GEOPHYSICS, VOL. 55, NO. 10 (OCTOBER 1990); P. 1389-1396, 15 FIGS.
`
`Encoding techniques for multiple source point seismic data acquisition
`
`J. E. Womack*, J. R. Cruz*, H. K. Rigdon:|:, and G. M. Hooveri
`
`ABSTRACT
`
`Recent advances in vibrator electronics have made the
`use of encoded sweeps for multiple source point data
`acquisition possible in an operational setting. Alternatives
`to existing operational multiple source point data acquisi-
`tion techniques, using complementary series and E-codes,
`are developed in this paper.
`Most existing techniques are, at each source point, a
`series of linear sweeps of predetermined polarity that
`enables the cancellation of the contributions from the
`other source points in processing. The complementary
`series techniques developed here also choose polarities
`such that the contributions from other source points can
`be cancelled. -Pairs of E-codes have been found that pro-
`duce no crosscorrelation, which makes it possible to use
`E-codes to produce a dual source point technique that is
`fundamentally different
`from the more conventional
`techniques.
`Field tests are carried out using E-codes in dual source
`point schemes. Records from the respective source points
`are readily separated from the composite data collected
`and compared with records produced by a linear sweep
`from a single source point. Harmonic distortion appears
`to be the major limiting factor; however, record quality
`indicates that E-codes can be used in operational multi-
`ple source point data acquisition.
`
`INTRODUCTION
`
`For over a decade there have been efforts to develop vibrator
`signals for seismic data acquisition schemes that allow more than
`one source point to be recorded simultaneously. In certain cases,
`such as vertical seismic profiling (VSP) and 3-D data acquisi-
`tion, the additional costs of data processing, field—crew members,
`and hardware for simultaneous source point acquisition are
`economically justifiable.
`
`Most existing multiple source point acquisition techniques rely
`on the use of linear sweeps of differing phase polarities. One
`such technique assigns to each source point a sequence of
`upsweeps, each of positive or negative phase polarity (Silvennan,
`1979). The idea behind this technique is to remove the signal
`due to all other source points by cancelling their contributions
`in the vertical stacking process. In practice, there is never com-
`plete cancellation because surface conditions differ and vibrator
`hardware cannot reproduce all sweeps identically. An extension
`to this technique that helps reduce these effects includes down-
`sweeps of opposite polarities as source point signals (Garotta,
`1983).
`Sometimes record degradation from these two techniques
`is of unacceptable levels. Therefore, investigations into new
`vibrator signals for use in multiple source point data acquisi-
`tion continue. Here we discuss two sweep encoding techniques
`that have been introduced as ways to reduce record correlation
`noise (Bernhardt et al., 1978; Edelman et al., 1983) as possible
`signals for use in multiple source point data acquisition (Womack
`et al., 1988).
`
`ENCODING SWEEPS FOR MULTIPLE SOURCE POINT SIGNALS
`
`The choice of upsweeps and downsweeps as signals for use
`in multiple source point recording is natural since these are the
`most commonly used signals in conventional recording. Also,
`vibrators are specifically designed to generate these signals.
`Recent vibrator electronics technology has made vibrators that
`generate a sequence of sweeps with no listening period between
`sweeps readily available. Proper encoding of such a sequence
`of sweeps provides a vibrator realization of complementary series
`(Golay, 1961) and E-codes (Welti, 1960) as shown in Figure 1.
`These codes have been investigated for use in data acquisition
`by Bernhardt and Peacock (1978).
`Each code of these encoding techniques consists of two
`separate sequences. They possess the property that if the auto-
`correlation of the first sweep-encoded sequence of the pair is
`added to that of the second sequence, a resulting correlation
`
`Manuscript received by the Editor January 13, 1989;_ revised manuscript received May 9. 1990.
`‘School of Electrical Engineering and Computer Science, University of Oklahoma, 202 West Boyd, Norman, OK 73019.
`1Phillips Petroleum Company, Bartlesville, OK 74004.
`©1990 Society of Exploration Geophysicists. All rights reserved.
`
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`

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`ENCODING TECHNIQUES
`
`Womack et al.
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`SEQUENCE 2
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`FIG. 1. Examples of a 4-bit complementary series code pair
`(top) and of a 4-bit E-code pair (bottom). The upward slope
`of the line in each element signifies an upsweep and downward
`slope a downsweep. The + and — in the circles indicate the
`polarity of the sweep. Each sweep is tapered.
