shooting seismic surveys in circles - schlumberger

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1. Camara Alfaro J, Corcoran C, Davies K, Gonzalez Pineda F,Hill D, Hampson G, Howard M, Kapoor J, Moldoveanu Nand Kragh E: “Reducing Exploration Risk,” Oilfield Review 19, no. 1 (Spring 2007): 26–43.

2. Production in the seismic sense means production ofseismic data. Therefore, productivity is the amount oftime a seismic vessel spends recording useful data relative to the total time the vessel is in the survey area,usually expressed as a percentage.

For help in preparation of this article, thanks to Tim Buntingand Mikael Garden, Kuala Lumpur; Iain Bush, Edward Hager,Hasbi Lubis, Gary Poole, Bruce Webb and Phil Williams,London; William Dragoset, Houston; and Jens Olav Paulsen,Oslo, Norway.Coil Shooting, Monowing, Q-Fin and Q-Marine are marks ofSchlumberger.

Shooting Seismic Surveys in Circles

Michele BuiaPablo E. Flores Eni E&PMilan, Italy

David Hill Ed PalmerRob RossRobin WalkerGatwick, England

Marianne HoubiersMark ThompsonStatoilHydroTrondheim, Norway

Sergio LauraEni IndonesiaJakarta, Indonesia

Cem MenlikliTürkiye Petrolleri Anonim Ortaklıgı (TPAO)Ankara, Turkey

Nick MoldoveanuHouston, Texas, USA

Earl SnyderJakarta, Indonesia

Traditionally, marine seismic data are acquired by a seismic vessel sailing in a straight

line over a target, then turning back to shoot another line parallel to the first. A new

technique acquires seismic data in continuously linked circles with little or no

nonproductive time. The results are high-quality data containing reflection information

from all azimuths. Test results indicate that the technique will be useful for improving

seismic imaging in several complex geological environments. The first full survey

using the technique was acquired for Eni in 2008 on the Tulip project in the Bukat

production-sharing contract block, offshore Indonesia.

Many of the world’s most significant technicaldevelopments were envisioned long before theybecame commercial. For example, Leonardo daVinci’s 15th century designs probably played apart in the development of the helicopter andscrew propeller. Within the E&P industry, recentdevelopments by WesternGeco in 3D seismicimaging systems apply fundamental principlesdefined decades ago that had to await advancesin technology to realize their potential. Thesedevelopments have now enabled circularshooting—another idea from the past—tobecome a reality.

The seismic industry performed early trials ofcircular acquisition geometries in the 1980s.However, it was not until 2006 that WesternGecorealized that it could make a circular acquisitiontechnique a practical option for 3D surveys.

Three-dimensional seismic surveys haveprobably done more than any other moderntechnology to increase the likelihood of explora -tion drilling success.1 Subsurface imaging using3D surveys has proved particularly succes sful forimaging clastic sediments; however, accurateimaging of sediments beneath hard sea floors,salt, basalt and carbonate layers remains achallenge, particularly in deep water.

In deep water, towed-streamer geometries arecurrently the only economically viable solutionsfor acquisition of large 3D seismic datasets.

Conventional marine 3D surveys, acquired as a setof adjacent parallel straight lines, have provedtheir value as exploration tools in a wide variety ofgeological settings. These surveys image thesubsurface with seismic raypaths that are alignedpredominantly in one direction. Complex geologyand highly refractive layers cause ray bending thatcan leave portions of the subsurface untouchedwhen recording seismic waves traveling in just onedirection. In such circumstances, conventional 3Ddata may provide ambiguous or misleadingimages, reducing confidence in explorationdecisions and potentially introducing uncer -tainties and inaccuracies in models used forreservoir development.

Wide-azimuth (WAZ) seismic surveys providedata from raypaths traveling in a wide range ofdirections. The method has been shown to deliverbetter illumination of the subsurface, highersignal-to-noise ratio and improved seismic resolu -tion in several complex geological environ ments,such as beneath large salt bodies with complexshapes. Until recently, in deep water, thisimproved method of imaging has required severalseismic vessels working together.

This article reviews developments in towed-streamer WAZ acquisition and introduces CoilShooting acquisition: a new technique that goesbeyond WAZ to acquire a full range of azimuths

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When the vessel gets back to the other edgeof the survey area, it turns to follow anotherracetrack-like course displaced laterally from thefirst. This continues until coverage of that part ofthe survey area is completed.

Usually, no data are recorded while the vesselis turning because, in conventional acquisitionsystems, streamers do not maintain their lateralseparation during turns, and the positions ofreceivers within the streamers cannot beaccurately calculated. Additionally, turning mayinduce increased lateral drag on the streamersas they move through the water, resulting inincreased levels of noise. Depending on surveydimensions, vessels can spend up to 50% of thetime available for production on line changes, soline changes represent a significant period ofnonproductive time (NPT).2 This limitation cancompromise data quality: to minimize NPT, andhence costs, some surveys are shot in the optimaldirection for efficiency but not the best directionto achieve geophysical objectives.

In conventional surveys, the direction, orazimuth, of a seismic ray traveling down from thesource into the subsurface then up to a receiver

using a single vessel shooting continuously with acircular or curved path. We describe modelingand feasibility tests, which indicate that thetechnique has considerable potential forefficiently addressing imaging challenges incomplex geological settings. Also presented aresome details of the world’s first full Coil Shooting project.

Wide-Azimuth Towed-Streamer AcquisitionConventional marine 3D surveys acquire datafrom a vessel sailing in a series of adjacentparallel straight lines. The vessel is typicallyequipped with one or two airgun source arraysand 8 to 10 streamers. When the vessel reachesthe edge of the defined survey area, it continuesin a straight line for one-half the length of astreamer, then turns around in a wide arc toposition itself for another straight line in theopposite direction, as if following the course of asimple racetrack (right).

