U.S. patent application number 13/185169 was filed with the patent office on 2012-01-19 for high density source spacing using continuous composite relatively adjusted pulse.
This patent application is currently assigned to CONOCOPHILLIPS COMPANY. Invention is credited to Joel D. Brewer, Peter M. Eick, Frank D. Janiszewski.
Application Number | 20120014213 13/185169 |
Document ID | / |
Family ID | 45466902 |
Filed Date | 2012-01-19 |
United States Patent
Application |
20120014213 |
Kind Code |
A1 |
Eick; Peter M. ; et
al. |
January 19, 2012 |
HIGH DENSITY SOURCE SPACING USING CONTINUOUS COMPOSITE RELATIVELY
ADJUSTED PULSE
Abstract
The invention relates to continuously or near continuously
acquiring seismic data where at least one pulse-type source is
fired in a distinctive sequence to create a series of pulses and to
create a continuous or near continuous rumble. In a preferred
embodiment, a number of pulse-type seismic sources are arranged in
an array and are fired in a distinctive loop of composite pulses
where the returning wavefield is source separable based on the
distinctive composite pulses. Firing the pulse-type sources creates
an identifiable loop of identifiable composite pulses so that two
or more marine seismic acquisition systems with pulse-type seismic
sources can acquire seismic data concurrently, continuously or near
continuously and the peak energy delivered into the water will be
less, which will reduce the irritation of seismic data acquisition
to marine life.
Inventors: |
Eick; Peter M.; (Houston,
TX) ; Brewer; Joel D.; (Houston, TX) ;
Janiszewski; Frank D.; (Richmond, TX) |
Assignee: |
CONOCOPHILLIPS COMPANY
Houston
TX
|
Family ID: |
45466902 |
Appl. No.: |
13/185169 |
Filed: |
July 18, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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61365631 |
Jul 19, 2010 |
|
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|
61365663 |
Jul 19, 2010 |
|
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61494952 |
Jun 9, 2011 |
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Current U.S.
Class: |
367/23 |
Current CPC
Class: |
G01V 1/3808
20130101 |
Class at
Publication: |
367/23 |
International
Class: |
G01V 1/38 20060101
G01V001/38 |
Claims
1. A process for acquiring seismic data and provide information
about geologic structures in the earth, wherein the process
comprises: a) providing a plurality of seismic receivers to receive
seismic energy; b) providing at least one pulse-type seismic source
to emit pulses of seismic energy into the earth; c) delivering a
distinctive series of pulses of seismic energy into the earth from
the at least one pulse-type seismic source to create a seismic
energy wavefield response from geologic structures in the earth
where the distinctive series of pulses of seismic energy are
delivered in a continual loop or near continual loop from said at
least one pulse-type seismic source in a planned order where the
loop is of sufficient length to provide listening time to receive
the wave field response from the geologic structures in the earth
from a portion of the loop defined as a composite pulse before the
distinctive series of pulses of the loop end and may be restarted
or have infinite length and wherein the series of pulses within the
loop are sufficiently distinctive such that portions of the loop
are recognizably distinct from other portions of the loop and the
distinctions are sufficient to distinguish the wavefield caused by
the loop from seismic energy in the environment that arises from
other sources; d) receiving seismic energy with the plurality of
seismic receivers including the seismic energy wavefield response
from the geologic structures in the earth; e) recording the seismic
energy wavefield response received by the seismic receivers to form
data traces; and f) processing the data traces of recorded seismic
energy to separately identify within the data traces each of the
composite pulses of the pulse-type seismic source when the
composite pulses were fired and to further separately identify a
number of segments of data within each loop where each segment
overlaps at least one composite pulse and by processing the
segments of data provides for greater data density of the geologic
structures in the earth.
2. The process according to claim 1 wherein at least one pulse-type
seismic source comprises a plurality of pulse-type seismic sources
and no more than half of the seismic sources are fired in
unison.
3. The process according to claim 1 wherein the step of firing a
distinctive series of pulses creates a first loop, and wherein the
process further comprises firing a distinctive series of pulses
from a second pulse-type seismic source which creates a second loop
wherein the first loop is distinctive from the second loop, and the
step of recording the seismic energy includes recording seismic
energy from wave fields created by the first loop and the second
loop and the step of processing further includes separating the
wavefield response in the data traces based on the source of the
first loop from the source of the second loop.
4. The process according to claim 1 wherein the loop comprises a
series of at least three separate distinctive composite pulses
wherein each composite pulse is fired within two seconds of the one
that precedes it.
5. The process according to claim 1 wherein the loop comprises a
series of at least three separate distinctive composite pulses
wherein each composite pulse is fired within four seconds of the
one that precedes it.