`
`ENCODING TECHNIQUES
`
`SWEEP LACS
`
`FIG. 3. The autocorrelation of the 4-bit E-code sequences in
`Figure 1: sequence 1 (top), sequence 2 (middle), the sum of the
`two autocorrelations (bottom).
`
`ENCODING faeflmqvss
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`F10. 4. Sequence of codes for dual source point data acquisition
`using 4-bit complementary series.
`
`However, these codes can still be used; an example for dual
`source point data acquisition is shown in Figure 4. As can be
`seen, four encoded sweeps will be applied at both source points
`1 and 2. It is clear that the last two sweeps for source point 2
`are merely phase-inverted versions of the first two. Therefore,
`this scheme is, in principle, equivalent to the Silverman tech-
`nique of cancelling the contributions of the signals from the
`other source points through vertical stacking. That
`is, by
`adding record 11 to record 21 and assuming that phase-inverted
`sweeps can be perfectly generated, then the contribution of
`source point 2 will be eliminated.
`Since vibrators have difficulty reproducing simple sweeps, it
`is unlikely that the more complicated encoded sweeps could be
`reproduced any better. However, if the errors in the reproduc-
`tion of sweeps are random, it may be possible for the encoded
`sweep, since it uses many short encoding sweeps, to reduce
`contributions from other source points in the correlation pro-
`cess. In such conditions, this benefit would have to be weighed
`against the problem of harmonic distortion almost certain to
`pose problems because of the short encoding sweeps used in the
`
`-2
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`FIG. 2. The autocorrelation of the 4-bit complementary. series
`sequences in Figure 1: sequence 1 (top), sequence 2 (middle),
`the sum of the two autocorrelations (bottom).
`
`wavelet confined to a time period twice the length of the sweeps
`used to realize the code is produced (Edelman et al., 1982). This
`can be observed in Figures 2 and 3. The bandwidth and tapers
`on each sweep in the encoded sequence will have a similar impact
`as on conventional sweeps.
`Complementary series are binary codes that can be realized
`with either linear or nonlinear upsweeps or downsweeps of a
`positive or negative polarity. If, for a dual-source scheme, two
`complementary series whose first sequences have a crosscorrela-
`tion equal and opposite to that of the second sequences could
`be found, then, by simple correlation with the proper reference
`before stacking, records from the individual source points would
`be recovered. Two complementary series with this property have
`yet to be found.
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`

`
`Encoding Techniques
`
`1391
`
`encoding process. Also, the additional storage costs of doubling
`the amount of field data, if the records are stacked in the field,
`and increasing processing by one correlation per source point
`have to be considered.
`
`For complementary series, the ill effects of error in sweep
`reproduction will be most prominent in the source wavelet
`around one encoding sweep width (see Figure 2). This will show
`up in the data as a multiple and can possibly be removed by
`standard multiple-removal
`techniques or by choosing the
`encoding sweeps to be longer, thereby moving the multiple down
`in the record. Also, in an extension to this technique, complemen-
`tary series can use codes comprising upsweeps at some source
`points and codes comprising downsweeps at others, as suggested
`by Garotta (1983).
`E-codes are quaternary codes and must be realized with
`upsweeps and downsweeps of both polarities. Pairs of E-codes,
`unlike complementary series, have been found such that the
`crosscorrelation of the first sequences added to the cross-
`correlation of the second sequences is zero. Therefore, encoding
`techniques can be devised (see Figure 5) using E-codes that are
`fundamentally different from any technique that cancels the
`contributions of other source points through phase inversion
`of sweeps.
`
`ENCODING TECHNIQUE
`
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`FIG. 5. Sequence of codes for dual source point data acquisition
`using 4-bit E-codes.
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`ENCODING Tzclvmlfiuzs
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`FIG. 6. The 8-bit E-code pairs whose crosscorrelations between
`code 1 of each pair and between code 2 of each pair sum to zero.
`
`There are only four 4-bit and eight 8-bit E-codes. From these
`there is a pair of 4-bit codes and two pairs of 8-bit codes which
`satisfy this property. The 4-bit codes are exhibited in Figure 5
`and two 8-bit pairs are shown in Figure 6. Through record
`cancellation, E-codes can be extended to encompass more source
`points than this dual source point scheme.
