(12)
`
`United States Patent
`de K0k
`
`(10) Patent N0.:
`(45) Date 0f Patent:
`
`US 6,545,944 B2
`Apr. 8, 2003
`
`US006545944B2
`
`(54) METHOD FOR ACQUIRING AND
`PROCESSING OF DATA FROM TWO ()R
`
`MORE SIMULTANEOUSLY FIRED SOURCES
`
`.
`(75) Inventor‘ 135m“ Jasper de Kok’ Houston’ TX
`(
`)
`(73) Assignee: Westerngeco L.L.C., Houston, TX
`US
`(
`)
`Subject‘ to any disclaimer, the term of this
`patent is extended or ad]usted under 35
`U_S,C, 154(b) by 0 days,
`
`( * ) Notice:
`
`(21) APPL No. 09/870,289
`(22) Filed:
`May 30, 2001
`(65)
`Prior Publication Data
`
`Us 2002/0181328 A1 Dec' 5’ 2002
`(51) Int. Cl.7 ................................................ .. G01V 1/00
`_
`_
`(52) US. Cl. .............. ..
`. 367/56 114/72 702/17
`(58) Field of Search
`i1 4 /2 42,3 67 /5 6_
`’ 702/17’
`
`"""""""""""""" "
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`3,744,021 A
`4,159,463 A
`4,168,485 A
`4,636,956 A
`
`7/1973 Todd ................. .. 340/155 TC
`6/1979 Silverman
`340/155 CR
`9/1979 Payton et a1. ............... .. 367/41
`1/1987 Vannier et a1. ........... .. 364/421
`
`4,715,020 A 12/1987 Landrum et a1. ........... .. 367/38
`4,953,657 A
`9/1990 Edington .................. .. 181/111
`
`. . . . .. 367/53
`2/1998 Beasley . . . . . . .
`5,717,655 A
`..... .. 367/41
`2/1998 Sallas et al. .
`5,721,710 A
`5,822,269 A 10/1998 Allen ..................... .. 367/41
`5,924,049 A * 7/1999 Beasley et a1. .......... .. 367/56
`5,973,995 A 10/1999 Walker et a1. .............. .. 367/20
`FOREIGN PATENT DOCUMENTS
`
`WO01/75481 A2 10/2001
`WO
`4 Cited by examiner
`
`M d
`a an,
`
`Primary Examiner—Gregory J. Toatley, Jr.
`74 A
`A
`F ' —D 'd S F'
`ttorney, gent, 0r zrm
`av1
`.
`igatner;
`Mossman & Sr1ram, PC.
`(57)
`ABSTRACT
`A method of seismic surveying and seismic data processing
`using a plurality of simultaneously recorded seismic-energy
`sources. An activation sequence for each of said plurality of
`Seismic _ene_rgy Sources may be determined Such that energy
`rom seismic sources ma e recor e s1mu aneous an
`f
`Y b
`d d
`R
`IV
`d
`seismic energy responsive to individual seismic sources
`separated into separate source records. The seismic sources
`are activated using an activation sequence, the recorded
`seismic energy in the shot recordings may be separated into
`source recordings responsive to individual seismic sources.
`The source records may be derived from the shot records
`using a combination of shot record summations, inversions
`d ?n .
`an
`enng'
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`WesternGeco Ex. 1003, pg. 1
`
`

`

`U.S. Patent
`
`Apr. 8,2003
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`WesternGeco Ex. 1003, pg. 2
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`

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`U.S. Patent
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`Apr. 8,2003
`
`Sheet 2 0f 6
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`WesternGeco Ex. 1003, pg. 3
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`

`

`U.S. Patent
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`Apr. 8,2003
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`US 6,545,944 B2
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`WesternGeco Ex. 1003, pg. 4
`
`

`

`U.S. Patent
`
`Apr. 8,2003
`
`Sheet 4 0f 6
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`US 6,545,944 B2
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`WesternGeco Ex. 1003, pg. 5
`
`

`

`U.S. Patent
`
`Apr. 8,2003
`
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`WesternGeco Ex. 1003, pg. 6
`
`

`

`U.S. Patent
`
`Apr. 8,2003
`
`Sheet 6 6f 6
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`US 6,545,944 B2
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`WesternGeco Ex. 1003, pg. 7
`
`

