Simultaneous sources: The inaugural full-field, marine seismic case history
`Craig Beasley* and Ian Moore, WesternGeco, David Monk and Laurence Hansen, Apache Energy Ltd
`
`Summary
`
`Simultaneous (blended) sources have attracted a great deal
`of attention recently because of their potential to increase
`significantly the rate at which seismic data can be acquired.
`The viability of the method was previously demonstrated
`through the use of small-scale tests on synthetic and field
`data. In this paper, we present a case history from Australia
`of the first field-development-scale use of this technology
`in the world.
`
`Concept studies involving simulations of simultaneous-
`source data from conventional data indicated that the
`proposed survey design would yield data that were
`separable into components for each source. The resultant
`data set contains twice as many traces as its conventional
`equivalent, and provides improved sampling for important
`processing steps such as coherent noise attenuation.
`
`Simultaneous-source acquisition requires quality control
`methods that are specific to the technique to ensure that the
`data are acquired as planned. New QC methods were
`developed specifically for this project, and showed that no
`problems related to the simultaneous-source technique were
`encountered.
`
`Data processing involved source separation at an early
`stage, after which a conventional processing sequence
`could be used on the resultant, densely sampled data set.
`Separation was performed using a sparse
`inversion
`technique, which proved very effective. Very little signal
`leakage was observed, and the interference was almost
`completely suppressed. Preliminary comparisons with
`existing data are encouraging with respect to the main
`objective of the project, which was to improve prestack
`amplitude fidelity for inversion.
`
`Through this case history, we demonstrate the viability of
`simultaneous sources as an effective marine seismic
`acquisition method.
`
`Introduction
`
`established
`an
`is
`acquisition
`Simultaneous-source
`technology for land data, and has a proven record of
`providing enormous increases in acquisition efficiency. A
`plethora of associated acronyms bear witness to the rapid
`development of the technology over recent years. Notable
`methods are slip-sweep (Rozemond, 1996), HFVS (Allen et
`al., 1998), DSSS (Bouska, 2010), ISS (Howe et al., 2008),
`and DSS (Bagaini and Ji, 2010). A useful summary was
`provided by Bagaini (2010). All of these methods provide
`
`efficiency gains through triggering two or more sources
`sufficiently close together in time that the recorded energy
`interferes. The interference is then handled in processing.
`
`The corresponding techniques for marine acquisition have
`seen somewhat slower development, despite being
`introduced over a decade ago (Beasley et al., 1998). The
`main reasons for this are consequences of the extra
`constraints marine acquisition places on its sources.
`Specifically, marine sources typically lack the ability to
`shape the source wavelet as can be done for land vibrators.
`Moreover, each source must move continuously at constant
`speed, and introducing extra source boats to achieve
`significant distance
`separation between
`sources
`is
`expensive.
`
`studies
`simultaneous-source
`Initially, most marine,
`involved wide-azimuth (WAZ) data (Stefani et al., 2007;
`Fromyr et al., 2008; Dragoset et al., 2009). WAZ
`acquisition typically involves multiple source vessels, and
`may require several passes for each line. Simultaneous
`sources can be used to reduce the shot interval for each
`pass,
`thereby reducing aliasing effects or improving
`efficiency by reducing the number of passes that are
`required.
`
`In this study, we consider the application of simultaneous-
`source acquisition and processing (SimSource+) technology
`to a conventional, narrow-azimuth (NAZ) survey known as
`Cambozola, and extend the material previously presented
`by Moore et al. (2012). Data from the region were
`previously acquired using a standard “flip-flop” technique,
`in which the sources fire alternately every 18.75 m, leading
`to a 37.5-m shot interval for each source line. The use of
`simultaneous sources, such that both sources are fired every
`18.75 m, halves the shot interval for each source line,
`thereby providing better-sampled data for coherent noise
`attenuation and imaging. No extra vessels are required, and
`the overall acquisition time is unaffected.
`
`In principle, the simultaneous-source methodology is very
`simple. The sources are dithered in time relative to one-
`another to enable separation using a sparse inversion
`technique (Moore et al., 2008; Akerberg et al., 2008). Once
`separated, the data can be processed conventionally, and
`will benefit naturally from the improved sampling. The key
`to success is, therefore, the quality of the source separation.
`To mitigate the risk associated with the separation process,
`concept studies were performed on similar data sets. These
`studies indicated that source separation was possible at the
`proposed shot interval.
