marine multicomponent seismology

Marine multicomponent seismologyhas seized the geophysical industry’sfancy since its semicommercial emer-gence in the North Sea in the autumnof 1996. The essence of this technologyis recording S-waves, in addition toconventional P-waves, on the seafloorwith sensor packages containinghydrophones and three-componentgeophones. New processing algo-rithms have had to be developed toproperly process these data.

Nearly 100 four-componentmarine surveys have been acquiredsince September 1996, the vast major-ity being 2-D. However, 4-C, 3-D sur-veys are growing at a faster rate than4-C, 2-D surveys. The processing ofthe first handful of 4-C, 3-D surveyshas been completed. Information con-tent has been very high and very use-ful. Four-component surveys havebeen conducted in the North Sea, theFar East, offshore West Africa, CookInlet, the Middle East, and in the Gulfof Mexico. Initial evaluation of thesedata suggests that quality in all loca-tions is relatively high, that acquisi-tion systems couple reasonably well tothe seafloor, and that most mode-con-verted S-wave energy derives from P-waves mode-converting to S at depthand not at the seafloor.

Acquisition of 3-D marine multi-component data is 1.5-4 times moreexpensive than conventional towedstreamer 3-D, but that is expected atthis stage of development. The costwill decrease fairly dramatically ascapacity and production rates increase.

What is marine 4-C seismology? Inthe context of this article, marine four-component seismology (M4C) is thetechnology that places four-compo-nent sensing systems on the seafloorto record the full vector wavefield ofpassing stress waves. In a simplisticsense, this means recording both com-pressional waves (P-waves) and shearwaves (S-waves). Further, M4C sup-plies sufficient data processing to makeinterpretation possible. Finally, it pro-vides for more complete interpreta-tion of reservoir properties, including

reservoir boundaries, than is providedby more conventional one- or two-component (P-wave) technology.

The four components of M4C area hydrophone, a vertical geophone,and two horizontal geophones ori-ented perpendicular to each other; allfour are included in each receivergroup. Conventional marine towedstreamers are one-component systems,use only hydrophones, and recorddirectly only P-waves. ConventionalOBC (ocean-bottom cable) systemsemploy hydrophones and vertical-axisgeophones (making them two-com-ponent systems capable of recordingthe vertical component of the vectorwavefield) and, again, record primar-ily P-waves. The reason for going tothe added effort and expense ofemploying M4C technology is torecord a wave not routinely recordeddirectly, the S-wave.

When a P-wave passes through

1274 THE LEADING EDGE NOVEMBER 1999 NOVEMBER 1999 THE LEADING EDGE 0000

Marine multicomponent seismologyJACK CALDWELL, Schlumberger, Houston, Texas, U.S.

Editor’s note: Much of the material in thispaper was prepared for presentation at the1999 Offshore Technology Conference inHouston, Texas, and is published in the OTCProceedings. The author has subsequentlyupdated some of the text for TLE publication.

Figure 1. A single node of theSumic-derived node system.(Photo courtesy of CGG.)

Figure 2. The three main cable types of marine multicomponent systems.The top two versions both derive from OBC technology.

Table 1. Comparison of S-waves and P-waves• • Most of the oil industry’s seismic work is done with P-waves, using single or dual sensor technology.• P-waves travel faster than S-waves, from roughly twice as fast at depth to as much as 8-10 times as

fast (occasionally even more) very near the seafloor.• S-waves can be created by conversion of P-waves at rock property boundaries, so conventional

marine air-gun arrays can be, and are, used to create S-waves.• S-waves cannot exist in fluids, but P-waves can and commonly do.• To first order, S-waves are not affected by the pore fluids in rocks, but P-waves are.• Taken together, these two types of energy can provide much more information about a reservoir than

can be provided by either alone.• To record S-waves in the marine environment, special recording equipment that contacts the seafloor is

required.

rock, its behavior is affected by boththe matrix of the rock (the solid part)and the pore spaces of the rock (thatportion filled with liquids and/orgases). To a first approximation, whenan S-wave passes through rock, itsbehavior is affected only by the matrixof the rock. Two other important prop-erties of S-waves are that they travelat roughly half the speed of P-wavesand they cannot exist in fluids—hencethe necessity of placing the sensors onthe seafloor. Recording both wavetypes makes it possible to infer muchmore information about the rock andits contained fluids than by recordingonly one. Also fundamental to the suc-cess of M4C is that S-waves can begenerated from P-waves impinging onboundaries that separate rocks pos-sessing different densities and/orvelocities. Table 1 summarizes themore relevant differences between P-waves and S-waves.

