U.S. patent application number 13/185195 was filed with the patent office on 2012-03-08 for tuning unique 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 | 20120057429 13/185195 |
Document ID | / |
Family ID | 45466902 |
Filed Date | 2012-03-08 |
United States Patent
Application |
20120057429 |
Kind Code |
A1 |
Eick; Peter M. ; et
al. |
March 8, 2012 |
TUNING UNIQUE COMPOSITE RELATIVELY ADJUSTED PULSE
Abstract
The invention relates to acquiring seismic data in either land
or marine environments, but typically marine environments where a
pulse-type source is fired in a distinctive composite pulse like a
distinctive rumble. In a preferred embodiment, a number of
pulse-type seismic sources, sometimes called an array, are fired in
a distinctive composite pulse to be able to identify within the
returning wavefield the energy resulting from the composite pulse.
Firing the pulse-type sources creates an identifiable signature so
that two or more marine seismic acquisition systems with source
arrays can be acquiring seismic data concurrently and the peak
energy delivered into the water will be less, which will reduce the
irritation of seismic data acquisition to marine life. In addition,
the composite pulse may be formulated by timing the firing of
several of the sources with respect to energy emitted by "ringing"
bubbles that attenuate within 100 to 300 ms to provide either or
both of low frequency pulses and high frequency pulses to provide
data for various processing and analysis of the data returned from
the subsurface. On land, the complicating factor to be addressed is
reverberation rather than bubbles.
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.: |
13/185195 |
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 ; 367/14;
367/38 |
Current CPC
Class: |
G01V 1/3808
20130101 |
Class at
Publication: |
367/23 ; 367/38;
367/14 |
International
Class: |
G01V 1/38 20060101
G01V001/38; G01V 1/00 20060101 G01V001/00; G01V 1/28 20060101
G01V001/28 |
Claims
1. A process for acquiring seismic data and providing information
about geologic structures in the earth, wherein the process
comprises: a) providing a plurality of seismic receivers to receive
seismic energy; b) providing a plurality of pulse-type seismic
sources to emit pulses of seismic energy into the earth; c)
developing a plurality of planned sequence arrangements for firing
each pulse-type seismic source in the plurality of pulse-type
seismic sources such that each sequence arrangement defines a
composite pulse and where each composite pulse is distinctive,
detectable and separable from other seismic energy by one or more
processing techniques; d) evaluating each composite pulse for its
energy levels across a frequency band; e) selecting one or more
composite pulses for use in a survey that emphasize one or more
desired frequency bands; f) firing each pulse-type seismic source
according to the selected composite pulses to deliver seismic
energy into the earth; g) receiving seismic energy at the seismic
receivers; h) recording seismic energy received by the seismic
receivers; and i) processing the recorded seismic energy to
separate the seismic energy to identify and separate the composite
pulse from other seismic energy and create an image of the geologic
structures in the earth below the seafloor.
2. The process according to claim 1 wherein the at least one
pulse-type seismic source comprises a plurality of pulse-type
seismic sources and no more than half of the plurality of seismic
sources are fired in unison.
3. The process according to claim 1 wherein the composite pulse
comprises at least three seismic source firings within one second
of one another.
4. The process according to claim 1 wherein the composite pulse
comprises at least three seismic source firings within two seconds
of one another.
5. The process according to claim 1 wherein the composite pulse
comprises at least three seismic source firings within four seconds
of one another.
6. The process according to claim 1, wherein the composite pulse is
made up by varying the order of firing of the pulse-type seismic
sources.
7. The process according to claim 1, wherein the composite pulse is
made up by varying the timing between the firing of each of the
pulse-type seismic sources.
8. The process according to claim 1, wherein the composite pulse is
made up by varying both the order and timing of the firing of each
of the pulse-type seismic sources.
9. The process according to claim 1 where a first seismic source is
towed by a vessel, and a second seismic source is towed by the same
vessel 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.