6. The process according to claim 1, wherein the series of pulses
are emitted by a plurality of different types of pulse-type seismic
sources and the loop is made distinctive by varying the order of
firing of the different types of pulse-type seismic sources.
7. The process according to claim 6, wherein the different type of
pulse-type seismic sources are air guns of different sizes or
designs.
8. The process according to claim 1, wherein the series of pulses
is made distinctive by varying the timing between the firing of
each pulse.
9. The process according to claim 1 where the pulse-type seismic
source comprises a plurality of pulse-type seismic sources towed by
a vessel and arranged in at least a first array and a second array
and the sequence of firing of the first array is distinct from the
sequence of firing of the second array.
10. The process according to claim 9 where the plurality of seismic
sources include at least a third array, and the sequence of firing
of the third array is distinct from the sequence of firing of the
other arrays.
11. The process according to claim 9 where the first and second
arrays are towed by different vessels.
12. The process according to claim 11 wherein the arrays are fired
in a synchronized order.
13. The process according to claim 11 wherein the arrays are fired
in a non-synchronized order.
14. The process according to claim 9 where the plurality of seismic
sources are towed by a plurality of seismic vessels, each seismic
vessel having at least one pulse-type seismic source and the
sequence of firing of each seismic source is distinct from the
sequence of firing of the other seismic sources.
15. The process according to claim 11 wherein at least one vessel
tows more than one array of pulse-type seismic sources where
vessels that tow more than one array have the arrays arranged in a
desired geometry so as to deliver seismic energy from spaced source
locations wherein the spaced source locations are also source
separable in the data traces by firing a distinctive series of
pulses from each array.
16. The process according to claim 1 wherein the sources are in the
water and the pulses create a rumble in the water.
17. The process according to claim 1 wherein the step of providing
at least one pulse-type seismic source more particularly comprises
moving a first pulse-type seismic source into a desired location
while also moving at least a second pulse-type seismic source into
a second desired location and the step of firing a series of pulses
further comprises each of said first and second sources firing a
series of pulses where the sequence of firing of the first seismic
source is distinct from the sequence of firing of the second
seismic source.
18. The process according to claim 1 where a first seismic source
is moved onto a desired location, and a second seismic source is
moved onto a desired location and the repeated composite pulse
firing sequence of the first source is distinct from the repeated
composite pulse firing sequence of the second source so that two
distinct pulse-type wavefields are produced.
19. The process according to claim 18 where a third seismic source
is moved onto a desired location and the repeated composite pulse
firing sequence of the third source is distinct from the composite
pulse firing sequence of the first and second sources.
20. The process according to claim 1 further comprising a plurality
of seismic sources that are moved onto desired locations and
wherein each seismic source has its own distinctive composite pulse
firing sequence and the sources are fired in a synchronized
order.
21. The process according to claim 1 further comprising a plurality
of seismic sources that are moved onto desired locations and
wherein each seismic source has its own distinctive composite pulse
firing sequence and the sources are fired in a non-synchronized
order.
22. The process according to claim 1 where the plurality of seismic
sources are moved onto a desired location and comprise a first
array, and a second array of seismic sources are moved onto a
desired location and the composite pulse firing sequence of the
first array is distinct from the composite pulse firing sequence of
the second array.
23. The process according to claim 1 where the plurality of seismic
sources are moved onto a desired location and comprise a first
array, and more then two additional arrays of seismic sources are
moved onto other desired locations and the composite pulse firing
sequence of the first array and all other arrays are distinct from
the composite pulse firing sequence of all other arrays.
24. The process according to claim 1 where the plurality of seismic
sources are moved onto a desired location and comprise a first
array, and more then two additional arrays of seismic sources are
moved onto other desired locations and wherein each array has its
own distinctive composite pulse firing pattern and the arrays are
fired in a synchronized order.
25. The process according to claim 1 where the plurality of seismic
sources are moved onto a desired location and comprise a first
array, and more then two additional arrays of seismic sources are
moved onto other desired locations and wherein each array has its
own distinctive composite pulse firing pattern and the arrays are
fired in a non-synchronized order.
26. The process according to claim 1 wherein the seismic source is
imparting seismic energy into the earth and the firing of the
plurality of seismic sources creates a rumble in the earth.
27. The process according to claim 1 wherein the pulses are created
by firing one or more seismic sources and the loop includes firing
of each seismic source at least three times.
