interactive fault mapping

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Interactive fault mapping:a case studyBy E. C. BREDE and STEPHEN W! THOMASLandmark Graphics CorporationHouston. Texas

The growing use of interactive seismic interpretationsystems epresentsan opportunity for innovation in inter-pretation tools and techniques. These systemsprovide anenvironment with fast graphics, computer power, andstorage for both seismic data and picks. Such resourcescan be used to construct interpretation tools which a reimpossible or impractical to create without them. Someby now well-known exam ples are interactive horizon flat-tening and attribute extraction, and mapping alongstructural boundaries. Horizon flattening p rovides an ap-proximate section as it would have been at the time ofhorizon deposition, and is a valuable structural interpreta-tion tool. Horizo n attribute extraction can be diagnosticof chan ges in both lithology and fluids along structuralboundaries, and is useful in stratigraphic interpretation.It is possible to imagine carrying o ut bo th of these opera-tions by hand (and flattening has been done, in the past,with judicious use of ra zor blades) but neither operationis practical without computer tools. The ongoing develop-ment of interactive interpretation technology prom ises omake many new tools available of which interactive faultmapping is another examp le.

The interactive fault mapping process described hereis based on tools providing m ultiple, but integrated, acces sto the many types of information about fau lts which arecontained in seismic data. Th e interpretational objectiveis to provide a correct (or at least mos t probable!) pictureof th e fault planes in the su rvey area, at lea st as far asthese planes delineate fault blocks of interest.F ault a basing : an exam ple. In many structural plays, thecorrec t association of fault picks into fault-horizon cutshas a profound effect on the resulting horizon structuremap. The significance of correct fault mapping is illus-trated by two structure maps resulting from alternativeinterpretations developed n this casestudy. Figures 1 and2 show two maps resulting from the same data and thesam e horizon picks, differing only in the way in whic hidentical fault picks are assoc iated nto planes. In Figure2 the white cross (cursor) points out a structural trapwhic h is present only a s a minor structural high on th eother map. This trap, if real, would be an attractive drill-ing target. Obviously, only one of these maps can be cor-rect, and a correct drilling decision may ride on knowingwhich one.How can two very different maps be consistent withthe same data? The answer lies in the phenomenon offau/t aliming. For many combinations of fault structureand seism ic ine sp acing (for two-dim ensional su rveys) hefault-cut d ata conventionally poste d to a shotpoint m ap

are not closely spac ed enough to uniquely define fault-horizon intersections. Viewed this way, the problem isone of spatial aliasing, whe re fault-horizon cuts are notsampled densely enough to reproduce the underlyingstructure.When fault aliasing is present it is one of the m ost dif-ficult p roblems in seismic interpretation. It is imperativethat all of the information available in the data, not justa fault-cut map , b e used in asso ciating seismic section

picks into planes. The tools describe d in this case studyprovide this capability.The data used n this study are from offshore Texaswherean elongated basement uplift trends southwest-northeastin the study area. Figure 3 showsa shotpoint map of thesegme nt of the survey selec ted. In this area, the linestrending north west-sou theast are dip lines for both thehorizons of interest and the faults.Two of these parallel d ip lines, 106 and 108, are shownin Figure 4 as migrated sections. The locations of th eselines are highlighted in green on the shotpoint map. Theinfluence, particularly in the fault stru cture, of the base-ment ridge can be seen near the center of both dip sec-tions. A reflector m apped in this study, designa ted theA Horizon, is shown picked between 1.6 and 2.0 set onthere sections. Strike sections n the area tend to be con-fused because of a large amount of sideswipe.F ault picks. Figure 5 showsa shotpoint map of dip linesinterpreted from the area, with associated ault cuts. Eachpair of tick marks in the figure representsa fault cut atthe level of the horizon, i.e., the intersection of the AHorizon and a fault pick on a seismic ine. The arrow oneach pair of ticks points to the down-thrown side of thefault. The arrows not connected o tick m arks in the figureshow dip of the A Horizon. This fault-cut map representsa starting point in assigning h e fault picks from individualsections into plan es. In a clearer situation, the one withfewer cuts or closer ines, the interpreter might simply con-nect the ticks with heave ines, the outlines of fault heavezones n the horizon, and proceed o contour the horizon.In this case, the co rrect association is ambiguous fromthe fault-cut map, and the need for examining the faultdata in other views is clear.The assignmentof fault picks on sections o fault planesis equivalent to th e assignm ent of horizon segm ents, be-tween fault cu ts, to fault blocks. For this reason the corre-lation of h orizon segm ents hape s rom line to line is oftenuseful in discerning a fault assignment. Figure 6 showsthe A Horizon as it app ears on seven dip SeC tiOnSrom

