United States Patent
`
`[191
`
`[11] Patent Number:
`
`4,823,326
`
`Ward
`
`SEISMIC DATA ACQUISITION TECHNIQUE
`HAVING SUPERPOSED SIGNALS
`
`Inventor:
`
`Assignee:
`
`Roger M. Ward, Carrollton, Tex.
`
`The Standard Oil Company,
`Cleveland, Ohio
`
`Appl. No.:
`Filed:
`
`65,557
`
`Jim. 23, 1987
`
`Related US. Application Data
`Continuation-in-part of Ser. No. 887,377, Jul. 21, 1986,
`abandoned, which is a continuation-in-part of Ser. No.
`813,593, Dec. 26, 1985, abandoned.
`
`[54]
`
`[75]
`
`[73]
`
`[21]
`
`[22]
`
`[63]
`
`[51]
`[521
`
`[53]
`
`[56]
`
`[45] Date of Patent:
`
`Apr. 18, 1989
`
`3,984,805 10/1976 Silverman.
`4,042,910
`s/1977 Rietsch.
`4,064,481 12/1977 Silverman .
`4,159,463
`6/1979 Silverman ... . .
`..... 367/59
`
`4,153,485
`9/1979 Payton et al..
`367/41
`4,295,213 10/1981 Mifsud ................................ .. 367/41
`
`Primaiy Examiner—Thomas H. Tarcza
`Assistant Examiner—Da.nie1 T. Pihulic
`Attorney, Agent, or Firm—Raymond F. Keller; David J.
`Untener; Larry W. Evans
`
`[57]
`
`ABSTRACT
`
`Vibrator seismic sources are operated concurrently to
`reduce the time required for acquiring the data for a
`seismic survey. Separate seismic records are produced
`for each of the concurrently operated vibrator seismic
`sources. Frequency sweeping pilot signals are gener-
`ated for each of the separate vibrator seismic sources
`concurrently in a sequence of at least four sweeps. The
`pilot signals for each vibrator source have a plurality of
`separate phase angles during the sweeps. The seismic
`signals produced by the concurrently operated vibrator
`seismic sources are detected for each of the sweeps. The
`detected seismic signals are correlated separately with
`each of the corresponding pilot signals which were used
`to drive the vibrator seismic sources that produced the
`detected seismic signal. The correlating process pro-
`duces first and second separate correlated records for
`each of the sweeps. The first correlated records from
`each sweep are summed to produce a first seismic re-
`cord which is derived only from a first of the concur-
`rently operated vibrator seismic sources. The second
`correlated records from the sweeps are summed to
`produce a second seismic record which is derived only
`from the second of the vibrator seismic sources. The
`harmonic distortion caused by the vibrator seismic
`sources is further reduced by having a plurality of uni-
`form phase differences for each of the pilot signals
`within each of the pilot sweeps for the vibrator sources.
`
`17 Claims, 2 Drawing Sheets
`
`Int. cu ............................................ .. G0lV 1/143
`U.S. Cl. ....................................... 367/41; 367/23;
`364/421
`Field of Search .................................... 367/38-42,
`367/50, 51, 59; 364/421
`References Cited
`U.S. PATENT DOCUMENTS
`1/1960
`4/1961
`6/1961
`5/1965
`11/1965
`7/1966
`11/1966
`6/1967
`7/1967
`7/1967
`1/1968
`4/1968
`6/1968
`6/1970
`8/1970
`5/1971
`1/1974
`2/1974
`5/1975
`5/1975
`7/1975
`
`2,920,306
`2,981,928
`2,989,726
`3,185,958
`3,221,298
`3,259,878
`3,288,243
`3,326,320
`3,332.51 1
`3,332,512
`3,363,720
`3,375,896
`3,386,525
`3,516,510
`3,523,277
`3,578,102
`3,786,409
`3,789.95 1
`3,883,725
`3,885,225
`3,895,343
`
`.
`
`.