28. The process according to claim 1 wherein the pulses are created
by firing one or more seismic sources and the loop unique sequence
of firing of the plurality of seismic sources includes firing each
seismic source at least ten times.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a non-provisional application which
claims benefit under 35 USC .sctn.119(e) to U.S. Provisional
Application Ser. No. 61/365,631, filed Jul. 19, 2010 entitled
"Unique Composite Relatively Adjusted Pulse" and U.S. Provisional
Patent Application Ser. No. 61/365,663, filed Jul. 19, 2010
entitled "Continuous Composite Relatively Adjusted Pulse" and U.S.
Provisional Patent Application Ser. No. 61/494,952, filed Jun. 9,
2011 entitled "High Density Source Spacing Using Continuous
Composite Relatively Adjusted Pulse", which are all incorporated
herein in their entirety.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] None.
FIELD OF THE INVENTION
[0003] This invention relates to emitting seismic energy into a
marine environment that is able to travel into the seafloor and
reflect from and refract through geological structures and be
received and recorded by hydrophones.
BACKGROUND OF THE INVENTION
[0004] It is very expensive to acquire seismic data in marine
environments. The cost of mobilizing vessels, equipment and people
can run in the several hundreds of thousands to millions of dollars
per day. Thus, once the survey is started, there is a lot of
pressure to acquire data twenty-four hours a day, seven days a
week. A problem arises when another survey crew is collecting data
in the same general area at the same time. The two operations may
contaminate one another and be forced to work out a time sharing
arrangement where only one crew acquires data for a period of time
and then waits while the other crew takes a turn. It is common to
time share seismic data collection in the North Sea off of
northwest Europe and in the Gulf of Mexico among other areas.
[0005] A second concern in the collection of seismic data in marine
environments is harm, injury or irritation of whales and other
marine life due to the intensity of the energy coming off the
seismic sources. Air guns are traditionally used in an array to
generate a single pulse powerful enough to get echo returns from
deep below the seafloor. The power of these pulses in the water is
presumed to be at least annoying to sea animals that use echo
location like whales, dolphins and others. Seismic surveying
techniques may cause these animals to leave the area and some
believe that it may be harmful to sea life.
[0006] The third concern in the collection of seismic data is the
sampling spacing. Conventional seismic acquisition fires an air gun
array and during the echo period no other sources can be acquired.
At the usual sailing speed of around 2.5 meters per second, and
given a normal 10 second record, the next shot point can't be any
closer then 25 m. Longer record lengths require even more time
between shot points so the sampling can be quite coarse between
successive firings. This is particularly bad in a wide azimuth
shooting where multiple vessels are towing guns and all guns are
fired in a round-robin fashion. It may be hundreds of meters
between successive shots of the same guns on the same sail
line.
[0007] A solution is needed for each of these issues. A solution
for all of the aforementioned concerns would be particularly well
received.
[0008] In one recently proposed technique for addressing the above
issues is to operate a system utilizing an assortment of airguns
which are discharged in a recognizable sequence that also reduces
peak energy input into the sea for minimizing impacts on marine
life. Two marine survey systems may operate concurrently in what
would be close proximity as long as their sequences are distinctive
from one another. In one improvement over such systems is to
provide a continuous or near continuous stream of airgun discharges
where each loop is distinctive or is made up of distinctive
segments.
BRIEF SUMMARY OF THE DISCLOSURE
[0009] The invention more particularly relates to a process for
acquiring seismic data and provides information about geologic
structures in the earth, wherein a plurality of seismic receivers
are provided to receive seismic energy and at least one pulse-type
seismic source is provided to emit pulses of seismic energy into
the earth. The at least one pulse-type seismic source is fired to
deliver a distinctive series of pulses of seismic energy into the
earth to create a seismic energy wavefield response from geologic
structures in the earth where the distinctive series of pulses of
seismic energy are delivered in a continual loop or near continual
loop from the one pulse type seismic source in a planned order. The
loop is of sufficient length to provide listening time to receive
the wave field response from the geologic structures in the earth
from a portion of the loop defined as a composite pulse before the
distinctive series of pulses of the loop end. The loop may be
restarted or may have infinite length. Moreover, the series of
pulses within the loop are sufficiently distinctive such that
portions of the loop are recognizably distinct from other portions
of the loop and the distinctions are sufficient to distinguish the
wavefield caused by the loop from seismic energy in the environment
that arises from other sources. The seismic energy is received by
the plurality of seismic receivers including the seismic energy
wavefield response from the geologic structures in the earth and
the seismic energy wavefield response received by the seismic
receivers is recorded to form data traces. The data traces of
recorded seismic energy are processed to separately identify within
the data traces each of the composite pulses of the pulse type
seismic source when the composite pulses were fired and to further
