SUMIC: Multicomponent sea-bottom seismic surveying in the North Sea-
`Data interpretation and applications
`
`Eivind Berg*, Bjornar Svenning, and James Martin, Statoil Research Center, Norway
`
`SUMMARY
`
`A new concept, named SUMIC (subsea seismics), of
`acquiring four component data at the seabottom has been
`developed. The development stage included an offshore
`acquisition of a 2D SUMIC seismic line using seismic detectors
`deployed at the seabottom. The survey was undertaken over a
`reservoir in the North Sea where the main objective Was to image
`the deep reservoir units through a gas chimney in the overburden
`above the reservoir. This has been an insoluble mapping problem
`for previously acquired conventional towed streamer 3D survey
`data.
`
`The image produced of the reservoir intervals from the
`vertical velocity component Vz data showed similar distortion to
`that obtainable from the conventional towed streamer data,
`because of P-wave nrisfocusing and attenuation introduced by
`propagation through the gas chimney. A more reliable image and
`interpretation of the resevoir intervals was produced from the
`X-component data which predominantly consist of irrlirre
`S-waves. A Graben structure was identified at the top of the
`reservoir structure using S-waves for the first time from a marine
`seismic survey. Shear—waves are less affected by the presence of
`gas than P-waves. Statoil has demonstrated that SUMIC
`surveying is an effective tool with significant advantages over
`conventional marine seisrrric methods. The results have very
`interesting strategic implications for the use of this technology for
`structural and stratigraphic
`definition of reservoirs and
`exploration prospects.
`
`INTRODUCTION
`
`A new concept, named SUMIC, of acquiring four
`component seismic data at the seabottom has been developed
`since 1988 and is the subject to four patents owned by Statoil.
`The SUMIC method involves measuring the X, Y & Z
`components of the velocity field at the sea-bed using three
`component geophones, and the pressure field using hydrophones.
`Assuming a homogeneous plane layered earth, the Z component
`is dominated by P-wave energy and the X and Y components by
`S, and Sh shear-wave energy respectively (Aki and Richards,
`1930).
`
`The development stage included a series of major
`experiments at many locations in the North Sea. We have gained
`much knowledge about S-wave generation and the applicability
`of SUMIC technology in the North Sea environment. We have
`also learned how to interpret best SUMIC data to enhance our
`structural interpretation in difficult data areas and directly detect
`hydrocarbons through the use of interpretation tools such as
`Vp/Vs-ratios (Ensley, 1984, Robertson and Pritchett, 1985,
`Tatharn, 1985).
`The SUMIC concept’s main goals concern the utilization
`of pressure (P) and shear (S) waves to: (1) reduce exploration
`costs by increasing the find rate through improved detection and
`
`mapping of stratigraphic traps, (2) reduce the number of dry
`exploration wells for both structural and stratigraphic prospects,
`(3) reduce reservoir estimate uncertainty at an earlier stage in the
`field development programme by improving the reservoir
`description, and (4) reduce the number of appraisal wells during
`field development.
`The data acquisition procedure required a two boat
`operation (see Figure 1). One boat, the receiver boat, remains
`stationary aside the seabottom geophone detector array taking
`care of geophone stick deployment, retrieval and data recording.
`The second boat, the shooting boat, tows and fires an airgun
`array along the shooting line to give a split-spread irrline
`cornmon- receiver gather.
`A North Sea reservoir was chosen by Statoil, as part of its
`on-going SUMIC development programme, to measure the
`potential of SUMIC seismic surveying as a means of providing
`cost effective solutions to specific geophysical problems which
`carmot be solved
`conventional seismic methods. The chosen
`reservoir has an exploration target which lies beneath a gas
`chimney. Previous conventional towed streamer seismic surveys,
`which rely on PP energy propagation, produced untenable
`images, because ofthe distortion and nrisfocusing introduced as
`the P-waves passed through the gas chinmey where some of the
`rock pore fluid has been replaced by the gas.
`During previous SUMIC experiments (Berg et al., 1994)
`we found that PSS energy was the dominant mode of propagation
`observed on the X and Y components in the North Sea, where
`the conversion from P to S takes place either at, or within the
`near surface of, the sea-bottom. The propagation velocity of such
`PSS energy is less affected by the presence of gas than that of PP
`energy. The seismic image resulting from a SUMIC PSS survey
`would, therefore, be less distorted than that from a PP survey by
`propagation through a gas chimney.
