United States Patent [19]
`Edingtou
`
`[11] Patent Number:
`[45] Date of Patent:
`
`4,953,657
`Sep. 4, 1990
`
`[54] TIME DELAY SOURCE CODING
`.
`[75] Inventor: Bruce L. Edmgton, Houston, Tex.
`[73] Assignee: Halliburtou Geophysical Services,
`Inc., Houston, Tex.
`
`[21] APPI- No-t 312,360
`[22] Filed:
`Feb. 14’ 1989
`
`[63]
`
`.
`.
`'
`7
`Related U's' Apphcahon Data
`Continuation of Ser. No. 126,346, Nov. 30, 1987, aban-
`d°n¢¢
`‘
`[51] Int. Cl.5 ............................................. .. G01V 1/00
`[52] US. Cl. ..................................... .. 181/111; 367/50
`[58] Field of swell .................. .. 181/111; 367/50, 55,
`367/47
`
`[55]
`
`References Cited
`U_S_ PATENT DOCUMENTS
`V
`183511)!“ ...............................
`2,322,332 1(5);
`4’159’463 6/1979 s?zzm
`" 367/59
`4:17o:oo2 10/1979 Strange
`All
`4,242,740 12/1980 111161116 ..
`367/36
`4,467,459 8/1984 Curn'e ........ ..
`. 367/23
`4,715,020 12/1987 Landrum, Jr. ...................... .. 367/38
`
`I
`
`Primary Examiner-Thomas H. Tarcza
`Assistant Examiner-Ian J. Lobo
`Attorney, Agent’ or Firm_Andrew J‘ Dillon
`[57]
`ABSTRACT
`
`In seismic exploration, a survey may be conducted
`using multiple seismic energy sources activated substan
`tially simultaneously. A series of shots is made at each
`shot point, with a determinable time delay between the
`activation of each source for each shot. There must be
`at least two different determinable time delays in each
`series of shots, The Seismic signals are recorded, A
`method of processing the signals to separate signals
`from each source is Provided" For each Signal receiver’
`the amplitudes “the Signals fmth each §h°t in the “ties
`‘5 Summed- The Signals are the“ tim Shtfted, 3° that tht
`signals from the second source to be activated are re
`ceived at the same time, and the amplitudes for the
`series are summed. This step is repeated for each subse
`quently activated source. Time domain operators are
`derived, the operators and the summations Fourier
`v transformed to the frequency domain’ in which the
`signals received from‘ each source are readily calcu
`lated. The separated signals may then be inverse Fou
`tier tmhsfmhed t° th't time. dQmahI'
`'
`10 Claims, 2 Drawing Sheets
`
`WesternGeco Ex. 1006, pg. 1
`
`

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`US. Patent Sep.4, 1990
`8%]
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`Sheet 1 of2
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`4,953,657
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`A 12
`$13. 1
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`2
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`A
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`Z2
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`“EM 5
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`\I E n in m
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`WesternGeco Ex. 1006, pg. 2
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`

`

`US. Patent Sep. 4, 1990
`
`Sheet 2 of 2
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`4,953,657
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`46
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`44
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`1:13. 5
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`WesternGeco Ex. 1006, pg. 3
`
`