> Typical conventional deepwater 3D acquisitionconfiguration. The acquisition path follows astraight line (blue arrow) then turns 180° toacquire data in the opposite direction (orangearrow). No data are normally recorded duringline turns (black).

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> Conventional single-vessel streamerconfiguration. A vessel tows an airgun sourcearray and 10 streamers, each containing about2,000 receivers. Streamers are typically 6 to 8 km[4 to 5 mi] long and 100 m [328 ft] apart.Downgoing seismic rays (green) emanate fromthe source. They reflect at the seabed and atsubsurface interfaces then travel upward (red)to the receivers where they are recorded. Theillustration shows only raypaths to the first andlast receivers in each streamer. The direction, or azimuth, between the seismic source andreceivers is close to the direction of the vesseltrack, particularly for far offsets—the datarecorded by receivers farthest from the source.

> A four-vessel wide-azimuth acquisition configuration. Two recordingvessels (left and right ), both equipped with an airgun source array and 10 streamers, are joined by two source vessels (center). The source on the left-hand vessel is fired, and data are recorded by streamers on bothrecording vessels, providing two areas of subsurface coverage (dark tan).

Sources are then fired in sequence by the other vessels, providing a widerarea of subsurface coverage (light tan) and a broader range of source-receiver azimuths than can be achieved by a single vessel. Sail lines maybe repeated with source vessels in different positions, providing differentranges of azimuths.


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will be close to the direction of the vessel track(previous page, top). Azimuths for far offsets willtypically be within ±10° of the vessel track. Nearoffsets will have a higher range of azimuthsbecause of the lateral displacement between thesource and the front of the streamers.

Complex geology and highly refractive layersbend seismic rays so that portions of thesubsurface may remain untouched by seismicwaves, particularly when source-receiver geome -tries provide only a narrow range of azimuths. Ananalogy to this phenomenon, whereby the azimuthof observation impacts the results of imagingthrough media with complex geometries, can bemade by viewing an object through textured glass(above). Viewing the glass from several differentdirections will enable the viewer to determine itscontents more accurately. Similarly, acquiringseismic data with a broad range of azimuths hasbeen shown to deliver more-accurate images ofthe subsurface in complex geologicalenvironments, such as those associated with saltbodies, by essentially “shining a light” on theformations from many directions.

Various towed-streamer acquisition geome triesthat increase the range of source-receiver azimuthshave been tested in several locations, including theNile Delta, the North Sea and, most prominently,the Gulf of Mexico. These surveys typically usethree or four seismic vessels, each shooting instraight parallel lines (previous page, bottom).

Since 2001, the WAZ technique has been usedin several surveys in which one or more recordingvessels, following essentially the same racetrack-like course as for a conventional 3D survey, arejoined by source vessels positioned in front ofand/or behind the streamer spread, and offsetlaterally at various distances on each side of the

spread. The technique typically provides analmost full range of azimuths at near offsets butonly ±30° for far offsets.

In 2006, WesternGeco acquired a survey inwhich a WAZ geometry was deployed in twoopposite sailing directions for each of threedifferent orientations—effectively covering thesubsurface six times. When combined, theresulting 3D dataset contained contributionsfrom a complete range of azimuths for mostoffsets. This survey, acquired over the Shenzifield in the Gulf of Mexico, was the world’s firstrich-azimuth (RAZ) seismic survey.3

A 2007 study concluded that these types ofsurveys, combined with accurate velocity modelsand high-fidelity migration algorithms, couldprovide a step-change improvement in subsaltillumination, signal-to-noise ratio and attenua -tion of multiples, compared with conventionalnarrow-azimuth (NAZ) surveys.4 The study foundthat wider crossline offsets led to better results,and that the best results were from data acquiredover a complete 360° range of azimuths, asdelivered by RAZ geometries.

The RAZ and WAZ towed-streamer geometriesdescribed above were all acquired as a series ofadjacent straight lines. As with a conventional3D survey, the time taken to turn the recordingvessel between lines is usually nonproductivetime. For multivessel operations and widestreamer spreads, this turning time is typicallymore than three hours per sail line, adding up toseveral weeks for a large survey. Anotherpotential problem with parallel geometries isthat they can leave undesirable acquisitionartifacts in the data, such as stripes in thedirection of sail lines that can be seen in theprocessed dataset.

3. Howard M, Harding C and Stoughton D: “Rich AzimuthMarine Seismic, A Cost Effective Approach to BetterSubsalt Images,” First Break 25, no. 3 (March 2007): 63–68.

4. Kapoor S, Moldoveanu N, Egan M, O'Briain M, Desta D,Atakishiyev I, Tomida M and Stewart L: “Subsalt Imaging:The RAZ–WAZ Experience,” The Leading Edge 26, no. 11(November 2007): 1414–1422.Multiples are seismic energy that has bounced betweenmore than one reflecting surface. A form of noise thatcan interfere with or obscure primary reflection events,multiples are suppressed by algorithms applied during processing.

> Image distortion. Refraction of light through the irregular surface of a glass causes parts of the spoon to be invisible or distorted. The image changes dependingon the direction of view. Similarly, the azimuth of observation impacts the results of seismic imaging through geologic media with complex geometries.

Circular AcquisitionWhile acquiring the Shenzi seismic survey,WesternGeco and the operator—BHP Billiton—agreed that it might be worthwhile to continuefiring sources and collecting data during line turns.The results of this experiment were extremelyencouraging. The first stage of processing for thesedata, acquired using Q-Marine single-sensortechnology, included an advanced noise-attenuation algorithm, which effectively addressedany increase in the levels of ambient noise toprovide useful data at the edge of the survey area.In addition, acquisition productivity was increasedthrough elimination of nonproductive time duringline changes.