`One advantage of E-codes over the complementary series is
`that the residual in the source wavelet caused by imperfect sweep
`reproduction by the vibrators will not be prominent around one
`encoding sweep width, and will not,
`therefore, cause the
`problems with multiples of the complementary series. E-codes
`
`can still suffer from harmonic distortion as with the complemen-
`tary series. Problems with harmonic distortion can be combated
`by making the encoding sweeps as long as possible. These effects
`may also be minimized by using harmonic distortion-reduction
`techniques (Martinez et al., 1987).
`
`FIELD TESTING
`
`Theoretical and modeling studies of complementary series
`and E-code encoding schemes previously described indicate that
`E-code techniques may be superior for dual source point record-
`ings. Field tests were performed using E-code techniques for fur-
`ther evaluation of these encoding schemes.
`Figure 7 shows the recording geometry used to field test the
`E-code techniques. Two vibrators, separated by 300 ft (91.5 m),
`were simultaneously recorded by a clamped downhole geophone
`at 500 ft (152.4 m) and by a series of surface geophones. Both
`the 4-bit and the 8-bit E-codes were tested.
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`field acquisition geometry for
`FIG. 7. Plane view of
`simultaneous vibrator testing.
`
`Records were generated from the vibrator located 900 ft (274.4
`m) off-end from the surface spread (VB1) and from a second
`vibrator
`(VB2), both using conventional
`(uncoded)
`linear
`downsweeps. Records were also generated with the vibrators
`operating simultaneously using E-code sweeps. The downhole
`geophone records are shown in Figure 8. The sweeps used for
`the encoded sweeps were 3.5 5 linear upsweeps and downsweeps
`over a frequency range of 10-96 Hz. A 0.2 s cosine-squared taper
`was applied to both ends of the sweeps. The conventional sweep
`was a 14 s linear downsweep over the 10-96 Hz bandwidth
`and also used a 0.2 s taper. A sum of four sweeps for both the
`sequential and simultaneous sources was used for each record.
`The downhole comparisons indicate that the encoded sweep
`records extracted have visually identical waveforms to the
`conventional sweep records for both vibrator locations. Differ-
`
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`

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`1392
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`Womack et al.
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`FIG. 8. The signals received at the downhole geophone from VB] (top) and VB2 (bottom) with the vibrator sweeps
`being: linear sweeps applied separately, 4-bit E-code sweeps applied simultaneously, and 8-bit E-code sweeps applied
`simultaneously.
`
`ences in signature shapes between the vibrator positions are
`attributed to differences in base-plate coupling.
`Figure 9 compares power spectra of the downhole recordings.
`These plots indicate that
`the records extracted from the
`simultaneous encoded sweeps and from the sequential uncoded
`sweeps have nearly identical power spectra.
`Figures 10-12 show records from the surface geophones using
`the same source vibrator parameters as Figures 7-9. Figure 10
`shows the single vibrator (VB2) conventional sweep record.
`Figures ll and 12 show the extracted VB2 records for the
`simultaneously recorded sweeps using the 4-bit and 8-bit E-codes,
`respectively. Each trace of the surface-spread records was
`acquired with a single 10 Hz geophone A 25-90 Hz band-
`pass filter has been applied to attenuate strong surface wave
`events and to enhance the reflection events for better comparison.
`A comparison of Figures 11 and 12 shows that the extracted
`4-bit and 8-bit E-code records are in good agreement and that
`both are similar to the sequential, standard-sweep record. Major
`coherent events (surface waves, air waves, reflection events, and
`reverberations) apparent on the conventional sweep recording
`are very similar on the 4-bit and 8-bit encoded recordings.
`Reflection events near 2000 ft
`(610 m) offset appear to
`demonstrate improved signal-to—noise ratio at reflection times
`
`of 0.98 s on the E-code records. A major difference between
`the conventional and E-coded records is the presence of a low-
`frequency event on the E-code measurements which does not
`appear on the conventional recording. This event tracks the
`surface wave in offset and time with a time delay of about 400
`ms and indicates that it is a correlation ghost associated with
`the upsweeps in the E-code sweep. A_ comparison of Figures 11
`and 12 indicates that correlation ghost noise is stronger on the
`4-bit E-code record. However, it is difficult to distinguish be-
`tween the effects of ghosting and the strong reverberations.
