Optimal marine streamer acquisition with High-Density 3D · 2015-07-29 · Optimal marine streamer acquisition with High-Density 3D Andrew Long, PGS Technology (Perth, Australia),

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Optimal marine streamer acquisition withHigh-Density 3DAndrew Long, PGS Technology (Perth, Australia), [email protected]

Bill Pramik, PGS Technology (Houston, Texas), [email protected]

Marie Dunn, PGS Marine Geophysical NSA (Houston, Texas), [email protected]

Introduction

Our goal with the reflection seismic experiment is to obtainthe most clearly defined, high-resolution image of thesubsurface geology, free of contaminating noise. This goalcan be achieved by acquiring seismic surveys with denseand even sampling in both time and space.

Historically though, 3D marine streamer acquisition hasbeen forced to compromise spatial sampling in the name ofefficiency and cost. This compromise translates to noiseand degraded resolution when processing the data. Asignificant component of the seismic “noise” contami-nating 3D images actually arises during processing, as anunfortunate and inescapable artifact from poor 3D spatialsampling. Aliased data create noise during the applicationof any multi-channel processing operation, notably pre-stack migration. If the cross-line acquisition dimensioncould be sampled at an equally small interval as the inlinedimension, a much larger frequency bandwidth thantypical of standard 3D marine streamer acquisition couldbe preserved throughout all stages of processing, free ofaliasing and free of artifacts.

The High-Density 3D (HD3D™) marine streamer acquisi-tion method explicitly addresses the issue of tight 3Dspatial sampling. HD3D acquisition offers a data productthat is properly sampled in both spatial directions, is ofhigh quality, and is cost efficient.

Since 2001 the ability to routinely tow 12 to 16 streamers atvery small separation (37.5 to 50 m) has delivered a greatleap forward in the marine streamer acquisition scenariospreviously considered impossible – all at a comparablecost. Single-source shooting delivers high fold, increasedsignal-to-noise (S/N) ratio data, and inline processingfidelity, while dual-source shooting delivers the tight 3Dcross-line spatial sampling required for high resolutionimaging of highly dipping events.

This article includes two recent high-density 3D casestudies, acquired by PGS in the NW Palawan Basin,Offshore Philippines, and in the North Madura Platform,East Java Sea area. Stringent resolution requirements andsteep dip imaging were survey objectives in both cases.Consequently, survey planning recommended that bothsurveys be acquired with 12 streamers at close separation,in dual-source shooting mode. This approach allowed therecording of steeply-dipping data free of aliasing in anyd i rection, and enabled a high resolution pro c e s s i n ga p p roach to preserve a large frequency bandwidththroughout all stages of processing. In both cases, data

quality and resolution are excellent, enabling the ability toconfidently interpret a highly complex set of geologicalfeatures.

A powerful demonstration is made that HD3D marinestreamer acquisition delivers the optimum foundation forhigh resolution, high quality 3D data – provided with effi-ciency (i.e. no loss in downtime) and with comparable costto standard marine streamer 3D acquisition andprocessing.

HD3D Fundamentals

Modern seismic vessels, such as the Ramform design(Figure 1), with their massive towing capacities, havechanged the way in which modern seismic data areacquired.

Dual-source shooting has historically dominated streameracquisition, by comparison to single-source shooting,because the streamer separation with single-sourc eshooting must be halved to preserve cross-line processingfidelity. In the days of limited streamer capacity, smallstreamer spreads using small streamer separation wereprohibitively expensive to deploy. Since 2001, the ability totow 12 to 16 long streamers at 37.5 to 50.0 m separation,with no loss of efficiency (i.e. no increase in downtime), hasreduced costs so that single-source acquisition is cost effec-tive (Hegna et al., 2001; Long et al., 2003). Such surveyshave benefits of increased fold, improved 3D spatial reso-lution, and improved imaging quality. At the same time,certain survey locations are more amenable to dual-source shooting in high-density (close streamer

F i g u re 1: PGS Ramform Ve s s e l

January 2004 CSEG RECORDER 31

Focus Article Cont’d


separation) 3D mode, where the gains in cross-line spatialsampling more than offset the compromise in fold.

As the best possible resolution for a given midpoint range canbe obtained with a 3D zero-offset gather (Vermeer, 1999) in thecase of no noise, full offset stack resolution will be inferior tothe resolution of that zero-offset gather. However, in a noiseenvironment, where fold is required, resolution is a function ofspatial sampling, frequency bandwidth, aperture size (Chenand Schuster, 1999), and subsurface illumination. Regone(1998) demonstrates that coarse spatial sampling in any direc-tion will compromise migration success far more seriously thanlimited fold.