Marine four-component activity todate. Marine multicomponent dataacquired over the Tommeliten Field(in the extreme southwest part of theNorwegian sector of the North Sea)by Statoil in the early 1990s was pre-sented at EAEG’s 1994 meeting. Thispresentation showed how the S-wavedata provided a picture of the reser-voir not available previously becausea gas chimney prevented a useful

image from P-wave data. The presen-tation generated high interest and, bythe autumn of 1996, four different sys-tems had been developed to acquirefour-component data at the seafloor.

By the end of 1996, marine multi-component acquisition had becomecommercial, several data sets havingbeen acquired in the North Sea and offthe coast of West Africa. Data pro-cessing took another few months to“become commercial” but, by mid-1997, a moderately strong demandexisted for this technology.

At least 95 marine four-componentprojects have been conducted since1996; about 80 are 2-D and approxi-mately 15 3-D. However, the percent-age rate of increase in the number of3-D, 4-C surveys is now higher thanthat of 2-D, 4-C surveys. The first 3-D,4-C processed results were deliveredin the middle of 1998. In general, over-all data quality has been very good,somewhat opposite to general experi-ence of the late 1970s and early 1980swith land S-wave data acquired using

dedicated S-wave sources. The over-all usefulness of the data, to this point,has not been heavily dependent onsophisticated data processing,although it is certain that more sophis-ticated processing will enhance theextractability of useful informationand increase cost-effectiveness. Andso far, independent of the location ofthe particular survey, the major usefulS-wave energy has been generatedfrom mode conversion of the down-going P-wave energy impinging onrock property boundaries in the sub-surface and not at the seawater/seafloor interface. Table 2 summarizesthe industry’s overall M4C experience.

Factors affecting data quality.Geographic location has not made anoticeable difference, to a first order,in data quality. However, differences(some larger, some smaller) in qualityappear when the data are examinedmore specifically. Some reasons aresomewhat obvious, although the rel-ative importance of each is not neces-


Figure 3. The three methods fordeploying seismic seabed cablesystems. Differences in data qual-ity may be associated with thedifferent methods of deployment,but more studies are needed.

Figure 4. This schematic illustrateshow a downgoing P-wave (red linesegments) generates an upgoing P-wave that is obliterated on encoun-tering a gas zone, and an upgoingS-wave (blue line segments) thatpasses relatively unscathedthrough the gas zone.

Figure 5. This example is from the Tommeliten Field in the Norwegian sectorof the North Sea, courtesy of Statoil. It is one of the earliest examples thatdemonstrated the usefulness of PS-wave data compared to PP for imagingthrough a gas cloud. Note how the gas chimney disrupts the P-wave (uppersection), while allowing the S-wave (lower section) energy to generate a use-ful image beneath the gas chimney.

Table 2. Summary of marine four-component activity•• ~ 45 proprietary 4-C surveys•• ~ 50 speculative 4-C surveys•• ~ 15 3-D 4-C surveys; the rest are 2-D•• ~ 10 3-D 4-C surveys have been completed through data processing.•• 4-C surveys have been completed in: the North Sea, offshore West Africa, the Gulf of Mexico,

Cook Inlet, offshore Indonesia, offshore Thailand, the South China Sea, and the Middle East.•• Data quality has been good, and comparable from area to area, with no noticeable systematic

differences related to gross geographic location.•• In all areas worked to date, the primary mode-converted S-wave energy is being generated at

depth and not at the seafloor.

sarily so obvious, unsurprisingly giventhe relative infancy of this technology.The more apparent reasons include (1)weather conditions; (2) conditions inthe water column above the recordingsystems; (3) conditions at the seafloor,immediately above it, and a small dis-tance below it; (4) design and con-struction of the sensor system; (5) themode of deployment; and (6) day-to-day practices of the crew.

