United States Patent
`Beasley et al.
`
`[19]
`
`[54]
`
`[75]
`
`METHODS FOR ACQUIRING AND
`PROCESSING SEISMIC DATA
`
`Inventors: Craig J. Beasley; Ronald E.
`Chambers, both of Houston, Tex.
`
`Assignee: Western Atlas International, Inc.
`
`Appl. No.: 09/016,679
`Filed:
`Jan. 30, 1998
`
`Related US. Application Data
`
`Continuation-in-part of application No. 08/829,485, Mar.
`28, 1997, Pat. No. 5,717,655, which is a continuation of
`application No. 08/423,781, Apr. 18, 1995, abandoned.
`
`Int. Cl.6 .................................................... .. G06F 19/00
`
`US. Cl. ............................................... .. 702/17; 367/56
`Field of Search ................................ .. 702/17, 16, 14;
`367/23, 53, 56, 57, 50
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`2,897,476 10/1959 Widess .................................... .. 702/17
`3,290,644 12/1966 Hoskins .................................. .. 367/21
`3,496,532
`2/1970 Thigpen.
`3,602,878
`8/1971 Sullivan ................................ .. 340/7 R
`3,687,218
`8/1972 Ritter ..
`. 181/107
`3,985,199 10/1976 Baird ..................................... .. 181/107
`4,042,906
`8/1977 EZell .............................. .. 340/15.5 TS
`4,159,463
`6/1979 Silverman
`..... .. 367/59
`4,224,474
`9/1980 Savit ........ ..
`370/68
`4,300,653 11/1981 Cao et a1.
`. 181/107
`4,486,864 12/1984 Ongkiehong et al.
`367/23
`4,797,861
`1/1989 Beasley ........... ..
`367/50
`4,823,326
`4/1989 Ward ....................................... .. 702/14
`4,914,636
`4/1990 Garrotta et a1. ........................ .. 367/56
`4,930,110
`5/1990 Bremner et al.
`367/56
`4,953,657
`9/1990 Edington ...... ..
`. 181/111
`4,970,696 11/1990 Crews et a1.
`367/56
`4,982,374
`1/1991 Edington et al.
`367/48
`5,430,689
`7/1995 Rigsby et al.
`367/56
`5,450,370
`9/1995 Beasley et al. ......................... .. 367/53
`
`US005924049A
`Patent Number:
`Date of Patent:
`
`[11]
`[45]
`
`5,924,049
`Jul. 13, 1999
`
`5,677,892 10/1997 Gulunay et al. ........................ .. 367/38
`5,717,655
`2/1998 Beasley ................................... .. 367/53
`
`OTHER PUBLICATIONS
`
`Beasley, Craig J ., Quality Assurance of Spatial Sampling for
`DMO, 63rd Annual Meeting of Society of Exploration
`Geophysicists, published in Expanded Abstrats, pp.
`544—547, 1993.
`Vermeer, Gijs J 0., Seismic Acquisition 3:3—D, Data Acqui
`sition, 64th Annual Meeting of the Society of Exploration
`Geophysicists, published in Expanded Abstracts, pp.
`906—909, 1994.
`Egan, Mark S.; DingWall, Ken; and Kapoor, Jerry; Shooting
`direction: A 3—D marine survey design issue, The Leading
`Edge, Nov. 1991, pp. 37—41.
`Primary Examiner—Donald E. McElheny, Jr.
`Attorney, Agent, or Firm—E. Eugene Thigpen
`[57]
`ABSTRACT
`
`A method for acquiring and processing seismic survey data
`from tWo or more seismic sources activated simultaneously
`or nearly simultaneously or for a single source moved to and
`?red at different locations. In one aspect such a method
`includes acquiring seismic survey trace data generated by
`the source or sources, attaching source geometry to the
`traces, soring the traces according to a common feature
`thereof, (eg to CMP order), interpolating data points for
`discontinuities on the traces, selecting tWo halves or tWo
`portions slightly more than half of the traces, ?ltering the
`trace data for each of the tWo portions to ?lter out data
`related to a second one of the tWo seismic sources, reducing
`the ?ltered trace data to tWo halves of the data and deleting
`interpolated data, and then merging the tWo halves to
`produce re?ned useful seismic data related to a ?rst one of
`the seismic sources. In one aspect the method includes
`re-processing the data and ?ltering out the trace data for the
`second seismic source to produce re?ned useful seismic data
`related to the second seismic source. In one aspect the
`sources are ?red temporally close together and, in one
`particular aspect, they are ?red substantially simultaneously.
`
`42 Claims, 8 Drawing Sheets
`
`ATTACH GEOMETRY
`
`ATTACH SOURCE/
`DETECTOR GEOMETRY
`TO TRACES
`
`CMP SORT
`WITHIN SOURCE!
`CABLE LINE
`
`TRACE INTERP
`
`INTERFOLATE INTERMEDIATE TRACES
`
`LIMIT OFFSETS
`TO FIRST HALF
`OF CABLE PLUS
`SOME EXTRA
`TRACES FOR AN
`
`OvERLAP ZONE. I
`
`TRACE SELECT
`
`TRACE SELECT
`
`OFFSET LIMIT
`
`DIP REJECT
`
`DIP REJECT
`
`MULTICHANNEL
`DIP REJECT OR
`PASS
`
`MULTICHANNEL
`DIP REJECT OR
`
`LIMIT OFFSETS
`To LAST HALF
`OF CABLE PLUS
`SOME EXTRA
`TRACES FOR AN
`OVERLAP ZONE.
`
`REJECT EVENTS
`COMING FROM
`SOURCE AT
`OTHER END OF
`CABLE
`
`PANEL
`A
`
`LIMIT TO FIRST
`HALF OF CABLE
`REJECTING
`OVERLAP ZONE
`INTERPOLATED
`TRACE DELETE
`IS OPTIONAL.
`
`TRACE SELECT TRACE SELECT
`OFFSET LIMIT
`AND DELETE
`INTERPOLATED
`TRACES
`
`OFFSET LIMIT
`AND DELETE
`INTERPOLATED
`TRACES
`
`LIMIT TO LAST
`HALF OF CABLE
`REJECTING
`OVERLAP ZONE’
`INTERPOLATED
`- TRACE DELETE
`ls OPTIONAL.
`
`MERGE PANELS TOGETHER
`
`WesternGeco Ex. 1004, pg. 1
`
`