46 GEOPHYSICS: THE LEADING EDGE OF EXPLORATION SEPTEMBER 1986


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Figure 5. Fault cuts from the study area dip lines.

pretation session.Typically the interpreter starts withunass igned egmen ts nly, in both sectionand map viewwindows.

Figure 6. Horizon correlation across dip lines.

By working hroughone planeat a t ime selectingmulti-ple views nteractively as neces sary, he interpreter triesout variouscombinationsof segments. segm ent an beassigned o, or deassignedrom, a fault plane by point-ing with the cursor. For example, the red-orange faultplaneat the northwestern nd of the survey s distinguishedby a numb er of features, ncluding:l Shapecorrelations f the horizon roll over in F igure6l Shape correlation of the fault segm ents hemse lvesl Ppence of distinguishingntithetic au lts in yellow)radiating from the b ase of the red-orange faultl Continuous shape of the resu lting fault plane

F ault plane perspe ctive. he co ntour map views alonecan be deceiving n view ing ault planes.For examp le, on-sider the yellow fault plane just southeastof the red-orange plane in Figure 7. From the contour map alone,this plane could be parallel o the others.Of course,studyof the sections eveals his fault to b e antithetic o the red-orange. A perspective iew, shown n Figure 8, make s tpossible o pick out these elationships irectly. Note thepresence f the north arrow in the top of the cube outlineused to orient the display.The orientation of the viewsis interactivelyselectable y the user.This particular viewis from the bottom up , and sho ws he yellow as clearlyantithetic o the red-orange ault. Note also he unassign edsegm ents h own in perspective iew as white lines.

Figure 7. Fau lt plane ma p and cross-section windows.

H o&on-fault intersections.Once fault plane assign-ments have been mad e, the intersections f theseplaneswith the horizon of interest in this case he A Horizon- can be computedand displayed.This display of hori-zon cuts is, in itself, a useful and im portant part of thefault assignm ent rocess. rial fault assignm ents us t eadto believablehorizon-fault cut patterns. Figure 9 show scompu ted fault-horizon intersections esulting rom thefault plane assignm entn Figure 8. Each intersection ineis automatically computed and sho wn n the color asso- Figure 8. Perspective view of assigned fault planes.



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ciated with the fault plane that produced t. Fault heaveline polygonshave been digitized as an overlay in Figure9, u sing he colored ault-horizon ntersections s a guide.The width of the fault-cut ticks, now show n in time-contour colors,providesa guide o establishinghe widthof the fault heave zones outlined by the polygons. Theunderlying shotpoint grid provides frame of reference.Once a consistentault plane assignment as been made,a faulted contour map can be generated.This is accomp-lishedby passing orizon picks, n this case he A Hori-zon, and fault heave line polygons, show n in Figure 9,to a contouring algorithm. The result s shown n Figure1. The contoursare time codedaccording o the color baron the left side of the figure. Note how the addition ofthe green color marker has clarified structural highs onthe map. The systemproducesmap s of the type show nin F igure 1 with very short responseime suitable or usein mak ing a judgm ent whether to proceed with furtherinteractive fault or horizon interpre tation.A n alternative nterpretation. An advantageof interac-tive interpretation echnology s the ability to quickly poseand testalternative nterpretation ypotheses. his processof testing multiple alternatives s beneficial in reducingthe risk of an incorrect drilling decision.An alternativefault plane interpretation,basedon the sameseismic ec-tion picks previously used, is sh own in Figure 10. Thelayout of this figure is the same as that of Figure 7.

Figure 9. Ho rizon-fault intersections nd heave ine polygons.