`

`1
`
`TIIVIE DELAY SOURCE CODING
`
`5
`
`10
`
`This application is a continuation of application Ser.
`No. 126,346, ?led 11/30/87, now abandoned.
`BACKGROUND OF THE INVENTION
`The present invention relates to a method of separat
`ing for analysis seismic signals received from multiple
`seismic sources which are activated substantially simul
`taneously.
`A common seismic survey method employs a single
`seismic source in conjunction with multiple geophone
`detector arrays arranged equally spaced along a line
`from the source. A number of shots are made, in order
`to improve the signal to noise ratio, and then the source
`and detector arrays are moved along the same line a
`short distance and the shooting procedure repeated.
`This procedure is repeated along the predetermined
`length of the survey line and other designated survey
`lines in the area to be surveyed. The recorded data from
`each shot is then processed, usually by a digital com
`puter at a headquarters processing department.
`When only one seismic source is used, the survey
`procedure can be very time-consuming. With modern
`signal processing methods, the most expensive part of
`25
`the seismic survey process has become the ?eld survey
`period. This period could be shortened if more than one
`seismic source could be used simultaneously. However,
`multiple sources can only be used if some means for
`distinguishing between signals emanating from the dif
`ferent sources can be provided.
`us. Pat. No. 3,885,225 to Anstey et a1 proposes a
`method to distinguish between multiple sources. This
`involves a frequency segmentation technique, applied
`to use of a pulse-compression system of seismic pros
`pecting (generally referred to in the art by the trade
`mark “Vibroseis”). Vibroseis involves the use of vibra
`tors emitting long, swept-frequency signals in the seis
`mic frequency range, the detection of emitted signals
`after re?ection and refraction within the earth by detec
`tors located on the earth’s surface in the survey area,
`and the cross correlation of the detected signals against
`the emitted signals. Anstey et al is directed to a method
`and apparatus for broad-line seismic pro?ling, using
`several vibrators simultaneously emitting signals. The
`45
`normal emission frequency bandwidth is divided into
`several parts which are allocated to individual vibrators
`in a sequence of separate emissions, in such a way that
`mutually exclusive frequencies are radiated by the sev
`eral vibrators at any one time. The detected signals are
`separated‘ on the basis of frequency to represent the
`individual signals from each vibrator. Apart from the
`fact that this technique is limited to one method of seis
`mic surveying, Vibroseis, the frequency limitation on
`each individual vibrator reduces sensitivity. Further,
`the ‘225 patent admits that harmonic distortion in the
`vibrators or their coupling with the ground can impair
`the capacity of the correlation process to separate the
`signals from different generators.
`
`4,953,657
`2
`In. particular, the method of obtaining the seismic
`data for a geophysical survey comprises shooting at
`least two seismic energy sources substantially simulta
`neously with a determinable time delay between the
`activation of each source, shooting the sources at least a
`second time substantially simultaneously with a differ
`ent determinable time delay between the activation of
`each source from the determinable time delay used in at
`least one previous shooting and, for each shooting,
`recording as a function of time the amplitude of the
`seismic signals must be received at at least one point in
`the survey area spaced apart from the seismic energy
`sources.
`In one embodiment, the seismic energy sources used
`are surface types of energy source. Preferably, at least
`one of the determinable time delays is preselected, and
`is selected so that the difference in time delay between
`any two shootings enables the signal received from the
`?rst activated source to be distinguished from the signal
`received from the second activated source. Preferably,
`the difference in time delay between any two shootings
`is selected to be not equal to an integral multiple of the
`period of a frequency component the seismic signals,
`most preferably being not less than about one-quarter
`and not more than about three-quarters of such period.
`In the preferred embodiment, the amplitude of the
`size of the signals is recorded in digital form.
`There is also provided a method of obtaining seismic
`data for a geophysical survey which comprises shooting
`at least two seismic energy sources substantially simul
`taneously with a determinable time delay between the
`activation of each source, shooting the sources at least a
`second time substantially simultaneously with a differ
`ent determinable time delay between the activation of
`each source from the determinable time delay used in at
`least one previous shooting, for each shooting record
`ing as a function of time the amplitude of the seismic
`signals received at at least one point in the survey area
`spaced apart from the seismic energy sources and ana
`lyzing the recorded seismic signals to separate signals
`originating from each seismic source.
`In the preferred embodiment, the seismic energy
`source is a surface energy source, and at least one of the
`determinable time delays is preselected. Preferably, the
`difference in time delay between any two shootings is
`selected so as to enable the signal received from the ?rst
`activated source to be distinguished from the signal
`received from the second activated source, preferably
`not being equal to an integral multiple of the period of
`a frequency component of the seismic signals. Most
`preferably, the difference in time delay between any
`two shootings is selected to be not less than about one
`quarter and not more than about three-quarters of the
`period of a frequency component of the seismic signals.
`In one embodiment, the amplitude of the seismic
`signals is recorded in digital form.
`In the preferred embodiment, the recorded signals are
`analyzed by a method comprising summing the ampli
`tude of the seismic signals from each shooting as a func
`tion of time for each point for which received seismic
`signals are recorded, then time shifting the seismic sig
`nals from each shooting so that the signal received from
`the second source to be activated is received at the same
`time for each shooting and then summing the amplitude
`of the time shifted seismic signals as a function of time.
`This time shifting step is repeated for each subsequently
`activated source, and for each summation derived from
`the above steps, and an appropriate time domain opera
`
`40
`
`SUMMARY OF THE INVENTION
`In accordance with the present invention there is
`provided a method of obtaining seismic data for a geo
`physical su'rvey in which two or more seismic energy
`sources are activated"substantially simultaneously for
`each shot, and a method of processing the data obtained
`in the survey so as to separate out the signals received
`from each seismic energy source.
`
`65
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`WesternGeco Ex. 1006, pg. 4
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`