At this stage it is worthwhile considering the merits of theinline shot density. In comparison to dual-source shooting,single-source shooting doubles CMP fold (increasing the S/Nratio by 40%) and halves the trace spacing in the common-offset, -receiver, and –midpoint domains, thereby optimizing allmulti-channel pre-stack processing operations. However, theassociated doubling of the cross-line bin size for a givens t reamer separation will compromise cross-line (and fullvolume) resolution unless the streamer separation is corre-spondingly halved. In the case of 37.5 m streamer separation,single-source shooting with 2:1 receiver summation typicallyyields 18.75 x 12.5 m bins at high fold. Alternatively, for 50 mstreamer separation, dual-source shooting with 2:1 receiversummation typically yields 12.5 x 12.5 m bins with halved fold(compared to single-source shooting), and doubled inline shotseparation. Overall, dense streamer acquisition provides tightreceiver sampling in all directions and tight shot sampling inthe shooting (inline) direction.

Many reservoir targets have complex structural settings and areassociated with significant diffraction energy emanating fromfaults and steeply dipping features. To properly image suchdata, it is critical that diffraction energy is not aliased. Ingeneral, on unmigrated seismic data, the energy that isreflected and scattered from near-surface features appears as aseries of linear events in the seismic data, being extensive acrossmost arrival times. In 3-dimensional space, these events areactually hyperbolae that have an apex at the point of scattering,i.e. the location of the near-surface discontinuity. The mosteffective means of removing these events from the target eventsis through proper 3D migration. The basic problem oftenencountered in the migration of these scattered events is that,because of their steep apparent dip on the unmigrated seismicdata, they are usually severely aliased. When migration ofthese aliased events is performed with most migration opera-tors, the operator can become aliased (Biondi, 1998), resultingin “noise” being introduced into the data, contaminating theseismic section and reducing the resolution of events. In areaswhere diffraction noise is prevalent, it is possible that much ofthe (random) noise inherent throughout the data may be theresult of migration operator aliasing. It should be rememberedthat the purpose of correctly migrating these diffraction eventsis not to correctly image the near-surface discontinuities thatgenerated them, but to remove the diffractions, and their asso-ciated migration noise, from the reflection events near the

target depth. Therefore, bin dimensions should be as small aspossible prior to migration. Figure 2 (collated from Yilmaz,1987) demonstrates the effects of migration aliasing.

Overall, the high fold and dense spatial sampling offered byHD3D acquisition leads to 3D data with higher resolution andhigher S/N ratio. Furthermore, significant improvements arepossible in many of the necessary steps in data processing suchas velocity determination, multiple removal, noise attenuationand migration imaging solutions.

Offshore Philippines HD3D Case Study

PGS acquired the 1100 km2 O ff s h o re Philippines multi-clientHD3D survey during 2002 (Figure 3). A s t ructurally complexcarbonate reservoir occurs at about 3.2 s TWT, below a veryrugose, deep, water bottom, and undercompacted, stratigraphi-cally complex overburden. Previous 3D data (refer to Figure 5)had failed to yield satisfactory images of the overburden, andmany structural ambiguities at the deep reservoir levelc o n f ronted the drilling and production development of reserves.

Pre-survey planning by both PGS and the multi-client surveyparticipants concluded that “strike” shooting would be usedwith anti-parallel sail line shooting to improve the uniformityof subsurface target illumination coverage. Strict vertical reso-lution requirements sought 70 – 80 Hz frequencies at the 3.0 –3.2 s TWT target level. Consequently, source/streamer depthsof 5.0/6.0 m were used. Modeling of the horizontal resolutionrequirements indicated that to image the steepest dips, the pre-migration bin size must be no larger than 12.8 m. In the pursuitof optimal resolution, it was decided that 12.5 m cross-linespatial sampling would be acquired.

The M/V Ramform Challenger was used to tow 12 x 4500 mstreamers at 50 m separation, shooting in dual-source mode at

Optimal marine streamer acquisition with High-Density 3DContinued from Page 30

F i g u re 2 A series of zero-offset synthetic stacks of dipping interfaces are plottedalong the top ro w, with trace spacings (bin sizes) of 25, 50, and 100 m re s p e c t i v e l yf rom left to right. Note the aliasing at large trace spacing, typical of conventionald u a l - s o u rce CMP gather trace spacing. Migration results are shown along thelower ro w. Note the extreme contamination by migration noise when tracespacing exceeds 25 m. Migration resolution is clearly a function of tight spatialsampling. From Yilmaz (1987).




