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Acquisition Design of the First Four Component 3D Ocean Bottom Seismic in the CaspianJack Bouska, Tom Lyon, Rodney Johnston, Dave Buddery, Dave Howe, Mike Mueller, Leon Thomsen, Dan

Ebrom , BP PLC.

Summary

The signal to noise problems inherent in towed streamer

data associated with mud volcanoes, subsurfaceheterogeneities and gas in the Azeri, and Gunashli

structures of the Caspian sea prompted the use of threedimensional four component ocean bottom seismic (3D/4C

OBS) to improve imaging. The introduction of severalinnovative enhancements to the traditional ocean bottom

cable technique, when applied cohesively across bothacquisition and processing, resulted in cost savings

compared to traditional OBS acquisition and improved

final data quality compared to towed streamer seismic.

Introduction

The Azeri-Chirag-Gunashli (ACG) is a world-class oilfielddevelopment (8 billion barrels oil in place) located in the

South Caspian Sea, offshore Azerbaijan (Fig. 1) The ACGanticline extends in a northwest to southeast direction, in

water depth of 120m to 350m. The structure is asymmetricwith steep dips (40deg.) on the north flank and gentler(25deg.) on the south flank. The reservoir consists of 9

laterally extensive stacked Pliocene sandstone intervals; inthe Pereriv and overlying Balakhany formations. Mud

volcanoes of varying size penetrate the structure near thecrest. The mud volcanoes are characterized by debris cones

on the seabed fed by over-pressured shale from stratabelow the target reservoirs. The existing towed streamer

seismic data, while generally of good quality, does contain

areas of weak reflections over the crest of the structures,especially in the vicinity of the mud volcanoes, to the

extent that accurate structural mapping over the crest hasbeen seriously impaired. (Fig. 2) The cause of the poor

data has been postulated as a combination of a number offactors:

P-wave absorption/attenuation through distributed gasin the overburden sediments

Disturbed/disrupted sediments in the vicinity of the

mud volcano plume (Fig. 2)

Backscattered shot generated noise from near surfaceheterogeneities.

The use of 3D/4C OBS was suggested by Mueller andLyon (2002) as a method to improve imaging, and S/N over

the central problem areas. Following a 2D test of 4Cseismic over Azeri in 2001 (Probert, et al. 2002), a pair of

3D/4C-OBS surveys were acquired in 2002, and processedin 2003, to image the Azeri (160 sq.km.), and Gunashli(114 sq.km.) areas of the ACG PSA. Caspian Geophysicalacquired the OBS surveys, with BP staff, in country, foracquisition supervision, and on-shore QC processing in the

Caspian Geophysical processing center. Full dataprocessing was performed in the WesternGeco Gatwickoffice, in the U.K.

The original intent of the 4C project was to acquire both aPZ (pressure phone & vertical geophone summation)

survey, as well as a PS (converted shear wave) survey. Theinitial belief was that the PS image would provide

improved data quality, by virtue of reduced gas induced

attenuation in the up going shear leg. Early processing

demonstrated that the P-wave image was of markedly betterquality than the existing towed streamer, to the point where

Figure 2: Examples of areas where towed streamer data iscompromised over the structure crest due to seabed scarps, mud

volcanoes, and near surface distributed gas.

Figure 1: Location of study area shown within red box: Azeri

Chirag Gunashli PSA, (ACG) Caspian Sea, Azerbiajan.

SEG Int l Exposition and 74th Annual Meeting * Denver, Colorado * 10-15 October 2004

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Design of the First 3D/4C OBS in the Caspian

the P-wave image from the 3D/4C OBS is vital to the

overall value derived from the seismic. (Fig. 3) This paperwill focus on the novel 3D/4C OBS acquisition design

techniques, which have resulted in net improvements to thedata quality, and imaging. In companion papers, Johnston,

et al, 2004 reports on the processing strategy that takesadvantage of this acquisition scheme, and Lyon, et al, 2004

describes the value derived from the interpretation of the

resultant OBS images.

Method

Data processing often receives insufficient attention during

survey design, because the cost of equipment deployment,and data collection dominates the overall budget. Thedanger of treating acquisition and processing as sequential

(separate) steps, is that various untested assumptions,

related to data processing requirements, can lead to anunbalanced design, with overemphasis of expensive

acquisition parameters. For example, a belief that noiseand multiple attenuation will fail, without tight spatial

sampling of both shots and receivers, will lead theacquisition design towards overly narrow source and

receiver line spacing.