`
`.
`.
`
`.
`
`.
`
`.
`Feagin et al.
`Crawford et al.
`Crawford et al.
`Masterson et a1.
`Burns .
`Mifsud .
`Silverman .
`Forester .
`Silverriian ............................. 367/59
`Sundt .
`Mifsud ct al.
`Beddo .
`Landrum et al.
`Cobum et al.
`.
`Landrum, Jr.
`.
`Ross etal. .
`Sorkin .
`Silverman .
`Fort et al.
`.
`Anstey et al.
`Farr .
`
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`PGS Exhibit 2016, pg. 1
`PGS Exhibit 2016, pg. 1
`WesternGeco v. PGS (IPR2015-00309, 310, 311)
`WesternGeco V. PGS (IPR2015-00309, 310, 311)
`
`

`
`US. Patent
`
`Apr. 18, 1989
`
`sheet 1 of 2
`
`4,823,326
`
`I72
`
`1
`
`'
`
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`
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`
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`I
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`
`‘l
`
`PGS Exhibit 2016, pg. 2
`PGS Exhibit 2016, pg. 2
`WesternGeco v. PGS (IPR2015-00309, 310, 311)
`WesternGeco V. PGS (IPR2015-00309, 310, 311)
`
`

`
`U.S. Patent
`
`Apr. 18,1989
`
`Sheet 2 of 2
`
`4,823,326
`
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`PGS Exhibit 2016, pg. 3
`PGS Exhibit 2016, pg. 3
`WesternGeco v. PGS (IPR2015-00309, 310, 311)
`WesternGeco V. PGS (IPR2015-00309, 310, 311)
`
`

`
`1
`
`4,823,326
`
`SEISMIC DATA ACQUISITION TECHNIQUE
`HAVING SUPERPOSED SIGNALS
`
`The present application is a continuation-in-part of 5
`previous application Ser. No. 887,377 filed July 21, 1986
`abandoned which is a continuation-in-part of previous
`application Ser. No. 813,593 filed Dec. 26, 1985 aban-
`doned.
`
`TECHNICAL FIELD
`
`10
`
`The present invention pertains in general to the field
`of seismic surveying and in particular to collection and
`processing of seismic signals.
`BACKGROUND OF THE INVENTION
`
`Seismic surveys are carried out on a wide scale
`throughout the world primarily for defining under-
`ground formations to detect and localize petroleum
`deposits. In past years the primary source of seismic
`energy for such surveys has been explosives, such as
`dynamite. However,
`the current dominant energy
`source for land seismic exploration is a seismic vibrator.
`The use of such a vibrator is generally identified by the
`trademark VIBROSEIS.
`The conventional method for collecting seismic re-
`cords by the use of a vibrator source is to lay out a line
`or multiple lines of detector geophones and sequentially
`operate the vibrator source along a series of points on a
`line of travel. The seismic geophone array and the vi-
`brator is then offset to a different line to collect more
`information. The cost for producing a seismic survey is
`closely related to the amount of time required to oper-
`ate the vibrator, collect the seismic records and move
`the detector geophones. A primary limiting feature in
`the speed of collecting seismic data is the use of only a
`single vibrator source.
`In a proposal by R. Garotta entitled “Simultaneous
`Recording of Several Vibroseis Seismic Lines” (S5.5)
`presented at the 53rd Annual International Meeting and
`Exposition of the Society of Exploration Geologists in
`1983, it is suggested that vibrators could be operated
`along parallel lines with either sequential or simulta-
`neous recording. However, the technique proposed by
`Garotta is severely limited since it provides incomplete
`separation of sources.
`It would be a more economical approach to the col-
`lecting of seismic data if there could be simultaneous use
`of a plurality of vibrator sources. However, the signals
`from the various vibrator sources would be mixed. Har-
`monic distortion produced by each of the sources
`would also be mixed. Therefore, there exists a need for
`a method for producing separate seismic records which
`are derived from a plurality of simultaneously operated
`vibrator seismic sources. Such a method could serve to
`substantially reduce the cost of producing seismic sur-
`veys.