separately identify a number segments of data within each loop
where each segment overlaps with at least one composite pulse and
by processing the segments of data provides for greater data
density of the geologic structures in the earth.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] A more complete understanding of the present invention and
benefits thereof may be acquired by referring to the follow
description taken in conjunction with the accompanying drawings in
which:
[0011] FIG. 1 is a schematic top view of a tow vessel towing two
seismic source arrays and streamers for acquiring seismic data in a
marine environment;
[0012] FIG. 2 is a schematic top view of an example source array of
air guns;
[0013] FIG. 3 is a chart showing two example loops of series of
pulses;
[0014] FIG. 4 is a chart showing a second example series of
pulses;
[0015] FIG. 5 is a chart showing one of the two example loops of
FIG. 3 identifying the composite pulses and examples of segments
that may be identified within the loops;
[0016] FIG. 6 is a schematic top view of a tow vessel towing two
seismic source arrays and streamers where the streamers are
flared;
[0017] FIG. 7 is a schematic top view of a tow vessel towing
seismic sources and streamers with additional source vessels towing
additional seismic sources operating in conjunction with the tow
vessel to acquire a higher volume of seismic data in one pass
through the survey area;
[0018] FIG. 8 is a chart showing a plan for several source arrays
where each source array delivers a series of distinctive composite
pulses and collect data in a single receiver array; and
[0019] FIG. 9 is a chart showing a comparison of the time and
intensity of the energy emitted with the firing of the same array
of air guns where two different composite pulses are undertaken,
Composite Pulse A and Composite Pulse B.
DETAILED DESCRIPTION
[0020] Turning now to the detailed description of the preferred
arrangement or arrangements of the present invention, it should be
understood that the inventive features and concepts may be
manifested in other arrangements and that the scope of the
invention is not limited to the embodiments described or
illustrated. The scope of the invention is intended only to be
limited by the scope of the claims that follow.
[0021] For the purpose of this discussion, an air gun seismic
source will be used as an example of an impulsive seismic source.
It should be understood that there are other impulsive sources that
could be used with this invention, for example sparkers, plasma
shots, steam injection sources or even explosive based sources. As
shown in FIG. 1, a seismic acquisition system is generally
indicated by the arrow 10. The system 10 includes a tow vessel 15
towing a number of streamers 18. Along each streamer 18 are a large
number of seismic receivers, each indicated by the letter "x" and
several guidance devices, also called "birds" that are indicated by
the circles along the streamers 18. The birds can be used for both
lateral and vertical streamer control as it transits the water. The
seismic sources are also towed behind tow vessel 15 in the form of
two source gun arrays, 20a and 20b. It is common to use air guns in
marine seismic acquisition and for each source gun array to
comprise a number of air guns where all the air guns are fired in
unison or at once to create a sufficiently powerful impulse to
create a return wavefield that is perceptible by the seismic
receivers along the streamers 18. It is also common to tow two sets
of source gun arrays forming the port and starboard gun array
set.
[0022] The current state of the art in seismic acquisitions
requires that all of the guns in the arrays fire at once. The
common timing spec is that all guns must fire within 1 ms of each
other. If all the guns don't fire within the 1 ms window, then the
array must be recovered, tuned and repaired until it meets the
required contract specifications. Normally, a source gun array will
be formed of 2 to 3 sub-arrays, and each sub-array will be made up
of around 10 individual air guns of varying sizes. In normal
operation, all 30 (in our example) of these guns will be fired
almost simultaneously to try and create a single, sharp peak of
energy. Great effort is spent on designing the size of the guns and
the spacing of the array to maximize the sharpness of the single
peak of energy. The varied sizes of the guns provide a large
composite peak of energy with little or no reverberation by firing
simultaneously and creating air bubbles that cancel each other out
so that the large composite peak will propagate through the sea and
into the seafloor. By conventional standards, this is the optimal
way of sourcing marine seismic data.
[0023] According to the present invention, the guns should not be
fired in unison, but are rather fired in a series of gun shots. The
series of airgun shots create a stream of pulses that result in
sustained rumbles in the water instead of the traditional crack of
the guns firing in unison so that there is no large composite peak
at the start of the source event.
[0024] With a distinctive design of the firing sequence of the
airguns including a reasonably precise delay between shots and a
distinctive order of big, small and medium shots, the distinctive
series of pulses may be recognized in the data record and isolated.
The isolated series of pulses may be called a composite pulse that,
as part of the processing, in a manner similar to the data
processing of sweep-type source data on land, the composite pulse,
is aggregated into a single data point. Recognizing that the marine
seismic system is moving, the horizontal earth location for this
single data point is assumed to be the midpoint between the
location of the airgun array when the series of pulses were emitted
and the hydrophone's location when received.