18.75 m shot interval. Thus, the natural CMP bin size (inline xcross-line) was 6.25 x 12.5 m, at 54 fold. In terms of trace densi-ties, the new HD3D survey yielded 691,000 traces/km2, incomparison to 95,000 traces/km2 corresponding to existing 3Ddata in the area.

Following the completion of acquisition, full Kirc h h o ff pre - s t a c ktime migration (PSTM) processing was employed. Data compar-ison to existing 1991 and 1993 3D data indicated significanti m p rovements in the structural imaging and resolution of thet a rget events (Figures 5 and 6). Figure 4 superimposes thecomparative amplitude spectra extracted from the data inF i g u res 5 and 6. At 70 Hz, the new HD3D data above the targ e tis 15 dB stronger than the existing 3D data – without any spectralwhitening or Q compensation being applied during pro c e s s i n g !

Furthermore, the 2002 PSTM data have better fault resolutionthan pre-stack depth migration (PSDM) versions of the earlier3D data sets. Such statistics are testament to the power of high-density 3D acquisition, even in an area containing rugose waterbottom and chaotic stratigraphic trends.

Why is the difference in frequency bandwidth between Figures5 and 6 so dramatic? Post-survey analysis of all the acquisitionparameters contributing to frequency content indicates that thedifference in cross-line spatial sampling is by far the mostimportant factor. In the original 3D case, steeply-dippingshallow events and diffraction energy cannot be imaged withfrequencies larger than about 60 Hz. In the HD3D case, morethan twice as many unaliased frequencies can be acquired andprocessed. At depth, the effects of anelastic Earth attenuationrestrict the highest frequency that can be used in both cases.F u r t h e r m o re, no data contamination from processing andmigration noise has affected the HD3D data. Overall, tight 3Dspatial sampling has allowed a much larger frequency band-width than normal to be acquired unaliased in the field andpreserved throughout all stages of processing – notably allthose steps that operate on more than one trace at a time (multi-channel processing). The net result is the significant improve-ment in resolution and interpretability observed in Figure 6.

East Java Sea HD3D Case Study

PGS acquired a 2729 km multi-client 2D survey in 2002 and a3963 km2 multi-client HD3D survey in 2003, both located in theEast Java Sea (Figure 7).

Interpretation of the 2D data allowed for the identification ofsource rock kitchens, migration fairways, and the main tectonicevents in the area. Several prospective features were inter-preted and influenced the location of the subsequent 3D survey.A complex series of shallow (Kujung) carbonate build-upsdominate the HD3D area and were given special attentionduring the pre-survey planning phase of the 3D survey. Inparticular, the ability to pursue high-resolution interpretationof shallow, steeply-dipping features demanded an emphasisupon very tight 3D spatial sampling.

The M/V Ramform Challenger was used to tow 12 x 3600 ms t reamers at 62.5 m streamer separation, shooting in


F i g u re 4. Superimposed amplitude spectra from the 1991 3D and 2002 HD3Ddata in Figures 5 and 6. The HD3D data are 15 dB stronger than the standard3D at 70 Hz.

F i g u re 3. Location map for the Offshore Philippines HD3D survey.





F i g u re 5. 1991 3D data from Offshore Philippines, acquired with 13.33 x 26.66 m bin size, 34 fold, 95,000 traces/km2, processed with a full PSDMflow (converted to TWT for display). Data courtesy of Shell.

F i g u re 6. 2002 HD3D data from Offshore Philippines, acquired with 6.25 x 12.5 m bin size, 54 fold, 691,000 traces/km2, processed with a Kirc h h o f fPSTM flow. In comparison to Figure 5, significant resolution improvements (up to 15 dB increase at 70 Hz) have been achieved at all depths.




dual-source mode at 12.5 m shot interval. Record length was4.0 seconds and nominal fold was 72. In comparison to stan-dard 3D acquisition, this represented a significantly increased

emphasis upon tight spatial sampling in all dire c t i o n scombined with maximum fold. In terms of trace densities, theHD3D survey with its 6.25 x 15.625 m CMPbins yields 3.2 timesthe trace density per square kilometer (737,280 vs. 230,400). Aspreviously established, high trace densities are an essentialp re requisite for successful pre-stack migration – a keyprocessing step in the East Java Sea HD3D survey.