Weather conditions. The effect ofweather on the quality of seaflooracquisition is much smaller than fortowed-streamer work. For seaflooracquisition, the cable is typically in aquieter environment than is a streamersubject to swell noise. Crew safety (notdata quality) usually determines whenthe weather will shut down seaflooracquisition operations.

Properties of the water column.Currents and their variability affectthe ability to place the sensor systemaccurately and efficiently. Variationsin salinity and temperature affect theaccuracy and precision with which thesystem’s actual position on the seafloorcan be determined, especially in deepwater. These variations introducebends (refracted paths) in the acousticpingers’ sound paths, causing locationerrors.

Conditions of the seafloor. Currentsright on the bottom can scour sedi-ments around the system or canmove/abrade the system. However,experience to date is limited (but notzero) with regard to areas that havebottom currents sufficiently strong todo these things. Do sediments coverthe seafloor? How hard are they or isit? How rough? The quantitativedescription of the properties of theseafloor and sediments very near thesurface where 4-C surveys have beenconducted to date has not been sys-tematically undertaken. This descrip-tion and an associated study of therelationship between seafloor proper-ties and data quality can be begun rel-atively easily given that most 4-Cprojects to date have been near pro-duction platforms for which detailedsite surveys/engineering studies exist.And the 4-C data themselves will yieldinformation about the very near sub-surface, as will the side-scan data oftenacquired in preparation for the 4-Csurvey.

Sensor system design and construc-tion. Today, there are two major cate-gories of systems: node type and cabletype.

Node-type systems (Figure 1) aredirect derivatives of the SUMIC sys-tem patented by Statoil. The four-com-


Figure 6. The PP image (left half of figure) and the PS image (right half)shown in this figure are from the Far East. The PP data are disruptedthroughout most of the section due to gas in shallow zones. The PS dataimage a shallow fault extremely clearly, as well as showing good eventcontinuity all the way across the section.

Figure 7. These data are from the Alba Field in the North Sea. The uppersection is the PP data, and the lower section is the PS data. The top of thereservoir is not visible in the PP data but is very evident in the PS data.The log curves on the left indicate why this is so: VP shows no change atthe top of the reservoir, but VS does.

Table 3. Applications of M4C• • 4-C Imaging 60%

Through gas clouds, chimneys, etc. 40%Higher S-wave impedance change than P-wave impedance change 10%Beneath salt 5%Beneath basalt 5%

• • 4-C Lithology & Fluid Prediction 40%Fluid identification 13%Discrimination of sand from shale 12%Mapping hydrocarbon saturation 8%Mapping OWC 5%Mapping fractures 2%

ponent sensors are housed in individ-ual packages, either cylindrical orhemispherical, which are stabbed intothe seafloor by remotely operatedvehicles (ROVs). An umbilical cableattaches one sensor container to thenext, provides power and communi-cation with the recording system, andis used to tow the overall receiverspread.

Cable-type systems (Figure 2) nowexist in three versions. One is based onconventional OBC technology, but twohorizontal geophones are added toeach hydrophone/vertical geophonepair. Sensors may be attached to theoutside of the cable or enclosed in amolded plastic cylinder with the geo-phones nestled in appropriatelyshaped hollows. Another version,based on logging technology, puts thesensors into steel cylinders that sit onthe seafloor. Sensor packages are con-nected by a high-strength, conductivecable. The third version, based onstreamer technology, puts the sensorsinside a fluid-filled cable. Just for con-venience, the first system will be calledan OBC type, the second a loggingtype, and the third a streamer type.

There has been a good deal of dis-cussion about differences in geophonecoupling to the seafloor by these dif-ferent systems, but little data andanalysis have been published. Some,if not all, seismic contractors and someoil companies have comparison datasets which are now being analyzed tosee what can be learned about the cou-pling characteristics of the differentsystems.

Deployment of the sensor system onthe seafloor. The actual method bywhich the sensor system is placed intocontact with the seafloor is known tochange the appearance of the data, butno generally preferred method has

been identified. For node-type sys-tems, it was found early on that insta-bilities associated with the conditionsof the seafloor or the stabbing into theseafloor or both created spurious res-onances. For instance, using the ROVto “nudge” the sensor package intothe most upright and level orientationmeant making the hole for its stakelarger than the stake itself, which, inturn, created instabilities. To rectifythis, a version emerged which hadthree stakes, and another versionemerged which has a special “skirt”added to the base of the cylindricallyshaped housing (Figure 1).