`

`U.S. Patent
`
`Jul. 13,1999
`
`Sheet 1 of8
`
`5,924,049
`
`Fig. 2
`
`Di+
`
`Fig. 3
`
`WesternGeco Ex. 1004, pg. 2
`
`

`

`U.S. Patent
`
`Jul. 13,1999
`
`Sheet 2 of8
`
`5,924,049
`
`(0E a
`
`UT
`
`CROSSLINE
`
`@ .wE
`
`CROSSLINE
`
`WesternGeco Ex. 1004, pg. 3
`
`

`

`U.S. Patent
`
`Jul. 13,1999
`
`Sheet 3 of8
`
`5,924,049
`
`-
`
`F1g. 7
`
`//
`
`/////
`“30!
`( DECODER )?aa
`
`I
`31—-<_DECODER )
`
`DOWN-DIP
`DOWN-DIP
`32 ~
`PROCESSING 36 PROCESSING
`/
`MERGE
`
`V
`
`M
`
`~34
`
`MODEL "38
`
`8
`
`to
`
`tn
`
`WesternGeco Ex. 1004, pg. 4
`
`

`

`U.S. Patent
`
`Jul. 13,1999
`
`Sheet 4 of8
`
`5,924,049
`
`K
`1 0 106
`H
`
`104
`
`102 100
`
`SOURCE
`—-—_—'>
`
`RECE|VERX><MIDPOINT
`I o o o o 0 o o 0 III
`I 00000000 I:
`I oooooooo III
`I oooooooo III
`I 00000000 Cl
`I oooooooo III
`
`QFFSET
`
`'
`
`Flg 9 <
`
`'
`
`I 00000000 [I
`I oooooooo III
`I oooooooo III
`I 00000000 I:
`I oooooooo III
`I oooooooo III
`I O O O O o o O O U
`I o o o o o o o 0 III
`I O O O O O O O 0 El
`
`0 DETECTOR POSITION
`III SOURCE POSITION 102
`I SOURCE POSITION 108
`\
`
`r
`SHOOTING DIRECTION
`
`'
`
`Flg'
`
`Flg
`
`'
`
`<
`
`—-———->
`120-1]
`
`F122
`
`oooofooosaeewswuv-u
`124 k+J
`PHYSICALLY SAME 124
`DETECTORS
`o DETECTOR POSITIONS SOURCE 120
`O DETECTOR POSITIONS SOURCE 122
`III SOURCE POSITION 120
`I SOURCE POSITION 122
`\
`
`135
`III-134
`(g
`131—~EI
`00000000 0000000
`oooooooooooooooo
`oooooooooooooooo
`oooooooooooooooo
`oooooooooooooooo
`oooooooooooooooo
`oooooooooooooooo
`oooooooooooooooo
`oooooooooooooooo
`OOOOOOOOOOOOOOOO
`oooooooooooooooo
`oooooooooooooooo
`oooooooooooooooo
`oooooooooooooooo
`oooooooooooooooo
`oooooooooooooooo
`
`WesternGeco Ex. 1004, pg. 5
`
`

