1. the need for seismic modeling of geologic structures

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PART I

Modeling Theory and Practice



CHAPTER 1

The Need for Seismic Modeling of Geologic Structures

THE PHOTOGRAPHIC IMAGE

Seismic eflectionprofiling s uniqueamong he geophysicalmethods n what it

proposeso achieve:a directcross-sectionalmageof the layeredportionsof theearth's crust. A measure of the success hat has been achieved toward this goal is

that a geologist,with little understandingf the magingmethod,can nterpret hedata in a way that useshis geologicexperience o solveproblems.Althoughwetake this adaptability or granted, here s a greatcontrastwith the data treatmentof othergeophysicalmethodsused n petroleumexploration;primarilygravity andmagnetics.n thesemethodsdetailed nterpretation equiresmodelingand inver-sion approacheswhich in turn requiregreatergeophysical ophistication n thepart of the interpreter.

For example, consider he sectionshown n Figure 1 from the East Texas saltbasin.Any geologist,given somenotionof scale,will recognize he presenceof apiercingsalt diapir in the middleof the section. f the geologists familiar withstructuralpatternsassociatedwith diapirs,he will recognize lankingwithdrawalsynclineswhich migrate toward the dome with time and signify an inwardlycollapsing alt reservoir. n addition, he interpreterwill observea low-relief saltpillow on the right which doesnot pierce the overlyingsection,and a moderate-relief structure to the left in which piercement evel is more difficult to judge.

All of these observationscan be made by simply accepting the section as a

picture of subsurface tructure n vertical cross-section.n other words, thesection s regarded s if it were a photograph f a vertical cliff face runningalongthe line of the seismic section. In the discussions that follow, this view of the

seismic section is referred to as the photographic image .

In summary, without any knowledgeof the imagingmethod, a well-informedgeologist an earnmuchby viewing he seismic ection s a photographicmage.This statementdoes not suggest hat the interpreter is better off without such

knowledgeonly that seismic maginghas progressed o far that much can bediscerned without it. Because of this success, accepting the section as a

photographicmage s properly he methodof first resortof the interpreter.Theburden of proof rests on the side of thosewho would contend hat a particularseismicevent is not properly magingstructure.But there are limits to the detailand accuracyof an interpretationmade solely by treating he seismicsection nthisway. These imitsare madeapparentwhenwe take a closer ook at the figuresreferenced here.

In Figure 1 the base-of-salt eflectionshowsseveralapparentbasementstruc-tures, each of which is localized beneath a salt structure. Most prominent s thebasement anticline below the diapir in the center of the section. This basement

anticline s a velocitypull-upwhich arisesbecause alt velocitiesare greater han



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Chapter I/Introduction 5

those of the surrounding section. Traveltimes to the base of salt are thereforeshorter.

Velocity anomalies can be used to infer something about overlying structure.

The pull-up below the structure on the left is clearly of lower magnitude than in

the central diapir. We may infer that the seismic wavefront traveled through a

smaller thickness of salt and that top of salt is therefore at a much lower level than

the central dome. In contrast, the low-relief structure on the right side of the

section, surprisingly, depicts a velocity sag rather than pull-up (we're assuming

that the depth structure of the base-of-salt s, in reality, planar). How can this be

explained?Note that the salt structure s of very low relief and the salt replaces

only narrow stratigraphic ntervals in the middle part of the stratigraphic section

(The missing beds are truncated by onlap in these intervals). A reasonable

explanation for the sag is that the interval velocity of this replaced section is

actually faster than that of the salt. (This section is presented, and these

distortions are pointed out in Tucker and Yorston, 1973).

Next, consider the section shown in Figure 17 (Case History 2, page 108) whichwas taken over a fold and thrust belt. Noted on the section four structures which

might be prospects. However, only two of these structuresare genuine while the

other two are a result of velocity effects. In their case history presented in Part II

of this volume, McClellan et al. relate how this distinction is made.Finally, consider the sections in Figure 13a (Case History 9, page 232) and in

Figure 64 (Chapter 4, page 74). Figure 13a depicts a deepwater salt sill with

prominent top-of-salt and base-of-salt reflections. Below the base-of-salt is a

reflection dipping steeply to the right. If the section s viewed as a photographic

image, then the subsalt section would be interpreted as rotated, truncated, and

highly disconformable with the base of salt. In fact, this reflection below the

base-of-salt is a second subsalt reflection recorded from out of the plane of

section, or in other words, a sideswiped reflection. (Sideswipe reflections are

discussed in detail in Chapter 4.) Proof of this is shown in Case History 9

presented in Part II.

Similarly Figure 64 (Chapter 4) shows two prominent reflections in the middle

of the section dipping to the left and adjacent to a fault-bounded uplift on the right.

These reflections are known to be associated with the same reflector and havebeen interpreted as evidence of repetition resulting from tectonic compression. n

fact, as is shown n Chapter 4, both of these reflections are sideswiped rom block

edges located in and out of the cross section, and the horizon is not structurally

repeated.