The interpretation ho wn n Figure 10 sdifferent mainlyin that a series f fault segm ents avebeenassembledntoa continuous ault plane , shown n yellow, cutting acrossthe study area. The associated ross-section isplays nthe figure show his interpretation s not impos sible. hisparticular fault plane interpretation producesa s ignifi-cant change n the structural picture of Horizon A, asshow n n Figure 2. This faulted contour map is computedand displayed n the sameway as Figure 1. The new faultinterpretation has introduced a much larger structuralclosure h an w as present on the earlier map. O n struc-tural groundsalone, the s potmarke d with the white cross(cursor) is a viable drilling location.T Figure 10. Alternative fault plane assignment.esting alternatives. The maps of Figures 1 and 2 areobviously inconsistent n a very s ignificant way. Figure2 supportsa drilling location where no trap is indicatedin Figure 1. Which interpretation is more correct? Thisis an example,albeit a dra matic one, of the many alter-natives he interpreter mus t continuously e st and acce ptor discard.Com puter-aided ault mappingmakes t possi-ble to test those alternatives nteractively,as ideascometo mind.One w ay to test the two interpretations eprese nted yFigures1 and 2 is to exam ine he evidence upportingkeyfault planes . The orange fault plane of Figures9 and 1and the yellow ault planeof Figures10 and 2 are mutuallyexclusiventerpretations. he yellow fault planeof the sec-ond interpretationcloses he suspe ctedrap that it show s.Figures 11 and 12 show fault plane maps and segm entassignmentsor the orange and the yellow fault planes,respectively.Figures 13 and 14 isolate perspective iewsof the orangeand yellow fault plane s.From these igures,it is possible o judge the relativeconformity of sh apeofthe fault segm ents n each plane, and the shap e and Figure 11. Fault plane map, orange plane, first interpretation.

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smoothness f the assigned ault planes hem selves. x-aminingFigure 13closely, t is possibleo see h at the faultsegm ent n line 108 s geometrically disjointed from therest of the fault plane. The yellow fault plane in F igure14, seenedge-on n the samecritical regionof the survey,sho ws etter geom etric onformity.The alternate nterpre-tation of Figures 12 and 14, which leads to the struc-tural closu re on Figure 2, m ay be judged the betterinterpretation.T*srticlepresents nly a sa mp ling f the interactiveaultinterpretationand mapp ing tools available,and an evensmallersampling f the many ways hese oolscan be com-bined in the interpretation process.The systemdemon-strated n this study allows h e user o constructhis owndata windows nd viewsat will and to interactwith severalviewssimultaneo usly. his capab ilityprovides remendousfreedom o exploreand understandhe data n the way bestsuited for each interpretationproblem. &

Figure12. Fault plane map, yellowplane,alternate nterpretation. (The authors thank Landmark GraphicsCorp. for permissionto publish thispapenand Grant-NorpacCorp. or permission tous eand display he seismic ata. The authorsalso wish o thankKevin Donihoo and J. A. Coffeen or their ideas and help.)

Figure 13. Perspective iew of orange plane.

Stephen W Thomas s vice-presidentorNew Business evelopment t LandmarkGmphics orporation wherehe s tespon-sible for developing new app licationsfor interactive nterpretation echnology.Prior to joining Land mark in 1985, Dr.Thomas was with Mobil Researchan dDevelopment Corporation as managerand technologyevaluation and researchconsultant. Dr. Thoma s conducted ormanag ed esearch nd development rojects n areas ncludinginteractiveseismic nterpretation, mageprocessing, omputermapping, seismicsignal processing, he ar-wave nterpretation,marine seismicdata acq uisition and n on-seismic xploration

As manager, TechnologyEvaluation, he was responsibleorevaluating capa bilitiesand planning development n all area sof explorationand production technology.He earned his doc-torate n electricalengineering nd computersciencerom Cor-nell University.

Figure 14. Perspective iew of yellow plane.

Chuck Brede is corpomte geophysicistwith Landm ark Gmphics Corporation.He hasmore than 30 yearsof explorationexperience, dvancingrom field work tosophisticated3-D interpretation. Bredesupervised om eof the irst field crews ouse digital equipm ent.Bmde became n-volved n pioneer3-D surveys n the early1970s.He servedas U S m arketing man-ager for Geophy sical Service Inc. forseveml ears b&on? eaving ha t firm, in 1982, o becomea con-sultant who specialized n 3-D interpretation.He joined Land-mark in 1985.


interactive fault mapping

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