`
`
`© 2012 SEG
`SEG Las Vegas 2012 Annual Meeting
`
`DOI http://dx.doi.org/10.1190/segam2012-0834.1
`Page 1
`
`Downloaded 09/18/15 to 64.124.209.76. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/
`
`PGS Exhibit 2018
`WesternGeco v. PGS (IPR2015-00309, 310, 311)
`
`

`
`Simultaneous sources: Inaugural case history
`
`
`
`
`
`
`
`Figure 1: Separation of part of a common offset plane. The input data (top left) are separated into S1 (top right) and S2 (bottom left) components,
`plus a residual (bottom right) of unseparated energy. The residual is small and contains mainly noise. The S2 data are shown in S1-time and the
`lack of coherent energy in this section indicates a low level of leakage. In S2-time, the separated S2 data look much like the separated S1 data due
`to the close proximity of the sources. About 3 s of data are shown, and the lateral extent is about 7.5 km.
`
`The simultaneous-source separation process depends on the
`dithers, and does not require that the sources be physically
`separated by a significant distance. In fact, the close
`proximity of the two sources for this survey ensured that
`the relative signal strengths were comparable, avoiding
`problems that can occur if the signal from one source
`dominates the record.
`
`Only minor software modifications to the point-receiver
`marine acquisition system were required to acquire data in
`simultaneous-source mode. However, some of the standard
`quality control processes were no longer applicable because
`of
`the
`interference between sources, and additional
`processes were required to check that the firing times and
`navigational data were correct. In practice, these processes
`worked well, and no specific problems were encountered.
`
`Processing is ongoing, but source separation results so far,
`both for prestack data and for stacks of imaged data,
`indicate that the separation step performs well, as expected
`from the concept studies.
`
`Concept studies
`
`Concept studies (Moore et al., 2012) were performed to
`assess the viability of the method for the proposed survey.
`In the primary study, a line was acquired in both
`conventional (single source) and simultaneous-source
`modes. Simultaneous-source data were simulated from the
`conventional data, separated, and the results compared with
`the acquired data. The results indicated that separability
`was good at the proposed shot interval (18.75 m), a
`conclusion that was enforced by results from the acquired
`simultaneous-source data.
`
`As a final test, separation tests were run on a simulated
`simultaneous-source line from the Cambozola survey area
`to investigate water depths and noise levels that were
`representative of the proposed survey. The separation
`results were also considered satisfactory.
`
`
`© 2012 SEG
`SEG Las Vegas 2012 Annual Meeting
`
`DOI http://dx.doi.org/10.1190/segam2012-0834.1
`Page 2
`
`Downloaded 09/18/15 to 64.124.209.76. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/
`
`PGS Exhibit 2018
`WesternGeco v. PGS (IPR2015-00309, 310, 311)
`
`

`
`Simultaneous sources: Inaugural case history
`
`
`
`Figure 2: Comparison of Minden (left) and Cambozola prestack depth migration (PSDM) stacks (in time). There is no evidence of interference on
`the simultaneous-source stack, indicating that the separation methodology has performed well.
`
`Data acquisition
`
`the use of
`from
`The acquisition geometry, apart
`simultaneous sources, was a conventional NAZ geometry
`using 10 x 6000-m point-receiver cables with 75-m
`separation. The two sources were fired simultaneously
`(apart from the dithers) every 18.75 m. One source was
`considered to be the “master” and shot on position. The
`“dithered” source fired with prescribed time differences
`(dithers) relative to the master source. The dithers were
`essentially randomly distributed over a small time window.
`
`Conventional QC products were used whenever they were
`appropriate. In addition, QC products were designed
`specifically to check that the firing times were in agreement
`with the planned dithers. The main methods used were
`visual checks of coherency on passively separated data and
`automated picking of the dither times on autocorrelations.
`More details are given by Moore et al. (2012).
`
`the
`the simultaneous-source aspects of
`In practice,
`acquisition proceeded without any problems. Some lines
`were reshot due to weather, and there was a small amount
`of infill.
`
`Method and results
`
`from
`differ
`data
`simultaneous-source
`Recorded
`conventional data in that each trace has two shot locations
`
`associated with it, as well as a dither time. At this stage, the
`data volume is the same as for the equivalent conventional
`survey. Any processes applied before separation must
`preserve the signal from both sources. The separation
`process cannot, of course, separate noise that is not source
`generated, and it is, therefore, desirable to remove this
`noise component prior to separation. Noise attenuation of
`this kind must generally be run in the common shot domain
`where the signal from both sources is coherent. In other
`domains, the signal from one source will not be coherent
`and the noise attenuation process is likely to attenuate that
`signal.
`
`Active separation was performed using a sparse inversion
`method (Moore et al., 2008) applied to common channels.
`Sparseness is promoted using a time-domain, linear Radon
`transform
`that effectively separates each
`trace
`into
`estimated components for each source, together with a
`(small) residual of unseparated energy. Figure 1 shows an
`example
`that demonstrates
`the effectiveness of
`the
`separation process. The residual contains both ambient
`noise and signal that has not been modeled, typically
`because it is weak or complex. To avoid attenuating this
`signal, the residual was added back to the separated data for
`both sources. After separation, the data volume is doubled
`and each trace is associated with only a single source.