`

`US 6,545,944 B2
`
`1
`METHOD FOR ACQUIRING AND
`PROCESSING OF DATA FROM TWO OR
`MORE SIMULTANEOUSLY FIRED SOURCES
`
`FIELD OF THE INVENTION
`
`This invention relates to the ?eld of geophysical pros
`pecting and, more particularly, to a method for generating
`seismic energy for seismic surveys.
`
`BACKGROUND OF THE INVENTION
`
`In the oil and gas industry, geophysical prospecting tech
`niques are commonly used to aid in the search for and
`evaluation of subterranean hydrocarbon deposits. Generally,
`a seismic energy source is used to generate a seismic signal
`that propagates into the earth and is at least partially
`re?ected by subsurface seismic re?ectors (i.e., interfaces
`betWeen underground formations having different acoustic
`impedances). The re?ections are recorded by seismic detec
`tors located at or near the surface of the earth, in a body of
`Water, or at knoWn depths in boreholes, and the resulting
`seismic data may be processed to yield information relating
`to the location of the subsurface re?ectors and the physical
`properties of the subsurface formations.
`US. Pat. No. 3,744,021 to Todd discloses the ?ring of loW
`energy shots for shalloW, high resolution pro?ling in com
`bination With high energy shots for deep seismic pro?ling.
`The method only alloWed for a small overlap of shalloW and
`deep pro?ling recording cycles, merely maximiZing the
`number of shots in a given period of time While minimiZing
`interference. US. Pat. No. 5,973,995 to Walker and Lindtje
`orn discloses a method for simultaneous recording of deep
`and shalloW pro?ling data. Their main objective Was to use
`different cables in one and the same shooting con?guration.
`US. Pat. No. 4,168,485 to Payton, et al, teaches a full
`simultaneous signal generation method. This patent imple
`ments orthogonal pseudorandom sequences for vibratory
`sources alloWing for the separation of the source signals
`during the correlation process. Experiments With pseudo
`random ?rings of airguns have also been conducted, hoW
`ever no global successes have been reported. Others have
`experimented With phase encoding of vibratory sources.
`Other patents attempting full simultaneous signal generation
`include US. Pat. No. 4,715,020 to Landrum and US. Pat.
`No. 5,822,269, to Allen. The problem With these types of
`encoding methods is that harmonic distortion is not rejected
`or is only partly rejected.
`US. Pat. No. 4,159,463 to Silverman describes the use
`multiple vibrators, repeatedly vibrating at stationary
`locations, generating opposite polarity sWeeps in encoded
`sequences. HoWever, Silverman does not include the use of
`vibrators for ?ring a single shot or sWeep at one set of
`locations While generating other polarity changing sWeeps at
`another set of locations.
`US. Pat. No. 5,721,710 to Sallas teaches a generaliZed
`method for the simultaneous use of an arbitrary number of
`vibrators, sWeeping a speci?ed number of times in constant
`geometry. In this method, the separation of sources is
`achieved through the repeated inversion of tWo-dimensional
`(source versus shot) matrices at constant frequencies.
`A general limitation When using pseudo-random
`sequences and sWeep signals is the length of the energy
`emission, rendering the method less attractive for dynamic
`(marine) recording. Methods that are applicable to explosive
`and implosive types of sources are limited. US. Pat. No.
`
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`5,924,049 to Beasley and Chambers teaches a processing
`method to separate the signals from different sources When
`?red simultaneously from tWo ends of recording cable(s).
`The method is not suitable for the simultaneous recording of
`signals arriving from approximately the same direction.
`A method disclosed by US. Pat. No. 4,953,657 to Eding
`ton discloses use of a suite of time delay differences betWeen
`sources. To enhance the signal from a particular source, the
`corresponding signals are aligned and stacked. The contri
`butions from the other source(s) are not aligned and do not
`stack to full strength. The remaining undesired energy is
`further attenuated in the frequency domain.
`The high cost of seismic acquisition necessitates that
`compromises in the ?eld be made, both on land and offshore.
`The common practice is to acquire data at a loW but still
`acceptable areal density of surface locations. On land both
`the source and the receiver deployment may be less than
`optimal While in the marine environment the source deploy
`ment is routinely compromised and often loWer than desir
`able. Often, data quality seems initially acceptable for the
`intended purpose, such as reconnaissance, neW ?eld
`exploration, Wildcat drilling, etc. HoWever, When more
`detailed studies like hydrocarbon identi?cation and reservoir
`characteriZation are needed at a later stage, the data quality
`proves insufficient. In both land and marine environments
`there is a compelling case for the efficient acquisition of
`seismic data at a denser grid of locations. The use of multiple
`sources ?ring simultaneously into the same recording sys
`tem is an attractive option to increase the ?eld efforts at
`relatively loW incremental cost. Simultaneous ?ring is par
`ticularly economical When additional sources can easily and
`cheaply be deployed, such as vibrator groups on land and
`airgun arrays in a marine situation. Unfortunately, the sepa
`ration of the information pertaining to the individual sources
`may be cumbersome and/or imperfect.
`It Would be desirable to have a method of simultaneous
`shooting With impulsive sources. The present invention
`satis?es that need.
`
`SUMMARY OF THE INVENTION
`A method of seismic surveying using a plurality of
`simultaneously recorded seismic energy sources. An activa
`tion sequence for each of said plurality of seismic energy
`sources may be determined such that energy from separate
`seismic source positions may be recorded simultaneously
`and seismic energy responsive to individual seismic sources
`separated into separate source records. The seismic sources
`are activated using an activation sequence, the recorded
`seismic energy in the shot recordings may be separated into
`source recordings responsive to individual seismic sources.
`The source records may be derived from the shot records
`using a combination of shot record summations, inversions
`and ?ltering.
`The present invention offers embodiments for simulta
`neous source separation applicable to both marine and land
`environments. One embodiment utiliZes source signals
`coded With positive and negative polarities, but Without the
`restriction of stationary locations and Without the restriction
`of vibratory sources. Another embodiment utiliZes source
`signals With time-delays betWeen source activation times.
`These embodiments achieve enhancement of the desired
`source energy through the alignment and combination of the
`coded signals. In the source discrimination process, an equal
`amount of data as recorded and of data, polarity changed in
`processing, may be used. This aspect alloWs that undesired
`source energy and distortion energy are effectively
`cancelled, Which are advantages over the prior art.
`
`WesternGeco Ex. 1003, pg. 8
`
`