[0025] Building on the concept of delivering seismic energy as a
stream of pulses, the next step would be to deliver a continuous
series of pulses. These pulses may be in the form of a loop that
comprises several composite pulses where each composite pulse is
distinct from other composite pulses in the loop. The loop is
carefully designed such that it is sufficiently long so that the
first composite pulse is able to travel to the maximum depth into
the earth for the survey and then return before the loop ends. As
such, when the loop ends, it is simply restarted in a seemingly
endless loop where each composite pulse is able to travel down and
return before the same distinctive composite pulse is delivered a
second time. As long as the series of pulses is not repeated in the
loop during the listen time for any one composite event, than the
data can be separated and is a separate shot event.
[0026] Turning back to FIG. 1, the source arrays are generally
indicated by the arrows 20a and 20b comprising two side-by-side
arrays. As shown in FIG. 2, source gun array 20a is shown with ten
individual air guns where the extra large guns are labeled A, the
large guns are labeled B, the medium guns are labeled C and the
small guns are labeled D. The two extra large air guns A provide
very low frequency seismic energy, the two large air guns B
generate low frequency energy, the two medium air guns C provide
more mid-frequency seismic energy and the four small air guns D
provide higher frequency seismic energy. This is very analogous to
a hi-fidelity stereo speaker system where the outputs are all tuned
to give a smooth broad band response. Normally, an array comprises
many more air guns and more air guns of different sizes. It is also
typical to have more small air guns than large air guns to make up
for the lower amount of energy that is released by one pulse of
each smaller air gun. This is all part of the traditional tuning of
the source to give the sharpest, cleanest peak with the minimal
bubble effects. It is also normal to put the biggest guns first for
ease of deployment and stable towing conditions through the water.
These are not requirements and are more a matter of convenience in
the operation at sea.
[0027] FIG. 3 illustrates representative loops of pulses created by
pulse firing sequences for both arrays 20a and 20b. Each bar in the
representative loop indicates the firing of a single airgun where a
taller bar indicates the firing of a larger gun A while a smaller
bar indicates the firing of a smaller airgun D and the larger of
the intermediate airguns is identified by the bars B and the
smaller of the intermediate airguns indicated by the bars C. It
should be recognized that although the representative arrays 20a
and 20b have the same arrangement of airguns, the loops are each
unique. Moreover, using a larger variety of airguns in the arrays
provides additional aspects for differentiation in the data created
by each array. FIG. 3 also shows that the loops are comprised of
composite pulses that are distinctive one from another where the
end/start points for the composite pulses are identified by the
taller dotted lines. It should be noted that the composite pulses
do not necessarily have the same time duration such that some are
longer and some are shorter. In FIG. 4, the time delay between the
firing of successive shots of the airguns is shown as varied such
that the loop of pulses may be designed with considerable variation
and uniqueness.
[0028] In FIG. 5, the loop for airgun 20b is shown where the
composite pulses are identified as 51A, 51B, 51C, 51D and 51E.
However, as an additional aspect of the present invention, there
are quite a number of distinctive segments that may be selected out
of the loop where example segments are identified by the brackets
52A, 52B, 52C and 52D. The segments overlap with at least one and
typically with two composite pulses. Considering that the segments
may be selected, there is an opportunity to select segments that
overlap one another as shown by segments 52B and 52C. So, in a
manner described above where the data from a composite pulse may be
identified, isolated and aggregated, so too, may segments be drawn
from the data, isolated and aggregated. So, in effect, the firing
of a single airgun may factor into the data from one composite
pulse and one, two and maybe more segments. Again, considering the
velocity of the system 10 over the seafloor, the aggregated data
from segments creates data at horizontal earth locations that are
between the earth points derived from the composite pulses.
Identifying data from segments that overlap at least the composite
pulses and possibly including other segments provides greater
effective density of earth points while NOT increasing acquisition
costs.
[0029] It should be emphasized that, contrary to conventional
operations, all source arrays are delivering seismic energy into
the water at the same time, but in a more muffled rumble. Each
airgun is recharged while others are firing rather than all firing
simultaneously and all recharging simultaneously. In the present
invention, the two arrays are operated together with each creating
a series of distinctive composite pulses continuously or near
continuously where no composite pulse is repeated more often than
the desired recorded record length.
[0030] Typically, a listen time is provided after each firing of
each composite pulse. However, considering that this example loop
is divisible into multiple composite pulses, the listen time for
the return for each composite pulse actually begins at the firing
of the first gun that forms part of the composite pulse. Thus, as
long as the entire loop of composite pulses is distinctive and does
not have repeating patterns within the loop and the loop is long
enough to provide sufficient listening time from the firing of the
last gun contributing to a distinctive subdivided sequence, the
guns may be fired in the loop, continuously and over and over.