Because sail lines acquired during linechanges approximate arcs of circles, they can beconsidered as a partial implementation ofcircular acquisition geometry. The success ofcontinuous acquisition during the Shenzi RAZsurvey convinced WesternGeco to investigate thepossibility of employing circular geometry forwide-azimuth towed-streamer acquisition.

Like many fundamental seismic acquisitiontechniques, some potential benefits of circulargeometry for marine acquisition have beenknown for many years. In the 1980s, it wasproposed that sailing in concentric circlesaround salt domes would improve structural


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5. French WS: “Circular Seismic Acquisition System,” US Patent No. 4,486,863 (December 4, 1984).

6. Durrani JA, French WS and Comeaux LB: “NewDirections for Marine 3-D Surveys,” ExpandedAbstracts, 57th Annual International SEG Meeting, NewOrleans (October 11–13, 1987): 177–181.

7. Ongkiehong L and Huizer W: “Dynamic Range of the Seismic System,” First Break 5, no. 12 (December 1987): 435–439.

8. Christie P, Nichols D, Özbek A, Curtis T, Larsen L,Strudley A, Davis R and Svendsen M: “Raising theStandards of Seismic Data Quality,” Oilfield Review 13,no. 2 (Summer 2001): 16–31.

9. Woodward M, Nichols D, Zdraveva O, Whitfield P andJohns T: “A Decade of Tomography,” Geophysics 73, no. 5 (September–October 2008): VE5–VE11.

10. Fold of coverage is the number of seismic traces thatmap to a defined area, typically 25 m by 25 m [82 ft by 82 ft].

11. Moldoveanu N: "Circular Geometry for Wide-AzimuthTowed-Streamer Acquisition," presented at the 70thEAGE Conference and Exhibition, Rome, June 9–12, 2008,paper G011.

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acquires short-offset data, something not possiblewith multivessel WAZ geometries. The methodalso delivers data sufficiently sampled in azimuththat the dataset can be split into differentazimuth ranges for building anisotropic velocitymodels and analyzing fractures (above).9

The Coil Shooting technique involves a singlevessel equipped with multiple streamers and aseismic source. The vessel sails along a pattern ofoverlapping circular or curved sail paths thatcover the survey area, shooting and recordingdata continuously. This continuous mode ofacquisition virtually eliminates NPT, so it is

imaging of the flanks of the domes and associ -ated faulting.5 Test surveys were acquired in theGulf of Mexico and in the North Sea usingconcentric circle acquisition.6 However, becausethe marine acquisition technology at that timedid not allow proper implementation of themethod, it was abandoned.

The Q-Marine system, introduced in 2000,has overcome many of the challenges inherentin circular acquisition geometries, enabling itsuse for several new applications. Q-Fin steeringdevices precisely control the depth and lateralposition of the streamers, making it possible tomaintain constant streamer separation.Monowing multistreamer towing technology ishighly effective at maintaining lateral streamerdisplacement while turning. Shaped like anaircraft wing, Monowing deflectors provide forceperpendicular to the sailing direction to keepthe wide streamer spread in position. Monowingdeflectors are mounted close to the edge of theouter streamer. By contrast, conventionaldeflection devices are mounted outside thestreamer spread, resulting in a much larger totalequipment spread that necessitates a largerturn radius.

An acoustic network provides accuratepositioning of the in-sea equipment. Large cali -brated source arrays deliver deep-pene trating

energy, and an advanced digital source controllerprovides a fully calibrated airgun sourcesignature for every shot.

In the presence of strong cross-currents,towing streamers in a curve can increase noiselevels in the raw field data. Q-Marine technologyleverages advances in electronics and fiber-opticnetworks to provide high channel-countrecording systems, enabling finely sampledsingle-sensor recording of both signal and noisein the seismic wavefield. Shell geophysicistsdocumented the potential of the single-sensormethod in the late 1980s, but limitations inhardware and processing capabilities at the timeprevented full realization of its benefits.7

Adequately sampling streamer noise allowstargeted signal-processing techniques to sup -press it while preserving the integrity of theseismic signal. Part of the digital group forming(DGF) process, this effective noise removalallows acquisition of high-quality seismic dataeven in poor weather or when towing streamersthrough strong currents.8

Using circular geometry for towed-streameracquisition offers benefits for both geophysicalanalysis and operational efficiency. The CoilShooting technique acquires well-sampled, full-azimuth (FAZ) data, providing more completeillumination of the subsurface and benefitingnoise reduction and multiple attenuation. It

> Comparison of acquisition geometries (bottom) and azimuth-offsetdistribution plots in rose diagrams (top). The number of traces recorded foreach offset-azimuth combination is color-coded in the rose diagrams.Offset corresponds to distance from the center of each circle. Azimuthcorresponds to the angle within each circle. Colors range in order frompurple and blue for a low number of traces, to green, yellow and red for ahigh number of traces. From left to right: Traditional marine surveys areacquired by one vessel in one azimuth and produce data with a narrowazimuth-offset distribution. Multiazimuth surveys are acquired by one

vessel sailing in multiple directions and have azimuth-offset distributionsclustered around the direction of the sail lines. Wide-azimuth surveys areacquired by several vessels, increasing the azimuth range for many offsets.Rich-azimuth surveys use several vessels shooting in several directions,combining the concepts of multiazimuth and wide-azimuth surveys todeliver contributions for most azimuth-offset combinations. The CoilShooting single-vessel technique delivers a high number of contributionsfor a complete range of azimuths for all offsets.