The high signal-to-noise quality and excellent resolution of theEast Java Sea HD3D data are testament to the virtues of tight 3Dspatial sampling possible with the HD3D acquisition method(Figure 8). The HD3D seismic data have revealed the presenceof numerous, often complex prospects on both the upper andlower K/0ujung levels as well as at the deeper basement levels.Some of these structures may have closures of greater than 100km2 (Johansen, 2003). Migration pathways can thus be inter-preted, making detailed prospectivity analysis possible.

A common frustration with 3D acquisition is that high-qualityresolution observed on existing 2D data is not observed at acomparable standard on new 3D data. Ideally, the structuralimaging power of the 3D method combined with high resolu-tion in all directions is desired. As standard 3D acquisitiontypically samples 4 to 6 times more finely in the shooting(inline) direction, resolution is compromised in the cross-linedirection. Figure 8 shows that the resolution of the HD3D datais significantly improved compared to the 2D data, demon-strating how 3D pre-stack migration benefits from tightlysampled, unaliased 3D data.

Figure 9 presents a migrated time slice at 0.15 s TWT. Note thenegligible acquisition footprint, even at such a shallow depth.The resolution and definition of complex meandering channelssystems is quite astounding, testament to the resolution powerof the HD3D acquisition method in dual-source shooting mode.

Figure 10 presents a deeper time slice (1.0 s TWT), intersectingthe Kujung carbonate features. Again, resolution is excellent,evidenced also in the interpreted 3D surface plotted in Figure11. Frequency analysis indicates primary event frequencies inexcess of 80 Hz throughout the target region before any Qcompensation or spectral whitening. Such processes will obvi-ously improve the frequency bandwidth even further.


F i g u re 7: Location map for the East Java Sea HD3D survey. The survey wasdesigned to cover several prospective leads identified on existing 2D data.

F i g u re 8. Compared to existing 2D data (upper section), the new HD3D data(lower section) yields a significant improvement in resolution. On this 25 kmdata extract, steeply-dipping Kujung carbonate reef flanks are crisply imaged onthe HD3D data, testament to the virtue of very tight 3D spatial sampling duringa c q u i s i t i o n .

F i g u re 9: Time slice from the East Java Sea HD3D survey at 0.15 s TWT.Resolution of the complex meandering channel system is excellent. Horizontalscale ª 15 km.





Looking back at Figure 8 serves as a reminder that seismic datawill always be heavily contaminated with complex 3D scat-tering and diffraction energy. It is no surprise that out-of-the-plane events have rendered the 2D data comparatively low inresolution. However, if tight 3D spatial sampling (closestreamer separation, and dual-source shooting) had not beenused in this area, the final 3D images would have been similarlydegraded in resolution.

Conclusions

We have seen that high-density 3D acquisition in dual-sourceshooting mode yields very tight 3D spatial sampling. Thebenefits for significantly increased data resolution (bothtemporal and spatial) are evident at all data depths and loca-tions. Small-scale faults and stratigraphic trends become quitedramatically evident, even at very shallow depths where effi-cient scale multi-streamer spreads might have historically beenregarded with caution. It is demonstrated that the ability toacquire and use a large frequency bandwidth from shallowerevents, free of aliasing, prevents processing noise fro mdegrading deeper data quality and resolution. At depth, theonly limiting factor upon frequency bandwidth is then naturalanelastic attenuation.

In the Off s h o re Philippines HD3D survey, the deep, stru c t u r a l l ycomplex carbonate reservoir was imaged with great clarity andresolution despite the severely rugose water bottom and strati-graphically complex overburden. In the East Java Sea HD3Ds u r v e y, very steeply-dipping, shallow carbonate flanks werealso imaged with great clarity and resolution, enabling thep recise mapping of the many small and large prospective stru c-t u res that are pervasive throughout the area. In addition,deeper migration pathways and source kitchens below thecomplex Kujung I, II, and III carbonate layers are imaged withhigh signal-to-noise quality, thanks to the imaging powerpossible with densely acquired data.

The significant increase in frequency bandwidth observed at allarrival times and the associated improvements in data resolu-tion and signal-to-noise quality are a direct consequence of thetight 3D spatial sampling inherent in the HD3D acquisition

method. In both the cases discussed here, a powerful demon-stration is made that very high resolution is only achievablewhen tight 3D spatial sampling is the centerpiece of acquisition.

