interval between successive source actuations is always large
16/04/14,ag op2320 amended speci & claim pages,8
enough to ensure that the compressor capability is adequate to ensure that the sources are
fully charged before actuation. In the present example illustration a minimum time interval
between successive source actuations of 6 seconds is used. However, the difference in
time interval between successive source actuations should be large enough so that the
distance between source positions at the respective actuation times is large enough to be
detectable. In one example, a mean number of source actuations along a selected travel
distance (e.g., 5 to 10 kilometers) using random spatial activation positions between source
actuations may be the same as an average number of source actuations using a same spatial
interval between successive actuations (i.e., using the technique known in the art having
regular source position spacing between actuations).
Using the foregoing source actuation technique it would be difficult to
record complete, discrete records of the detected seismic energy from each source
actuation. Recording of the detected seismic energy in the present example is facilitated by
continuous recording of detected seismic signals. Individual source actuation (“shot”)
records may or may not overlap in time depending on the actual positions at which the
respective seismic sources are actuated.
Another possible implementation of source position spacing randomization
may include actuation of particular air guns within the source array. It is known in the art
for each source array (e.g., SA1, SA2 in to be composed of several (typically 3)
“sub arrays” which are deployed with a selected separation in the direction perpendicular
to the vessel heading . Such separation is typically 10 meters. In conventional seismic
data acquisition all subarrays are typically activated simultaneously; however if a smaller
source energy is sufficient for the acquisition of suitable seismic signals, then not all of the
subarrays need to be activated simultaneously for each shot. As an alternate configuration,
extra sub arrays may be deployed wherein not all of them are actuated for each particular
shot.
To illustrate the foregoing concept, the source array (e.g., SA1 and SA2 in
, and referring to the source array SA1 may be considered to be composed
of, for example, three sub arrays, AR1-1, AR1-2, AR1-3 separated from each other
laterally (transverse to the vessel heading) by, for example, 20 meters. Corresponding sub
16/04/14,ag op2320 amended speci & claim pages,9
arrays are shown at AR2-1, AR2-2 and AR2-3 for the second source array SA2. It would
therefore be possible to activate the center (AR1-2) and port (AR1-1) sub array, the center
(AR1-2) and starboard (AR1-3) sub array, or the two outer (port and starboard sub arrays
(AR1-1 and AR1-3). If the sub arrays are substantially identical in configuration, and
sufficiently laterally separated such that there is negligible interaction between them when
actuated, then the same far field vertical energy signature would result from each
combination of activated sub arrays, but the effective position of the source would vary in
a direction perpendicular to the vessel track 5. If the actuations were determined on a
random basis, then each source composed of 3 sub arrays may be activated in one of 3
possible crossline positions in a random sequence. Randomization of both the inline and
crossline positions of the effective array could now be achieved. The foregoing is
illustrated in The sub array position at the time of each actuation is shown by the
square and diamond symbols in It should be noted that 3 sub arrays is not a limit to
the configuration of the source array, and more subarrays may be deployed in other
implementations. Using more sub arrays increases the number of possible activation
positions of the source.
Following the acquisition of continuous data records using randomly
spatially distributed source actuations there may be a need for extraction of the recorded
signal data to convert the data to discrete “shot” records for subsequent processing. Such
extraction and conversion will restore the recorded signals to a time index of zero and
source/sensor geodetic positions existing at the time of each source actuation (firing).
Note that the selected time limit of the extracted recordings can be any
selected value. There is essentially no limit to the extracted recording length. It can be
short or long. The longer the recording time is the more overlapping data from multiple
source activations, and consequent interfering data, there are likely to be.
While a single source cannot be activated more frequently than is possible
in view of the compressed gas source (compressor) capability where air guns are used, this
time is often less that the time it takes to traverse the regular source activation distance.
When activating the sources at randomized spatial positions, it is possible that a second
source may be activated twice before the first source is activated again. If the compressors
16/04/14,ag op2320 amended speci & claim pages,10
can fill an entire air gun array in, for example, 6 seconds, it is therefore possible, for
example, if 4 sources are deployed,, to have all 4 sources activated within a 6 second time
window. TABLE 1 illustrates a series of activation positions based on random sampling
and the associated firing times for the sources. The basis for source actuation is that the
sources would normally be fired on a regular 25 meter interval “flip flop” scheme (i.e. each
source is fired every 50 meters). In the present example the sources are actuated randomly
in space (and thus in time) but with the same average shot density (number of actuations
over a selected length of travel of the source towing vessel). An example of a section of a
long continuous record with a plurality of source firings spaced at random intervals is
shown in
A first step in computing discrete shot records from a continuous data
recording would be to extract fixed time records (which may or may not overlap) from the
continuous signal recording. Each time record may be initiated at the time of a source
being activated. There may or may not be interfering source energy from previous or
subsequent source actuations. For short shot record lengths, that is the length of time
between source actuation and the last signal recorded at the most distance seismic sensor
(e.g., four seconds or less) there will be very few interfering signals, and for long shot
record lengths (e.g., ten seconds or more) there will be more.
Data from interfering records may be removed, for example, by sorting the
data to a common sensor or common midpoint trace arrangement where the interfering
data will be random, and can then be attenuated by conventional noise attenuation
processes, for example, F-X deconvolution.
There are several issues to be considered in the present method. The first is
that the data recorded from actuation of one source may have interfering energy from other
sources present. The interfering energy may be expected to be random with respect to each
record for two reasons. The first is that the source has been activated in a spatially random
sense. Therefore, the same source may be fired again before the end of the record
generated by the first firing. The interfering energy may be from the subsequent or prior
actuation of a single source, but may also be from a second source. The second reason is
16/04/14,ag op2320 amended speci & claim pages,11
that the interfering source has also been activated with random intervals between
actuations of the first source.