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highly cost-effective for acquiring data withenhanced azimuthal contributions, particularlyfor appraisal and development projects, and forparts of the world where it is impractical tomobilize several vessels. For parallel geometries,vessels are typically productive about 45% of thetime they are in a survey area. With circulargeometry, this can double to 90% of availableacquisition time. For parallel geometries, thelast half-streamer length of each straight lineacquires low fold of coverage in a “taper-off”mode.10 This typically means that 3 km to 5 km[1.9 mi to 3.1 mi] of each straight line are ofdegraded quality. By contrast, the Coil Shootingmethod provides well-sampled data close to theedge of the whole survey area. Continuous lineacquisition is also highly efficient for conven -tional 3D projects, in which shooting whileturning provides valuable additional data for arelatively small additional effort.

Gulf of Mexico Feasibility TestBased on the successful acquisition of dataduring line turns for the 2006 Shenzi survey, aCoil Shooting feasibility test was performedduring 2007 in an area of the Gulf of Mexicopreviously covered by a parallel WAZ survey.11 Aprimary objective of this first test was todetermine whether it is feasible to sail using acircular geometry while maintaining constantstreamer separation and accurately measuringreceiver positions. Another objective was toprovide an indication of any relationshipbetween the curvature of streamers while towedin a circular trajectory and the levels of noiseintroduced. The test would also determine if CoilShooting data could be processed and imaged,and give an early indication of the effectivenessof the method for delivering high-quality, full-azimuth data.

Prior to acquiring the test data, WesternGecomodeled the coverage, offset and azimuthdistribution for circular geometry assuming asurvey area of 42 km x 42 km [26 mi x 26 mi] anda vessel equipped with a single source and 10streamers, each 7 km [4.4 mi] long separated by120 m [394 ft]. This streamer configuration istypical of a WAZ acquisition geometry. The modelassumed that data would be acquired by a vesselfollowing a set of circles, separated from eachother by a fixed distance in both x and ydirections (right).

Fold of coverage is highest over the targetarea in the middle of the survey and decreasestowards the fringes. Azimuth-offset distributionacross the modeled circular geometry survey was

analyzed for three different areas, presented as“rose diagrams.” This analysis shows that thecircular geometry provides full-azimuth

distribution over the target area and a reducingrange of azimuthal contributions in a fringearound the edges of the survey area.

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> Azimuth and offset distribution for a Coil Shooting 3D survey. Data are acquired by a vessel sailingin a circular path (top right ). When transit of one circle is complete, the vessel moves to a secondcircle separated by a fixed distance from the first, and this is repeated until the survey area is covered.The figure on the top left shows a grid of circles covering a survey area. The second diagram on theleft is a coverage plot, displaying the trace density across the modeled survey area. This shows a highnumber of traces in the middle of the survey area (color-coded red), reducing to fewer traces at theedge of the area (green, blue and purple). On the right are rose diagrams indicating average azimuth-offset distribution for three parts of the survey area. Red indicates a high number, decreasing throughyellow, green and purple. The area at the center of the survey has a complete range of azimuths andoffsets. The area at the edge of the survey receives no azimuths between 90° and 270°. The region inthe corner of the survey receives azimuths mostly from 0° to 90°. The bottom-left diagram is a “spider”plot illustrating the offset and azimuth of all traces within one typical 25-m x 25-m bin at the center ofthe survey area. Distance from the center of the circle corresponds to offset, and angle within thecircle corresponds to azimuth.


For the purpose of comparison, analysis ofthe azimuth-offset distribution was also per -formed for a parallel WAZ geometry surveyacquired with a four-vessel configuration. Nearoffsets are better recorded with circular geome -try than with the parallel WAZ geometry becauseof the high lateral displacement between sourceand recording vessels in parallel geometries.Near offsets are needed to image shallow over -burden, and fully sampled data, both in azimuthand offset domains, enable more effectiveapplication of processing algorithms that removenoise and improve resolution.

The high density of shot locations achievedthrough continuous acquisition results in highertotal fold of coverage than could typically beachieved within the same number of days using aparallel WAZ geometry survey acquired with twosource vessels and two recording vessels with thesame streamer configuration. Well-sampled datawith higher fold result in higher signal-to-noiseratio in the processed dataset.

Following the modeling exercise, a field testwas performed to investigate the practicalities ofacquiring data using the modeled circulargeometry. Four overlapping circles with different

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12. Palmer E, Bacon J, Toygar AR, Uygun S and Menlikli C:“A Complex, Deepwater Seismic Survey Produces HighQuality Results,” World Oil 229, no. 7 (July 2008): 89–93.

Wide-Azimuth Data Circular-Geometry Data

> Comparison between E-Octopus 3D prestack depth migration of full-aperture, full-fold WAZ data(left ) and circular-geometry test data (right ). The Coil Shooting data exhibit some indications ofimproved imaging, such as the higher amplitude and better continuity of the salt overhang in the top-left area of the section.

Gulf of Mexico Coil Shooting Test

> Acquisition geometry for the Gulf of Mexico Coil Shooting test. The plotshows areas containing traces from each of four circles acquired withdifferent radii, colored orange, yellow, green and blue.

radii were acquired in April 2007 by the seismicvessel Western Regent over an area covered bythe E-Octopus project (left). E-Octopus is aWesternGeco multiclient program coveringseveral hundred blocks in the Green Canyon andadjacent areas of the Gulf of Mexico. The projectis designed to deliver more-precise base andedge of salt definition through the integration ofmagnetotelluric, gravity and Q-Marine WAZseismic data.