`Two-dimensional power spectra of the surface-recorded
`records in Figure l0—l2 are shown in Figures 13-15. Maximum
`power levels are indicated by the darkest shading, and power
`levels of -60 dB are represented by the lightest area. The time
`transform was limited to a 0.4 to 0.8 5 window.
`Analysis of the f-k spectra indicates that the E-code records
`have similar power spectral properties to the conventional sweep
`record. The E-code records appear to contain a slightly greater
`proportion of energy with sweep velocities corresponding to
`signal, reverberation, or surface waves in the 30-75 Hz band-
`width. This indicates a slightly larger coherent-to-background
`noise ratio for the E-code records. However, it is difficult to ascer-
`tain whether this benefit can be realized in operational use.
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`
`

`
`Encoding Techniques
`
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`FIG. 9. Power spectra for the signals shown in Figure 8.
`
`CONCLUSIONS
`
`A significant residual in the source wavelet at one encoding
`sweep width using complementary series may result and cause
`a multiple in the data of one encoding sweep width. The
`multiples will be more prominent in the data that are deeper
`and at farther offsets.
`E-code techniques do not produce a prominent residual in the
`source wavelet located around one encoding sweep width and
`do not rely on source point cancellation techniques. For this
`reason, E-codes are considered, for most uses, the most prom-
`
`ising of the two encoding techniques introduced here for
`multiple source point acquisition. The field tests show that
`records can be readily separated using 4-bit and 8-bit E-codes
`for the dual source point scheme. A slightly larger coherent—to-
`background noise ratio in records obtained with E-codes as
`compared to conventional single source recording was evident.
`Record degradation due to the effects of relatively large
`encoding sweep tapers required to implement E-code techniques
`was not noticeable in the field test data but requires further
`investigation. The major limiting factor in these techniques may
`be the ill effects of harmonic distortion on records due to the
`
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`
`

`
`1394
`
`womack et al.
`
`use of short encoding sweeps. Collection of data at greater
`depths is required to better define the relative strengths and
`weaknesses of E-codes and conventional sweep technologies;
`however, there are indications that Ecodes can be used in opera—
`tional multiple source point data acquisition.
`
`ACKNOWLEDGMENTS
`
`The authors would like to thank Tom Thomas of Phillips
`Petroleum Co. for his time and efforts in processing the field
`data.
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`FIG. 10. Record recorded with surface geophone spread using
`VB2 with conventional linear downsweep.
`
`11. Record extracted for VB2 recorded with surface
`FIG.
`geophone spread using both VBI and VB2 and 4-bit E-code
`sweeps.
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`FIG. 12. Same as Figure 11 using 8-bit E-code sweeps.
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`WesternGeco Ex. 1013, pg. 6
`
`

`
`FIG. 13. Frequency—wavenum—
`ber (f-k) power spectrum of
`record in Figure 10, 0.4-0.8 s
`time window.
`
`FIG. 14. Frequency-wavenum-
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`record in Figure 11, 0.4-0.8 s
`time window.
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`
`WesternGeco Ex. 1013, pg. 7
`
`

`
`1396
`
`Womack ei al.
`
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`Bernhardt, T., and Peacock, J. H., 1978, Encoding techniques for the
`Vibroseis system: Geophys. Prosp., 26, 184-192.
`Edelman, H. A. K., and Werner, H., 1982, Combined sweep signals for
`correlation noise suppression: Geophys. Prosp., 30, 786-812.
`The encoded sweep technique for Vibroseis: Geophysics, 47,
`809-818.
`,
`Garotta, R., 1983, Simultaneous recording of several Vibroseis seismic
`lines: 45th Eur. Assn. Expl. Geophys., Expanded Abstracts, 35; and
`53rd Ann. Intemat. Mtg., Soc. Explor. Geophys., Expanded Abstmcts,
`308-310.
`
`Golay, M. J. E., 1961, Complementary series: Inst. Radio Eng. Trans.
`Inf. Theory, 7, 82-87.
`Martinez. D. R.. and Crews. G. A., 1987, Evaluation of simultaneous
`Vibroseis recording: 57th Ann. Internat. Mtg., Soc. Expl. Geophys.,
`Expanded Abstracts, 577-580.
`Silverman, D., 1979, Method of three-dimensional seismic prospecting:
`U.S. patent 4 159 463.
`Welti, G. R., 1960, Quaternary codes for pulsed radar: Inst. Radio Eng.
`Trans. Inf. Theory, 6, 400-408.