The Azeri, Gunashli acquisition / processing / interpretation

steps were considered holistically, as a single integrated,iterative, system. The tasks of Acquisition design,

processing management, and interpretation are carried outby a consistent team of specialists, who were focused onthe project from inception to completion. This unifiedapproach alleviated the typical loss of continuity that can

occur when each step of a project is managed by separateteams, and resulted in two notable advantages:

Knowledge of the inevitable deficiencies inherent in

the acquisition design which arise as a result of budgetconstraints, or limited equipment availability, can be

carried seamlessly forward in the project, allowing

processing flow, and program parameter adjustments,avoiding known problems

Innovations, which are intentionally imbedded in the

acquisition design, are well understood during the

processing stage, and can be properly exploited tomaximum advantage.

The remote location of the Caspian Sea exaggerates thetypically high cost of 3D/4C OBS acquisition. A strong

desire to adhere to a realistic budget prompted innovationin acquisition design that would accommodate the

conflicting requirements of tight spatial sampling (highfold) over the crest of the structure, while maintainingadequate aerial coverage over the migration aperture extentdemanded by the steeply dipping reservoir strata at depth.

Apart from air gun sources, and a marine setting for 3D/4C

OBS surveys, the underlying acquisition design, andprocessing techniques have many features predominantly incommon with land 3D seismic, hence procedures adaptedfrom tow streamer processing may not be appropriate.

Several of the innovations in the Caspian 3D/4C OBSacquisition and processing are derived from experiencewith BP's 3D seismic surveys in the thrust belts of North

and South America.

Figure 4: Acquisition pattern for Azeri and Gunashli OBS

surveys.

Figure 3: OBS vs Tow streamer data comparison.


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Numerous parameter optimizations were applied duringacquisition and processing however six major areas ofadvance stand out as unique innovations, pioneered in the

Caspian 3D/4C OBS surveys:

1. Variable cross-line spatial sampling via receiver line

interlacing. Deployed during acquisition to induce foldvariability generating high fold on the crest (to

improve S/N) grading to lower fold on the down dipflank to expand migration aperture.

2. Uniformly sampled shot wave field comprised of awide patch (wide aperture), 75m x 75m grid of source

points (4km X 10.4km) surrounding each receiver linepair, forming one half of a 3D symmetric sampled

wavefield (Vermeer 1994)

3. First break refraction tomography (made viable withthe wide patch acquisition scheme) used to estimate P-

wave (and indirectly S-wave) receiver statics, and nearsurface velocity model definition for joint inversion

depth migration.4. Pre-stack noise attenuation in the common receiver

domain using 3D-FXY-Decon (3D Random noise

attenuation, RNA, made possible via the large,regularly sampled source grid around each receiver

point.)5. Additional pre-stack noise attenuation via a second

pass of 3D RNA in the single fold common offsetdomain, a technique borrowed from land processing.

6. Kirchoff 3D pre-stack time migration, with pre-migration fold normalization, and post migration

offset dependant fold weight restoration, providingimproved attenuation of backscatter noise (Bouska1998), acquisition/processing footprint, and multip les.

The six acquisition and processing techniques listed above

represent the first known application on any of BP's 3D/4COBS surveys conducted worldwide.

Examples

The Azeri and Gunashli

OBS surveys were designedto use an interlaced patchlayout, instead of uniformreceiver line spacing,creating a distribution of

high fold coverage over thedifficult zone near the crestof the structure, grading tolower fold over the betterquality, deeper data in the

flanks of the structure. (Fig5.)

Arranging a greaterconcentration of receivers

over the poor data quality area served two purposes: first,to help attack some of the noise associated with

backscatter, and compensate for weak signal penetration.Second, to maintain stack fold consistent along

stratigraphy, rather than constant at one depth. The latter isimportant when imaging over high relief structures, where

the processing mute generally opens to include wider

offsets with depth. Survey designs with uniform linespacing, will produce excessive stack fold in the synclines,

where the target is deepest. This effect is also exaggeratedwith wide patch, multi-azimuth acquisition, where

cumulative fold increases as the square of the maximumoffset range.

The elemental receiver patch was held constant across thesurvey area, and consisted of two lines (700m or 720m

apart), surrounded by an aerial grid of shots, 75m x 75mand 4km wide, 10.4km long, used for all two line patches,

(apart from variations due to rig obstructions, and permitboarders) (Fig. 4 and Fig 5)

The spatial sampling / fold variation was achieved by

interlacing, or inter-fingering the pair of receiver lines. The

Azeri and Gunashli surveys used two different styles ofinterlacing, to accommodate the different migration

aperture requirements of the subsurface. (Fig 4.).Operationally, the widely spaced pair of lines is overlapped

by deploying the lines with a partial lateral shift, such that aportion of the receiver patch falls in-between the wider line

spacing of the previously recorded receiver patch. Thiscreates an effective receiver line spacing in the 350-36mrange over the crest, while the line spacing in the synclines

remained the same as the elemental recording patch (700-720m). (Fig. 5.)

Figure 5: Detail of reciever line-pair interlacing used for Azeri (left) ,Gunashli (middle) with Azeri fold

illustrated on far right.