The Coil Shooting acquisition test resultsproved that, using Q-Marine technology, it isfeasible to sail along circles while maintainingconstant streamer separation and achievinghighly accurate receiver positioning. After theDGF process, ambient noise levels wereacceptable for all the test data and comparableto those encountered in straight-line parts of the test.

The Coil Shooting test data were processedusing the same sequence as applied to theparallel WAZ data, including single-sensorcoherent noise attenuation, DGF at a 12.5-m [41-ft] group interval, shot-by-shot bubbleremoval and anomalous noise attenuation.Three-dimensional prestack depth migration wasperformed with the same velocity model used forparallel WAZ data. Despite low fold and limitedmigration aperture, the four-circle test datacompare favorably with the full-aperture, full-fold parallel WAZ data (left).

Black Sea TestThe second field test of the Coil Shootingtechnique was in an area of the Black Sea knownto have strong sea currents and subsurfaceimaging challenges related to complex geology.This test built on the success of the Gulf ofMexico test, which had already confirmed thefeasibility of acquiring and processing seismicdata with a circular geometry. The objective ofthe Black Sea test was to build experience andidentify technological developments and proce -dures required to better support the method,particularly in terms of survey design, acquisitionoperations, quality control and data processing.

WesternGeco acquired a conventional NAZ3D survey between September 2006 and January2007 for Türkiye Petrolleri Anonim Ortaklıgı(TPAO) in the Kozlu area of the Black Sea (nextpage, top).12 The NAZ survey was acquired by theseismic vessel Western Pride, using Q-Marinetechnology. Water depth in the area ranges from


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1,100 to 2,200 m [3,600 to 7,200 ft]. The target forthis exploration seismic survey included poten -tial reservoirs related to a complex sequence oflimestone reefs and shales at depths of 3,500 to4,000 m [11,500 to 13,100 ft], overlying volcanicstructures. Parts of the target strata have lowacoustic impedance contrast with rocks aboveand below, so create only weak reflections. Theoverburden includes layers of gas hydrates,which can inhibit transmission of seismic energy.The seabed is rugose, and complex near-seabedgeology results in strong diffractions. In someareas, the target is masked by strong water-bottom multiple energy, including multiples ofthe diffractions generated near the seabed.

The Q-Marine data, combined with anadvanced processing sequence to address thestrong multiples and other noise, delivered ahigh-quality seismic image. Experience gained inacquiring and processing this dataset suggestedthat the geological and geophysical challenges ofthe area would benefit from an enhanced-azimuth seismic imaging approach such as theCoil Shooting technique.

During acquisition of the NAZ survey, stronglocalized currents were encountered; these werethought to be related to the undulating seabed.These currents tested the Q-Marine streamer-steering system and also provided anotherlearning opportunity for the subsequent CoilShooting test. The currents were strong enoughto cause the spread of eight streamers, each 6 kmlong, to bend and deflect, sometimes increasingfeather angle by more than 15° within the spaceof 6 km (right). Despite these strong currents, thestreamer array remained parallel and under goodcontrol throughout the survey.

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> Location and design of the Kozlu Coil Shooting test. The Kozlu survey area is in Turkish waters inthe southwestern part of the Black Sea. The test data were acquired as a set of nine intersectingcircles in a “half-dahlia” pattern (top right ).

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. The effects of strong currents in the Kozluarea. The illustration at top left shows the shapeof the eight streamers for one example locationof the 2006/2007 Kozlu NAZ 3D survey. Acrossline offset of zero represents the directionof sailing. Localized currents caused significantbending and deflection, or feathering, of thestreamer spread from the direction of sailing.The same currents were observed during the2007 Coil Shooting test. The middle figuresdisplay streamer positions in three parts of oneacquired circle. Current direction is shown bythe red arrows. When shooting perpendicular tothe current direction (left and right ), streamersare deflected. When shooting parallel to thecurrent direction (center), streamers follow thecourse of the vessel around the circle. Thegraph (bottom) plots noise in the field dataagainst the azimuth of the vessel. Noise ismeasured as the root-mean-square (RMS)amplitude of data between 0.5 and 2 s two-waytime, which is earlier than the first reflections.The graph indicates that the direction of towingclearly impacts noise levels in the streamers.


TPAO agreed to have WesternGeco perform aCoil Shooting test survey over its acreage. Vesselcommitments meant that only a four-day timeslot was available for acquisition of the test datavolume. A suitable survey design had to bederived that would allow efficient acquisition of afit-for-purpose dataset in this short time frame.To meet the objectives of the test, the datasetwas required to have full-azimuth and close tofull-fold coverage over a geographical areasufficiently large to enable generation of a 3D-migrated image suitable for analysis andcomparison with data from the NAZ survey.

The geometry modeled for the previous Gulf ofMexico test involved a series of circles thattranslated laterally by a fixed distance. Thisdesign is not efficient for a small test area. Aninnovative solution was required to quickly

achieve coverage of a small, full-azimuth, high-foldarea. One proposal was for the vessel to follow thepattern of the edges of the petals of a flower suchas a dahlia. The design ultimately selected for theBlack Sea test was a “half-dahlia,” comprisingnine circular coils rotated about a fixed point. Thiswas expected to deliver a 2-km x 5-km [1.3-mi x3.1-mi] area of adequate coverage within theallocated time frame.

The Kozlu Coil Shooting test data wereacquired within three days during December 2007by the seismic vessel Western Monarch. As withdata from the Gulf of Mexico test and later CoilShooting projects, levels of noise in the field datawere observed to fluctuate considerably withineach of the nine circles acquired (above).Experience to date indicates that tow ing a

streamer in a curved trajectory will increaseaverage noise levels relative to shooting in astraight line. A lesson learned from the Kozlu test,and supported by other Coil Shooting projects, isthat current direction has a signifi cant impact onhow and when noise levels increase.