`

`U.S. Patent
`
`Jul. 13, 1999
`
`Sheet 5 0f 8
`
`5,924,049
`
`—> OFFSET
`
`—--> OFFSET
`
`
`
`-<— TIME
`
`Fig. 12
`
`PANEL A
`
`Y
`PANEL B
`
`Fig. 14
`
`—>-
`
`OFFSET
`
`Fig. 13
`
`
`
`<— TIME
`
`
`
`<—— TIME
`
`WesternGeco Ex. 1004, pg. 6
`
`

`

`U.S. Patent
`
`Jul. 13,1999
`
`Sheet 6 of8
`
`5,924,049
`
`WesternGeco Ex. 1004, pg. 7
`
`

`

`U.S. Patent
`
`Jul. 13,1999
`
`Sheet 7 0f 8
`
`5,924,049
`
`SPATIAL
`OVERLAP
`ZONE
`_,_
`OFFSET ’—H
`
`SPATIAL OVERLAP ZONE.
`DATA WILL BE MERGED
`AT DASHED LINES. __
`Rh“ OFFSET
`
`TIME \
`
`
`
`-<— TIME
`
`<—
`
`I
`PANEL A
`Y
`PANEL B
`
`Fig. 17
`
`WesternGeco Ex. 1004, pg. 8
`
`

`

`U.S. Patent
`
`Jul. 13, 1999
`
`Sheet 8 0f8
`
`5,924,049
`
`Fig. 19
`
`@
`
`GEOMETRY
`ATTACH GEOMETRY
`
`ATTACH SOURCE /
`.lqg-IjriiI-coERseEoME-I-RY
`
`CMP SORT
`
`CMP SORT
`
`IIIIILEIERCE’
`
`TRACE INTERP
`
`INTERPOLATE INTERMEDIATE TRACES
`
`I—I——I
`
`TRACE SELECT TRACE SELECT
`OFFSET LIMIT
`OFFSET LIMIT
`
`LIMIT OFFSETS
`TO FIRST HALF
`0|: CABLE PLUS
`SOME EXTRA
`TRACES FOR AN
`OVERLAP ZONE.
`
`LIMIT TO FIRST
`HALF 0|: CABLE
`REJECT'NG
`OVERLAP ZONE-
`INTERPOLATED
`TRACE DELETE
`IS OPTIONAL.
`
`LIMIT OFFSETS
`TO LAST HALF
`OF CABLE PLUS
`> SOME EXTRA
`TRACES FOR AN
`OVERLAP ZONE.
`
`REJECT EVENTS
`COMING FROM
`SOURCE AT
`OTHER END OF
`CABLE
`
`LIMIT TO LAST
`HALF OF CABLE
`REJECTING
`OVERLAP ZONE.
`INTERPOLATED
`TRACE DELETE
`IS OPTIONAL.
`
`DIP REJECT
`DIP REJECT
`MULTICHANNEL MULTICHANNEL
`DIP REJECT OR DIP REJECT OR
`PASS
`PASS
`
`PANEL
`A
`
`PANEL
`B
`
`TRACE SELECT TRACE SELECT
`OFFSET LIMIT
`OFFSET LIMIT
`AND DELETE
`AND DELETE
`INTERPOLATED INTERPOLATED
`TRACES
`TRACES
`
`L__'__J
`
`MERGE
`MERGE PANELS TOGETHER
`
`i)
`
`WesternGeco Ex. 1004, pg. 9
`
`