`

`US 6,545,944 B2
`
`3
`One embodiment of the present invention described in
`this disclosure is based primarily on source polarity
`encoding, While another embodiment is based primarily on
`source time shift encoding. It should be noted here that in the
`?rst part of the detailed description, the source signal has
`been simpli?ed to a spike representation, or in other Words
`it has been deconvolved for the direct source signature. This
`implies that the schematic data shoWn in the relevant ?gures
`should be convolved With the direct source signature to
`obtain the actual responses.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The present invention and its advantages Will be better
`understood by referring to the folloWing detailed description
`and the attached draWings in Which:
`FIG. 1A illustrates a doWnWard ?ring source comprising
`tWo source elements.
`FIG. 1B illustrates an upWard ?ring source comprising
`tWo source elements.
`FIG. 2 illustrates a tWo source shooting geometry and
`simultaneous signal-coding scheme.
`FIG. 3 illustrates discrimination of simultaneous sources
`by data processing in the FK domain.
`FIG. 4 illustrates a four source shooting geometry and
`simultaneous signal-coding scheme.
`FIGS. 5A, 5B and 5C illustrate a source discrimination
`scheme for tWo sources.
`FIGS. 6A and 6B illustrate an alternate source discrimi
`nation scheme for adding a third source to the tWo sources
`of FIGS. 5.
`FIGS. 7A and 7B illustrate a three-source four-shot
`sequence With varying amplitudes and time delays.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`The present invention is a method for acquiring seismic
`data using simultaneously activated seismic energy sources.
`This invention may enable seismic surveys to be acquired
`more rapidly than conventional surveys. Other advantages
`of the invention Will be readily apparent to persons skilled
`in the art based on the folloWing detailed description. To the
`eXtent that the folloWing detailed description is speci?c to a
`particular embodiment or a particular use of the invention,
`this is intended to be illustrative and is not to be construed
`as limiting the scope of the invention.
`EXcept for the seismic vibrator, changing the polarity of
`a seismic source has not generally been considered to be a
`viable option and hence polarity encoding has not been used
`in all seismic acquisition environments. HoWever, in the
`marine situation, incorporating the negative sea surface
`re?ection into the method can approximate a polarity
`reversed impulse. Referring to a standard airgun array, the
`far ?eld source signature is composed of a positive pressure
`pulse folloWed by a negative pulse from the sea surface
`re?ection. The negative pulse, called the ghost, is time
`separated from the positive pulse by a time shift that may be
`referred to as the ghost time delay. This makes the far-?eld
`signal look like an anti-symmetric Wavelet. The ratio of the
`constituent positive to negative peaks can, hoWever, be
`changed by using vertically staggered arrays operated in so
`called ‘end ?re’ mode. By directing energy doWnWards or
`upWards, the positive direct pulse or the negative re?ected
`pulse can be enhanced respectively. The left side of FIG. 1A
`shoWs a doWnWard ?ring source comprised of tWo source
`elements. Time is the horiZontal aXis 111; depth is the
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`vertical aXis 113. The shalloWer source element 101 is
`activated at time t1, emitting energy in all directions, thereby
`creating a pressure Wave front. At the moment the pressure
`Wave front arrives at the deeper source element 103, that
`element is activated at time t2, thereby enhancing the doWn