Following a conventional processing step of attenuating random noise, the
data will still be distributed randomly in space, that is, the seismic sensor locations at the
time of energy detection will be spatially distributed in essentially the same manner as the
source positions. At this stage, the recorded data may be interpolated to finer spatial
positions on regular interval spacing for further processing. The interpolation and
regularization should also be expected to attenuate residual energy that is not coherent.
Typical regularization techniques (See, Sheng Xu and Yu Zhang (2010) Seismic data
regularization for marine wide azimuth data, Proceedings of the 2010 annual meeting,
Society of Exploration Geophysicists), may include interpolation over several different
dimensions/directions such as; common sensor, common source, common offset, common
depth point and time. Interfering energy in any seismic sensor record will only be coherent
in the common shot domain.
An example of the activation positions and times for sources fired in this
random spatial position sense is shown below in TABLE 1. Note that the positions are
generated randomly, and are not set. Further note that in this example, many of the shots
are fired in a way that there is no interfering energy on extracted shots.
16/04/14,ag op2320 amended speci & claim pages,12
TABLE 1
vessel speed 4.5 knots
extracted record length 8 seconds
minimum shot interval 18.5184
regular shot interval 25
Normal Regular Position Random Positions Activation Times
Source 1 Position Source 2 Position Source 1 Siource 2 time1 time2
x y x y Y Y
0 -25 25 25 0 25 0 10.80007
50 -25 75 25 33.12853 86.36322 14.31162 37.30915
100 -25 125 25 106.4594 143.2652 45.99075 61.89094
150 -25 175 25 132.2147 168.3899 57.11711 72.74492
200 -25 225 25 183.8826 210.5364 79.43781 90.95232
250 -25 275 25 226.8013 240.2097 97.9788 103.7712
300 -25 325 25 286.322 266.4713 123.6919 115.1163
350 -25 375 25 312.9564 340.5364 135.198 147.1127
400 -25 425 25 349.3745 376.6552 150.9307 162.7161
450 -25 475 25 415.6553 417.7835 179.5642 180.4836
500 -25 525 25 486.0781 458.2179 209.9871 197.9514
550 -25 575 25 531.4911 494.5444 229.6056 213.6445
600 -25 625 25 585.2791 565.7238 252.8422 244.3943
650 -25 675 25 649.8403 592.8224 280.7328 256.1009
700 -25 725 25 684.9578 637.505 295.9037 275.4039
750 -25 775 25 748.1958 696.8316 323.2226 301.0332
800 -25 825 25 796.556 756.4485 344.1144 326.7878
850 -25 875 25 868.7287 824.3228 375.2932 356.1097
900 -25 925 25 897.8333 858.0764 387.8665 370.6914
950 -25 975 25 948.2315 911.4307 409.6386 393.7406
1000 -25 1025 25 1010.093 941.9803 436.3629 406.9381
1050 -25 1075 25 1071.264 1006.485 462.7888 434.8043
1100 -25 1125 25 1122.44 1075.195 484.8973 464.4873
1150 -25 1175 25 1155.968 1126.908 499.3813 486.8276
1200 -25 1225 25 1227.983 1168.112 530.4919 504.6276
Actuation of a seismic energy source using random spatial positions
between successive source actuations may provide improved seismic data quality than that
using conventional, regular spatial source activations.
While the invention has been described with respect to a limited number of
embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that
other embodiments can be devised which do not depart from the scope of the invention as
disclosed herein. Accordingly, the scope of the invention should be limited only by the
attached claims.
16/04/14,ag op2320 amended speci & claim pages,13
Throughout this specification and the claims which follow, unless the
context requires otherwise, the word "comprise", and variations such as "comprises" and
"comprising", will be understood to imply the inclusion of a stated integer or step or group
of integers or steps but not the exclusion of any other integer or step or group of integers or
steps.
The reference to any prior art in this specification is not, and should not be
taken as, an acknowledgment or any form of suggestion that the prior art forms part of the
common general knowledge.
16/04/14,ag op2320 amended speci & claim pages,14
Claims (4)
1. A method for acquiring marine seismic data, including: towing at least one seismic energy source in a body of water at a substantially constant speed; actuating the at least one seismic energy source at a plurality of spatial positions, a distance between each of the plurality of actuations being randomly different than any other such distance, the difference between successive distances being sufficiently large to enable detection of corresponding differences between a position of the at least one seismic energy source corresponding to each actuation; and recording seismic energy detected by at least one seismic sensor substantially continuously through a plurality of actuations of the at least one seismic energy source, the recording comprising recording a geodetic position of the at least one seismic energy source and the at least one seismic sensor at each actuation.
2. A method according to claim 1, further including: actuating at least a second seismic energy source towed in the body of water at a known positional relationship with respect to the at least one seismic energy source; actuating the at least a second seismic energy source, a distance between each of the plurality of actuations being randomly different than any other such distance; and recording seismic energy detected by a plurality of seismic sensors substantially continuously through a plurality of actuations of the at least one seismic energy source and the at least a second energy source, the recording comprising recording a geodetic position of the seismic energy sources and the seismic sensors at each actuation.
3. A method according to either claim 1 or claim 2, further including extracting seismic signals from the recorded seismic energy corresponding to individual actuations of the at least one seismic energy source.
4. A method according to claim 3, wherein the extracting includes sorting the recorded seismic energy to at least one of common sensor position records and a common midpoint position records and attenuating energy in such records resulting from interfering source actuations by a random noise attenuation process.
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