`

`1
`METHODS FOR ACQUIRING AND
`PROCESSING SEISMIC DATA
`
`RELATED APPLICATION
`
`This is a continuation-in-part of US. application Ser. No.
`08/829,485 ?led Mar. 28, 1997, now US. Pat. No. 5,717,655
`Which is a continuation of US. application Ser. No. 08/423,
`781 ?led Apr. 18, 1995, noW abandoned, and Was
`co-pending therewith. US. application Ser. No. 08/829,486
`and US. application Ser. No. 08/423,781 are both entitled
`“A METHOD FOR PROVIDING UNIFORM SUBSUR
`FACE COVERAGE IN THE PRESENCE OF STEEP
`DIPS”, and both are co-oWned With the present invention
`and incorporated fully herein for all purposes.
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`The present invention, in certain aspects, is directed to
`seismic survey systems and methods in Which tWo or more
`seismic sources are ?red simultaneously, or signi?cantly
`close together temporally, but Which is, in one aspect,
`signi?cantly spatially separated, and resulting seismic data
`is processed meaningfully utiliZing data generated by both
`(or more) seismic sources.
`3-D marine seismic surveys entail toWing a sWath of
`elongated seismic sensor arrays. Subsea formations are
`acoustically illuminated to produce seismic re?ection data
`that are detected and processed by the arrays and associated
`ancillary equipment. In the presence of steeply-dipping
`subsea formation, this invention corrects the non-uniform
`illumination of the formations due to the backWard geometry
`caused by the steeply-dipping Wave?eld trajectories.
`2. Description of Related Art
`The prior art discloses seismic survey systems and meth
`ods employing tWo or more seismic sources ?ring simulta
`neously. In order to make meaningful use of resultant
`seismic data, each source is initially encoded differently [e.g.
`signals at different frequency bands or phases (orthogonal)]
`so that resulting seismic data contains a signature indicating
`to Which source the data is related. Such encoding requires
`corresponding decoding When processing the data. Often, in
`actual practice, the level of separation achievable is not
`satisfactory. Also, encoding is impractical for some source
`con?gurations.
`There has long been a need, noW recogniZed and
`addressed by the present invention, for seismic survey
`methods in Which multiple seismic sources ?ring simulta
`neously or temporally close together may be used effectively
`and ef?ciently. There has long been a need for such methods
`Which do not require individual encoding or other separate
`identi?cation of each of tWo or more seismic sources.
`In 3-D marine operations, a seismic ship toWs a sWath
`including a plurality of parallel seismic streamer cables
`along a desired line of survey, the cables being submerged
`by a feW meters beneath the Water surface. The number of
`cables that make up a sWath depends only on the mechanical
`and operational capabilities of the toWing ship. There may
`be six or more such cables, spaced about 50 to 100 meters
`apart. The respective cables may be up to 3000 meters long.
`Each streamer cable typically includes about 120 spaced
`apart seismic detector groups. Each group consists of one or
`more individual interconnected detectors, each of Which
`services a single data channel. The group spacing is on the
`order of 25 to 50 meters longitudinally along the cable. The
`seismic detectors are transducers that perceive the mechani
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`55
`
`60
`
`65
`
`5,924,049
`
`2
`cal activity due to re?ected acoustic Wave?elds and convert
`that activity to electrical signals having characteristics rep
`resentative of the intensity, timing and polarity of the
`acoustic activity as is Well knoWn to the art. The detectors
`are operatively coupled to data-storage and processing
`devices of any desired type.
`An acoustic source such as an array of air guns, is toWed
`in the Water by the ship near the leading end of the sWath of
`seismic streamer cables. As the ship proceeds along the line
`of survey, the source is ?red (activated) at selected spatial
`intervals equal, for example, to a multiple of the seismic
`detector group spacing, to acoustically illuminate (insonify)
`the subsurface formations. Assuming the ship travels at a
`constant velocity such as six knots, the source may be
`conveniently ?red at selected time intervals such as every
`?ve seconds, assuming a 50-meter group interval. The
`Wave?eld emitted by the source travels doWnWardly to be
`re?ected from subsea earth formations, Whence the Wave
`?eld is re?ected back to the Water surface Where the re?ected
`Wave?eld is received by the detectors and converted to
`electrical signals as previously explained. The detected
`electrical signals are transmitted to any Well-knoWn signal
`recording and processing means for providing a physical
`model of the subsurface.
`For a better understanding of a problem to be solved by
`this disclosure, FIG. 1 shoWs a source, S, at or near the
`surface 10 of the Water 12. Detectors Din, DHZ, Di+3 are
`disposed near the Water surface above a ?at-lying formation
`F. AWave?eld emitted from S folloWs the indicated ray paths
`to the respective detectors as shoWn. For example, the ray
`path from S to Di+3 is re?ected from incident point IP on
`formation F. The incident angle 4),, relative to the perpen
`dicular to F at IP or Zero-offset point Z, must equal the angle
`of re?ection q), as in geometric optics, assuming the earth
`material is isotropic. The surface expression of the subsur
`face re?ection point, R, the midpoint betWeen S and Dig, M,
`and the Zero offset point, Z, are coincident. The incident
`points of all of the raypaths are evenly distributed along the
`line as shoWn.
`In regions of steep dip, the symmetrical picture of FIG. 1
`is distorted as shoWn in the 2-D illustration of FIG. 2. are,
`With a dip of 45°, While the angles of incidence and
`re?ection q), and q), are equal, the Zero-offset point Z, is
`up-dip of the midpoint M. The surface expression R, of the
`re?ection point (incident point IP) lies not betWeen the
`source and detector as in FIG. 1, but up-dip of the source S.
`FIG. 3 traces a number of raypaths from a source S to
`detectors Di_1, Din, DMZ, Dig, Dim for a 45°-dipping bed
`F. The important point to observe in this Figure is the
`non-uniform spacing of the incident points. Because reci
`procity holds, assuming that the earth materials are isotropic,
`the source and detectors can be interchanged. It is thus
`evident that When shooting doWn-dip, the incident points
`tend to bunch up. Shooting up-dip results in a spreading
`apart of the incident points. Because of the complex non
`uniform subsurface illumination, signi?cant undesirable
`shadoW Zones are formed. The problem becomes particu
`larly troublesome Where multiple cables are used in a 3-D
`sWath, due to the additional aWkWard lateral geometry.
`One method for minimiZing shadoW Zones is taught by C.
`Beasley (co-inventor in the present invention) in US. patent
`application Ser. No. 08/069,565 ?led May 28, 1993, entitled,
`“Quality Assurance for Spatial Sampling for DMO”,
`assigned to the assignee of this invention and issued Sep. 12,
`1995 as US. Pat. No. 5,450,370 Which is incorporated fully
`herein for all purposes. That application is the basis for a
`
`WesternGeco Ex. 1004, pg. 10
`
`