Typical listening times are between 6 and 15 seconds. With a loop
that is as long or longer than the listening time plus the duration
of the composite pulse, the only limitation is the recharging
ability of the compressor and the ability to deliver the compressed
air to the air guns fast enough. The elapsed time between each air
gun firing in the inventive system is typically between about ten
milliseconds up to several hundreds of milliseconds, but typically
in the twenty to five hundred ms range. From a practical
standpoint, as long as the loop is unique, computer analysis of the
return wavefield will be able to identify the composite pulses from
the loop of composite pulses contained in the returned wavefield as
distinct from pulses from any other source of pulses. With a
continuously emitting seismic source, a continuously recording
system and a continuously moving tow vessel and source and receiver
arrays, the density of data in the data record will be substantial
when coupled with a continuous recording system or near continuous
recording system.
[0031] Continuing to study the FIG. 3 example, the air guns in the
array 20a and 20b including the firing of each gun in a loop of
five distinct composite pulses over slightly more than 18 seconds.
Due to the limitations of the drawing, the sources are being fired
at 200 ms intervals with no variation in time spacing except that
between composite pulses where an extra 200 ms gap is shown to help
separate the composite pulses within the loop and a dashed line is
placed. It would generally be preferred that the delays are between
about 20 ms and 500 ms and structured for increased uniqueness or
distinctness of the composite pulses and the loops. Moreover, the
guns do not need to fire alone. Certainly, multiple guns may fire
concurrently, but it is preferred that the guns have individual
signatures (be different in size or character) for signal
separation. The first composite pulse of the loop for array 20a
starts with the firings of the extra large guns A with 200 ms gaps,
followed by the medium guns C, followed by the small guns D and
then the large guns B. The second composite pulse in array 20a of
the loop begins at about the four second mark. It should be
appreciated that a longer gap in the loop may be used or the next
composite pulse may begin right at the end of the previous
composite pulse as long as the composite pulses are distinct from
one another within the loop. Also, it should be noted that there
may be other composite pulses that can be created within a designed
loop if it is considered that the qualification for a composite
pulse is that it be distinct from any other composite pulse within
the loop or any other pulses from a nearby source that might fire
within an associated listening time.
[0032] Associated with the firing of each composite pulse within
the loop, there is a listening time that starts with the initial
firing time of the first gun in the composite pulse and recognizing
that the listening includes reference to the arrangement of guns
fired following the composite pulse to identify within the data
traces which gun at which location was fired to produce the
specific data trace. When utilizing a continuous or near continuous
seismic recording system, the zero time used for setting the
extraction of individual seismic records is set by the initial
firing time of the first gun contributing to the particular
composite pulse being extracted. The extracted record length would
then be the desired listening time that is less than or equal to
the length of the full loop minus the length of the particular
composite pulse. This extracted record would be one input to the
process of separating the wavefield associated with this particular
composite pulse. The implication of the continuous or near
continuous seismic recording and the subsequent extraction of
seismic records associated with each composite pulse within the
source firing loop coupled with the fact the tow vessel generally
acquires data a speed of between 4 to 5 knots results in the
creation of a dense inline spatially sampled source data set. The
advantages gained from this dense source sampled data set are
numerous when processing the data set and include benefits in such
processing steps as noise attenuation, multiple attenuation,
velocity analysis, frequency content and overall subsurface
resolution.
[0033] Continuing with the explanation of FIG. 3, the second
composite pulse immediately follows the first, but is distinctly
different than the first composite pulse and one that is readily
identifiable in post gathering processing. The second composite
pulse comprises extra large and medium guns firing in alternation
at 100 ms intervals until all of those sized guns within the array
20a is fired, followed by an alternating series from two smaller
guns and one large gun at 100 ms intervals. This second composite
pulse is completed at about eight seconds. The third composite
pulse in array 20a includes pairs of equal sized guns firing in
sequence beginning with extra large A to medium C to small D to
large B and finally to small D again: A, C, D, B, D. This concludes
at about the twelve second mark. The fourth composite pulse begins
with an extra large gun A and then follows with a descending size
succession through a large B, medium C and two small guns D: A, B,
C, D, D. This descending succession is repeated four times until
all of the guns in the array 20a have fired which occurs just
beyond the sixteen second mark. The next and final composite pulse
in the 18.5 second loop is similar to the fourth composite pulse
except that the firing of the two small guns D is separated by the
medium gun rather than both following the medium gun: A, B, D, C,
D. The array 20b is fired near simultaneously with array 20b but
with a distinctly different firing pattern that yields five
distinct composite pulses that form a distinct 18.5 second loop
from the array 20a. In practice the source arrays 20a and 20b would
be spatially separated to produce wavefields that illuminate
different subsurface areas or the same subsurface area but from
different orientations.