`Ward traveling signal. The activation sequence time delay is
`computed by determining the time difference betWeen t1 and
`t2, the pressure Wave front travel time betWeen source
`elements. The upWard traveling Wave fronts from the time
`separated sources propagate such that the Wave fronts do not
`reinforce. The impulse responses are displayed With arbi
`trary amplitude 114 on the right hand side of FIG. 1A, With
`time Zero t, (reference time) chosen to correspond to the sea
`surface. The impulse responses at the right hand side of FIG.
`1A are as measured vertically beloW the sources, but reda
`tumed such that time Zero corresponds to the sea surface.
`In the upWard ?ring source, as depicted in FIG. 1B, the
`guns of the deeper source element 107 ?re ?rst While the
`shalloWer guns 105 are time delayed until the upWard
`traveling Wave-front has arrived at that shalloWer depth
`level. This Way the up-going Wave front is enhanced. Again,
`on the right side of FIG. 1B, the reference time tr in the
`impulse response of the up-going Wave front is chosen to be
`the sea surface. Although more than tWo source elements can
`be deployed for creating enhanced positive and negative
`signals, and for enhancing directionality of signals, only tWo
`are used in the illustration of FIG. 1A and FIG. 1B.
`FIG. 2 shoWs the sequences of tWo sources ?ring simul
`taneously With polarity coding. It should also be noted that
`the number of sources is not limited to tWo. Larger numbers
`of sources can be used in combination With longer source
`activation or coding sequences. FIG. 2 shoWs a source vessel
`201 toWing sources 203 and 205. A source vessel 201 may
`also toW a streamer containing sensors for receiving source
`signals, for eXample streamer 207. In FIG. 2, Source 203
`emits S1, in Which positive (P) and negative (N) polarity
`source signals alternate as depicted by the positive and
`negative polarity representation through time. S1 may cor
`respond to the eXample of alternately using a pair of source
`elements in the con?guration shoWn in FIG. 1A With a pair
`of source elements in the con?guration shoWn in FIG. 1B.
`Source 205 emits positive signals S2 only. S2 corresponds to
`using a pair of source elements in the con?guration shoWn
`in FIG. 1A. The sources may have arbitrary positions in a
`seismic survey, but in the eXample of FIG. 2 they are located
`closely together like in a ?ip-?op shooting con?guration.
`The seismic energy returned from shot records containing
`multiple source position energy must be separated into
`source records containing energy responsive to the indi
`vidual seismic sources. The separation of individual source
`contributions into source records (as opposed to shot
`records) is achieved during processing, preferably in the
`common mid-point (CMP) domain but any other domain
`Where the contributions from successive shot records are
`present may be used. In the domain selected, a form of
`miXing or ?ltering may be applied to remove the undesired
`source position contribution seismic energy. In the eXample
`of FIG. 2 for instance, a tWo-trace miX With equal Weights
`or a three trace running miX With Weights 1/2, 1 and 1/2 may
`greatly attenuate the polarity changing signals from Source
`S1. It should be noted here that all multi-trace operations
`such as DMO, stacking, migration, etc. have a miXing effect
`on the data. Thanks to the polarity changing nature of the
`undesired data, these multi-traces processes Will also reduce
`undesired source contributions.
`In order to enhance the signal from Source S2 and
`attenuate signals from Source S1, successive shots are
`
`WesternGeco Ex. 1003, pg. 9
`
`