`
`SUMMARY OF THE INVENTION
`
`A selected embodiment of the present invention is a
`method for producing separate seismic records derived
`respectively from a plurality of concurrently operated
`vibrator seismic sources. The seismic records thus pro-
`duced also have reduced harmonic distortion. The
`method comprises the following steps. Frequency
`sweeping pilot signals are generated for respectively
`driving at least two vibrator seismic sources concur-
`rently in a sequence of at least four sweeps. The pilot
`
`20
`
`25
`
`30
`
`35
`
`45
`
`50
`
`55
`
`65
`
`2
`signal for each vibrator has a plurality of separate phase
`angles during the sweeps. One seismic signal is detected
`which is produced by the concurrently operated vibra-
`tor seismic sources. One seismic signal is produced for
`each of the sweeps. Each of the detected seismic signals
`is correlated separately with each of the corresponding
`pilot signals which were used to drive the vibrator
`seismic sources that produced the detected seismic sig-
`nal. The step of correlating generates at least first and
`second separate correlated records for each of the
`sweeps. The first correlated records for each of the
`sweeps are summed to produce a first seismic record
`derived only from a first of the concurrently operated
`vibrator seismic sources. The second correlated records
`for each of the sweeps are summed to produce a second
`seismic record derived only from a second of the vibra-
`tor seismic sources. Thus, separate seismic records are
`derived from concurrently operated vibrator seismic
`sources.
`
`A further embodiment of the present invention com-
`prises a method for producing separate seismic records
`derived respectively from a pair of concurrently oper-
`ated vibrator seismic sources. The seismic records pro-
`duced by this method have reduced harmonic distor-
`tion. The first step comprises generating at a first of the
`vibrator seismic sources a sequence of eight frequency
`modulated pilot signals. Each of the pilot signals com-
`prises a sweep and has an increasing frequency through
`the sweep. The pilot signals for the first seismic source
`serve to drive the first vibrator seismic source. These
`pilot signals have respective phase angles of 0°, 45°, 90°,
`135°, 180°, 225°, 270° and 315°. The next step comprises
`generating for a second of the vibrator seismic sources
`a sequence of eight frequency modulated pilot signals
`which also have an increasing frequency through the
`sweep. The pilot signals for the second vibrator seismic
`source are generated concurrently and respectively
`with the first vibrator seismic source signals. The pilot
`signals are therefore generated concurrently in pairs to
`produce eight pairs of sweeps. The pilot signals for the
`second vibrator seismic source serve to drive the sec-
`ond vibrator seismic source and have respective phase
`angles of 0°, 225°, 90°, 315°, 180°, 45°, 270° and 135°. A
`reflected seismic signal is detected following generation
`of each pair of the sweeps thereby producing eight
`seismic signals. Each of the detected seismic signals is
`correlated with the respective one of the first seismic
`source signals which serve to produce the correspond-
`ing detected seismic signal. This results in the produc-
`tion of eight correlated records for the first seismic
`source. Likewise, each of the detected seismic signals is
`correlated with the respective one of the second seismic
`source signals, which served to produce the corre-
`sponding detected seismic signal. This produces eight
`correlated records for the second seismic source. The
`eight correlated records for the first seismic source are
`summed to produce a first seismic record which is de-
`rived almost exclusively from the first vibrator seismic
`source. Finally, the eight correlated records for the
`second seismic source are summed to produce a second
`seismic record which is derived almost exclusively from
`the second vibrator seismic source. Therefore, through
`this method, separate seismic records are produced
`from concurrently operated vibrator seismic sources.