`

`0
`
`4,953,657
`3
`tor is derived from the determinable time delays. The
`Fourier transform to the frequency domain of each time
`domain operator and to the frequency domain of each
`summation are then derived, the signals received from
`each energy source in the frequency domain for each
`frequency are calculated and the inverse Fourier trans
`form to the time domain of the signals received from
`each energy source is derived.
`The present invention also provides a method of
`analyzing seismic signals from two or more shootings of
`at least two seismic energy sources activated substan
`tially simultaneously with a determinable delay between
`the activation of each source so as to separate the sig
`nals originating from each seismic source. This method
`comprises, for each point for which received seismic
`signals are recorded, summing the amplitudes of the
`seismic signals from each shooting as a function of time
`and time shifting the seismic signals from each shooting
`so that the signal received from the second source to be
`activated is received at the same time for each shooting,
`then summing the amplitudes of the time shifted seismic
`signals as a function of time. The time shifting step is
`repeated’ for each subsequently activated source. An
`appropriate time domain operator is derived from the
`determinable delays for each summation derived from
`the above steps, and the Fourier transform to the fre
`quency domain of each such time domain operator and
`of each summation is derived. The signals received
`from each energy source in the frequency domain for
`each frequency are calculated, and then the inverse
`Fourier transform to the time domain of the signals
`received from each energy source is derived.
`
`4
`sources. Referring to FIG. 3, two seismic energy
`sources, 24 and 26 are shown, located on the earth’s
`surface 28. For simplicity, a single detector 30 is shown,
`but it will be readily understood that this can be re
`placed by a geophone group conventional in the art. In
`FIG. 3 the reflection paths of seismic waves originating
`from energy sources 24 and 26 and re?ected off geolog
`ical stratum 32 after passage through a homogenous
`layer bounded by the surface 28 and stratum 32 to de
`tector 30 are shown.
`Seismic energy sources 24 and 26 are preferably sur
`face energy sources, such as weight drop or Vibroseis,
`well known in the art. In the preferred embodiment,
`activation of sources 24 and 26 is controlled by a com
`puter in the survey control truck by way of radio signals
`or by cable.
`It is conventional to perform a number of shots at
`each shotpoint in order to build up the signal-to-ambient
`noise ratio. vPreferably, the seismic sources are in the
`same position for each of these shots. The decision as to
`how any shots at each shotpoint depends on the circum
`stances, and is readily made by persons of ordinary skill
`in the art. The present invention can be used with two
`or more shots, but for the purposes of this description it
`will be assumed that eight shots are made at each shot
`point.
`In the method of the present invention, seismic en
`ergy sources 24 and 26 are activated with a short time
`delay between the activation of 24 and of 26. This time
`delay must be measured and recorded, which may be
`done by any conventional means.
`Preferably, seismic energy sources 24 and 26 are
`activated at certain variable times relative to time zero
`on the recording system. The activation time relative to
`time zero of the recorder may be repeated at a speci?ed
`shot point from shot to shot for either source. However,
`for each shot point there must be some variation in the
`activation time delay between sources 24 and 26 for
`different shots, and this delay is preferably different for
`each shot made at one shot point.
`In the preferred embodiment, the times of activation
`of the sources are predetermined, and the sources oper
`ate with suf?cient accuracy to allow these predeter
`mined times to be used in the separation calculations
`made according to the method of the present invention.
`However, for sources which exhibit considerable ran
`dom variation in operation from the selected activation
`time, the true time of activation should be measured and
`recorded to improve the accuracy of the separation
`process.
`In order to satisfactorily detect the difference be
`tween the signals from sources 24 and 26 at the detector
`30 for each frequency in the desired seismic frequency
`band, for any two shots in the series the relative activa
`tion time delays should differ by a non-integral fraction
`of the period of that frequency. It is most preferred that
`this difference should be between one quarter and three
`quarters of the period of the frequency concerned.
`However, the actual fraction of the period needed to
`compute the signal separation to the required accuracy
`for a particular frequency is a function of the signal to
`noise ratio, and determining the optimum difference in
`activation time delays for each pair of shots may readily
`be performed by a person of ordinary skill in the art.
`The seismic signals from the two sources 24 and 26
`are received by detector 30 and transmitted to a storage
`means. This storage means may be a recorder of a con
`ventional type, which may be either‘ analog or digital or
`
`35
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 illustrates a ?eld layout for a seismic survey
`according to the present invention.
`FIG. 2 illustrates vertical seismic pro?ling.
`FIG. 3 illustrates a typical path for seismic waves in
`a survey as shown in FIG. 1.
`FIG. 4 shows schematically signals received by a
`detector from a series of seismic source shootings ac
`cording to the present invention.
`FIG. 5 shows the effect of time-shifting the signals of
`FIG. 4 according to the method of the present inven
`tion.
`
`40
`
`45
`
`DETAILED DESCRIPTION
`One application for which the present invention is
`particularly useful is three dimensional seismic survey
`ling. A ?eld layout for this type of survey is shown
`schematically in FIG. 1. Seismic detectors 2 are spaced
`along a survey line 4. These detectors are preferably
`clusters of geophones. Energy source 6 is located on the
`survey line 4 at a distance from the nearest detector 2,
`and seismic energy sources 8, 10 of the same type as
`source 6, are located on either side of line 4, spaced
`apart from the line.
`Another useful application for the present invention
`is vertical seismic pro?ling, shown schematically in
`FIG. 2. A tool 12 containing geophones is located in
`well bore 14, in contact with the geological stratum 16.
`Seismic energy sources 18 are located at the surface 20.
`Signals from the geophones are transmitted to the sur
`face through leads 22.
`For purposes of simpli?cation, the method of the
`present invention will be described for two seismic
`energy sources, but ‘those skilled in the art will readily
`recognize that the method can be used with multiple
`
`55
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`WesternGeco Ex. 1006, pg. 5
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`