`

`US 6,545,944 B2
`
`5
`polarity reversed during processing. Hereby signals from S1
`Will become positive in polarity for all shots, While the
`signals from Source S2 Will alternately become positive and
`negative.
`Although data mixing can take place at various stages
`during processing, the preferred domain is the CMP gather
`Where the desired data are sorted to offset and normal
`move-out (NMO) corrected as shoWn schematically on the
`left panel 301 in FIG. 3. The ?rst panel 301 of FIG. 3 is a
`depiction of an NMO corrected CMP gather shoWing ?at
`tened re?ection events R1, R2 and R3. There are also
`seismic event contributions 309 from tWo additional sources
`located at the tail end of the cable, for instance sources 413
`and 415 as depicted in FIG. 4. The NMO corrected data are
`transformed to the Frequency-Wavenumber
`domain
`representation 303 Where attenuation of undesired energy
`can take place by passing Wavenumber
`values around
`the K=0 axis only. Panel 305 is the FK domain representa
`tion after ?ltering; panel 307 is the CMP gather after
`transforming from the FK domain back to CMP containing
`re?ection events R1, R2 and R3 With other unWanted
`seismic energy suppressed or absent.
`FIG. 4 depicts a four source (203, 205, 413, 415) shooting
`arrangement With tWo receiver cables (407, 409). Front
`source 203 emits signal sequence S1 and 205 emits signal
`sequence S2, may be as previously shoWn in FIG. 1A and
`FIG. 1B. Source vessel 201 may also toW tWo streamers
`containing sensors for receiving source signals, for example
`streamers 407 and 409. FIG. 4 shoWs tWo sources, 413 and
`415, folloWing the streamers. Source 413 may have a signal
`sequence S3 as depicted in FIG. 4. TWo shots of one polarity,
`for instance positive polarity, are folloWed by tWo shots of
`negative polarity before tWo shots of positive polarity are
`again initiated. The signal sequence S4 for another source,
`for instance 415, is the same as S3 except the polarity
`sequence is offset one step either direction compared to S3.
`In the con?guration of FIG. 4 the tWo sources 203 and 205
`preceding streamers 407 and 409 are relatively close to each
`other, and also sources 413 and 415 at the back of the
`streamers 407 and 409 are in relatively close proximity. As
`a consequence, When processing for desired front source
`205, the data from undesired source 203 have moveout that
`is approximately the same as that of the desired front source
`205. HoWever, the corresponding energy from each front
`source projects at different locations in the FK domain.
`Because the undesired data is changing in polarity as out
`lined above, it has Zero average amplitude and thus has no
`energy at K=0. Instead, the polarity sWapping data represent
`a periodic function in X and projects onto constant K values,
`311 in FIG. 3, pertaining to the length of a period. Trans
`formation back to the TX domain, after any necessary FK
`?ltering, yields the desired data absent the undesired source
`contributions, for example as is shoWn on panel 307 in FIG.
`3.
`After polarity reversing successive shot recordings during
`processing, this process can then be repeated for the other
`sources. For instance, ?ipping alternate records Will cause
`signals from source 203 to be in phase and those from other
`sources to be out of phase. Flipping (inverting) the signals
`from the 3rd and 4”1 shot and the 7”1 and 8th shot and so on
`Will cause signals from source 413 to be in phase and those
`from the other sources to be out of phase in successive
`recordings. There are four separable sources shoWn in the
`con?guration of FIG. 4, separable by using stacking, ?lter
`ing or combinations of both stacking and ?ltering.
`Another embodiment of the present invention entails time
`delay encoding. The time delay encoding technique relies on
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`programmed time delays in the ?eld and polarity decoding
