United States Patent [19J
`Beasley et al.
`
`111111
`
`1111111111111111111111111111111111111111111111111111111111111
`US005924049A
`[11] Patent Number:
`[45] Date of Patent:
`
`5,924,049
`Jul. 13, 1999
`
`METHODS FOR ACQUIRING AND
`PROCESSING SEISMIC DATA
`
`5,677,892 10/1997 Gulunay et a!. .......................... 367/38
`5,717,655
`2/1998 Beasley ..................................... 367/53
`
`[54]
`
`[75]
`
`Inventors: Craig J. Beasley; Ronald E.
`Chambers, both of Houston, Tex.
`
`[73]
`
`Assignee: Western Atlas International, Inc.
`
`[21]
`
`Appl. No.: 09/016,679
`
`[22]
`
`Filed:
`
`Jan. 30, 1998
`
`[63]
`
`[51]
`[52]
`[58]
`
`[56]
`
`Related U.S. 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
`U.S. Cl. ................................................. 702/17; 367/56
`Field of Search .................................. 702/17, 16, 14;
`367/23, 53, 56, 57, 50
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`2,897,476
`3,290,644
`3,496,532
`3,602,878
`3,687,218
`3,985,199
`4,042,906
`4,159,463
`4,224,474
`4,300,653
`4,486,864
`4,797,861
`4,823,326
`4,914,636
`4,930,110
`4,953,657
`4,970,696
`4,982,374
`5,430,689
`5,450,370
`
`10/1959 Widess ...................................... 702/17
`12/1966 Hoskins .................................... 367/21
`2/1970 Thigpen .
`8/1971 Sullivan .................................. 340/7 R
`8/1972 Ritter ...................................... 181!107
`10/1976 Baird ....................................... 181!107
`8/1977 Ezell ................................ 340/15.5 TS
`6/1979 Silverman ................................. 367/59
`9/1980 Savit .. ... ... ... ... .... ... ... ... ... ... .... ... . 370/68
`11/1981 Cao et a!.
`............................... 181!107
`12/1984 Ongkiehong eta!. .................... 367/23
`1!1989 Beasley ..................................... 367/50
`4/1989 Ward ......................................... 702/14
`4/1990 Garrotta eta!. .......................... 367/56
`5/1990 Bremner eta!. .......................... 367/56
`9/1990 Edington ................................. 181!111
`11/1990 Crews eta!. .............................. 367/56
`1!1991 Edington eta!. ......................... 367/48
`7/1995 Rigsby eta!. ............................ 367/56
`9/1995 Beasley eta!. ........................... 367/53
`
`1/0
`
`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.O., Seismic Acquisition 3:3-D, Data Acqui(cid:173)
`sition, 64th Annual Meeting of the Society of Exploration
`Geophysicists, published
`in Expanded Abstracts, pp.
`906-909, 1994.
`Egan, MarkS.; 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
`fired 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, (e.g. to CMP order), interpolating data points for
`discontinuities on the traces, selecting two halves or two
`portions slightly more than half of the traces, filtering the
`trace data for each of the two portions to filter out data
`related to a second one of the two seismic sources, reducing
`the filtered trace data to two halves of the data and deleting
`interpolated data, and then merging the two halves to
`produce refined useful seismic data related to a first one of
`the seismic sources. In one aspect the method includes
`re-processing the data and filtering out the trace data for the
`second seismic source to produce refined useful seismic data
`related to the second seismic source. In one aspect the
`sources are fired temporally close together and, in one
`particular aspect, they are fired substantially simultaneously.
`
`42 Claims, 8 Drawing Sheets
`
`MERGE PANELS TOGETHER
`
`1/0
`
`PGS Exhibit 1005
`PGS v. WG
`
`

`
`U.S. Patent
`
`Jul. 13, 1999
`
`Sheet 1 of 8
`
`5,924,049
`
`IP
`
`Fig. 1
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`PGS Exhibit 1005
`PGS v. WG
`
`

`
`U.S. Patent
`
`Jul. 13, 1999
`
`Sheet 3 of 8
`
`5,924,049
`
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`PGS Exhibit 1005
`PGS v. WG
`
`