`A continuous and regular 2D seabottom line of 12 km
`length passing over two wells was acquired. The well data will
`enable us to correlate and calibrate our multicomponent data,
`improving the reliability of cm interpretations. Pressure data from
`a conventional towed hydrophone streamer were also acquired
`over the same seismic line to provide a direct comparison with
`the SUMIC seabottom survey.
`The data from the SUMIC acquisition phase were
`somewhat oversampled, with respect to fold, inter-trace spacing
`within each comrnon-mid-point gather and offset range, when
`compared with conventional marine 2D and 3D surveys. One of
`the purposes of our survey, however, was to provide an
`oversampled SUMIC dataset so that a number of degradations
`could be undertaken during data processing, to test their impact
`on the structural and lithological interpretability of the data. A
`fairly conventional data procwsing sequence was applied to the
`full dataset to achieve the main geophysical objective of the
`survey, namely to “see through” the gas chimney lying above flre
`reservoir’s target level. The conventional towed streamer dataset
`which provided the benchmark, was processed using a similar
`sequence with only necessary data dependent parameter changes.
`
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`

`
`SUMIC: Multicomponent seismic surveying
`
`RESULTS
`
`In general, the quality of the multi-component data was
`excellent at all locations along the 2D line as the seabottom,
`geological conditions and water depth varied.
`The seabottom data were of significantly better quality
`than the surface towed hydrophone data. Observations of the
`seabottom geophone components:
`the vertical particle velocity
`Vz and the horizontal particle velocities Vx and Vy indicate a
`clear and distinct separation of the pressure (P) and shear (S)
`wave modes in their respective common receiver gathers. The
`common receiver gathers of the vertical partical velocity Vz are
`dominated by pressure wave data while the inline Vx horizontal
`particle velocity gathers are dominated by shear waves. The other
`horizontal component, Vy, shows mainly weak out of plane shear
`wave energy with a low signal/noise ratio.
`Low frequency and dispersive boundary waves (Scholte
`waves) propagating along the seabottom, similar to ‘ground roll’
`seen during land seismic exploration, were found to be extremely
`weak on all SUMIC components.
`The component separated data have undergone a
`conventional processing sequence and CMP velocity analysis.
`The SUMIC and conventional data reflection events have been
`calibrated and correlated using well log data We have been able
`to identify major lithological boundaries and the SUMIC survey
`had enabled us to extrapolate our detailed log interpretations to
`more distant locations.
`It is quite clear from these analyses that the main shear
`wave mode is SS and conversion of pressure energy fiom the
`airgun source takes place at the seabottom.
`Figure 2 shows a migrated stacked section of the
`conventional 2D towed streamer data. The reservoir targets of
`interest are the reflections between 3s and 3.5s in the Top
`Ekofisk chalk interval and possible Jurassic prospects below 3.7s
`in the mid-part of the section. But as illustrated,
`it is quite
`difficult to produce reliable structural interpretations from these
`data. The reflections loose continuity and are completely broken
`up the middle part of the section. The reflection collapse is
`caused by the gas chimney which rises from the domed and
`faulted reservoir sediments above an intruding salt dome and
`extends to approximately 1.0s in the shallow section. These
`effects of the gas chimney are caused by propagation of the
`P-Waves through sediments with only 2-4% gas saturation which
`heavily misfocuses the ray paths because of the rapid and
`“stochastic” behaviour of the velocity field above the reservoir
`level.
`
`On Figure 3, the V‘ component SUMIC data acquired in
`the surveyed area is shown as the migrated stacked version. As in
`Figure 2,
`the distortions of reflectivity pattern in the reservoir
`interval of interest are the same even if the gas chimney
`influenced area is narrower than on the conventional section. This
`is achieved due to more optimal acquisition with the split-spread
`configuration compared to the surface conventional data.
`Otherwise, outside the disturbed gas chimney, the data quality
`and continuity of the reflections are comparable and m parts
`better than the surface conventional data shown in Figure 2. All
`events observed on the surface conventional data are recognized
`on the V, P-wave section in Figure 3. The structural image
`quality in Figures 2 and 3 is comparable to that seen from the
`conventional 3D survey data.
`
`the stacked V, component SUMIC data
`In Figure 4,
`illustrates clearly another situation compared with Figures 2 and
`3. The first 95 of the data are shown, but they are plotted at half
`scale with respect to the conventional data shown in Figure 2.