ReferencesBiondi, B., 1998, Kirchhoff imaging beyond aliasing; Stanford Exploration Project,SEP-97, Stanford University, 13-35.

Chen, J., and Schuster, G.T., 1999, Resolution limits of migrated images ; Geophysics,64, 1046-1053.

Hegna, S., Krokan, B., and Selbekk, T., 2001, Single source 3D acquisition - a highquality and cost-effective alternative; Annual Meeting Abstracts, SEG, 48-51.

Hoffmann, J., Rekdal, T., & Hegna, S., 2002, Improving the data quality in marinestreamer seismic by increased cross-line sampling; Annual Meeting Abstracts, SEG,Session ACQ 3.7.

Johansen, K.B., 2003, Depositional geometries and hydrocarbon potential within Kujungcarbonates along the North Madura Platform, as revealed by 3D and 2D seismic data;29th Annual IPA Convention & Exhibition, Indonesia, October 14-16, 2003.

Long, A.S., Ramsden, C.R.T., & Hoffmann, J., 2003, On the issue of strike or dips t reamer shooting for 3D multi-streamer acquisition; Exploration Geophysics,accepted for publication.

Regone, C.J., 1998, Suppression of coherent noise in 3-D seismology; The LeadingEdge, 17, 11, 1584-1589.

Reksnes, P.A., Haugane, E., & Hegna, S., 2002, How PGS created a new image for theVarg field; First Break 20, 773-777.

Vermeer, G.J.O., 1999, Factors affecting spatial resolution; Geophysics, 64, 942-953.

Widmaier, M., Hegna, S., Smit, F., and Tijdens, E., 2003, A Strategy for OptimalMarine 4D Acquisition, Annual Meeting Abstracts, SEG.

Yilmaz, 0., 1987, Seismic data processing, Investigations in Geophysics, 2, SEG,Tulsa, 526 p.

Acknowledgements

Thanks to PGS Marine Geophysical for permission to publishthese data, and to the members of the East Java Sea HD3D andOffshore Philippines HD3D QC committees for their projectmanagement.

HD3D and holoSeis are registered marks of Petroleum Geo-Services.

F i g u re 11: 3D perspective display from the holoSeisT M immersive visualizationand interpretation system. The Top Kujung I surface interpreted here (~ 1.0 sTWT) reveals the density and complex distribution of carbonates throughout theHD3D survey are a .

Figure 10. Time slice from the East Java Sea HD3D survey at 1.0 s TWT. TheKujung carbonate features are highly complex in distribution, demanding a high-resolution 3D seismic acquisition and processing strategy. Refer also toFigure 11.




Andrew Long has aPh.D. ingeophysics, and 15years of experiencein both the seismicindustry andacademia. Hejoined PGS

Technology (Perth) in 1997, where heis currently Geophysical Manager,being responsible for all geophysicalsupport to PGS throughout the Asia-Pacific region. He is a member ofSEG, EAGE, ASEG, PESA, andSEAPEX. His main interests areseismic modelling, seismic surveydesign, seismic imaging, seismictechnology, and rock physics.

Marie Dunn is Vice President of PGS Marine Geophysical NSA inHouston, TX. She received her degree in Geology from RiceUniversity. Marie spent 7 years with Schlumberger Geco-Prakla indata processing and Exploration Services (multi-client) prior tojoining PGS in 1995 and building the project management teamwhich is responsible for the design and data quality of North andSouth American multi-client datasets.

Bill Pramik is TeamLeader of theG e o p h y s i c a lSupport Group atPGS’s Houstonoffice. He receivedhis degree inGeophysics fro m

Vi rginia Polytechnic Institute andState University. He has representedPGS Geophysical technologythroughout the world and works toconstantly ensure that PGS tech-nology is appropriately applied tothe client’s needs and requirements.Bill started his career with theAmoco Production Company intheir New Orleans Regional office in1979 and moved to their researchcenter in Tulsa, Oklahoma in 1989where he specialized in seismic dataacquisition technology. He hascontinued his work in seismic acqui-sition with PGS since joining them in1995.

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Optimal marine streamer acquisition with High-Density 3D · 2015-07-29 · Optimal marine streamer acquisition with High-Density 3D Andrew Long, PGS Technology (Perth, Australia),