`

`5,924,049
`
`3
`paper delivered in 1993 at the 63rd Annual meeting of the
`Society of Exploration Geophysicists and published in
`Expanded Abstracts, pp. 544—547. That invention provided
`a method for examining the geometry of the disposition of
`a plurality of sources and receivers over an area to be
`surveyed With a vieW to optimiZing the array to avoid
`shadoW Zones in the data and to optimiZe the resulting
`seismic image. The method depends upon studying the
`statistical distribution of dip polarity in dip bins along
`selected CMP aZimuths. The method Was implemented by
`rearranging the geometrical disposition of the sources and
`receivers. It Was not directed to the per se problem of
`non-uniform subsurface coverage and shadoW Zones in the
`presence of steep dips.
`Another discussion directed to symmetric sampling is
`found in a paper entitled, “3-D Symmetric Sampling” by G.
`Vermeer, and delivered in 1994 in a paper at the 64th Annual
`Meeting of the Society of Exploration Geophysicists,
`Expanded Abstracts, pp 906—909. Here, the authors revieW
`the various different shooting geometries involved in land
`and marine surveys including 2-D, 3-D and 5-D con?gura
`tions. The presence of non-uniform subsurface insoni?ca
`tion is recogniZed and the need for symmetric sampling to
`prevent aliasing is emphasiZed.
`M. S. Egan et al., in a paper entitled, “Shooting Direction:
`a 3-D Marine Survey Design Issue”, published in The
`Leading Edge, November, 1991, pp 37—41 insists that it is
`important to maintain consistent source-to-receiver trajec
`tory aZimuths to minimiZe shadoW Zones, imaging artifacts
`and aliasing in regions of steep dips. They are particularly
`concerned about 3-D marine surveys in areas Where the
`proposed seismic lines are obstructed by shipping, offshore
`structures and other cultural obstacles.
`There is a need for equalizing the density of the subsur
`face coverage provided by Wide, toWed sWaths of seismic
`streamer arrays in the presence of steeply-dipping earth
`formations in the circumstance Where the acoustic source is
`located at an end of the sWath.
`
`SUMMARY OF THE INVENTION
`
`This method may be applied to any form of seismic
`operation, be it on land or on sea. HoWever for convenience,
`by Way of example but not by Way of limitation, certain
`disclosures are explained in terms of a marine seismic
`survey.
`The present invention, in certain aspects, discloses a
`seismic survey system for use at sea or on land With tWo,
`three, four, or more seismic sources (or one source moved
`form one location to another and ?red at multiple locations)
`for generating an acoustic Wave?eld (e.g., but not limited to,
`acoustic sources, e.g. air guns); a plurality of spaced-apart
`seismic detectors for discrete sampling of the acoustic
`Wave?eld re?ected and/or refracted from earth layers (e.g.,
`but not limited to geophones or hydrophones); and, at sea, a
`vessel or vessels for carrying or toWing the seismic sources
`and, in one aspect, the detectors. In one aspect, the seismic
`sources are activated simultaneously at a knoWn location
`With the seismic sensors at a knoWn location. In another
`aspect, the seismic sensors are activated over a relatively
`short time period, e.g., but not limited to, Within 25 seconds
`and preferably Within 15 seconds. In one aspect, the seismic
`sources’ signals are “plain,” e.g. they bear no encoding or
`individual identifying signature. In another aspect, methods
`according to the invention are used With encoded signals.
`Resultant seismic Wave?elds (e.g. resulting from re?ec
`tion and/or refraction from sub-surface strata) are sensed as
`
`4
`seismic data and transmitted from the seismic sensors to
`knoWn apparatus for receiving, storing, transmitting, and/or
`processing such data (signals). In one aspect, each seismic
`sensor senses, from an earth layer, a part of a resulting
`acoustic Wave?eld generated by each seismic source.
`The resulting seismic data contains re?ections,
`refractions, etc., due to each source and is processed to
`separately distinguish data related to each source. For
`example, in one method according to the present invention,
`seismic data from a marine streamer geometry With tWo
`sources ?ring simultaneously off of both ends of a single
`streamer cable is recorded onto a single shot record. The shot
`record contains information from both sources and the
`record is processed tWice. With tWo passes through the
`process the information from each particular source is
`separated from the signal from the other source. To separate
`the sources’ data, the record is updated With one source’s
`geometry information (e.g. x, y location coordinates and
`time of day identi?ers, e.g. SEG standard format
`information, are attached to the seismic data traces by
`knoWn methods, e.g. a header With the desired information
`is applied to a trace tape); optionally sorted to order, e.g. by
`knoWn common mid-point (CMP) sorting methods or
`knoWn methods such as common shot order, common detec
`tor order or common offset order and/or combinations
`thereof; optionally trace interpolated to theoretically pro
`duce a Well-sampled curve betWeen knoWn data points by
`knoWn methods, and spatially paneled, i.e., a portion of the
`data is isolated that includes data from both sources. Each
`panel of data is then dip ?ltered by knoWn methods to
`remove the effects of the signal from the other source. The
`panels are then merged together producing the seismic data
`related to only one of the seismic sources. The interpolated
`traces are removed if created. The process is then re-done
`With the attachment of the other source’s geometry produc
`ing the seismic data related to the other seismic source. After
`the tWo passes, there Will be tWice as many shot records than
`before the process; e.g. for tWo sources and one initial shot
`record, tWo data records are produced; or, in other aspects,
`multiple shots, e.g. three, four, ?ve, etc. or more.
`In an aspect of this invention, there is provided a method
`for providing a more uniform insoni?cation of subsurface
`earth formations for the purpose of minimiZing shadoW
`Zones. To that end, a sWath of parallel, elongated seismic
`cables, each including a plurality of spaced-apart seismic
`detectors, are advanced along a line of survey. A ?rst
`acoustic source is positioned near the leading end of the
`sWath and a second acoustic source is located near the
`trailing end of the sWath. At alternate timed intervals,
`substantially simultaneously, or Within at least 25 seconds of
`each other and, in one aspect, Within at least 15 seconds of
`each other, the sources launch a Wave?eld that is re?ected
`from the subsurface earth formations to provide ?rst and
`second seismic-signal data sets. Means, operatively coupled
`to the detectors, process and merge the ?rst and second data
`sets to provide a uniformly-insoni?ed model of the subsur
`face earth formations substantially free of shadoW Zones.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`The novel features Which are believed to be characteristic
`of the invention, both as to organiZation and methods of
`operation, together With the objects and advantages thereof,
`Will be better understood from the folloWing detailed
`description and the draWings Wherein the invention is illus
`trated by Way of example for the purpose of illustration and
`description only and are not intended as a de?nition of the
`limits of the invention:
`
`10
`
`15
`
`25
`
`35
`
`45
`
`55
`
`65
`
`WesternGeco Ex. 1004, pg. 11
`
`