`A still further embodiment of the present invention is
`a method for producing separate seismic signals from a
`plurality of concurrently operated seismic sources. This
`embodiment differs from the above described embodi-
`
`PGS Exhibit 2016, pg. 4
`PGS Exhibit 2016, pg. 4
`WesternGeco v. PGS (IPR2015-00309, 310, 311)
`WestemGeco V. PGS (IPR20l5-00309, 310, 311)
`
`

`
`3
`ment in regard to the production of the pilot signals.
`For this embodiment each of the pilot signals for the
`first seismic source is uniformly increased by a predeter-
`mined phase increment from the previous pilot signal.
`Pilot signals for the second seismic source are uniformly
`decreased by the same phase increment from one pilot
`signal to the next. As with the other embodiments, the
`pilot signals for the two seismic sources are generated
`concurrently and are utilized to drive the correspond-
`ing vibrator seismic sources.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`For a more complete understanding of the present
`invention and the advantages thereof, reference is now
`made to the following Description taken in conjunction
`with the accompanying Drawings in which:
`FIG. 1 is a schematic illustration of a conventional
`technique for collecting seismic data which comprises
`moving a single vibratory source along a track,
`FIG. 2 is a schematic illustration of a technique of the
`present invention for collecting seismic data by the use
`of simultaneously operated vibrators,
`FIG. 3 is a schematic illustration of an alternate tech-
`nique for collecting seismic data by the use of concur-
`rent vibrators in accordance with the present invention,
`FIG. 4 is a block diagram illustrating apparatus used
`in accordance with the present invention for collecting,
`processing and recording seismic data from a plurality
`of concurrently operated seismic sources, and
`FIG. 5 is a graph illustrating the pilot signals used for
`driving each unit of a pair of concurrently operated
`vibrator sources during a sequence of eight sweeps.
`DETAILED DESCRIPTION
`
`The present invention is directed to a method of
`collecting seismic data by the use of concurrently oper-
`ated vibratory seismic sources. The conventional prac-
`tice for collecting seismic data is by the use of a single
`seismic source. This process is schematically illustrated
`in FIG. 1. A vibrator source 10 is placed at sequential
`locations along a line 12. The vibrator source locations
`are indicated by the letter X. Line 12 also includes a set
`of geophones which are indicated by small circles, such
`as geophone 14. The geophones are also placed at uni-
`form intervals along the line 12. In the majority of appli-
`cations the geophones are placed along a single line.
`Parallel and offset from the line 12 there may also be
`provided geophone lines 16 and 17, each of which has a
`plurality of geophones placed at uniform intervals. The
`vibrator source 10 is activated to produce a seismic
`signal at a first location and then moved to the next
`location to produce a similar seismic signal. For each
`generation of a seismic signal, the entire set of geo-
`phones, which could possibly be 120, receive the re-
`flected seismic signals produced by the vibrator source
`10. After the vibrator source 10 is moved a certain
`distance along the line 12, the geophones are moved
`ahead to cover a new area. After an entire line of opera-
`tion has been completed, the sets of geophones are
`moved to an offset parallel location, such as to lines 16
`and 17, and the vibrator source 10 is worked along a
`new line.
`
`5
`
`l0
`
`15
`
`in
`
`25
`
`30
`
`35
`
`45
`
`50
`
`55
`
`The conventional process for collecting seismic data
`is quite time consuming and expensive. The expense
`becomes even greater when the area of operation is an
`adverse terrain or climate, such as an extremely cold
`region. One of the primary cost factors is the amount of
`time required for operating the vibrator source 10. In
`
`65
`
`4,823,326
`
`4
`many applications the cost of a seismic survey is
`charged on the number of hours that the crew and
`equipment are operating in the field. As can be seen in
`the description of the conventional process shown in
`FIG. 1, the primary "restraining function is the time
`required to operate the vibrator source 10 and to move
`it to new locations.
`
`A principle object of the present invention is to make
`possible the concurrent use of a plurality of vibrator
`sources so that the overall time required to produce a
`seismic survey can be substantially reduced.