10. The process according to claim 9 where a third seismic source
is towed by the vessel, and the repeated composite pulse firing
sequence of the third source is distinct from the composite pulse
firing sequences of the first and second sources.
11. The process according to claim 1 where a first seismic source
is towed by a first vessel, and a second seismic source is towed by
a second vessel and the repeated composite pulse firing sequence of
the first source is distinct from the firing composite pulse
sequence of the second source.
12. The process according to claim 1 further comprising a plurality
of vessels where each vessel is towing at least one seismic source
in the water and wherein each seismic source has its own
distinctive composite pulse firing sequence and the sources are
fired in a synchronized order.
13. The process according to claim 1 further comprising a plurality
of vessels where each vessel is towing at least one seismic source
in the water and wherein each seismic source has its own
distinctive composite pulse firing sequence and the arrays are
fired in a non-synchronized order.
14. The process according to claim 1 where the plurality of seismic
sources are towed by a vessel and comprise a first array, and a
second array of seismic sources are towed by the same vessel and
the composite pulse firing sequence of the first array is distinct
from the composite pulse firing sequence of the second array.
15. The process according to claim 1 where the plurality of seismic
sources are towed by a vessel and comprise a first array, and more
then two additional arrays of seismic sources are towed by the same
vessel 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.
16. The process according to claim 1 where the plurality of seismic
sources are towed by a vessel and comprise a first array, and a
second array of seismic sources are towed by a second vessel and
the composite pulse firing sequence of the first array is distinct
from the firing sequence of the second array.
17. The process according to claim 1 further comprising a plurality
of vessels where each vessel is towing at least one array of
seismic sources in the water and wherein each array has its own
distinctive composite pulse firing pattern and the arrays are fired
in a synchronized order.
18. The process according to claim 1 further comprising a plurality
of vessels where each vessel is towing at least one array of
seismic sources in the water and wherein each array has its own
distinctive composite pulse firing pattern and the arrays are fired
in a non-synchronized order.
19. The process according to claim 1 wherein the seismic source is
in the water and the firing of the plurality of seismic sources
creates a rumble in the water.
20. 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.
21. The process according to claim 20 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.
22. 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.
23. 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.
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 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.
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 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.
26. 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.
27. 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.
28. 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.
29. The process according to claim 1 wherein the composite pulse
includes time separation of at least 20 ms before and after the
firing of at least one of the seismic sources to provide at least
one distinct high frequency pulse within the composite pulse to
provide higher resolution of the image of the subsurface structure
of the earth.
30. The process according to claim 1 wherein a low frequency pulse
created in step d) and used to analyze density of geologic
structures in the earth.
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 active parts
of the world.
[0005] A second concern in the collection of seismic data in marine
environments is potential harm, injury or irritation of whales and
other marine life due to the intensity of the energy coming off the
conventional seismic sources. Air guns are used in an array formed
from multiple air guns synchronized in a way to generate a single
sharp pulse with short duration powerful enough to get echo returns
from deep below the seafloor. The power of these pulses in the
water are probably 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 even be harmful to sea life.
[0006] A further challenge for seismic prospectors relates to the
wavelengths of the seismic energy and attenuation through the
earth. High frequency energy provides sharpness and detail,
especially for small scale geological structures. However, the
earth tends to rapidly attenuate higher frequency seismic energy.
On the other hand, low frequency data is helpful, such as for
calculating formation densities and velocities of seismic waves
through such layers. Salt formations are also typically quite
challenging for seismic prospectors and low frequency energy is
best for defining the size and shapes of salt domes.
[0007] A solution is needed for each of these issues. A solution
for both would be particularly well received.
BRIEF SUMMARY OF THE DISCLOSURE
[0008] The invention more particularly relates to a process for
acquiring seismic data to provide information about geologic
structures in the earth where a plurality of seismic receivers are
provided to receive seismic energy. At least one pulse-type seismic
source is provided to emit pulses of seismic energy into the earth.