`

`4,953,657
`5
`it may be any kind of data storage used in conjunction
`with a computer. In the preferred embodiment, a digital
`recording system is employed, with a two millisecond
`digital sampling period. The signal received by detector
`30 is shown schematically as the trace 34 which is num
`bered “1” in FIG. 4. Blip 36 corresponds to the signal
`from the ?rst activated source 24, while blip 38 corre
`sponds to that from source 26. For the purposes of
`simplifying this description it is assumed that these sig
`nals are received after passage through a homogenous
`layer bounded by 28 and 32 and after re?ection from a
`feature 32 parallel to the surface 28 and that detector 30
`is spaced equidistant from sources 24 and 26, so that the
`time delay between blips 36 and 38 is equal to the acti
`vation delay between sources 24 and 26. However, the
`invention is applicable to any type of subsurface struc
`ture and any placement of the detectors relative to the
`sources.
`For the second shot of the series, the time delay be
`tween the activation of the ?rst and second sources
`must differ from the activation delay of the ?rst shot,
`and must also be measured and recorded. It is preferred
`that this time delay be predetermined, and that it be
`greater than the ?rst activation delay by a determinable
`increment. This process is repeated for each of the shots
`in the series, with a different measured time delay for
`each shot. Preferably, the time delay between the acti
`vation of the ?rst and second sources is increased for
`each subsequent shot by a constant amount, k, so that
`the time delay t,, for the nth shot is given by the formula
`
`20
`
`25
`
`30
`
`where to is the activation time measured relative to the
`start of the recorder for the second source on the ?rst
`shot.
`It will be observed that, in this preferred embodi
`ment, when the traces for each shot are aligned with
`blips 36 on straight horizontal line 40, the blips 38 plot
`ted with respect to time lie on a straight line 42 which is
`at an angle to line 40.
`In general, this procedure is repeated at multiple
`locations in the survey area. In the preferred embodi
`ment, the analysis of the data gathered by the above
`procedure is performed after all the data has been ob
`tained, but the method of the present invention may also
`be used to analyze the data as it is gathered.
`The ?rst step in analyzing the data is to sum the traces
`in the series. This summation may be expressed by the
`equation
`
`35
`
`40
`
`45
`
`NXi+
`
`1
`0
`
`where N is the total number of shots in the series, x,- are
`the individual signals 36 from energy source 24 and Yi
`(P-1)K are the individual signals 38 from energy source
`26.
`The signals shown in FIG. 4. are then time shifted as
`shown in FIG. 5. so that the signals 38 are aligned on
`straight line 44 and signals 36 are on sloping line 46, and
`the‘ time shifted signals are summed. This summation
`may be represented by the equation
`
`55
`
`65
`
`6
`Appropriate operators H and K are derived from the
`time delays in summation (1) and (2) respectively, an
`operation well known to those of ordinary skill in the
`art of operational mathematics and Fourier analysis. H
`and K are such that
`
`K’xi+NY.-=NY?
`
`(4)
`
`Where “‘ represents convolution.
`The Fourier transform to the frequency domain of
`equation (3) and (4) yields
`
`MF(Io-F<{xi}>+NF{Y.})=NF({Ya>
`
`(6)
`
`where F represents the Fourier transform and M is the
`dimension of the Fourier transform.
`These two equations may be solved for the two un
`knowns F ({x,}) and F({Y,-}) for each frequency, which
`are then inverse Fourier transformed to the time do
`main.
`It will be obvious to those skilled in the art that these
`equations cannot be solved if any determinant on the
`left hand side is equal to zero, so the time delays should
`preferably be selected so that no determinant is equal to
`zero. In practice, it is generally possible to select the
`time delays in such a manner that all the determinants
`are non-zero for frequencies within the frequency band
`of most interest in seismic surveys. However, it may be
`necessary or desirable under certain conditions to allow
`some singularities within this seismic frequency band,
`for example, in order to reduce the number of shots
`needed at each shot point.
`These singularities, whether within, or outside the
`seismic frequency band, can be handled by known
`methods used in geophysical analysis for stabilization of
`inverse operators. Preferably, a “white noise” method is
`used. Essentially, in this method a matrix which is sin
`gular or close to singular is replaced by a matrix closer
`to the identity matrix by increasing the size of the ele
`ments on the main diagonal by adding a constant factor.
`In practice, it is preferred to produce an equivalent
`result by instead dividing each of the elements not on
`the main diagonal by 1 +0, where c is a small positive
`constant quantity. For frequencies outside the seismic
`frequency band, the application of a bandpass ?lter may
`also be used to prevent the buildup of noise by the sepa
`ration process. Another method is to set a minimum
`absolute value for the determinant. When the determi
`nant has a value smaller than the minimum absolute
`value, the actual value is replaced by this minimum
`value with the proper sign attached. This method is
`suitable for frequencies outside the seismic frequency
`band.
`The above analysis may be carried out by any con
`ventional method, although it is preferred to use a pro
`grammed digital computer. Programs for performing
`the summations, time-shifting and Fourier transforms
`may be readily obtained or prepared by those of ordi
`nary skill in the art of computer programming.
`The signals from the individual energy sources, once
`separated by the above method of analysis, may then be
`used in conventional methods of obtaining geophysical
`information from seismic data.
`
`WesternGeco Ex. 1006, pg. 6
`
`