`By using the technique of the present invention, fur-
`ther described below, it is possible to operate two or
`more vibrator seismic sources concurrently. However,
`there are many configurations for collecting seismic
`data in this way. Two such configurations are shown in
`FIGS. 2 and 3.
`Referring now to FIG. 2, there are illustrated two
`vibrator sources 20A and 20B which are respectively
`worked along lines 22 and 24. Each of the vibrator
`sources 20A and 20B are positioned at locations shown
`by a small X along the lines 22 and 24 where they are
`operated concurrently. The seismic signals produced by
`the sources are received by an array of geophones.
`Geophones, indicated by small circles, are placed at
`uniform locations along the lines 22 and 24 as well as
`along parallel geophone lines 26, 28 and 29. The concur-
`rent operation of the sources 20A and 20B permits a
`seismic survey to be completed in approximately one
`half the time required for a conventional seismic survey,
`thereby substantially reducing the overall cost of the
`survey. Further, note for FIG. 2 that after the vibrator
`sources 20A and 20B have completed the operation
`along their respective lines, the sources together with
`the geophones are moved to a parallel area to cover
`another region of the survey.
`Another technique for simultaneous operation of
`vibrator sources is illustrated in FIG. 3. Vibrator
`sources 30A and 30B are worked side by side along a
`line 34 at the locations indicated by the small X’s. In this
`configuration one of the vibrator sources 30 or 32 is a
`source of compressional waves while the other is a
`source of shear waves. The line 34 is also a line for a set
`of geophones which are marked by circles such as 36.
`On opposite sides of the line 34 there are provided geo-
`phone lines 38 and 40 which are provided with uni-
`formly spaced geophones. After the sources 30A and
`30B have progressed along the line 34, they can be
`moved to a parallel line where they are again worked
`along the line for similar operation. The geophones may
`be placed only along the line being worked or may be
`placed along a plurality of parallel lines.
`FIGS. 1-3 are schematic illustrations which show
`techniques and relative locations. These figures are not
`intended to be a scale or model representation.
`Referring to FIG. 4 there is illustrated apparatus 44
`for carrying out the present invention. The apparatus 44
`illustrated in FIG. 4 is for an embodiment of the present
`invention utilizing two vibrator sources. It is to be un-
`derstood that the present invention can be expanded to
`use more than two vibrator sources. A vibrator source
`46A is used in collecting data for a data set A and a
`vibrator source 46B is used in collecting data for a data
`set B. A sweep generator 50A produces a pilot signal A
`for driving the source 46A. A sweep generator 50B
`produces a pilot signal B, which is used to drive the
`vibrator source 46B.
`
`PGS Exhibit 2016, pg. 5
`PGS Exhibit 2016, pg. 5
`WesternGeco v. PGS (IPR2015-00309, 310, 311)
`WestemGeco V. PGS (IPR20l5-00309, 310, 311)
`
`

`
`4,823,326
`
`5
`A sweep generator 54A, for data set A, generates a
`pilot signal which is provided to an input of a dual
`correlator 56. The sweep generator 54A is connected
`through a communication link 58A, typically a radio
`charmel, for triggering the operation of sweep genera-
`tor 50A.
`
`A sweep generator 54B, for the data set B, produces
`a pilot signal which is provided to an input of the dual
`correlator 56. The sweep generator 54B is further con-
`nected through a communication link 58B, typically a
`radio channel, to trigger the operation of sweep genera-
`tor 50B.
`The dual correlator 56 includes an A correlator 60A
`which receives the A pilot signal A from sweep genera-
`tor 54A and includes a correlator 60B which receives
`the pilot signal B from sweep generator 54B.
`In a typical seismic survey there are laid out 120
`geophones for a single line of geophones for receiving
`the reflected seismic signals. The group of seismic
`traces produced by the geophones are input through a
`channel 66 to the inputs of correlators 60A and 60B.