`Conventional processing can be used from this point
`onwards.
`
`© 2012 SEG
`SEG Las Vegas 2012 Annual Meeting
`
`DOI http://dx.doi.org/10.1190/segam2012-0834.1
`Page 3
`
`Downloaded 09/18/15 to 64.124.209.76. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/
`
`PGS Exhibit 2018
`WesternGeco v. PGS (IPR2015-00309, 310, 311)
`
`

`
`Simultaneous sources: Inaugural case history
`
`expectation is that the better sampled data set will have
`correspondingly better amplitudes.
`
`The second data set (Orcus) was acquired within three
`months of the Cambozola survey and had a similar
`geometry, though the line orientations were not the same
`and the processing was performed separately. Nevertheless,
`it provides a comparison data set for which the main
`difference
`is
`the
`simultaneous-source methodology.
`Although comparisons are not yet available for publication,
`we expect, based on previous experience, to see that the
`simultaneous-source acquisition yields better attenuation of
`the coherent noise, and that doubling the fold leads to an
`improved signal-to-noise ratio in the image.
`
`Conclusions
`
`Although data processing is not yet complete, a preliminary
`conclusion is that the simultaneous-source acquisition
`technique was successful. No issues were encountered
`during the data acquisition phase, indicating that the
`method is sufficiently robust to be used on full-scale 3D
`surveys. The critical source-separation step
`in
`the
`processing sequence performed well. There was very little
`signal leakage between sources, and results so far have
`been in accordance with expectations based on the concept
`studies and on preliminary comparisons with existing data.
`
`This project involved the use of advanced technology in a
`new area, and minimizing the associated risks was
`extremely important. The preliminary concept studies were
`very valuable in providing confidence that the method
`would work, and in assisting with the survey design.
`Concept studies such as these are highly recommended,
`especially when the proposed simultaneous-source survey
`is in a new area, or utilizes a new acquisition geometry.
`
`Acknowledgements
`
`The authors acknowledge the contributions made to the
`acquisition and processing of the simultaneous-source data,
`as well as to the preparation of this manuscript, by Jim
`Ross, Paul Anderson and Rob Kneale (Apache), and by
`Bart Szydlik, Jason Gardner, Richard Bisley, Chris
`Semeniuk, Dea Mustafa Hudaya, Ted Phillips and Morten
`Svendsen (WesternGeco), as well as the crew of the
`Western Spirit.
`
`We also thank the JV partners for blocks WA-290-P and
`WA-450-P, namely Apache Energy, Finder Exploration,
`Santos, OMV Australia, JX Nippon Oil & Gas Exploration
`Corporation and Tap Oil, for permission to publish.
`
` Mark of Schlumberger
`
` +
`
`
`Figure 3: Back-to-back comparison of Minden (left) and
`Cambozola PSDM gathers (in time). The longer offsets (red lines)
`and improved sampling of the simultaneous-source data set allows
`us to have greater confidence in the amplitude fidelity and
`suitability for AVO analysis and inversion. The blue line indicates
`the mute used to generate Figure 2.
`
`Comparisons with existing data
`
`Two existing data sets overlap the Cambozola survey area.
`The Minden data set (1995) covers most of the area, but
`had 12 times lower sampling density overall, of which only
`a factor of two was due to simultaneous-source acquisition.
`In addition,
`the Minden and Cambozola processing
`sequences were very different. It is, therefore, not possible
`to attribute any differences in the results directly to
`simultaneous-source
`technology. Comparisons
`do,
`however, provide validation that the simultaneous-source
`methodology is viable, and give an indication as to whether
`the survey objectives are likely to be achieved.
`
`Figures 2 and 3 compare depth-migrated stacks and gathers,
`respectively, between Minden and Cambozola. We neither
`expect, nor observe, major differences in the stacks, which
`is reassuring. The expectation is that the amplitude fidelity
`in the gathers will be improved. Further analysis is
`necessary to determine whether this is indeed the case,
`though it is clear that there are differences in the AVO
`behavior between
`the surveys, and
`the
`theoretical
`
`© 2012 SEG
`SEG Las Vegas 2012 Annual Meeting
`
`DOI http://dx.doi.org/10.1190/segam2012-0834.1
`Page 4
`
`Downloaded 09/18/15 to 64.124.209.76. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/
`
`PGS Exhibit 2018
`WesternGeco v. PGS (IPR2015-00309, 310, 311)
`
`

`
`http://dx.doi.org/10.1190/segam2012-0834.1
`
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`© 2012 SEG
`SEG Las Vegas 2012 Annual Meeting
`
`DOI http://dx.doi.org/10.1190/segam2012-0834.1
`Page 5
`
`Downloaded 09/18/15 to 64.124.209.76. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/
`
`PGS Exhibit 2018
`WesternGeco v. PGS (IPR2015-00309, 310, 311)