`Even if we didn’t have well control with S-wave logs,
`the
`correlation of events between the P-wave sections (Figure 2 and
`3) and S-wave section (Figure 4) is easily seen for the main
`events and structural features.
`In the reservoir zone a more reliable image from S-wave
`data in the central part of the section is achieved. The reservoir
`zone lies between 5.5s to 6s and a faulted pattern can be indicated
`across the crest of the dome. Shear wave propagating will only be
`slightly distorted by the presence of gas,
`their velocity of
`propagation through a given medium being affected only by the
`density changes occuring when the “fluid” Within the medium is
`altered.
`
`Figure 5 shows the stacked migrated version of the V,
`component SUMIC data with the improved structural definition
`of the reservoir zones at 5.55 in the middle part of the section.
`The top of reservoir,
`the top Ekofisk horizon (A), can be
`interpreted through the reservoir with a downfaulted Graben
`structure at the top of the dome.
`The results of o11r SUMIC experiments and surveys show
`that this technology can be used to solve a series of related
`mapping problems in bad data areas where for instance we have
`hard seabottom, shallow gas and gas chimneys.
`In other experiments, we have also experienced that
`multicomponent SUMIC data have a potential to be used to
`predict
`lithology and directly detect hydocarbons, even in
`stratigraphic traps with limited well control. We are continuing to
`develop further and implement novel applications of SUMIC
`technology.
`
`CONCLUSIONS
`
`The results from the application of the SUMIC concept of
`acquiring multicomponent seismic data at the seabottom have
`demonstrated that it is possible to “see through” gas chimneys
`and produce a more reliable seismic interpretation of a Graben
`structure at the top of the reservoir for the first time from a
`remote seismic survey.
`The seismic quality of the multicomponent seabottom
`data, as also confirmed by other experiments,
`is better than
`conventional marine seismic surveys with respect to S/N-ratio
`and resolution
`As the main objective of the SUMIC method is to reduce
`risks in the exploration phase and reduce uncertainty in reservoir
`mapping, the data results demonstrate that the first application of
`the SUMIC technique has shown significant advantages over
`conventional
`seismic methods.
`The results from the experiments demonstrate the
`tremendous potential of SUMIC technology. SUMIC can be used
`to address a series of related mapping problems where pressure
`waves have so far been the only data upon which to provide
`structural and stratigraphic definition of reservoirs and
`exploration prospects.
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`ACKNOWLEDGEMENT
`
`shooting Boat
`
`_
`Recording Boat
`
`$UIl|cCMuMuolti:onmpdsI:umunsqrisgn ic s u rveyi n g
`
`The authors would like to thank Den norske stats
`)ljeselskap a.s. (Statoil) for the permission to publish this paper.
`
`Airgun array
`
`REFERENCES
`
`Aki, K. and Richards, P.G., 1980, Quantitative seismology,
`Theory and methods, Volume I, W.H. Freeman & Co.
`
`Berg, E., Sverming, B. and Martin, J., 1994, SUMIC - A new
`srategic tool for exploration and reservoir mapping, presented at
`he 56th annual EAEG meeting.
`
`Ensley, R.A., 1984, Comparison of P- and S-wave seismic data:
`A new method for detecting gas reservoirs. Geophysics, 49,
`1420-1431'
`Robertson, J.D. and Pritchett W.C., 1985, Direct hydrocarbon
`detection using comparative P-wave and S-wave seismic sections.
`Geophysics, 50, 383.393.
`
`Tatham, R.H., 1982, VP/V! and Lithology Geophysics, 4’7, 336.
`
`Figure 1. Two boat operation for SUMIC 2D-line data
`acquisition‘
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`streamer data acquired over the surveyed reservoir.
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`Figme 3- Migmted Stacked 5991310“ 0f the SUMIC V2‘
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`Sumic: Multicomponent seismic surveying
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`Figure 5_ Mgmmd sgackgd secfion of 1113 SUMIC v_.componem
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`Eivind Berg, Bjesmar Svenning, and James Martin (1994) SUMIC:
`
`Multicomponent sea-bottom seismic surveying in the North Sea—Data
`
`interpretation and applications. SEG Technical Program Expanded Abstracts
`1994: pp. 477-480.