Cable systems can be placed onthe seafloor in at least three ways: (1)by dragging (the cable is laid on theseafloor some distance from where itis intended to actually record data, andthen dragged to its intended position);(2) by draping the cable in place, butkeeping it under tension the entiretime; and (3) by looping or draping thecable in place, but not maintaining itunder tension (Figure 3). Differencesin amplitude spectra for some Gulf ofMexico data sets have been notedbetween data acquired with thestreamer-type system when the cablehas been dragged into place and whenit has been draped under tension.

Crew behavior. The day-to-day per-formance of the crew will affect dataquality, just as is true for conventionalland and marine acquisition.

Observing best practices, paying atten-tion to detail, and learning quicklyfrom a relatively short history of M4Cmay have a large effect on quality.

Applications of M4C—why go to thetrouble? The applications of M4C fallinto two major categories, imaging(about 60% of all 4-C projects) andlithology/fluid prediction (about40%). A specific application includesimaging beneath gas chimneys andgas clouds. This has been the “no-


Figure 10. Curved arrows indicateseismic events associated with thehorizon of interest, an oil-bearingsand extending from the fault onthe right until location 1550, wherethe sand becomes water-bearing.The bottom graph plots the maxi-mum amplitude of that event.Beginning on the right, the PP andPS amplitudes are similar untillocation 1550, where they diverge.

Figure 8. Upper left is the PPamplitude volume for amplitudesabove a certain threshold, and thelower right is the PS amplitudevolume also for amplitudes abovea certain threshold. The PP dataprovide no indication of the sandchannel, while the PS data clearlyshow the channel geometry of thesand reservoir.

Figure 9. These Far East data show the difference in response between thePP section on the left and the PS section on the right. The bright spot indi-cated by the arrow shallow on the PP section is a gas accumulation thatobscures the data beneath it. The gas accumulation has no amplitudeanomaly, as expected, on the PS section. The two sections taken togethersuggest a gas accumulation, and the PS section provides a clearer picture ofthe underlying structure and stratigraphy.

brainer” application of M4C. It isrobust and requires no novel process-ing capabilities or new interpretivetechniques. In all examples of which Iam aware, a downgoing P-wavemisses the gas zone, the reflected P-wave is disrupted by the gas zone, butthe upcoming (mode-converted) S-wave passes through the gas zonelargely unaffected and generates animage of reflectors directly beneath thegas (Figure 4).

This technique has been successfulin the North Sea, offshore Indonesia,offshore China, the Gulf of Mexico,and offshore West Africa. Figures 5and 6 show examples from the NorthSea and the Far East. The PS data yielda much clearer image than the PP datain both cases. Since processing of mul-ticomponent data is still evolving,these images were achieved with“basic” processing, hence my beliefthat imaging beneath gas is achievedrelatively easily.

Large S-wave impedance contrastassociated with small P-wave imped-ance contrast. Figure 7 shows datafrom a 4-C, 3-D survey over Chevron’sAlba Field in the North Sea. The reser-voir is an oil-filled sand where the P-wave impedance contrast between thetop of the reservoir and the overlyingrocks is essentially nil. The sonic logand density data also indicate nochange in VP. However, the PS sectionclearly shows the top-of-reservoirevent, and the sonic log shows a largechange in VS at depth. The oil-watercontact is visible in both sections.

Figure 8 shows PP and PS volumecubes in which only amplitudes abovea certain threshold are plotted. Thehigh amplitudes in the PS data delin-eate the sand channel, but those in thePP data show nothing discernible.

Lithology/fluid discrimination. Thearrow in Figure 9 shows an amplitudeanomaly on a PP section that is not pre-sent on the corresponding PS data. Thelogical first interpretation is that thisis a gas zone, since the P-wave wouldexperience a large impedance contrastat the top of the gas and the S-wavewould be largely unaffected in goingfrom a brine-filled rock to a gas-filledrock. However, without the S-wavedata, the interpreter could not rule outa change in porosity, a hard streak, oranother change in lithology as thecause of the P-wave anomaly. While acursory look at just the amplitude isnot sufficient to nail down the gasinterpretation, it was the correct analy-sis in this example.