`in the processing center. The time shifts for encoding may be
`arbitrarily chosen per source, but they should preferably be
`equal to the ghost time delay in the marine case. In the land
`case the delay should be less than the reciprocal of the
`maximum frequency of interest. The enhancement of data
`pertaining to a particular desired source is accomplished
`through equaliZing the polarity of corresponding signal
`components and to align and average (mix or stack) the
`responses. This principle is illustrated With the impulse
`response representations of FIG. 5A for a marine applica
`tion. The impulse response representations may be, for
`example, Where the sea surface re?ection having opposite
`polarity folloWs a primary source impulse. Here, the primary
`source is represented as a positive polarity folloWed by a
`ghost of negative polarity With a delay time. In FIG. 5A (and
`in FIG. 6 and FIG. 7), only the individual source ?ring
`sequences are shoWn and not their combined responses. In
`FIG. 5A, Source TD1 and Source TD2, being sequential
`series of simultaneous shots, have different delay codes for
`successive shots (numbered 1 to 4 for each simultaneously
`activated source). The time delays in these ?gures are
`relative to an arbitrary reference, here labeled t,=0, repre
`sented by the vertical dashed lines. For example, simulta
`neously ?red shot 1 from TD1 (501) and shot 1 of TD2 (511)
`are initiated With no relative time delays betWeen them, but
`shot 2 from TD2 (513) is initiated before shot 2 of TD1
`(503), the time separation betWeen the initiation of shot 2 of
`TD2 (513) relative to shot 2 of TD1 (503) being the time
`delay determined or chosen for the acquisition program,
`Which may for example, be the ghost delay. In FIG. 5A for
`Source TD1 the shot 1 (501) and shot 2 (503) representations
`are of positive delay times relative to the reference time. The
`FIG. 5A Source TD1 shot 3 (505) and shot 4 (507) repre
`sentations are for negative delay times. For FIG. 5A Source
`TD2 shot 1 (511) and shot 3 (515) have positive delay times
`While shot 2 (513) and shot 4 (517) have negative delay
`times. The polarity of a particular source is determined by
`the polarity of the impulse coinciding With the reference
`time t,=0. The source With no delay is considered positive,
`the source With a negative delay is considered negative.
`Here the result of polarity decoding to enhance and
`separate energy for the Source TD1 shot series from the shot
`series of Source TD2 consists of reversing the contributions
`from shot 3 (505, 515) and shot 4 (507, 517), Which causes
`energy from Source TD1 to reinforce and that of Source TD2
`to cancel after mixing, K-?ltering or stacking (also here the
`CMP gather may be the preferred domain to execute the
`source discrimination). This result is demonstrated in FIG.
`5B Where the impulse response of the processed shot series
`is shoWn. In FIG. 5C, shot 2 (503, 513) and shot 4 (507, 517)
`are reversed causing energy from Source TD2 to reinforce
`and energy from Source TD1 to cancel.
`When using a sequence of four shots as in FIG. 5A and
`FIG. 5B, the method can accommodate three different
`sources. The coding of a third source, Source TD3, is shoWn
`in FIG. 6A and FIG. 6B and consists of positive delay times
`for shot 1 (601) and shot 4 (607) With negative relative delay
`times for shot 2 (603) and shot 3 (605). In this case, the
`decoding for Source TD3, the third source, is achieved by
`inverting shot 2 (503, 513 and 603) and 3 (505, 515 and
`605). It may be observed that both Source TD2 in FIG. 6A
`and Source TD1 in FIG. 6B do not pass energy after mixing,
`K-?ltering or stacking. If it is desirable to use more than
`three simultaneous sources, the sequence can be changed to
`consist of cycles longer than four shots.
`Although the signals used in FIG. 5A, FIG. 5B, FIG. 6A
`and FIG. 6B have equal amplitude and ghost delays, this is
`
`WesternGeco Ex. 1003, pg. 10
`
`