`
`U.S. Patent
`
`Jul. 13, 1999
`
`Sheet 4 of 8
`
`5,924,049
`
`10 106
`
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`
`PGS Exhibit 1005
`PGS v. WG
`
`

`
`U.S. Patent
`
`Jul. 13, 1999
`
`Sheet 5 of 8
`
`5,924,049
`
`-
`
`OFFSET
`
`-
`
`OFFSET
`
`w
`::2:
`1-
`
`w
`::2:
`1-
`
`Fig. 12
`
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`v---_____,
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`
`Fig. 14
`
`-
`
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`
`-
`
`Fig. 13
`
`PGS Exhibit 1005
`PGS v. WG
`
`

`
`U.S. Patent
`
`Jul. 13, 1999
`
`Sheet 6 of 8
`
`5,924,049
`
`-OFFSET
`
`-OFFSET
`
`w
`~
`1-
`
`Fig. 15
`
`PANEL A
`
`PANEL A
`
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`
`Fig. 16
`
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`
`PANEL B
`
`PANEL B
`
`PGS Exhibit 1005
`PGS v. WG
`
`

`
`U.S. Patent
`
`Jul. 13, 1999
`
`Sheet 7 of 8
`
`5,924,049
`
`SPATIAL
`OVERLAP
`ZONE
`----
`OFFSET ~
`
`SPATIAL OVERLAP ZONE.
`DATA WILL BE MERGED
`AT DASHED LINES. - (cid:173)
`~ OFFSET
`
`w
`~
`1-
`
`~ -----~)
`PANELA
`V
`PANEL B
`
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`PANELA
`V
`PANEL B
`
`Fig. 17
`
`Fig. 18
`
`PGS Exhibit 1005
`PGS v. WG
`
`

`
`U.S. Patent
`
`Jul. 13, 1999
`
`Sheet 8 of 8
`
`5,924,049
`
`Fig. 1
`9
`
`)
`
`1/0
`I
`GEOMETRY
`ATTACH GEOMETRY
`I
`CMP SORT
`CMP SORT
`I
`TRACEINTERP
`
`CH SOURCE I
`} ATTA
`CTOR GEOMETRY
`DETE
`TOT
`RACES
`
`} CMP SORT
`URGE/
`WITHIN SO
`E
`CABLE LIN
`
`INTERPOLATE INTERMEDIATE TRACES
`I
`
`LIMIT OFFSETS
`TO FIRST HALF
`OF CABLE PLUS
`SOME EXTRA
`TRACES FOR AN
`OVERLAP ZONE.
`
`{
`
`I
`TRACE SELECT
`
`OFFSET LIMIT
`I
`I
`DIP REJECT
`
`I
`TRACE SELECT
`
`OFFSET LIMIT
`I
`I
`DIP REJECT
`
`MULTICHANNEL MULTICHANNEL
`DIP REJECT OR DIP REJECT OR
`PASS
`PASS
`
`I
`I
`PANEL
`A
`
`I
`I
`PANEL
`B
`
`TRACE SELECT
`
`TRACE SELECT
`
`OFFSET LIMIT
`AND DELETE
`INTERPOLATED
`TRACES
`I
`
`OFFSET LIMIT
`AND DELETE
`INTERPOLATED
`TRACES
`I
`
`I
`MERGE
`
`MERGE PANELS TOGETHER
`I
`( 1/0
`
`)
`
`LIMIT TO FIRST
`HALF OF CABLE
`REJECTING
`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.
`
`-'
`
`PGS Exhibit 1005
`PGS v. WG
`
`