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The use of interlacing allows coverage of the full survey

area with 25% fewer receiver patches than would berequired for a constant line spacing design. This results in

acquisition cost savings proportional to the reduction in

patches.

Part of the motivation for deploying a continuous 75m x75m grid of sources can be traced back to the North SeaHod 3D/4C OBS survey (Kommedal et al, 2002), which

contained two sets of source lines; one set orthogonal andthe other set parallel to the receiver cables. The Hod studyillustrated that either type of acquisition would be adequatefor PZ OBS surveys, however PS images were notablybetter with the inline style of acquisition. To garner the best

of both worlds, the source grid for the Caspian OBSsurveys was chosen to be inline with the receiver lines, but

with sufficient cross line dimension to provide good multi-azimuth distribution for P-wave processing, anisotropyanalysis, and 3D prestack noise attenuation in the common

receiver domain.

The use of receiver line interlacing also demands specificattention to the design of the source grid, so that adequate

cross line source-receiver offset is maintained over the zoneof wide line spacing. The choice of wide aperture shot

patches also has the advantage of generating a broad

distribution of source receiver offsets and azimuths, whichcan benefit the PZ prestack imaging step.

Harvesting the advantages of wide azimuth acquisition

design also requires careful treatment, and retention of thefar offsets, during the processing step, as these traces

comprise the bulk of the prestack-migration fold, or in

other words, the majority of the useful reflected energy isderived from the mid and far offsets, in this case, offsets

greater than 1500m.

The receiver spacing along the cable sets the inline bin size,and the distance between flip and flop source lines sets the

cross line bin size, resulting in a natural bin dimensions of12.5 (dip) x 37.5 (strike). The inline direction can record

strata dipping at greater than 45 degrees, however the

predominant angles of strata in the strike direction arelower than 20 degrees, allowing the wider bin size in this

direction. Natural variation in line and shot spacing alsohelp to inject midpoint scatter in the final survey, which

spreads the true reflection points across the subsurface.When these are collected, or imaged into output bins during

migration, the midpoint scatter provides some spatial anti-alias protection, which helps guard against adverse effectsof the large cross line bin dimension.

Both the wide line spacing, and wide source patch, aspectsof the Caspian OBS surveys were designed to facilitate ouruse of new processing techniques such as:

- First break refraction tomography.

- 3D prestack random and aliased noise attenuation.- Offset distribution biased to far offsets for improved

multi-azimuth illumination, and multiple suppression

- Stronger backscatter attenuation in prestack imaging.- Undershooting near surface low velocity anomalies

(trapped gas, mud plumes, etc)

Conclusions

Treating acquisition design and processing as a singlefunction can benefit both cost and quality by allowinginnovations to be applied seamlessly across the wholetechnical project. Exploitation of the unique advantages of

OBS in the future may require a move away from themarine processing paradigm which mandates application of

2D linear dip filters for removal of noise and multiples, andtowards a broader view of acquiring and processingcoarsely sampled 3D wave fields.

References

Bouska, J., 1998, The other side of the fold: THE LEADING EDGE, 17, no. 01,31-35.

Kommedal, J. H., Ackers, M., Folstad, P. G., Gratacos, B. and Evans, R.,2002, Processing the Hod 3D multicomponent OBS survey, comparingparallel and orthogonal acquisition geometries: THE LEADING EDGE, 21,no.8, 795-801

Michael Mueller & Thomas Lyon, AZERI FIELD 2D 4C OBS TESTRESULTS AND 3D 4C OBS BUSINESS CASE EVALUATION, BakuGeophysical Conference 2002

Probert, T., Bryan, R., Underwood, D., Mueller, M., Lyon, T. and Rowson,C., 2002, Multicomponent Seismic Challenges on a Mud Volcano - Imagingthe Azeri Field, 64th Mtg.: Eur. Assn. Geosci. Eng., F017.

Vermeer, G. J. O. and Shell Research, , 1994, 3-D symmetric sampling,64th Ann. Internat. Mtg: Soc. of Expl. Geophys., 906-909.

Acknowledgments

BP operates ACG field on behalf of the shareholders of theAzerbaijan International Oil Company (AIOC) which include thefollowing companies: BP 34.14%, UNOCAL 10.28%, SOCAR10%, INPEX 10%, Statoil 8.56%, ExxonMobil 8%, TPAO 6.75%,

Devon 5.63%, Itochu 3.92% and Amerada Hess 2.72%.

The authors would like to thank the AIOC shareholders forpermission to publish this case study and their input to theplanning and execution of the project.

We also acknowledge the dedication and skill of those individualsin Caspian Geophysical and Western Geco who acquired andprocessed the OBS survey.

Many colleagues in BP helped to make the Azeri OBS a successfulproject; in particular we thank Jan Kommendal and Richard

Seaborne,


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