`

`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`5
`FIG. 1 shows acoustic raypaths in the presence of Zero
`dip;
`FIG. 2 provides de?nitions for certain data-processing
`terms;
`FIG. 3 demonstrates the non-uniform insoni?cation of the
`subsurface in the presence of steep dips;
`FIG. 4 is a plan vieW of the con?guration of a typical
`sWath of cables and associated acoustic sources such as may
`be used in 3-D marine seismic surveying;
`FIG. 5 shoWs the surface expression of subsurface re?ec
`tion points and the shadoW Zones associated With steep dips
`With respect to a sWath Where the source is positioned near
`the leading end of the sWath;
`FIG. 6 shoWs the surface expression of subsurface re?ec
`tion points and the shadoW Zones associated With steep dips
`With respect to a sWath Wherein the source is located near the
`trailing end of the sWath;
`FIG. 7 is a schematic ?oW diagram of the data processing
`method; and
`FIG. 8 is a timing diagram for controlling the activation
`sequence of the acoustic sources.
`FIG. 9 is a schematic representation of a prior art marine
`seismic streamer system useful in methods according to the
`present invention.
`FIG. 10 shoWs schematically a prior art marine seismic
`streamer system useful in methods according to the present
`invention.
`FIG. 11 is a schematic representation of a land-based
`seismic system used in methods according to the present
`invention.
`FIG. 12 is a graphical representation of a marine streamer
`shot record produced With one seismic source by methods
`according to the present invention. The vertical axis is a time
`axis With time increasing from top to bottom. The horiZontal
`axis is an “offset” axis (distance from a seismic source to a
`seismic detector) With distance increasing from left to right.
`These axes are the same in FIGS. 13—17.
`FIG. 13 is a graphical representation of a marine streamer
`shot record produced With tWo seismic sources by methods
`according to the present invention. For a source to the right,
`the offset distance increases from right to left in this ?gure.
`FIG. 14 is a graphical representation of data as in FIG. 13
`shoWing the selection of tWo overlapping data sets, Panel A
`45
`and Panel B, the data sets based on offset increasing from
`left to right (for the left source only).
`FIG. 15 is a graphical representation of ?ltering of the
`data of Panel A(of FIG. 14) to reject events (data) resulting
`from one of tWo seismic sources.
`FIG. 16 is a graphical representation of ?ltering of the
`data of Panel B (of FIG. 14) to reject events (data) resulting
`from one of tWo seismic sources.
`FIG. 17 is a graphical representation of the data resulting
`from the ?ltering of Panels A and B.
`FIG. 18 is a graphical representation of the data resulting
`from the ?ltering of Panels A and B With left source data
`rejected.
`FIG. 19 is a schematic diagram of a method according to
`the present invention.
`
`50
`
`55
`
`60
`
`DETAILED DESCRIPTION OF THE
`EMBODIMENTS PREFERRED AT THE TIME
`OF FILING FOR THIS PATENT
`
`65
`
`Please refer noW to FIGS. 3 and 4. FIG. 4 is a plan vieW
`of a 3-D sWath 13 of six parallel seismic cable arrays A1—A6
`
`5,924,049
`
`6
`Which are being toWed through a body of Water by a ship 14.
`(It should be understood that, if land operations are under
`consideration, the cables could, according to the present
`invention, be toWed by one or more trucks or could be laid
`out by cable trucks using roll-along techniques in a manner