`

`US 6,545,944 B2
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`7
`not a requirement for the method. All sources may be
`different both in amplitude and ghost delay. This implies that
`simultaneous shooting for deep and shalloW pro?ling is also
`feasible.
`Also, simultaneous shooting of land sources other than
`Vibroseis is feasible With the time delay coding method. The
`signal response after the application of this method is similar
`to that of that of the ghost in a marine environment, i.e., the
`original response compounded With a delayed (or advanced)
`opposite response.
`In FIG. 7A and FIG. 7B three sources Without ghosts are
`shoWn. All three sources have different amplitude and have
`been coded using different time delays. Compare FIG. 6A
`and FIG. 6B With FIG. 7A and FIG. 7B. In FIG. 7A and FIG.
`7B the polarity decoding (1—2—3+4) is shoWn Which pre
`serves energy for Source TD6. Polarity decoding (1—2+3—4)
`preserves energy from source TD5 While polarity decoding
`(1+2-3-4) preserves energy from Source TD4.
`Provided the pre-stack data are of suf?cient quality,
`source separation can be further improved through interpo
`lation techniques, not only in the CMP domain but in other
`domains as Well. As an example, interpolation in the com
`mon offset domain Will be demonstrated. The prestack data,
`sorted into common offset range gathers, consist of the
`contribution of many shots. The eXamples shoWn here have
`been sequences of 4 shots, like source TD4 in FIG. 7,
`numbered shot 1, shot 2 etc. In each shot, three delay coded
`sources (source 1, source 2 and source 3) ?re simulta
`neously. The period of the sequence consists of four shots.
`During the ?fth shot, the sources ?re With the same time
`coding as in shot 1, and also the other shots in the cycle
`repeat. Shots having the same source time coding Will be the
`series of shots 1, 5, 9, 13 and so on. The other series Will be
`2, 6, 10, etc., 3, 7, 11, etc., and 4, 8, 12 and soon.
`The ?rst step in the interpolation method is to sort the
`common offset sequence gathers into depopulated data sub
`sets that contain only those shots that have the same source
`time coding, for eXample shots 1, 5, 9 and so on, for four
`shot sequences. Of course, there are 4 different Ways in
`Which the sources are combined in a shot, corresponding to
`the four different source time coding sequences, so four
`different depopulated subsets are formed.
`The process then continues With the interpolation, by any
`knoWn method, of three neW traces in betWeen tWo subse
`quent traces of the depopulated subset of a common offset
`series. For example, three traces betWeen shot 1 and shot 5
`create traces corresponding to neWly interpolated traces
`from shots 2, 3 and 4. Another eXample is betWeen shot 8
`and shot 12 creating neWly interpolated traces corresponding
`to shot 9, 10 and 11, and so on, such that the common offset
`data sets are repopulated With interpolated traces. In this
`manner four interpolated common offset data series traces
`pertaining to the same offset have been generated. At each
`trace location in addition to the one original, three interpo
`lated traces have been created. The process may then be
`continued and completed With the polarity decoding for
`particular sources such that the sources are separated by the
`summation of the four interpolated common offset data
`series in the manner previously described.
`It is possible that methods based on subtraction of data
`from different shots fail in the land situation due to the
`changing coupling conditions for both sources and detectors.
`HoWever, the use of surface consistent deconvolution can
`provide a ?rst order correction, not only increasing the
`consistency of the data but also improving the conditions for
`discrimination of the simultaneous sources.
`
`10
`
`15
`
`25
`
`35
`
`45
`
`55
`
`65
`
`8
`A cause of cross talk in Vibroseis encoding methods is the
`distortion caused by the vibrator and the poor vibrator-earth
`coupling. The use of the present invention could alleviate
`this problem because the undesired source suppression is
`based on processing equal numbers of positive (as recorded)
`traces With negative traces (inverted in processing). The
`result of this encoding method is cancellation of all undes
`ired source data, including the stationary part of the distor
`tion. In addition the polarity reversal method cancels even
`harmonics in the desired source data. In contrast, the sum
`mation of positive and negative sWeeps as generated in the
`?eld fails because the even distortion from the tWo sWeeps
`is of same polarity. Subtraction of similar sWeeps during
`processing is essential to combat the distortion problem.
`No method Works perfectly on all data cases, but there are
`advantages to both the polarity coding and the time delay
`coding methods, and their combination, that lead to sup
`pression of any undesired energy that may leak through from
`the associated processing steps. Routine multi-trace process
`ing algorithms may suppress some of this ‘leakage’ very
`effectively.
`Persons skilled in the art Will understand that the inven
`tion described herein is not to be unduly limited to the
`foregoing Which has been set forth for illustrative purposes.
`Various modi?cations and alternatives Will be apparent to
`those skilled in the art Without departing from the true scope
`of the invention, as de?ned in the folloWing claims.
`What is claimed is:
`1. A method of seismic surveying using simultaneously
`recorded seismic energy sources, the method comprising:
`(a) selecting a plurality of seismic energy sources to be
`used for surveying, each of said seismic energy sources
`containing a plurality of source elements;
`(b) positioning said plurality of seismic energy sources
`and associated seismic energy receivers at locations