`doi: 10.1190;’1.1932132
`
`SUMIC: Multicomponent sea-
`
`transition zone
`
` bottom seismic surveying in the
`' North Sea—Data interpretation and
`applications
`
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`
`SUMIC: Mullicomponentsea-bottom seisrniosurveying inlhe N orlh Sea—Data interpretation and applications : SEG Technical Program Bqaanded
`114259014
`I | 1-311! WIIISI I2. SUIIIILJ
`
`mu1ticomgonent|
`‘ Title Information
`sea bottom | AND
`An where: seismi
`
`EG Technical Program Expanded Abstracts 1994
`cm
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`sur... {std - 281
`o Anywhere: marin
`QSSQ reflections
`bottom velocigg
`transition zone std
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`Publication Data
`
`£1
`
`ISSN (print): 1052-3812
`ISSN (online): 1949-4645
`Copyright Year: 1994
`Pages: 1679
`Publisher: Society of Explorati on Geophysicists
`Authors: Eivind Berg, Bjomar Svenning, James Martin
`Statoil Research Center, Norway
`Perm alinkz httg:!fdx.doi.org:‘10. 1 l90fl. 1932132
`SUMIC: Multieorn
`ponent sea-bottom seismic surveying in the NoI1h Sea-
`Data interpretation and applications
`
`Eivind Berg‘. Bjnmtr Svenning. and James Martin. Stator‘! Research Center. Nomviy
`mapping of straugraphtc Inns. (2) reduce the number of thy
`SUMMARY
`etptoration wells for both stmntinl and stratiyaphie prospects.
`t3)rethrv:reser\'oirestirtatetncertaintyatanexlierstagciiIlie
`field development programn: by improving the reservoi
`tkscfption ml t4) redutr the mmberotappraisd tells duriig
`field dcwlcpmcnt.
`The data acquisition procedure required a two hour
`operation (see Figure I). One bod. the receiver bol. remains
`stlionary aside the seabottom gcotitonc dctccttr array taking
`an: of gonphmc stick drployrnerl. ruricnl and tin toomlirg.
`11:: second bout. the shooting boat. tows and tires an airgtm
`army along the shooting line to give rt split-spread inline
`eummurv receiver pha.
`:\NorthSearesaVoirI'asdtosenbyStatoi.aspond'its
`on-going SUMIC development programme. to measure the
`potential of SUMIC
`surveying as a mars of providing
`costeffeetive solutiorn to specific geophysial problems which
`anrntbcsolvedusirrgcornentiornl sc-‘umicmcthods Thechoscn
`reservoir has an exploration target which lies beneath a gas
`dtirrney. Previous contentions med streamer seismic stncys.
`which rely on PP energy propagation. produced untenable
`irnages. because of the distortion and rnisfocmirtg introduced 5
`tlteP-wavesprassedthrottglttltegaschinltcy vultcresorneofthe
`mckporellridltmbeenreplarvedbytltegatt
`I991)
`During previous SUMIC csperineno (Bag ct d..
`vwfoundthl P&$ertergyw:rstlretlr1ttittuInttxletl‘propog;:ti<a1
`obsened on the X and Y cornponerls it the North Sea. where
`the conversion from P to S takes place either at. or within the
`notrsiirtaeeot.tltes«a-hottrintihepropagaionselticityolsttch
`P$Senergyislessatl’eetotlhythepre~enceulpsthanthtofPP
`enu'gy.1he
`irnge resulting from a SUMIC PS8 atncy
`\\ould.therefote.l:elessdistomdthantlmt'rtInaPl’surveyby
`propqatiat tluuuph a gas chimney.
`A continuous and regular 20 sedronom line oi I2 km
`length passing over two vtells was acquied. The well data will
`enable us to correlate and calibmc our rmhicomponent data.
`inptoving the teliitiliy ot our itlerpetziots. Pressure data from
`a conventional towed ltydtofltone streamer one also acquired
`over the same seismic line to provide a diner conparison with
`the SUMK.‘ seabotntn surwy.
`The data from the SUMIC acquisition phase were
`sotnewfnt osrmrnpled. with respect to fold. inlrr-trace spring
`within each common-mid-point pther and oflaaet range. when
`cornpued with converttiornl maine 2Dand 3D tancys. One of
`the purposes of our survey. however. was to provide an
`mersarnpled SUMIC thtaset so that a number of dcgndaions
`owld be undertaken thring data processing. to test their impact
`on the structural aid lithological interpretability of the dam A
`fairly conventional dun poccssing sequence was applied to the
`full dataset to achieve the main geophysical objective of the
`IlI\V|II
`truvu-Iv tn '\» lhvnn-It" the iris rhinvv-v lvita IMVF th-
`
`A new concept. mined SUMIC (subsea seislnics). of
`acquiring four component data at the seahottom has been
`developed. The development stage included an ofishon:
`acquisition of a D SUMIC seismic line using seismic detectors
`deployed I the seabottom The suney was undendcen over a
`reser\uiriadteNonhSawhereiterninohjeaitevmtoitmge
`he deep resenuir units mug. a gas dtimney in the ox-erhurden
`drove the rtrentir. ‘nix Ins heen an insohble mapping prudent
`for previously acquired eonvernioml toned streamer 3D survey
`«tau.