`

`4,953,657
`7
`8
`It is an advantage of the present invention that the
`at least one point within said geophysical survey com
`prises digitally recording said amplitudes.
`?eld survey time can be substantially shorter using mul
`tiple shots ?red substantially simultaneously than with
`6. A method of obtaining seismic data for a geophysi
`cal survey, said method comprising the steps of:
`the conventional single shots, and this produces con
`activating a ?rst seismic energy source having a se
`comitant cost savings. In the case of vertical seismic
`lected frequency component;
`pro?ling, normal well operations can often be inter
`receiving seismic signals from said ?rst seismic en
`rupted for only a limited time, making it important to
`ergy source at at least one point within said geo
`complete the survey process as quickly as possible, so
`physical survey;
`the present invention is particularly advantageous.
`activating a second seismic energy source after a ?rst
`Additional advantages and modi?cations will be
`selected time delay from activation of said ?rst
`readily apparent to those skilled in the art. The inven
`seismic energy source and while said seismic sig
`tion in its broader aspects is therefore not limited to the
`nals from said ?rst seismic energy source are being
`speci?c details, representative apparatus or the illustra
`received, said ?rst selected time delay being a non
`tive example shown and described. Accordingly, depar
`integral multiple of the period of said selected fre
`tures may be made from the detail without departing
`quency component;
`from the spirit or scope of the disclosed general inven
`receiving seismic signals from said second seismic
`tive concept.
`'
`energy source at said at least one point within said
`What is claimed is:
`geophysical survey;
`1. A method of obtaining seismic data for a geophysi
`activating said ?rst seismic energy source a second
`cal survey, said method comprising the steps of:
`time;
`activating a first seismic energy source having a se
`receiving seismic signals from said second activation
`lected frequency component;
`of said ?rst seismic energy source at said at least
`receiving seismic signals from said ?rst seismic en
`one point within said geophysical survey;
`ergy source at at least one point within said geo
`activating said second seismic energy source after a
`physical survey;
`second selected time delay from said second activa
`activating a second seismic energy source after a ?rst
`tion of said ?rst seismic energy source and while
`selected time delay from activation of said ?rst
`said seismic signals from said second activation of
`seismic energy source and while said seismic sig
`said ?rst seismic energy source are being received;
`recording, as a function of time, the amplitudes of the
`nals from said ?rst seismic energy source are being
`seismic signals received at said at least one point
`received, said ?rst selected time delay being a non
`within said geophysical survey for each activation
`integral multiple of the period of said selected fre
`of said ?rst and second seismic energy source;
`quency component;
`summing the amplitudes of the received seismic sig
`receiving seismic signals from said second seismic
`nals from each activation as a function of time;
`energy source at said at least one point within said
`utilizing said selected time delays to time shift the
`geophysical survey;
`received seismic signals from each activation so
`activating said ?rst seismic energy source a second
`that the signals received from said second source
`time;
`are received at the same times for each activation;
`receiving seismic signals from said second activation