During acquisition of the Kozlu test dataset,the dominant current was flowing in anapproximately northeasterly direction. Whentowed along part of a circle perpendicular to thiscurrent, streamers were observed to feather tothe northeast. When towed with or against thecurrent, streamers remained close to the track ofthe vessel. Plotting average noise levels in all theacquired test data against source-receiverazimuth—approximately the towing direction—shows clearly that noise levels are highest whentowing perpendicular to the current.

Q-Marine field data are recorded with singlesensors at 3.125-m [10.25-ft] intervals along eachstreamer. This spacing is sufficiently small tosample most noise, enabling it to be recognizedand removed by data-adaptive algorithms applied within the DGF process. Shots with lowlevels of ambient noise are free of noise after theDGF process. For shots with higher levels ofambient noise, some residual noise may remainafter DGF, but this is attenuated duringsubsequent processing.

The Coil Shooting test data were processedusing a sequence comparable to that applied tothe previously acquired NAZ data, includingprestack time migration. A new technology, 3Dgeneral surface multiple prediction (3D GSMP),was applied to the test data volume.13 Thismethod is effective for attenuating multiples,while preserving the integrity of primary energy.The technique is useful in areas of complexgeology, such as rugose surfaces and crosslinedip, where the geophysical assumptions of manyother demultiple techniques break down. Unlikemany other algorithms, 3D GSMP can be appliedto multiazimuthal data, as generated by CoilShooting acquisition.

The Kozlu Coil Shooting test achieved itsobjectives. The innovative survey design enableda fit-for-purpose dataset to be acquired withinthe allotted time. It also provided valuableknowledge about the effects of ocean currents onnoise levels. The test dataset was small and maynot include sufficient migration aperture toaccurately image all reflection events. Neverthe -less, the processed results compare favorablywith the previous NAZ survey (next page, top).The Coil Shooting data show more continuity atthe top and bottom of the carbonate reef targetand improved fault resolution in several parts of

26 Oilfield Review

Trace number1 1,801901

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> Kozlu field data. At top left is a shot record with low levels of noise. Atbottom left is a shot record showing high levels of noise. The first curved linebelow 2.6 s on the near (left-hand) trace is the water-bottom reflection. In aperfectly noise-free environment, amplitudes above this would be zero. Onthe right are the same two shot records after digital group forming (DGF),which greatly attenuates the noise. Further noise reduction is achievedduring subsequent processing stages.


Autumn 2008 27

the section. The results indicate that full-azimuth acquisition will improve the quality andreliability of seismic imaging in the area.

Full Deployment in IndonesiaIn October 2007, WesternGeco presented the CoilShooting technique at the Milan headquarters ofthe E&P division of Eni, a company with experi -ence in several projects involving wide- andmultiazimuth imaging and with a history ofdesigning and applying innovative marineacquisition techniques. The Eni E&P geophysicalteam immediately recognized the benefits of thetechnology and worked to identify a suitablelocation where the technique could be expectedto resolve imaging challenges better than othertechniques. Eventually the company chose theTulip structure in its Bukat production-sharingcontract block, offshore Indonesia.

The Tulip field has complex geology, and thetarget has low P-wave impedance, so only weakseismic reflectivity. By contrast, the waterbottom is strongly reflective, and there is abottom-simulating reflector (BSR) across thewhole survey area.14 Parts of the existing 2Dseismic data exhibit up to six reverberations ofwater-bottom and BSR-related multiples, whichcontaminate the weak primary reflections. Theseafloor topography is highly undulating, withridges and canyons. Anomalies close to theseabed create diffractions that propagatethrough the section. In addition, a gas hydrateBSR in the overburden attenuates P-wave energy,obscuring the geology below.

A team of Eni and WesternGeco geophysicistsevaluated possible towed-streamer acquisitionconfigurations using a structural model of theTulip field based on existing information,including bathymetry, 2D seismic data and a 3Dvelocity-depth model. The team compared thesubsurface illumination, in terms of incidenceangles and azimuth, that would result from a CoilShooting geometry; conventional NAZ surveys,considering four different shooting directions;and multiazimuth (MAZ) acquisition, which isthe combination of several NAZ surveys shot indifferent directions (right). Multivessel WAZ andRAZ options were not considered because it wasimportant to record near offsets to image theundulating seabed.

13. Moore I and Dragoset B: “General Surface MultiplePrediction: A Flexible 3D SRME Algorithm,” First Break 26(September 2008): 89–100.

14. A bottom-simulating reflector (BSR) is a seismicreflection often seen in seismic sections from deepwaterareas. Studies indicate that it is primarily due to theacoustic impedance contrast in areas where free gas istrapped at the base of a gas hydrate stability zone.

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> Comparison between NAZ 3D survey (left ) and Coil Shooting test results (right ). The topcomparison shows better imaging of faults in the shallow section of the Coil Shooting data. The bottomcomparison displays improved continuity and more detail at the top and base of the reservoir (blackarrows) in the Coil Shooting data.

1 1,180Number of traces

NAZ 35° Acquisition MAZ 35°, 80°, 125°, 170° Acquisition Coil Shooting Acquisition

> Presurvey illumination tests for the Tulip project. NORSAR-3D software was used to predict theillumination of a subsurface target. The colors represent the number of traces mapping to each partof the target horizon, ranging from blue (low) to red (high). Areas of the target are poorly illuminatedwith conventional narrow-azimuth acquisition (left ). Multiazimuth acquisition in four directions(middle) provides much better illumination but requires four times the acquisition effort of theconven tional geometry. Coil Shooting acquisition (right ) illuminates the target effectively and is moreefficient to acquire.