The process further includes the developing of a plurality of
planned sequence arrangements for firing each pulse type seismic
source in the plurality of pulse type seismic sources such that
each sequence arrangement defines a composite pulse and where each
composite pulse is distinctive, detectable and separable from other
seismic energy by one or more processing techniques. Each composite
pulse is evaluated for its energy levels across a frequency band
and one or more composite pulses are selected for use in a survey
that emphasize one or more desired frequency bands. Each pulse type
seismic source is fired according to the selected composite pulses
to deliver seismic energy into the earth and seismic energy is
received at the seismic receivers. The seismic energy received by
the seismic receivers is recorded and the recorded seismic energy
is processed to separate the seismic energy to identify and
separate the composite pulse from other seismic energy and create
an image of the geologic structures in the earth below the
seafloor.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] 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:
[0010] FIG. 1 is a schematic top view of a tow vessel towing
seismic sources and streamers for acquiring seismic data in a
marine environment;
[0011] FIG. 2 is a schematic top view of an example source array of
air guns;
[0012] FIG. 3 is a chart showing an example series of pulses;
[0013] FIG. 4 is a schematic top view of a tow vessel towing
seismic sources and streamers where the streamers are flared;
[0014] FIG. 5 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;
[0015] FIG. 6 is a chart showing a plan for several source arrays
to deliver unique shot sequences and collect data in a single
receiver array; and
[0016] FIG. 7 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 firing sequences are undertaken,
Sequence A and Sequence B.
[0017] FIG. 8 is a chart showing the corresponding power spectrum
for Sequences A and B.
[0018] FIG. 9 is a chart showing a comparison of the energy emitted
subtracting the energy emitted from Sequence B from energy emitted
by sequence A.
DETAILED DESCRIPTION
[0019] 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.
[0020] 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 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, not specifically indicated. The tow vessel 15 also tows
an example array 20 of seismic sources. It is common to use a
number of air guns 22 in the array as the seismic sources where all
the guns are fired in near unison or at once to create a powerful
short period impulse of less than about twenty milliseconds (20 ms)
to create a return wavefield that is perceptible by the seismic
receivers along the streamers 18.
[0021] The current seismic acquisition state of the art require
that all of the air guns in the arrays fire nearly at once with
only minimal delays to tune the array to maximize the sharp peak at
the beginning of the pulse. A common industry standard 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 and repaired until it meets the required specification.
Normally a gun array will be formed of 2 to 3 sub-arrays, and each
sub-array will be made up of around 8 to 10 individual guns or guns
clusters of varying sizes. In normal operation, all 30 odd of these
guns will be fired almost simultaneously to try and create a
single, sharp peak of energy. The varied sizes of the guns provide
a large composite peak of energy with little or no reverberation by
firing near simultaneously and creating air bubbles that tend to
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.
[0022] According to the present invention, the guns within an array
should not all be fired in unison or near unison, but are fired in
a coded and time delayed sequence that is unique or at least
distinctive so that it can be distinguished in the return wavefield
from other coded sequences. The sequence or series of pulses define
a composite pulse which is like a short rumble instead of the
traditional crack of the guns firing in unison and there is no
large composite peak at the start of the source event. In a
preferred arrangement as shown in FIG. 2, and example array 20
comprises air guns 22A, 22B, 22C, 22D, 22E, 22F, 22G and 22H.
Typically, an array of air guns may exceed eight and may be as many
as 11 per sub-array. This results in 24 guns to 33 guns per source.
In the example array 20, air guns 22A and 22B are extra large
volume air guns that tend to provide more low frequency seismic
energy, air guns 22C and 22D are large volume air guns that
generate a lower range frequency end, 22E and 22F are medium sized
air guns that provide more mid-frequency seismic energy and 22G and
22H are small volume air guns that provide higher frequency seismic
energy. In practice air guns are broad band but tend to have
different amplitude energy levels at different frequency bands that
in general can be related to the gun's volume size. Normally, the
array comprises many different volume sizes of air guns and it is
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. The bubble effects are the trailing small energy
pulses that occur after the first main sharp energy pulse and are
the result of oscillation in the actual bubbles created by the
rapid high pressure air release that occurs when a air gun fires.