`
`5,924,049
`
`1
`METHODS FOR ACQUIRING AND
`PROCESSING SEISMIC DATA
`
`RELATED APPLICATION
`
`This is a continuation-in-part of U.S. application Ser. No.
`08/829,485 filed Mar. 28, 1997, now U.S. Pat. No. 5,717,655
`which is a continuation of U.S. application Ser. No. 08/423,
`781 filed Apr. 18, 1995, now abandoned, and was
`co-pending therewith. U.S. application Ser. No. 08/829,486
`and U.S. application Ser. No. 08/423,781 are both entitled 10
`"A METHOD FOR PROVIDING UNIFORM SUBSUR(cid:173)
`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
`
`2
`cal activity due to reflected acoustic wavefields and convert
`that activity to electrical signals having characteristics rep(cid:173)
`resentative of the intensity, timing and polarity of the
`acoustic activity as is well known to the art. The detectors
`5 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 fired (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
`15 conveniently fired at selected time intervals such as every
`five seconds, assuming a 50-meter group interval. The
`wavefield emitted by the source travels downwardly to be
`reflected from subsea earth formations, whence the wave-
`field is reflected back to the water surface where the reflected
`wavefield 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 Di+l' D;+2 , D;+3 are
`disposed near the water surface above a fiat-lying formation
`F. A wave field emitted from S follows the indicated ray paths
`to the respective detectors as shown. For example, the ray
`path from S to D;+3 is reflected from incident point IP on
`formation F. The incident angle <!J;, relative to the perpen(cid:173)
`dicular to F at IP or zero-offset point Z, must equal the angle
`of reflection <Pr as in geometric optics, assuming the earth
`35 material is isotropic. The surface expression of the subsur(cid:173)
`face reflection point, R, the midpoint between Sand D;+3 , 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
`reflection <jl; and <Pr are equal, the zero-offset point Z, is
`45 up-dip of the midpoint M. The surface expression R, of the
`reflection point (incident point IP) lies not between the
`source and detector as in FIG. 1, but up-dip of the sourceS.
`FIG. 3 traces a number of raypaths from a source S to
`detectors D;_ 1 , D;+1 , D;+2 , D;+3 , Di+n 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
`55 tend to bunch up. Shooting up-dip results in a spreading(cid:173)
`apart of the incident points. Because of the complex non(cid:173)
`uniform subsurface illumination, significant undesirable
`shadow zones are formed. The problem becomes particu(cid:173)
`larly troublesome where multiple cables are used in a 3-D
`60 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 U.S. patent
`application Ser. No. 08/069,565 filed May 28, 1993, entitled,
`"Quality Assurance for Spatial Sampling for DMO",
`assigned to the assignee of this invention and issued Sep. 12,
`1995 as U.S. Pat. No. 5,450,370 which is incorporated fully
`herein for all purposes. That application is the basis for a
`
`25
`
`1. Field of the Invention
`The present invention, in certain aspects, is directed to
`seismic survey systems and methods in which two or more 20
`seismic sources are fired simultaneously, or significantly
`close together temporally, but which is, in one aspect,
`significantly 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 reflection data
`that are detected and processed by the arrays and associated
`ancillary equipment. In the presence of steeply-dipping 30
`subsea formation, this invention corrects the non-uniform
`illumination of the formations due to the backward geometry
`caused by the steeply-dipping wavefield trajectories.
`2. Description of Related Art
`The prior art discloses seismic survey systems and meth(cid:173)
`ods employing two or more seismic sources firing simulta(cid:173)
`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
`configurations.
`There has long been a need, now recognized and
`addressed by the present invention, for seismic survey
`methods in which multiple seismic sources firing simulta(cid:173)
`neously or temporally close together may be used effectively 50
`and efficiently. There has long been a need for such methods
`which do not require individual encoding or other separate
`identification 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(cid:173)
`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 65
`order of 25 to 50 meters longitudinally along the cable. The
`seismic detectors are transducers that perceive the mechani-
`
`40
`
`PGS Exhibit 1005
`PGS v. WG
`
`