`The output of correlator 60A is provided to a summer
`68A and the output of correlator 60B is provided to a
`summer 68B. The output from summer 68A is trans-
`ferred to a recorder 72A for the data set A. Likewise,
`the output from summer 68B is transferred to a recorder
`7213 for the data set B.
`i
`The correlators 60A and 60B together with the sum-
`mers 68A and 68B comprise dual correlator 56 which
`may be, for example, a Schmitt Correlator, model SC 6.
`This equipment is manufactured by Schmitt Consulting,
`which is located in Columbus, Kans.
`The operation of the apparatus 44 of the present in-
`vention is now briefly described in reference to FIG. 4.
`The vibrator sources 46A and 46B are operated concur-
`rently, such as shown in the FIGS. 2 and 3 described
`above. For each sweep, the pilot signals for driving the
`sources 46A and 46B are generated by the correspond-
`ing sweep generators 50A and 50B. These generators
`are in turn triggered respectively for concurrent opera-
`tion by the sweep generators 54A and 54B. The re-
`flected seismic signals produced by the vibrators 46A
`and 46B are collected and passed through the channel
`66 to the inputs of the correlators 60A and 60B. Each of
`these correlators receives the same collection of seismic
`traces.
`
`The sweep generators 54A and 54B generate the
`respective pilot signals A and B. These pilot signals are
`stored in the respective correlators 60A and 60B. The
`pilot signals stored in the correlators correspond to the
`same pilot signals that are generated during each sweep
`to drive the vibrator sources 46A and 46B. For each
`sweep, the collected seismic traces are correlated with
`the pilot signals which were used to drive the vibrators
`46A and 46B. Thus, for data set A, the pilot signal used
`by correlator 60A is the same as the pilot signal used for
`driving the vibrator source 46A. For data set B, the
`pilot signal input to the correlator 60B is the same as the
`pilot signal produced by generator 50B for driving the
`vibrator source 46B.
`The correlators 60A and 60B produce respective first
`and second correlated records which are then trans-
`ferred into the respective summers 68A and 68B. Within
`the summers, the correlated records are point-by-point
`summed with respect to a common starting time for all
`of the sweeps taken while the vibrator sources, 46A and
`46B are operated at one pair of locations. The resulting
`summed seismic records are then transferred to the
`
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`15
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`20
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`25
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`30
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`35
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`45
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`50
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`55
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`65
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`6
`recorders 72A and 72B. Both the correlation and sum-
`ming operations can be carried out by use of the
`Schmitt Correlator described above.
`Detailed operation of the present invention is further
`described below.
`The various sweep signals used by the concurrently
`operated vibrator sources and correlators for a selected
`embodiment of the present invention are illustrated in
`FIG. 5. When the two vibrators, identified as A and B,
`are positioned at a particular location they carry out
`eight sweeps which are numbered 1-8. For each of the
`vibrator sources, during each of the sweeps, there is
`shown the phase angle, direction of sweep (increasing
`frequency or decreasing frequency) and an illustrative
`waveform for the pilot signal that drives the vibrator
`source. The start of the first pilot signal is defined to be
`the reference zero phase angle.
`Referring further to FIG. 5, sweep 1 has pilot signal
`80 for data set A and pilot signal 82 for data set B. Note
`that both signals 80 and 82 start at the 0 phase reference.
`Signal 80, for data set A, is an up sweep going from 8
`hertz up to 80 hertz. Signal 82 for data set B is a down
`sweep going from a high frequency of 80 hertz down to
`8 hertz.