The bubble effects usually attenuate and have no significant energy
after approximately 100 ms and are greatly attenuated within 200
ms. However, when successive shots of air guns are undertaken prior
to the attenuation of the air bubbles, the timing of such shots
with respect to the size oscillations or vibrations of the
attenuating bubbles may create an additive or subtractive effect on
the energy delivered within the composite pulse. This also creates
distinctness in the resulting pulse.
[0023] Continuing the explanation of the present invention as
related to the first example air gun firing composite pulse,
referring now to FIG. 3, is an air gun firing sequence for the air
gun array designed so that the composite pulse will be unique as
compared to the firing sequence of any other sources in the
vicinity. These other sources could be towed by the same or other
vessels which offer great flexibility in acquisition designs and
field operations. An example composite pulse is shown in FIG. 3
where A indicates the firing of air gun 22A and B indicates the
firing of air gun 22B and so on. The elapsed time between each air
gun firing is typically measured in tens to hundreds of
milliseconds, but as long as it is unique, computer analysis of the
return wavefield will be able to identify pulses from the example
array 20 as distinct from pulses from any other source. The order
of firing of the individual guns can be used to help encode the
source to make it a unique or distinctive composite pulse along
with variations in the timing delays between guns. Additionally the
same gun may be used multiple times with the gun cycle time being a
restriction to the minimum time before a gun could be reused.
[0024] The present invention takes this concept of the composite
pulse and then tunes the output of the composite pulses to create
the wavelet one would desire. For example, if a lower frequency
wavelet was desired, one could take a escalating series of gun
sizes and fire them sequentially from the smallest to the largest
and then back down to the smallest. This would create a composite
broad low frequency pulse. On the other hand if a higher frequency
pulse is required, the array could have some of the bigger guns
removed and replaced with smaller high frequency guns that when
fired would skew the bandwidth of the composite pulse toward a
higher frequency result. The present invention creates an
opportunity to tune the wavelet going into the earth by adjusting
the firing order, gun size and delays to create a very unique
signal. At the same time, the invention creates a balancing issue
that if multiple gun arrays are in operation on the survey that
they all have the same composite bandwidth. So, by delivering a
composite pulse or series of shots in close time proximity, there
is an aspect of tuning the composite pulses to best take advantage
or to avoid the disadvantage of the energy continually emitted by
the "ringing" bubbles. Slight timing variations can alter the
energy delivered into the earth by the same number of shots
delivered by the same air guns. Knowing this, one can now optimize
the timing and effectively increase the energy and shift the
effective frequency of the composite pulses.
[0025] In a particular example of the present invention is the
tuning of the firing of the guns to deliver pulses within the
distinctive composite pulse that have prescribed frequencies for
analyses of the formations. For instance, the composite pulse may
be designed with the concurrent firing of several large volume
airguns to create a combined low frequency pulse within the series
of pulses that make up the composite pulse. The composite pulse
will continue to have the quality of a rumble in the water with a
stronger pulse within the rumble. At the same time, the composite
pulse may also be designed with a number of smaller guns firing
with sufficient time delay that the combination of the firings have
minimal compounding effect. The intent of having extended
separation of the firings is to increase the amount of high
frequency data in the returning wavefield for analyses, especially
when considering that high frequency data provides higher
resolution.
[0026] The composite pulses, as noted above, would still be
designed to be distinctly different from seismic energy from other
vessels and from other local sources so that the data record would
not be contaminated by other nearby seismic survey efforts. The
composite pulses would comprise pulses from big guns and smaller
guns and other size guns. Further, while it is conventional to have
an assortment of different sizes, not all arrays have to have the
same number of air guns, the same sizes of air guns, multiple air
guns within an array may be fired simultaneously, the time
difference between shots is clearly variable, and there are many,
many possibilities for unique coding. Given the possibilities for
unique coding, there is little to stop seismic acquisition teams
from firing many, many arrays in the same close area, where they
could all be uniquely encoded and separable in processing.
