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<br />imaging requires full volume 3D
<br />PSDM, with its six to eight migration
<br />iterations.
<br />The first iteration uses the velocity
<br />field produced during the time migra-
<br />tion steps described earlier. In this
<br />case, data from 16 offshore blocks,
<br />involving about 23 million traces,
<br />were input into the full volume 3D
<br />prestack.
<br />The iteration is nn the shallow ve-
<br />locities down to the top of salt. Fig. 11
<br />shows the improvement that results
<br />between the first and third iterations.
<br />Proper imaging of the base of salt and
<br />subsalt reflectiyih' twill require four or
<br />five more iteratons.
<br />A major complication of salt imaa
<br />ing is the irregularity of the top of salt,
<br />a result of the interaction between salt
<br />and (cults in adjacent sediment..
<br />Fig. L' demonstrates salt's dcna-
<br />mism with hvo 3D PSD>I sections
<br />spaced only 1,~OU ft apart.
<br />The images shmc the interaction
<br />beh. een salt and faults in oterlving
<br />sediments. The differences between
<br />them-the change in salt shape over a
<br />relatively small distance-attest to the
<br />importance to telncih• analysis of
<br />close spacing.
<br />Accurate velocities
<br />Since all this work hinges on accu-
<br />rate velocities, it is important to know
<br />when telocities are, indeed, accurate.
<br />The test comes on full, 3D prestack
<br />depth-migrated, common depth point
<br />(CD[) gathers, examples of which ap-
<br />pear in Fig. 13. Offsets range from 200
<br />m to 6,000 m, which is important.
<br />As with the stack panels in time-
<br />domain velocih• analysis, the tech-
<br />nique is to find the velocitq that re-
<br />motes normal moceout cunature-
<br />Flattens the data-for specific events.
<br />It is unusual to perform this velocity
<br />analysis step with CDP gathers from
<br />fulh migrated 3D data, common as it
<br />is in cont'entional 2D processing.
<br />A lacer-stripping approach is essen-
<br />tial.
<br />The interpereter (first flattens events
<br />dotcn to perhaps 3,000 1t, then does
<br />so down to 6,000 ft, then again down
<br />to 9,000 ft and w on (or three or (our
<br />more iterations.
<br />lYhat the interpreter cannot do is
<br />try to Flatten all events at all depths at
<br />once.
<br />A small error high in the record
<br />becomes a Jorge error at greater depth.
<br />This is where long offsets become
<br />Trucial. The}' ensure multiplicity of
<br />data front hise-u(-salt and subsalt re-
<br />flections, tchich, becmisv they tend to
<br />occur at depth, may appear on rela-
<br />tively few traces when offsets are
<br />short Events that appear on relatively
<br />
<br />IYrlrrr
<br />Da: i. Raldift is :itt.ly nuknl of Dm-
<br />wrnrJ GnpLV+;rnl Srr, rrr G'rp. cord
<br />Ira, dnC of tlrr rsrnay &nur+i Din-
<br />nuad Gv!nn¢r Rccn:h Cnrp. Hr has
<br />6ror ur,vlrnl m ~rrldm.aal mm~iu(
<br />n/ n'mpln' >al! gn+lrjir < ;roddn•idr for
<br />Ihr pr.o-l 70 umr: cord 4a: nlq+hr.l 3D
<br />iumSiu~ IiilwoloC4 rc nowt rLan 700 soh
<br />:hurfure> Fq~+r'r jriuio~ Di.vmuld. hr
<br />:ra< <upn^iav of ~ngr4 vied kdmnlogv
<br />Jrv Ponom Pradurlirn; Q+.'s U.5..4. and
<br />Eurrpmn crl lnrnlimr hu:nlrss unit.
<br />RnftlifJ Loki: n 65 n: umlhrnmtirs
<br />from !hr Unia'r: Try of ,\'ra' Orlrm;i. Hr
<br />is ILi ]P9! rrripicnl o; Ihr Snciau a/
<br />E,p!,nalion Vr+grLy+ic=r; l'irSil f:.vrff-
<br />man Geld M:dn! n;rrtr,l for lus cork rn
<br />3D scot, lurnl ima~in~.
<br />Do;vrl /. It'rdn is prniJnrl of Din-
<br />nurnd Gmldry<iral 9: rtrr Corn. and
<br />rLnf c,n ufi;r n)frcrr rl DuvnnuJ Gro-
<br />.nrmr P.nmrdr Cop+. Hr 1vr:vnniv
<br />•rrrknl I_> ur ors for Trn:ar Gruphv:i-
<br />ral'> Jra~r;>h; nnJ ur'rn:aliurrnl c,;nna:
<br />data pn•rC•in~ rnrlrn. rhr Iu;1 S umri
<br />u< dirrrlor of !r[Inuo;! rr;, u'4RinS.