`
`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 5
`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 10
`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 configura(cid:173)
`tions. The presence of non-uniform subsurface insonifica(cid:173)
`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(cid:173)
`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(cid:173)
`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 m 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 fired at multiple locations)
`for generating an acoustic wave field (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 field reflected 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 60
`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 wavefields (e.g. resulting from reflec(cid:173)
`tion and/or refraction from sub-surface strata) are sensed as
`
`5,924,049
`
`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 wavefield generated by each seismic source.
`The resulting seismic data contains reflections,
`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 firing 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
`15 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 identifiers, e.g. SEG standard format
`20 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-
`25 tor order or common offset order and/or combinations
`thereof; optionally trace interpolated to theoretically pro(cid:173)
`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
`30 panel of data is then dip filtered 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
`35 with the attachment of the other source's geometry produc(cid:173)
`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,
`40 multiple shots, e.g. three, four, five, etc. or more.
`In an aspect of this invention, there is provided a method
`for providing a more uniform insonification of subsurface
`earth formations for the purpose of minimizing shadow
`zones. To that end, a swath of parallel, elongated seismic
`45 cables, each including a plurality of spaced-apart seismic
`detectors, are advanced along a line of survey. A first
`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,
`50 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 wavefield that is reflected
`from the subsurface earth formations to provide first and
`second seismic-signal data sets. Means, operatively coupled
`55 to the detectors, process and merge the first and second data
`sets to provide a uniformly-insonified model of the subsur(cid:173)
`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-
`65 trated by way of example for the purpose of illustration and
`description only and are not intended as a definition of the
`limits of the invention:
`
`PGS Exhibit 1005
`PGS v. WG
`
`

`
`5,924,049
`
`5
`FIG. 1 shows acoustic raypaths in the presence of zero
`dip;
`FIG. 2 provides definitions for certain data-processing
`terms;
`FIG. 3 demonstrates the non-uniform insonification of the 5
`subsurface in the presence of steep dips;
`FIG. 4 is a plan view of the configuration 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 reflec(cid:173)
`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 reflec- 15
`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 flow 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 figure.
`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 filtering 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 filtering 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 filtering of Panels A and B.
`FIG. 18 is a graphical representation of the data resulting
`from the filtering of Panels A and B with left source data
`rejected.
`FIG. 19 is a schematic diagram of a method according to 60
`the present invention.
`
`50
`
`DETAILED DESCRIPTION OF 1HE
`EMBODIMENTS PREFERRED AT THE TIME
`OF FILING FOR THIS PATENT
`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
`
`25
`
`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
`10 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
`reflection points tend to converge when shooting down-dip.
`20 In the case of a 3-D operation, employing the swath of FIG.
`4, the laterally-distributed, crossline lines of midpoints cor(cid:173)
`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 reflection points for every
`12th detector of a 120-detector swath of six cables repre(cid:173)
`sented as straight, evenly-spaced, horizontal lines A1-A6.
`With the cables spaced 100 meters apart, the solid lines
`30 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 reflection points tend to
`35 converge down-dip along the inline direction. Crossline, the
`subsurface reflection points do not stray far from the inner
`central-cable midpoint lines M3 and M4. But the subsurface
`reflection points for the outer midpoint lines M1, M2, M5
`and M6, corresponding to cable A1, A2, AS and A6 converge
`40 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 fill 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 wavefield 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 reflecting 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
`55 M1-M6 represent the midpoint lines that make up swath 13.
`Here again, the subsurface reflection points for the two
`middle lines M3 and M4 are nearly coincident with the
`midpoint lines although significant up-dip in-line and
`crossline divergence is present. Crossline, the subsurface
`reflection points diverge well outside the lateral limits of the
`swath as demarcated by lines 23 and 25, leaving a non-
`uniformly insonified up-dip zone as indicated by arrows 31
`and 33.
`Comparison of FIGS. 5 and 6 show that the crossline
`65 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
`
`PGS Exhibit 1005
`PGS v. WG
`
`

`
`5,924,049
`
`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 insonification at the outer
`crossline swath limits, created by single-ended source
`activation, can be virtually eliminated when the resulting 5
`data are properly processed and merged. By this teaching, a
`model of the subsurface earth formations results, with the
`shadow zones filled 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 10
`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(cid:173)
`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(cid:173)
`dure is impractical. The physical configuration 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
`first 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 first and second sources are activated at timed intervals
`in alternate cycles to provide first and second reflected
`wavefields. The reflected wavefields are detected and con(cid:173)
`