[0027] This 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 speakers 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. In the case of an air gun pulse source, there
are modeling packages that create composite pulse signatures by
allowing the user to choose the type and size of air gun while also
adjusting the firing time of individual guns. A simple method for
creating the different composite pulses is to modify this type of
modeling software and apply the basic understanding of the pulse
signature that any one air gun will contribute. In this way several
composite pulse signatures can be quickly created and analyzed for
uniqueness and frequency band width. A similar method uses similar
modeling techniques but automates the creation and analysis of the
composite pulse by using Monte Carlo simulation methods to vary the
air guns used and the firing timing. In this way, thousands of
composite pulse signatures are evaluated and signatures that best
satisfy the set criteria such as desired bandwidth and uniqueness
are found.
[0028] 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 unique source signature and then shape
filter, correlate or even invert for the actual shot record and the
desired output listen time. These processes like inversion are
standard for the ZenSeis.TM. acquisition technique and there are
many related patents on the art of this technique.
[0029] 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 the 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 and the
rumble is never the loud crack even if you are close to the 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.
[0030] Turning now to FIG. 4, a marine seismic acquisition system
60 with a flared streamer array is shown that is comparable to the
system 10 in FIG. 1. The flared streamer array 60 is preferred in
that the risk of gaps of coverage in both the near receivers
(closest to the tow vessel) and far receivers (farthest from the
tow vessel) is reduced. Turning to FIG. 5, a marine seismic
acquisition system is indicated by the arrow 70. In system 70, a
streamer array 78 is towed by a tow vessel 75. Tow vessel 75
includes a source array 80 that comprises a plurality of pulse type
seismic sources such as air guns that are arranged to be fired in
the manner described above where the air guns in the air gun array
are fired in a sequence that is uniquely coded and identifiable in
the return wavefield where the energy is spread out over time.
However, the system 70 includes auxiliary source vessels 84, 86 and
88 and their source arrays 85, 87 and 89, respectively, arranged to
follow the tow vessel 75 on either side of the streamer array 78
and, optionally if chosen, behind the streamer array 78. Each
auxiliary source vessel has its own unique air gun firing sequence
whether the source array is identical to any other source array. As
such, acquiring seismic data with the system 70 may include a
unique rumble from the array 80 followed by a unique rumble from
array 85, followed by array 87 and thereafter followed by array 89.
The seismic receivers on the streamers 78 are continuously
recording seismic data along with their location based on GPS data.
While each source array is waiting for its turn to rumble, a
suitable listening time elapses so that signals from one rumble do
not provide errant information into a second rumble of the same
source array. Actually, the rumbles from various source arrays do
not need to wait on one another but are only ordered within system
70 so as to minimize the peak energy delivered into the sea
irritating the marine life. As such, source arrays 80, 85, 87 and
89 could fire simultaneously or near simultaneously or overlapping
or whatever. Such an arrangement where the source array is waiting
for its turn to rumble may be understood when referring to FIG. 6
where the timing of each rumble is shown on the timeline along with
listening time.
[0031] Referring specifically to FIG. 6, the four source arrays 80,
85, 87 and 89 are set to fire in a synchronized pattern where
source array 80 begins its composite pulse at time zero as
indicated at S80 and completes the composite pulse in less than a
second. A ten second listening time is allowed for the return of
the wave field from source array 80 before the same source array is
fired again. Meanwhile, the composite pulse for source array 85 is
initiated as indicated at S85 which is shown to be after the
completion of the composite pulse for source array 80. It may be
preferred that the composite pulses of two or more source arrays
are not overlapping so as to minimize the energy introduced into
the water in respect of marine life. However, as long as the
composite pulses are unique, overlapping is not a problem for data
collection. Continuing with the explanation, the composite pulse
for source array 87 as shown at S87 is initiated and thereafter the
composite pulse for source array 89 as shown at S89 is initiated.