[0034] As an example of greater variability within a composite
pulse, FIG. 4 shows a complete single composite pulse undertaken in
just under three tenths of a second. This is probably more
compressed than preferred recognizing that for the next composite
pulse, each of the guns will need to recharge with compressed air
but it is demonstrative of the variability that can be created
using this technique. This FIG. 4 is an idealized display where
FIG. 9 shows two guns firing actual composite pulses as recorded by
a seismic receiver located with the airguns.
[0035] The unique signal can be analogized to being in a crowded
room with a lot of people talking and a person being able to lock
his hearing into one person talking just based on some uniqueness
of that person's voice. Not necessarily because that person is
talking louder than others, but because of some combination of tone
or frequency or amplitude variations of the speaker's voice. There
are some very key analogs that can be derived from this concept of
a crowded room and trying to listen to a conversation. One is that
the source must put out a sufficient volume to be detected. But at
the same time just going louder tends to encourage other sources to
also get louder which provides no advantage. Another observation is
that the more unique a person's voice is, the easier it is to sort
out or distinctly hear that person's voice from the others in the
room. Thus, the number of alternative noise sources that are active
in the room, the more unique the person's voice should be to hear
it. Returning to the sequence of firing a source array, the
variations in size, timing and duration of the firing of the coded
shot should be carefully designed prior to acquisition. To a
certain extent, the various unique composite pulses that may be
used might also be site specific and variable from site to site.
There may not be one "perfect" answer but this can easily be
modeled and tuned for different situations.
[0036] The first benefit of delivering seismic energy into the
marine environment in this manner is that it would allow two or
three or even many different survey teams to operate at essentially
the same time in the same area. This is a breakthrough for field
operations and acquisition as it completely eliminates the
traditional time share problem of the conventional sharp peak air
gun sourcing. This also allows for wide azimuth acquisition in a
cost effective manner as we can now source many different lines at
the same time and at much tighter station spacing with minimal to
no contamination. This can be done because the unique signature of
the pulses can be identified by each system and will ignore the
other pulses as noise. This can be done through the inversion
process of the data. Essentially, the processing would involve
taking a block of simultaneously recorded data starting at the time
zero for a particular composite pulse within a loop and then one
could shape filter, deconvolve or even invert for the actual shot
record and the desired output listen time. These processes are well
documented and used in the ZenSeis.TM. acquisition technique and
there are many related patents on the art of this technique.
[0037] The second benefit of delivering seismic energy into the
marine environment in this manner is that it distributes the energy
into the water over time in such a manner that peak energy is
significantly less. Actually, based on current methods of
calculating energy emitted into a marine is based on measurement of
peak signal as compared to bubble size created by each pulse.
Bubbles created by air guns are very elastic in water and appear to
bounce in size from a large bubble to a small bubble and back to a
large bubble. As the bubble created by one air gun is created,
another air gun is fired such that the ratio actually may be
negative. A negative ratio would imply that sound is actually being
taken out of the water, but that is an artifact of the calculation.
What is important is that with the present invention, what would
have been a very loud crack or bang becomes a more tolerable
background rumble that should be much less irritating to marine
life. A very good analogy to this is listening to the thunder. When
one is close, it can be quite scary and quite a shock as it is
quite loud and forms a strong pulse. On the other hand, due to
interactions of the thunder crack with the earth effects, at long
distances thunder is just a low rumble which is much more
tolerable. The invention takes the sharp crack of thunder and turns
it into a rumble that is uniquely tuned to each source. Thus,
seismic surveying in a marine environment becomes multiple rumbles
occurring at once and each can easily be sorted out to know where
it came from.
[0038] Turning now to FIG. 6, a marine seismic acquisition system
60 with a flared receiver array 68 is shown that is comparable to
the system 10 in FIG. 1. The flared receiver array 68 is preferred
in that the risk of gaps of coverage in both the near receivers
(closest to the tow vessel 65) and far receivers (farthest from the
tow vessel 65) is reduced. Side by-side dual source arrays 66 are
shown between the middle two streamers of receiver array 68
representing conventional flip flop shooting style acquisition.