`During sweep 2 there is a pilot signal 84 for data set
`A. Signal 84 starts at a phase reference of 90° and is an
`up sweep. Pilot signal 86 starts at a reference angle of
`270° and is a down sweep. For sweep number 3, a pilot
`signal 88 starts at a reference phase of 180° and is an up
`sweep. The data set B pilot signal 90 also starts at a
`phase angle of 180° but is a down sweep. For sweep
`number 4 an data set A pilot signal 92 starts at a phase
`angle of 270° and is an up sweep. The B data set signal
`for sweep no. 4 starts at a reference angle of 90° and is
`a down sweep. Sweep 5 has a data set A pilot signal 96
`which starts at a 0 phase angle and is a down sweep. A
`data set B pilot signal 98 is an up sweep which starts at
`a 0 degree phase angle. For sweep 6, an data set A pilot
`signal 100 starts at a phase angle of 90° and is a down
`sweep. A data set B pilot signal 102 starts at a phase
`angle of 270° and is an up sweep. For Sweep 7, a data set
`A pilot signal 104 starts at a phase angle of 180° and is
`a down sweep. A data set B pilot signal 106 starts at a
`phase angle of 180° and is an up sweep. The sweep
`number 8 has a data set A pilot signal 108 which starts
`at a phase angle of 270° and is a down sweep. A data set
`B pilot signal 110 starts at a phase angle of 90° and is an
`up sweep.
`‘
`All of the up sweeps are 8-80 hertz and all of the
`down sweeps are 80-8 hertz. It is understood that differ-
`ent bandwidths and sweep ranges can also be used.
`The present invention is now described in detail in
`reference to FIGS. 4 and 5. The method of the present
`invention produces separate seismic records which are
`derived from a plurality of concurrently operated vibra-
`tor seismic sources. The present invention further pro-
`vides for reducing the harmonic distortion which is
`produced as an inherent part of the operation of a vibra-
`tor seismic source. Concurrent pilot signals are gener-
`ated for driving at least two vibrator seismic sources
`concurrently for a sequence of at least 4 sweeps. The
`pilot signals for each vibrator source have a plurality of
`separate phase angles during the four sweeps. A seismic
`signal is detected which is a result of the concurrently
`operated vibrator seismic sources, with one seismic
`signal being detected for each of the four sweeps. The
`detected seismic signals are correlated separately with
`each of the corresponding pilot signals which were used
`
`PGS Exhibit 2016, pg. 6
`PGS Exhibit 2016, pg. 6
`WesternGeco v. PGS (IPR2015-00309, 310, 311)
`WestemGeco V. PGS (IPR20l5-00309, 310, 311)
`
`

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`4,823,326
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`5
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`25
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`8
`eliminating the odd harmonics. This is done through the
`production of specific pilot signals in the following
`sequence:
`S1 = >SA° & S120
`32: >5/490 & SB270
`S3: >5/1180 & 33180
`54: >5/1270 & 5390
`By use of the above pilot signals for the A and B data
`sets,
`there can now be produced correlated records
`10 RCA and RC}; which comprise only the fundamental plus
`the alternate odd harmonics for the corresponding
`channels. The symbol * represents correlation of the
`adjacent quantities. This result is shown below:
`Rc,1(Fund+5th+9th .
`.
`.
`)=R1 * SA°+R2 *
`sA90+R3 at sA180+R4 an sA270
`Rc3(Fund+5th+9th .
`.
`)=R1 * SB°+R2 *
`sB27o+R3 n: sBiso+R4}= 5390
`By adding additional sweeps and reducing the steps
`between the phase angles, the present invention can be
`20 expanded to reduce even more of the odd order har-
`monies. However, in practice the fifth harmonic is of
`such a small magnitude that its impact upon the result-
`ing record is very small.
`Going further with the concept of the present inven-
`tion, the phases of the sweep can be alternated with up
`and down frequency sweeping to further enhance the
`resulting record. To achieve dual source separation
`together with greater harmonic suppression and corre-
`lation noise reduction, an optimum sweep sequence is
`30 shown below. This is a dual, polarity encoded, eight
`sweep sequence of variable phases. The number of
`sweeps can optionally be increased to have varying
`bandwidths and center frequencies to enhance still fur-
`ther the resolution of the records. The sequence is as
`follows:
`
`7
`to drive the vibrator seismic sources that produced the
`detected seismic signal. The separate correlation of the
`detected seismic signals generates respective first and
`second separate correlated records for each of the
`sweeps. The first of the correlated records for each of
`the sweeps is summed to produce a first seismic record
`which is derived from a first of the concurrently oper-
`ated vibrator seismic sources. The second correlated
`record from each of the sweeps is summed to produce a
`second seismic record which is derived from a second
`of the vibrator seismic sources. The result is the produc-
`tion of separate seismic records derived respectively
`from concurrently operated vibrator seismic sources.