`
`Theirnageprotlueedoftheresenoirimcrvalsfrontdte
`venicd vdodty component V1 du shtned similar distortion I)
`that obtainable from the eonventioml trwted slimmer data.
`because of P-wave rtistocusing and nttentntion introduced by
`popngatiortthtoughthe;asdtiirutey.Antotereli$lcirItqead
`interpretation of the resevoir intervals was produced from the
`Xeornponent data which predominantly consist ot inline
`S~wavex A Grabcn structure was identified at the top of the
`roenoir stmuttre usiru S-vnvet tor the first time from a main:
`seismic stlvey. Shea-waves are less affected by the presence of
`gas titan P-waves. Statoil hm demonstnbd that SUMIC
`surveying is an cffectite tool with sigrilimnt advantages over
`eonventioml marine seismic methods. The resuls have very
`irlerestitg stnlegic implicaions for the use of this teclltology for
`structural and stratigraphic
`definition of
`and
`eatpltntim pmspeux
`
`INTRODUCTION
`
`A new concept. nxned SUMIC. of acquiring four
`component teismic data at the sewottutn has been developed
`since I988 and is the ruhject to fair patents owned by Statoil.
`1'he SUMIC method involves measuring the X. Y & 2
`components of the velocity field at the sea-bod using three
`eorrponert goophoncs. and the gesture field using Itydrophates.
`Assuming: homogeneous plane layered carth.thc Zcontponent
`isdotrinat:dbyP-v~avcencrgyairdtleXuIiYctrrnpatnrtstry
`S, and 8. shear-wave energy respectively (Ali and Riclurds.
`I980).
`
`The development stage included a series of major
`atpaitncntsatmmy |ot:ationsinthcNmhScaWchaw gined
`much kntmlcdpc wool S-wave genaation and the applicability
`ot SUMIC technology in the North St: environment. We have
`also lrarned how to iiterprot best SUMIC data to athunce oil
`structtli interpretlion in difftcul din arms and directly detect
`hydrocarbons through the use ofintapretation tools such as
`Vp/V9-ratios (Enslcy. I98-I. Robertson and Pritchett. I985.
`Tatham. I9S5l.
`hltp:.tflibrary.s eg.or gldoir‘ahs!10.1 19.1l1.1Q32132
`
`WesternGeco Ex. 1010, pg. 10
`
`

`
`11959914
`
`SU MIC: M ulticom ponem 5 ea- boflom s eis mic s unteyi ng in the N orth Sea—D ata inter pr etation and applications 2 SE6 Technical Program Expanded Ab. ..
`nan’. nun-nu; m K; uu\:\g.n um. Eu‘ \.lIlIIHL_1 n_uu5 nuns; un.
`méisiwc‘ «mph mm mm mm-cm «tr uunm‘-m
`rc\a\'oi'\ mm M. m mnvmmrul mm! mm dalavtt
`"f P'‘“‘-'“‘ ‘P’ 4"“! ‘M1 ‘S’ “'““*“ '05 ‘ I’ W‘-'“°'-‘ °‘P‘l‘7"°"
`which pmvidcd Ihc hcnchnlark. was processed using a similar
`00*“ ‘F i"¢'V=‘i‘8 "W “M “'3 'hfl‘“$h i“P'*M‘d 4~"'~‘“i’“ -W‘
`vapctwc nit: only mccsury dun ck-pant-nl panmcia chang-:x
`
`477
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`http:.0‘li b! ary.s eg.orgfdoi/absJ10.119Ji‘1.1Q32132
`
`WesternGeco Ex. 1010, pg. 11
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`

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`11/250014
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`SU MIC: Multicomponent sea- bottom seismic surveying in the North Sea—Data interpretation and applications : SEG Technical Program Expanded Ab...
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`Literatum
`
`http:l/Iibrary.seg.org’doilabs/10.1 190/1 .1 9321 32
`
`WesternGeco Ex. 1010, pg. 12