[0039] Turning to FIG. 7, a marine seismic acquisition system is
indicated by the arrow 70. In system 70, a receiver array 78 is
towed by a tow vessel 75. Tow vessel 75 includes source arrays 72
that comprise a plurality of pulse type seismic sources such as air
guns that are arranged to be fired in the manner described above
where the array is fired in a composite pulse that is uniquely
coded and identifiable in the return wavefield where the energy is
spread out over time. In this Figure, the source arrays 72 are
shown as three in-line arrays instead of the more common dual,
side-by-side arrays or single array that could be used. In
addition, the system 60 includes auxiliary source vessels 74a and
74b and their source arrays 75 and 77, respectively, arranged to
follow the tow vessel 75 on either side of the receiver array 78.
The reason for this inline arrangement is that it can be used in
two methods. Either it can be used to create a normal composite
pulse as described above, or the sources can be fired continuously
to allow for a much shorter bin size due to a short shot point
increment as compared to other industry techniques.
[0040] Another optional arrangement is to tow a source array behind
the receiver array 78. Each auxiliary source vessel has its own
loop of distinctive composite pulses whether the composition of the
source array is identical to any other source array. As such,
acquiring seismic data with the system 60 may include concurrent
rumbles from the source array 70 while distinctive rumbles emanate
from source arrays 75 and 77. This is illustrated in FIG. 7 where
each line represents one full loop and the beginning of a second
loop. The seismic receivers on the streamers 68 are continuously
recording seismic data along with their location based on GPS
data.
[0041] Continuing with the description of FIG. 8, each horizontal
bar represents a composite pulse where S72A is the first composite
pulse of source 72 for the loop that source 72 will emit. S72B is
the second composite pulse and S72C is the third composite pulse
and so on. The time that elapses after S72A has been emitted until
the loop begins again with S72A is the available listening time for
S72A. An essentially equivalent listening time will be provided for
each composite pulse. Similarly, it should be seen that all of the
source arrays will be emitting their loops in a generally
concurrent arrangement where the signals overlap. However, since
each composite pulse is distinctive from all other composite pulses
in all of the loops, post recording processing may source separate
the signal received by each receiver in the receiver array. It
should also be recognized that the various composite pulses may be
synchronized such that one composite pulse from one vessel may end
at nearly the precise moment another source array begins to emit
its composite pulse. Thus, the various loops may be choreographed
so that continuous data is collected, but that the energy in the
water is managed.
[0042] It should further be understood that prior to undertaking
the data collection, the composite pulses should be designed and
analyzed for their distinctness. There are many methods of creating
distinctiveness and it is believed that distinctiveness can be
designed such that every composite pulse can be provided with no
more than two discrete pulses in sequence that will be the same and
that any three discrete pulses in a row can be made
distinctive.
[0043] Two separate crews using the inventive techniques may
overlap signals, however, care should be taken in designing
composite pulses to try and collect data with a conventional sharp
pulsed air gun system while an inventive system is in the area. The
conventional system will not interfere very much with an inventive
system, but the conventional system will likely have difficulty
identifying their generic return wavefield from the returning
wavefields from the inventive system.
[0044] It should be noted that the invention is described as having
a plurality of pulse-type seismic sources which are most commonly
air guns. Other types of pulse-type sources are available.
Moreover, a plurality of pulse-type sources are not necessarily
required to practice the broadest form of the present invention.
Specifically, given a very short cycle time between successive
firings of the same device, a single, highly controlled pulse-type
source device may create the composite pulses and the loops without
having to have additional such devices. While it is preferred to
have a variety of reasonably different sources, as long as the
energy is emitted in a manner that is a distinctive series of
pulses, the broadest aspect of the invention may be practiced.
[0045] Moreover, this type of seismic data acquisition should not
be limited to a marine environment. While pulse type sources are
commonly used in marine environments, pulse type sources may be
used on land, too. As such, a land application using pulse type
sources with distinctive composite pulses for source separation
should be equally useful and beneficial on land. Land examples of
pulse type sources are accelerated weight drops, explosives,
thumper trucks and even conventional vibes if properly set up.
[0046] In closing, it should be noted that the discussion of any
reference is not an admission that it is prior art to the present
invention, especially any reference that may have a publication
date after the priority date of this application. At the same time,
each and every claim below is hereby incorporated into this
detailed description or specification as additional embodiments of
the present invention.
[0047] Although the systems and processes described herein have
been described in detail, it should be understood that various
changes, substitutions, and alterations can be made without
departing from the spirit and scope of the invention as defined by
the following claims. Those skilled in the art may be able to study
the preferred embodiments and identify other ways to practice the
invention that are not exactly as described herein. It is the
intent of the inventors that variations and equivalents of the
invention are within the scope of the claims while the description,
abstract and drawings are not to be used to limit the scope of the
invention. The invention is specifically intended to be as broad as
the claims below and their equivalents.
* * * * *