`The method of the present invention is now described
`in a mathematical context using the following terminol- 15
`ogy:
`S,4=>a sweep produced by a data set A vibrator
`seismic source.
`S1 = >the composite of the sweeps produced by the
`vibrator seismic sources for a first sweep.
`S,1+,— = >sweep by a source A having a 0° (+) or
`180° (—) phase angle.
`R1: >the record produced by the sweep S1.
`The most general aspect of the present invention
`comprises a group of four sweeps S1, S2, S3 and S4
`having the polarities of the pilot signals as follows:
`Si + > SA + & S134"
`S2 + > SA + & SB ’'
`S3 + > SA ‘ & SB 4'
`$4 + > SA ‘& SB‘
`Note that both of the vibrator sources have a plural-
`ity of phase angles during the group of four sweeps. The
`uncorrelated records corresponding to sweeps S1—S4
`are defined as R1 through R4. The record correspond-
`ing to the data set A vibrator source is designated as RA. 35
`Likewise, the record corresponding to the vibrator data
`set B source is designated as R3. The fundamental and
`odd harmonics for each of these records is produced by
`summing and differencing the uncorrelated records R1
`through R4 in the manner shown below:
`RA (Fund. & odd har.)=R1+R2 —-(R3 +R4)
`R1; (Fund. & odd har.)=R1—R2+(R3-R4)
`The mathematical process described above for sum-
`ming and differencing the various records is presented
`to explain the theory of the present invention. How-
`ever, when the detected traces are first correlated prior
`to the step of summation, the mathematical effect is
`carried out without the need for the various summations
`and differencing of the records.
`Note that in the above mathematical example, the
`uncorrelated field records are summed and differenced
`to produce the separate A and B data set field records.
`In a selected aspect of the invention, which is further
`described below, the phase angle of the pilot signals is
`changed in incremental steps through a plurality of 55
`sweeps. This aspect of the invention is that described
`above in reference to FIGS. 4 and 5 wherein the field
`records are immediately correlated with the corre-
`sponding pilot signals. This immediate correlation and
`summation replaces the above described summing and
`differencing of the field records.
`Note that by use of the plurality of phase angles for
`both the A and B data sets that there has been produced
`almost total separation of the A and B sources. How-
`ever, the odd harmonics produced by the correspond-
`ing channel vibrator sources are still present. These
`harmonics can produce substantial harmonic distortion
`which can adversely affect the survey record. A further
`aspect of the present invention includes substantially
`
`50
`
`45
`
`S1=>SAT°& SBi°
`s2=>5/‘T903; 531270
`S3=>SAT180& 531180
`g,=>5AT27o& 53190
`S5=>SA T°&S13l°
`S6=>5AT9o& 331270
`s7=>s/‘T1803; 531130
`S3=>5AT27o& 33190
`The above sweeps are used in the same manner as
`described in reference to FIG. 4.
`A further embodiment of the present invention is a
`sweep sequence which comprises a series of eight
`sweeps. This sequence is defined below.
`S1=>S,4 T 0& SBT0
`52=>sAT4s& 531225
`S3=>SAT9°& SBT9°
`s4=>gAT135& SB 1 315
`S5___.>SAT13o& 331180
`56=>SA T 22555 53145
`S7= >SA T 27° & SB T 27°
`s8=>sA T 315 & 331 135
`This sweep sequence differs from the previously de-
`scribed sequence iri having all of the sweeps being in-
`creasing frequency sweeps. In a preferred embodiment
`60 thes