`Womack, J. E., Cruz, J. R., Rigdon, H. K., and Hoover, G., 1988,
`Simultaneous Vibroseis encoding techniques: 58th Ann Intemat. Mtg.,
`Soc. Expl. Geophys., Expanded Abstracts, 101-104.
`
`LEGEM)
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`3; :° -222:: ,.,E-=A
`-43 to -48db :,,::
`-49 to -54db I’?-*,
`
`KJXII-0UNI
`
`
`
`FREQUENCY(HZ)
`
`WAVELENGTH (FT)
`
`FIG. 15. Frequency-wavenumber (f-k) power spectrum of record in Figure 12, 0.4-0.8 s time window.
`
`\ E
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`O.ODQ)
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`WesternGeco Ex. 1013, pg. 8
`
`

`
`112512014
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`Encodi ng techniqus for mulfi ple source point s eism ic data acquisifi on : GEOP HYS IC 5: Vol. 55. N o. 10 (S ociety of Exploration 6 eophys icists)
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`A new look at
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`A new look at
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`GEOPHYSICS
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`< Previous Article
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`Volume 55, Issue 10 (October 1990)
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`0 Abstract
`0 PDF
`0 CitedB
`
`J. E. Womack, J. R. Cruz, H. K. Rigdon, and G. M. Hoover (1990). ”Encoding
`techniques for multiple source point seismic data acquisition.” Encoding
`techniques for multiple source point seismic data acquisition, 55(10), 1389-
`1396.
`doi: 10.1190/1.1442787
`
`GEOPHYSICAL STUDIES
`
`Encoding techniques for multiple
`source point seismic data acquisition
`
`Article History
`
`Received: 13 January 1989
`Revised: 9 May 1990
`
`Publication Data
`
`A M
`
`ulticomponent
`
`sea-bottom seismicISSN (print): 0016-8033
`ISSN (online): 1942-2156
`surveying in the
`North Sea—Data
`Publisher: Society of Exploration Geophysicists
`interpretation and CODE“? gPYSa7
`J. E. Womack*, J. R. Cruz*, H. K. Rigdoni and G. M. Hooveri
`applications
`>I<
`Marine PSSP
`reflections with a
`http ://I ibr ary. seg. org/doi /abs/1 0.1 190/1 .1442767
`
`School Of Electrical Engineering and Computer Science, University of Oklahoma, 202 WCSt
`Ilnnrl 7\T,.......... n11 71010
`
`WesternGeco Ex. 1013, pg. 13
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`

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`1 1/25/2014
`
`Encoding techniques for multiple source point seismic data acquisition : GEOPHYSICS: Vol. 55, No. 10 (Society of Exploration Geophysicists)
`L)LJ_)’bI, LVUI lllilll, lJ.[\
`I JUJ_/
`
`bottom velocity
`transition zone
`
`:i:Pl1illips Petroleum Company, Bartlesville, OK
`
`H
`
`Recently Searched
`
` Recent advances in vibrator electronics have made the use of encoded sweeps
`COMPUTATION r multiple source point data acquisition possible in an operational setting.
`M Alternatives to existing operational multiple source point data acquisition
`APPLICATIONS
`
`techniques, using complementary series and E-codes, are developed in this
`
`paper. Most existing techniques are, at each source point, a series of linear
`sweeps of predetermined polarity that enables the cancellation of the
`contributions from the other source points in processing. The complementary
`series techniques developed here also choose polarities such that the
`
`Anywhere:
`encoding
`contributions from other source points can be cancelled. Pairs of E-codes have
`.
`.
`.
`.
`It 1
`t
`h '
`een found that produce no crosscorrelation, which makes it possible to use E-
` cm ue.S mu 1
`source point
`codes to produce a dual source point technique that is fundamentally different
`seismic data
`
`from the more conventional techniques. Field tests are carried out using E-
`acquisition (std -
`codes in dual source point schemes. Records from the respective source points
`L)
`A are readily separated from the composite data collected and compared with
` records produced by a linear sweep from a single source point. Harmonic
` llI' SStd - 644 distortion appears to be the major limiting factor; however, record quality
` indicates that E-codes can be used in operational multiple source point data
`mu1t1component|
`acquisition.