`Well-knoWn to the seismic industry.) Signals from the
`respective cable arrays A1—A6 are fed over a data-signal
`manifold 20 to a processing means 22 of any Well-knoWn
`type, installed on ship 14 and operatively coupled to means
`22 by electrical lead-ins 16 and 18. A discrete acoustic
`source SL is toWed by ship 14 near the leading end of sWath
`13, substantially at the center of the sWath. More than one
`discrete source such as SL‘ and SL“, offset from the center
`line may be used if desired.
`Dashed line M3 is a line of midpoints that might be
`associated With seismic cable A3 positioned toWards the
`center of the sWath such as suggested by FIG. 3 for a 2-D
`slice of the earth Where it Was shoWn that the subsurface
`re?ection points tend to converge When shooting doWn-dip.
`In the case of a 3-D operation, employing the sWath of FIG.
`4, the laterally-distributed, crossline lines of midpoints cor
`responding to detector cables A2 and A1 are shoWn as
`dashed lines M2 and M1. Similar lines (not shoWn) may be
`draWn for cables A4—A6.
`FIG. 5 shoWs, as small rectangles, the surface expression
`of steeply-dipping subsurface re?ection points for every
`12th detector of a 120-detector sWath of six cables repre
`sented as straight, evenly-spaced, horiZontal lines A1—A6.
`With the cables spaced 100 meters apart, the solid lines
`represent the lines of midpoints for the respective cables and
`are 50 meters apart, each cable being 3000 meters long. The
`source SL is at the leading or left hand end of the sWath;
`up-dip and direction of advance of the ship are to the left. As
`Would be expected from FIG. 3, the re?ection points tend to
`converge doWn-dip along the inline direction. Crossline, the
`subsurface re?ection points do not stray far from the inner
`central-cable midpoint lines M3 and M4. But the subsurface
`re?ection points for the outer midpoint lines M1, M2, M5
`and M6, corresponding to cable A1, A2, A5 and A6 converge
`toWards the center line of the sWath 13 by 25 to 30 meters,
`creating doWn-dip crossline shadoW Zones marked by the
`arroWs 27 and 29 at the right hand end of the sWath 13.
`Under conventional practice, to ?ll in the shadoW Zones,
`the operator Would be obliged to resurvey the region by
`making a second pass over the region. That process is
`decidedly uneconomical.
`Please refer noW to FIGS. 4 and 6. A second ship 24,
`toWing an acoustic source ST launches a Wave?eld from the
`trailing end of sWath 13. Here, also, more than one discrete
`source such as ST‘ and ST“ may be used. FIG. 6 shoWs the
`subsurface re?ecting points (small rectangles as for FIG. 5)
`associated With every 12th detector for sWath 13 When
`source ST is actuated. As before, the straight horiZontal lines
`M1—M6 represent the midpoint lines that make up sWath 13.
`Here again, the subsurface re?ection points for the tWo
`middle lines M3 and M4 are nearly coincident With the
`midpoint lines although signi?cant up-dip in-line and
`crossline divergence is present. Crossline, the subsurface
`re?ection points diverge Well outside the lateral limits of the
`sWath as demarcated by lines 23 and 25, leaving a non
`uniformly insoni?ed up-dip Zone as indicated by arroWs 31
`and 33.
`Comparison of FIGS. 5 and 6 shoW that the crossline
`subsurface coverage provided by the innermost cables A3
`and A4 does not depart very much from the line of midpoints
`regardless of the source location With respect to the leading
`
`WesternGeco Ex. 1004, pg. 12
`
`