`summing the amplitudes of said time shifted seismic
`of said ?rst seismic energy source at said at least
`signals as a function of time;
`one point within said geophysical survey;
`utilizing said selected time delays to derive an appro
`activating said second seismic energy source after a
`priate time domain operator for each summation;
`second selected time delay from said second activa
`deriving the Fourier transform to the frequency do
`tion of said ?rst seismic energy source and while
`main of each time domain operator;
`said seismic signals from said second activation of
`deriving the Fourier transform to the frequency do
`said ?rst seismic energy source are being received;
`main of each summation;
`recording, as a function of time, the amplitudes of the
`calculating the signals received from each seismic
`seismic signals received at said at least one point
`energy source in the frequency domain for each
`frequency; and
`within said geophysical survey for each activation
`deriving the inverse Fourier transform to the time
`of said ?rst and second seismic energy source; and
`analyzing said recorded seismic signal amplitudes
`domain of the signals received from each seismic
`utilizing said selected time delays in order to sepa
`energy source.
`7. A method of obtaining seismic data for a geophysi
`rate signals originating from each of said seismic
`cal survey according to claim 6 wherein said ?rst and
`energy sources.
`second seismic energy‘ sources are surface energy
`2. A method of obtaining seismic data for a geophysi
`sources.
`cal survey according to claim 1 wherein said ?rst and
`8. A method of obtaining seismic data for geophysical
`second seismic energy sources are surface energy
`survey according to claim 6 wherein said second se
`sources.
`lected time delay comprises a non-integral multiple of
`3. A method of obtaining seismic data for geophysical
`the period of said selected frequency component.
`survey according to claim 1 wherein said second se
`9. A method of obtaining seismic data for geophysical
`lected time delay comprises a non-integral multiple of
`survey according to claim 6 wherein said ?rst selected
`the period of said selected frequency component.
`60
`time delay comprises not less than one-quarter and not
`4. A method of obtaining seismic data for geophysical
`more that three-quarters of the period of said selected
`survey according to claim 1 wherein said ?rst selected
`frequency component.
`time delay comprises not less than one-quarter and not
`10. A method of obtaining seismic data for geophysi
`more that three-quarters of the period of said selected
`cal survey according to claim 6 wherein the step of
`frequency component.
`“
`recording the amplitudes of the seismic signals received
`5. A method of obtaining seismic data for geophysical
`at said at least one point within said geophysical survey
`survey according to claim 1 wherein the step of record
`comprises digitally recording said amplitudes.
`ing the amplitudes of the seismic signals received at said
`
`50
`
`*
`
`i
`
`i
`
`i
`
`i
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`5
`
`25
`
`45
`
`65
`
`WesternGeco Ex. 1006, pg. 7
`
`

`

`UNITED STATES PATENT AND TRADEMARK OFFICE
`CERTIFICATE OF CORRECTION
`
`PATENT NO. : 4,953 , 657
`DATED
`: Septanber 4, 1990
`
`INVENTOR(S) : Bruce L. Eddington
`
`It is certified that error appears in the above-identi?ed patent and that said Letters Patent is hereby
`corrected as shown below:
`
`In Column 4, Line 21, the word "any" should read ——many——
`
`Signed and Sealed this
`
`Twenty-third Day of June, 1992
`
`Attest:
`
`Arresting O?icer
`
`Acting Commissioner of Patents and Trademarks
`
`DOUGLAS B. COMER
`
`WesternGeco Ex. 1006, pg. 8
`
`