`AND |Anywhere:
`4b°“°m AND _Permal1nk. http.//dx.do1.org/10. 1 190/1 . 1442787
`|Anywhere: seismic
`sur...(std-281
`,
`Anywhere: marineclted
`pssp reflections
`Woohyun Son, Suk'oon Pyun, Changsoo Shin, Han-Joon Kim. (2014) An
`
`bm't,V_OttOmV610Ci
`transition Z0116 (Stdalgorithm adapting encoded simultaneous-source full-waveform inversion to
`11
`marine-streamer acquisition data. GEOPHYSICS 79:5, R183-R193.
`Online publication date: 1-Sep-2014.
`Abstract 1 Full Text 1 PDF (6368 KB) 1 PDF W/Links (774 KB)
`Jeff Godwin, Paul Sava. (2013) A comparison of shot-encoding schemes for
`wave-equation migration. Geophysical Prospecting
`61: 10.1 1 l 1/gpr.2013.6l.issue-sl, 391-408.
`Online publication date: 1-Jun-2013.
`CrossRef
`
`J.W. (Tom 1 Thomas, Dana M. Jurick, Dwight Osten. 2012. Vibroseis as an
`impulsive seismic source - 3D field testing Permian Basin Texas. SEG
`Technical Program Expanded Abstracts 2012, 1-5.
`Abstract 1 PDF ( 1034 KB) 1 PDF w/Links (665 KB) 1 Supplemental Material
`Ying Ji, Ed Kragh, Phil Christie. 2012. A new simultaneous source separation
`algorithm using frequency-diverse filtering. SEG Technical Program
`Expanded Abstracts 2012, 1-5.
`Abstract 1 PDF (997 KB) 1 PDF w/Links (794 KB)
`Kees Wapenaar, Joost van der Neut, Jan Thorbecke. (2012) Deblending by
`direct inversion. GEOPHYSICS 77:3, A9-A12.
`
`Online publication date: 1-May-2012.
`Abstract 1 Full Text1 PDF ( 1361 KB:
`Gbovega Aveni, Ali Almomin, Dave Nichols. 2012. On the separation of
`http:/Aibrary.seg.org/doi/abs/1 0.1 190/1 . 1442767
`
`WesternGeco Ex. 1013, pg. 14
`
`

`
`Encodi ng techniqus for multi ple s ource point 5 eis mic data acquis ii on: G EDP HYS IC 8: Vol. 56. N o. 10 (S ociety of Expl or aii on G eoptrys icists)
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`simultaneous-source data by inversion. SEG Technical Program Expanded
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`Abstracts 2011, 20-25.
`
`Abstract | PDF (1544 KB} |PDF w:‘Links
`Gbogega Ayeni. 2012. Seismic reservoir monitoring with permanent encoded
`seismic arrays. SEG Technical Program Expanded Abstracts 2010, 4221-4226.
`Abstract | PDF §3150 KB} |PDF w:’Links
`I. W. {Tom} Thomas, Bob Chandler Dwight Osten. 2012. Galcode:
`Simultaneous Seismic Sourcing. SEG Technical Program Expanded Abstracts
`2010, 86-90.
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`Abstract | PDF [1773 KB} |PDF W:"Linl{s | Supplemental Material
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`Gbogega Ageni, Yaxun Tang, Bindo Biondi. 2012. Joint preconditioned least-
`
`squares inversion of simultaneous source time-lapse seismic data sets. SEG
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`Technical Program Expanded Abstracts 2009, 3914-3918.
`Atzstnactl PDFJMIZKBI I EDDmCLink.& I SJ.Lm2l.:m£nLaLMa.Lm.al
`Qr_a1g__T__Be,a_fl_:1, 2012. Simultaneous sources: A technology whose time has
`come. SEG Technical Program Expanded Abstracts 2008, 2796-2800.
`Ahslmctl PDF_CiQ2KBl|
`B.iu;hJ..2:u1. . 2012. Flam —
`A simultaneous source wide azimuth test. SEG Technical Program Expanded
`Abstracts 2008, 2821-2825.
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`Ahmantl PDP_Cli32.KBl I EDbflm
`£2rai2.l_B.easlfiJL. &znalsl.E.£haml;u:rs.. Z.1:r2n2..Iian.z. 2012.-"~neW100k at
`simultaneous sources. SEG Technical Program Expanded Abstracts 1998, 133-
`135.
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`Encoding techniques for multiple source point seismic data acquisition : GEOPHYSICS: Vol. 55, No. 10 (Society of Exploration Geophysicists)
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