Mapping hydrocarbon saturation.Anintriguing possibility is that hydro-carbon saturation may be mappedusing PP and PS amplitude ratios.Figure 10 shows a North Sea examplein which the PP and PS amplitudestrack each other along the portion ofthe seismic line where sands are oil-bearing and then diverge where thesands become water-bearing.

Table 3 summarizes how M4C hasbeen used over the last 2.5 years. Thelist of applications is relatively longand should become longer soon. It ishoped that M4C will help delineateshallow water-flow zones, but that willrequire further research. It is quite clearthat combining information from S-waves with that from P-waves gives abetter and more complete picture ofthe reservoir.

Where from here? The future of M4Clooks very bright, but some develop-ments must come to pass fairlyquickly to address the industry’srequirements. Seabed acquisition sys-tems need to deliver quality data atwater depths up to 3000 m to func-tion in the hottest new areas of theGulf of Mexico and other prospectiveregions. It may take different systemsto efficiently and cost-effectively han-dle the necessary water depths. Cable-type systems (and even theSUMIC-derivative systems use cablesfor retrieval) must have: (1) sufficientstrength to lift themselves over thevertical extent of the maximum waterdepth; (2) seals that withstand thepressures at depth; (3) an efficient,robust, and safe method for handlinglarge amounts of cable, and (4) light-weight sensor packages for high-fidelity data (which implieslightweight cables or sensor packagesdecoupled from the cables or stand-alone sensor packages like the ocean-bottom seismometers used by theacademic community for manyyears). Additionally, if yet-to-be-gained experience by the industry dic-tates data quality that can bedelivered only by dragging, not drap-ing, cables into place, then systemswhich can be efficiently dragged invery deep water would have to bedeveloped. It is not likely that thatbridge will have to be crossed, but itis one to keep in mind in these earlydays when the industry is evaluatingwhat techniques deliver what qualitydata at what expense . . . and whatthat implies for the future. Presently,the OBC-type systems can operate inwater depths down to about 150 mand streamer-type systems to about

1000 m. The logging-type method hasno inherent depth limitation relatedto the strength of the system.

All systems are in the same boatwhen it comes to determining exactlywhere they lie on the seafloor.Currently, positioning accuracy is onthe order of 0.5%, which means know-ing the location of a station to within5 m at 1000 m. If that same accuracyis required for deep water (whichimplies knowing station location towithin 15 m at 3000 m), then advancesmust be made in routinely used tech-nologies. This is because there may berelatively substantial vertical and lat-eral variations in water temperatureand salinity, which will cause unac-ceptable error in pinger-type locationsystems. In shallow water, those vari-ations are small or nonexistent and notoften a problem.

The processing of four-componentdata is advancing rapidly. It has beendemonstrated that 3-D, 4-C data setscan be effectively processed, whichwas a question that the industry hadnot answered until the second half of1998. PS prestack depth migration hasbeen shown to be necessary in somesituations, and that capability doesexist, including the ability to handlevertically transverse isotropy.Anisotropy will be a more importantconsideration with converted wavesthan for P-waves (and the industry israpidly coming to the conclusion thatanisotropy must be dealt with for P-waves), so developments in this areawill occur rapidly.

Suggestions for further reading.“SUMIC—a new strategic tool for explo-ration and reservoir mapping” by Berget al. (1994 EAEG Meeting). “Converted-wave imaging of Valhall reservoir” byThomsen et al. (1997 EAGE Meeting).“Application of marine 4-C data to thesolution of reservoir characterizationproblems” by Kristiansen (SEG 1998Expanded Abstracts). “Marine 4-compo-nent seismic test, Gulf of Mexico: Subsaltimaging at Mahogany Field” byCaldwell et al. (SEG 1998 ExpandedAbstracts). “Subsalt imaging usingprestack depth migration of convertedwaves: Mahogany Field, Gulf of Mexico”by Kendall et al. (SEG 1998 ExpandedAbstracts). LE

Acknowledgments: Tony Johns, PaalKristiansen, and Jason Robinson have beenextremely helpful in putting this materialtogether.

Corresponding author: J. Caldwell, [email protected]


marine multicomponent seismology

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