Two additional observations should be made at this point. First,
the composite pulses are not necessarily of the same length as
shown by different length bars in FIG. 8. Secondly, with long
listening time, and no limit on firing other unique source arrays,
there is a lot of time available to perform seismic surveying while
avoiding the energy peaks of current technology. While the sources
may be fired in a serial manner as described above or fired
asynchronously (without respect of another firing) or even
synchronously (where the firing of sources is synchronized with
other sources). There are no real limitations of how the various
sources are fired other then operational issues such as where it is
desired to maintain a relative spacing between the sources and the
receivers. The only requirement is that the receivers are recording
continually or sufficiently continually to capture the wavefield,
because we know where the individual shots were started and what
unique coding was applied with a specific source array.
Additionally, it is within the scope of the invention that several
composite pulses are designed and implemented for use with the
sources where the different composite pulses have different
frequency content. Indeed, there is no reason or technical
limitation for firing the sources to create composite pulses with
frequency content that are the same or very similar. The composite
pulses may be tuned to emphasize different frequency bands to
expand the bandwidth of the resulting recorded seismic data.
[0032] Two separate crews using the inventive techniques may
overlap signals, however, it would not be advised 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.
[0033] While the impulses of the various air guns have been shown
in a simplistic form in the previous drawings, the charts in FIG. 7
shows two example composite pulses that have distinctive signatures
created by multiple delayed firings of one or more air guns or
other impulse type sources. Sequences A and B are analyzed to
assess dB amplitude across the frequency spectrum in FIG. 8. While
the two composite pulses may appear as squiggly lines to an
ordinary observer, the location and intensity of the peaks and
their decay pattern will be more distinctive than two very
dissimilar fingerprints to an FBI lab technician. When the two
composite pulses are used simultaneously or near simultaneously,
the distinctive differences will result in a combined source
wavelet that will propagate through the earth and back to the
seismic receivers where seismic wavefield is recorded. By the use
of match filtering techniques, inversion techniques or other
processing techniques the recorded wavefield can be separated back
into two wavefields, each from one of the two original composite
pulses.
[0034] In FIG. 9, the differences between Sequences A and B are
more apparent when B is subtracted from A and there are clear
differences in the frequency band from about 8 Hz to 14 Hz
continuously and from 8 Hz to up beyond 20 Hz where the dB
amplitude is well above 10 dB at several frequencies. It is clear
from FIG. 9 that the two composite pulses are tuned to emphasize
different frequency bands. One might evaluate many proposed
composite pulse arrangements prior to the survey and select the
composite pulse to actually use based on this type of tuning. This
allows for the tuning of the composite pulse to better satisfy the
specific resolution needs of a specific geologic target. In FIG. 9
it is shown that Pulse A has a significant higher dB level than
Pulse B over the frequency range of about 8 Hz to 20 Hz. This is
significant since this is a primary frequency band for imaging
below salt structures.
[0035] It should further be understood that prior to undertaking
the data collection, the composite pulses that will be used 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 consecutive pulses that will be
exactly the same and that any three pulses in a row can be made
distinctive in a large number of sequences based on time delays
between pulses and the uniqueness of various individual sources or
guns.
[0036] 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 sequences for source separation should be
equally useful and beneficial on land. Thus, the same techniques
for simultaneous sourcing, compositing pulses and unique or
distinctive encoding described above can easily be performed on
land also. Conceptually, it is easier to explain the intricacies of
the technique in a near homogenous medium like seawater with air
guns and the bubble effect then to explain the same results with an
impactive source and the natural plate bounce that occurs when say
a hammer hits a baseplate in contact with the earths surface on
land and where there is an elastic transfer of energy to the
ground. The same approach to coding the composite pulses and
maintaining distinctive composite pulses can be applied to land
impactive sources for example accelerated weight drops, thumpers,
explosives or even a conventional land vibrator with certain
modifications to the hardware.
[0037] 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.
[0038] 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.
* * * * *