<br />l11lrr, ;rlrrr LuIJ: a 65 f~rnn L+ursi~
<br />ann SLM Unr,'rr:il v. I:.I+ I+ru iu;vl;rd
<br />in u¢nr Ilru'r _'SO 3D !;::our armrvs
<br />:nvlA;rr.iL irrrLeliu~ ,: ~.aa; cord yro-
<br />rn?urS r( rrr faun nr ae.l rvndnr.ur-
<br />I.,v<.
<br />fete traces are difficult to flatten; being
<br />difficult to see un CDP Gathers, thec
<br />racy seem to IloRen nt several veloci-
<br />ties.
<br />The greater nuduplicity of data that
<br />results from long offsets also helps
<br />improve stacking of the base of salt
<br />and subsalt events.
<br />~~
<br />.rne tuture
<br />The importance of full volume 3D,
<br />PSDM to salt imaging is evident in the'
<br />subsalt discoveries made so far in the'
<br />Gulf of Mexico. And 3D PSDM tech-•
<br />nology will help explorationists learn'
<br />much more about subsalt prospecting,;
<br />It is not enough to run PSD~1 algo.•
<br />rithms on standard 3D data voh:mes.:
<br />Success depends on acquisitian of~
<br />close!}' spaced, long-offset da:,-. and
<br />careful application of conventional
<br />processing and interpretation tech-
<br />niques to ensure accurate velocity in-
<br />formation.
<br />The process is expensive, time-con-
<br />suminG, and difficult. But it is a major
<br />step (onward in structural imaging
<br />rechnologv with immediate practical
<br />benefits, including optimizing tcell lo-
<br />cations, reducing subsalt exploration
<br />ricks, and importing subsalt he: '. de-
<br />velopment.
<br />Fig. ]4 shotcs what can be :: ~.om-
<br />plished-and what's possible iil the
<br />future. It includes automatic gain rnn-
<br />trol and relative amplitude sections of
<br />a salt body in the South Timbalier area
<br />of the Gulf of Mexico.
<br />Top and base of salt are distinct on
<br />these sections. Three subsalt sediment
<br />packaGes also appear. What's more,
<br />relative amplitudes are presen~ed be-
<br />low salt.
<br />l1'ithin the next couple of tears,
<br />geophysicists should be able to r.~.uge
<br />stratigraphic details and subtle i_ul[-
<br />ing below salt. The technology i; at
<br />hand (or !his and just needs to be
<br />applied.
<br />As the technology progresses, geo-
<br />phcsicists trill be able to do more and
<br />more faith amplitudes, leading even-
<br />tually to subsalt AVO analysis.
<br />And benefits will extend becond
<br />structural imaging. l\'hat geophysi-
<br />cists learn from 3D PSDAi trill help
<br />push seismic technology along anoth-
<br />er crucial front resen~oir characteriza-
<br />tion.
<br />There, as in 3D structural ima;ing,
<br />technology succeeds where the t eloci-
<br />h• model is accurate and fails where it
<br />is not.
<br />Acknowledgments
<br />The authors wish to thank PGS Ten-
<br />sor for 3D time and 3D depth imaging
<br />technology applications; specifically,
<br />Karen Checis, Kurt Sellers, John An-
<br />derson; and Kenny Lambert.
<br />The authors also thank PGS E\plo-
<br />ration for the 3D subsalt aalui: i'f+n
<br />techniques used over the t,lahr.: ~:t'
<br />salt sill 3D project; namely, .`.".no
<br />Sauer, Etienne Marc, And}' Cunning
<br />hant, Paal Slaatsceen. Oddcar Olaus-
<br />sen, Kjell Karlsson, and Aage Fismn.'
<br />P
<br />~'
<br />Ryuhei Mu
<br />etsmic
<br />intpor
<br />(or oil
<br />ampli'
<br />anomalies al
<br />. cause the cl
<br />ante sugges
<br />the change ,
<br />~~~ ed b}' an A\%
<br />the existencr
<br />On the otl
<br />spots and A
<br />relate to ht
<br />pedance am
<br />dvect indica
<br />are indvect
<br />There are
<br />these phys
<br />. chows the r
<br />saturation, ,
<br />..~on physical
<br />:;.~voir. In this
<br />~- ties are divir
<br />,-,physical pn
<br />Gthologies I
<br />Based on a poi
<br />East Geosc+en
<br />April 2527, in !
<br />sp Oil 8 Gas Journal • Oct. 2~, 199a OGJ SPECIAL ~ OGJ SPECIAL
<br />rardiry
<br />
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