`

`10
`
`15
`
`35
`
`7
`or trailing end of the swath. But FIGS. 5 and 6 suggest that
`by insonifying the sWath from both ends in alternate cycles,
`the gaps due to non-uniform insoni?cation at the outer
`crossline sWath limits, created by single-ended source
`activation, can be virtually eliminated When the resulting
`data are properly processed and merged. By this teaching, a
`model of the subsurface earth formations results, With the
`shadoW Zones ?lled in completely, as may be seen readily by
`superimposing (merging) FIG. 5 over FIG. 6. The proposed
`method is therefore an economic alternative to a resurvey
`operation that Was previously required.
`It might be suggested that a single acoustic source could
`be positioned at the geometric center of sWath 13 such that
`a single activation of a source Would produce both an up-dip
`and a doWn-dip component such as provided by a conven
`tional split-spread. That process is useful With single cables
`or Widely-spaced dual cables. But for large-scale 3-D sWaths
`or patches that use many closely-spaced cables, that proce
`dure is impractical. The physical con?guration of the cables
`cannot be accurately controlled Within the required tolerance
`in actual operation nor could a ship, Which itself may be 20
`meters Wide, be safely stationed in the middle of the sWath
`Without causing cable damage.
`In the presently-contemplated best mode of operation, the
`sWath 13 of parallel elongated seismic cables is effectively
`advanced along a desired line of survey either physically as
`by toWing or by use of Well-known roll-along methods. A
`?rst acoustic source (or sources), SL is located near the
`leading end of the sWath. A second acoustic source (or
`sources) ST is positioned near the trailing end of sWath 13.
`The ?rst and second sources are activated at timed intervals
`in alternate cycles to provide ?rst and second re?ected
`Wave?elds. The re?ected Wave?elds are detected and con
`verted to ?rst and second data sets of re?ected signals. The
`?rst and second data sets of electrical signals are processed
`and merged as indicated in the How diagram of FIG. 7, to be
`described later, to provide uniformly-insoni?ed subsurface
`re?ection points along the line of survey. Preferably, the
`sWath is advanced along the line of survey at a constant
`velocity. The lengths of the ?rst and second timed intervals
`are substantially constant and designed to alloW the sWath to
`advance spatially, at the selected velocity of advance, by
`some desired multiple of the spacing betWeen detector
`groups in the seismic cables.
`In the event that several discrete acoustic sources are used
`at each end of the sWath, such as SL, SL‘, SL“ and ST, ST‘,
`ST“, the sources may be activated in some desired alternat
`ing sequence such as SL-S