The study concluded that a Coil Shootingsurvey would provide the best illumination of thetargets. As a result, Eni Indonesia, on behalf ofthe Bukat Joint Venture, awarded WesternGecothe world’s first commercial full-azimuth towed-streamer survey using the technique. The seismicvessel Geco Topaz acquired the 563-km2

[220-mi2] survey during August and September2008, equipped with eight streamers, each 6 kmlong, separated by 100 m (above).

Because of ocean currents and other factors,acquisition cannot exactly match planned sourceand receiver positions, so the actual subsurfacecoverage was monitored as the surveyprogressed. The presurvey evaluation used thecommercial NORSAR-3D modeling package toevaluate illumination of the subsurface targetfrom planned acquisition geometries.15 The samepackage was used onboard Geco Topaz duringacquisition to model the illumination based onactual source and receiver positions (aboveright). Comparison of the predicted illuminationfrom planned versus actual positions highlightedgaps in coverage requiring additional data,known as “infill lines,” to be acquired.

The field data exhibit low levels of noise,building confidence that acquiring data whiletowing streamers in a circular trajectory does notcompromise data quality (right). The Tulip fieldCoil Shooting survey was completed in 49 days.By comparison, a three-azimuth MAZ survey waspredicted to require 60 days, and a four-azimuthsurvey 75 days (next page, top).

28 Oilfield Review

15. NORSAR-3D is a product of NORSAR.16. Houbiers M, Arntsen B, Thompson M, Hager E, Brown G

and Hill D: “Full Azimuth Seismic Modelling at Heidrun,”presented at the PETEX conference, London, November 25–27, 2008.

Tulip Survey Plan

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> Planned source positions for the Tulip survey.The survey plan required the vessel to sail along145 circles covering the survey area.

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> Onboard quality control of illumination. As data were acquired over the Tulip field, actual sourceand receiver positions were used to model illumination of the subsurface target horizon usingNORSAR-3D software. The left plot shows the number of traces mapping to the horizon at each partof the survey area, ranging from blue (low) to red (high). The plot on the right displays the ratio ofactual to planned coverage. A value of 1 (green) means that coverage matches the survey plan. Redindicates higher coverage than planned. Areas in dark blue signify lower coverage than planned.Based on this, some additional “infill” data were acquired over areas of low coverage within the mainsurvey area and away from the edge, where variable fold is to be expected.

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> Onboard quality-control (QC) displays for the Tulip survey. The example “pie bin” stack section (top), produced during acquisition of one circle, iscontinuous: if rolled on a cylinder, the start and end of the section will match.The time slice at 2 s (bottom) was extracted from the 3D QC stack volume builtonboard during acquisition. This display was made after about 90% of theprogrammed circles were acquired.


Autumn 2008 29

Norway Survey Design Study and TestStatoilHydro included the Coil Shooting option ina simulation study performed during 2008 thatinvestigated whether a full- or wide-azimuth

acquisition geometry could resolve seismicimaging challenges at its Heidrun field, locatedin 350-m [1,150-ft] water depth in the NorwegianSea, 100 km [62 mi] from the coast of Norway.16

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> Comparison of acquisition duration for three single-vessel geometriesconsidered for the Tulip survey. The Coil Shooting survey (right ) wascompleted more quickly than was predicted, and more quickly than estimatesfor MAZ surveys using either a three- or four-azimuth parallel-line geometry.

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> Heidrun modeling results. The depth-velocity model (top) displays thefaulted reservoir, coal markers and a small salt body. In the lower panels,the synthetic seismogram (far left ) shows the 1D zero-offset reflectivityconvolved with the wavelet for the middle part of the model. It representsthe simplest seismic response. Note the pull-up of events below the saltbody, caused by the high P-wave velocity in the salt. This pull-up

disappears when the section is converted to depth. The red arrows markthe horizon used for the amplitude analysis shown in the figure on the nextpage. The simulated NAZ data (middle left ) exhibit distortion in reservoirreflections under the salt body. The WAZ (middle right) and Coil Shootingdata (right ) have better continuity and better imaging of steeply dippingevents. Sea-surface multiples are included in all three simulations.

Heidrun came on stream in 1995, and productionin 2006 was estimated at 3 million m3 [106 MMcf]of gas and 22,260 m3 [140,000 bbl] of oil per day.

The reservoir consists of sandstones of Early and Middle Jurassic age at about 2,300-m[7,500-ft] depth below the Base CretaceousUnconformity (BCU). The reservoir is heavilyfaulted, and parts of the field are not fullyunderstood because of imaging problems. Inparticular, a dome-shaped feature in one areacauses a highly disturbed image. One explana tionfor this dome is that it is formed by a salt diapirsourced by Triassic salt. Another imaging problemis that, in some areas of the field, faults and dipdirections below the BCU are unclear andconflicting. Despite several vintages of seismicdata, it has not been possible to obtain a clearseismic image of these complex areas. An ocean-bottom seismometer (OBS) survey pro videdbetter attenuation of multiples caused by strongimpedance variations around the dome than wasachieved by conventional towed-streamer surveys.The geometry of an OBS survey provides data froma full range of azimuths but at a significantlyhigher cost than a towed-streamer survey.

Geoscientists generated a 3D geologic model,covering a rectangular area of about 200 km2

[78 mi2] to a depth of 3,800 m [12,500 ft], torepresent the field (left and below). It includedan overburden with weak lateral velocityvariation, a faulted reservoir section and asection below the reservoir. The faults includedin the reservoir section represent the two mainfault systems in the field, crossing each other at

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an angle of about 45°. Apart from two faults withlower average dip, all faults had an average dipbetween 40° and 60°. Below the reservoir, twothin coal markers were included, one of which isthe base of the reservoir. A small salt body wasincluded at the location of the dome. Each layerand faulted subdivision of the structural modelwere allocated densities and P-wave velocitiesbased on a depth-migration model from 3Dseismic data and well data.

Three types of acquisition geometries weresimulated: conventional NAZ; a WAZ four-vesselconfiguration; and a Coil Shooting survey, usingabout 170 coils to provide a full 360° range ofazimuths over the modeled area. Modeling was

performed using a one-way wave-equationmigration approach that included simulation ofseabed multiples. Geoscientists evaluated simu -lated results for each of the acquisitiongeometries, comparing modeled vertical sectionsand also maps of peak amplitude extracted fromone of the horizons in the overburden (above).

Compared with the WAZ and Coil Shootingresults, the NAZ design exhibited more noise,especially around steep faults and in areas wherethe seabed has small bumps. The NAZ resultsalso have artifacts below the salt that were moreanomalous. The NAZ configuration showedconflicting dips resulting from contamination by

energy from multiples. The seabed multiple andmultiples of the coal markers were betterattenuated in the WAZ and Coil Shooting designsthan in the NAZ design. A steep flank of thestructure, visible in the WAZ and Coil Shootingdesigns, was invisible in the NAZ geometry. Thisflank was also invisible when modeling wasperformed without multiples. Artifacts related tothe acquisition footprint can be seen in theshallow part of the WAZ data, and to a lesserextent in the NAZ design. The Coil Shootingresults exhibit less acquisition footprint.

The Coil Shooting geometry showed moreconsistent amplitudes along the analyzed horizonthan the NAZ or WAZ designs, and these

30 Oilfield Review

NAZ Simulation

WAZ Simulation Coil Shooting Simulation

Synthetic Seismogram

> Heidrun modeled horizon amplitudes. Amplitudes were extracted by autotracking theamplitude peak on the horizon marked by the red arrows on the previous figure.Amplitudes in the simulated Coil Shooting data (bottom right ) match amplitudes of thesynthetic seismic section (top left ) better than do the NAZ (top right ) and WAZ (bottomleft ) geometries.


Autumn 2008 31

amplitudes were comparable to the expectedvalues of the reflection coefficient at that horizon.The study concluded that a survey design with anincreased range of azimuths and a higher fold ofcoverage would enable imaging of steeplydipping flanks that are invisible in a NAZ design.Such a survey would also deliver fewer artifacts,better noise suppression and better multipleattenuation. The study indicated that a CoilShooting design would also provide moreconsistent and accurate amplitudes.

To validate the conclusions of the modelingexercise, StatoilHydro acquired a Coil Shootingtest dataset over the field. The seismic vesselWestern Monarch acquired the data during afour-day period in September 2008. Survey designapproximated the dahlia pattern, with 18 coilsintersecting over the target area (right). Thecoils are slightly irregular in order to avoidsurface obstructions in the area, including atension leg platform and two loading buoys. Onestraight line was also acquired to provide someazimuths that were missing because of theobstructions. The test survey geometry wasdesigned to provide approximately 3 km x 3 km offull-azimuth, high-fold data, plus sufficientsurrounding aperture to provide 3D migratedimages that could be compared with previousseismic surveys and the modeling results. At thetime of writing, the data were being processed inpreparation for this analysis.

Worldwide PotentialWide- and rich-azimuth seismic techniques havebeen proven to deliver superior subsurfaceimages and better attenuation of coherent noiseand multiples relative to conventional narrow-azimuth acquisition. Experience gained sincetrials starting in 2006, including the firstcommercial deployment in 2008, indicates thatthe Coil Shooting technique is a highly efficientand effective method of acquiring data with a360° range of azimuths across the full offset-range over a survey area. A key benefit of thetechnique is that it requires only one vessel,negating the need for deployment of additionalsource and streamer vessels. This is particularlyattractive for acquiring small to medium-sized3D datasets, or for projects in remote areas,where it may not be practical to mobilize severalseismic vessels at the same time.

For very large surveys, deploying multiplevessels may be a more effective option, becauseevery shot is recorded by a large number ofstreamers (bottom right). WesternGeco multivesselsurveys in the Gulf of Mexico typically deploy a

total of 20 streamers, compared with 10streamers expected for typical Coil Shootingprojects. In practice, few surveys are likely to belarge enough to warrant deploying severalvessels, and these will normally be practical onlyin highly active areas such as the Gulf of Mexico.

Compared with a single-vessel three-azimuthMAZ survey, for the same sailing distance, CoilShooting technology delivers more shots, a largerimaged area and better sampling of offset andazimuth. The Coil Shooting technique provides acost-effective solution for better illumination andimproved seismic imaging in complex geologicalenvironments around the world. —JK

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> Heidrun acquisition geometry. The test data were acquired as 18 intersectingcoils plus one straight line oriented at a heading of approximately 60°. Thenumber of traces (fold) mapping within 25- x 25-m bins is color-coded. Foldrises where circles intersect and is highest in the center of the survey.

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> Estimate of acquisition duration for surveys of various sizes. This chartcompares the predicted time required to acquire different sizes of surveyusing three techniques: the Coil Shooting method, a three-azimuth single-vessel MAZ geometry and a multivessel parallel WAZ geometry. Estimates forthe WAZ geometry assume two recording vessels and two source vessels.The predicted time to complete the survey is shown for all three geometries.For the WAZ survey, the chart also shows “vessel days,” in which the time isincreased by a factor of three—a rough indicator of the increased costincurred though deploying multiple vessels.


shooting seismic surveys in circles - schlumberger

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