These examples demonstrate that there is a point at which the seismic image

departs from a photographic image. In order to obtain structural information

beyond this point, the manner in which the structure was imaged must beconsidered.

THE IMAGING MARGIN

The degree that a section departs from a photographic image depends on thecomplexity of the structure to be imaged and the effort applied to the imaging.

Structures which (a) incorporate units with highly contrasting velocities, (b) have

steep dips, (c) are composedof closely spaced olds and faults, and (d) do not have

well-defined strike directions all cause imaging problems. The section departs

from a photographic image because, the ability of seismic reflection imaging to

handle these problems is limited. These limits can be broadened, but never

eliminated, with the application of higher-effort imaging approaches such as 3-D

seismic, depth migration, or prestack migration.

The term complex structure is used here for those structures which cannot



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be easily imaged because of the structural characteristics described. Petroleum

explorationoccurs n many settingswhich includecomplexstructures.The mostcommon examples are mobilized salt provinces, fold and thrust belts, andextensional glide-plane provinces.

In eachof these ectonicsettings, xploration dvancesrom the highlyvisibleto the slightlyvisible argets.The development f new prospects epends n theexplorationist's bility to see or infer something reviouslyunrecognized. orthese easons, xplorationn complex tructure reassoonconcentrateslonganimagingmargin alongwhich he seismic videnceor the prospects slight. n

areasof salt mobilization, he imagingmargin and exploration)progressesromdepicting argetsalong he crestof nonpiercing tructures,o increasinglymoresteeply dipping targets along the flanks of diapirs, to targets beneath saltoverhangs.Current and future efforts are likely to concentrateon targetscompletely overedby salt sills.Similarly, n thrust-belt xplorationhe imagingmargin dvancesrom surface nticlineso increasingly orecomplex nddeeperimbricates.

THE ROLE OF SEISMIC MODELING

As exploration oncentrateslong he magingmargin, he needdevelops or ananalysis which derives structure without relying on the seismic section as a

photographicmage.This need s satisfied y the role of seismicmodeling.Theapproachused, shown diagramatically n Figure 2, is to treat the seismicsection

as a displayof recorded eflections ather than as a photographicmage. Bypredicting he reflections hat shoulddevelop rom a particulardepth modeland

INITIAL DEPTH MODEL

FORWARDODELING

REFLECTION SYNTHETICINTERPRETATION SECTION

ACCEPT FINAL

DEPTH MODEL

Fig. 2. Seismic modeling diagrammed.



Chapter 1/Introduction 7

comparinghese eflectionswith the appropriateeal data he interpreter s able odeterminewhether the depth model s compatiblewith the data. If not, the modelis modifiedand, by successiveterations,a structurederived where the seismicresponseclosely recreates hese reflections.

In a sense his approach s a throwback o the way seismic eflectiondata weretreated at their inceptionand even into the postwar era. Prior to the advent of

digital recordingand CDP acquisition,data displayedalong analogue apes werevisuallyexamined o estimatearrival times o key reflections o determinegrossstructure. Reflectionswere migrated to their proper lateral position by graphic

methods (see the representativepapers in Gardner, 1985 for examples). Traces

were generallynot compositedo create a subsurfacemage. n a similar way,modeling tilizesan approachwhichdirects he interpreter o examine he sectionas a compositeof reflections ather than as an image of geologicstructure.

The developmentof digitally recorded data and the migration algorithms ooperateon themmade hese echniquesargelyobsolete, nd he modeling f moststructuresunnecessary.For example, it would not be a productive use of the

interpreter's time to model a bow-tie structure observed on an unmigratedsection, n order to derive a syncline hat is likely to be accurately maged on the

migrated section.

When should he interpreter suspect naccurate magingof a structure?The twomost mportant imits for seismicmigrationprograms re their inability to imagesideswipe eflections if the data are 2-D) and their inability to imagereflectionswith complex aypaths. f eitherof theseproblems indersatisfactory efinitionofa prospect, he use of seismicmodelingshouldbe considered.

Seismicmodeling s also used o designacquisitionprogramswhich will ensurethe recordingof reflections rom key portions of structures. n particular, theeffect of varying ine, spread,and array lengthscan be evaluated or a specificstructure. In a similar manner, modeling s being increasingly used to support the

processing f seismicdata over complex-structurereas.Justas the effectivenessof interpretation s limited without knowledgeof the imagingprocessemployed,so the ability to effectivelyprocess s greatly imited without bringing o bear onthe attemptwhat is knownabout he structure.The model s the vehicleby which

this is done and the approachcan be thoughtof as model-guidedprocessing .May and Covey (1983)presented n exampleof this approachas part of an effortto image a salt dome flank. Canal and Diet (1987) present an example of howmodeling s used o establish nd portray a velocity field for 3-D depth migration.In these cases the model was used to define processing parameters, such as

stackingand migrationvelocities, and as a monitor of processing erformance.The goal of this book is to demonstrate,by example, how this technologycan

be used to define hydrocarbon raps in complex structure areas. The book isalmostentirely devoted o ray-tracemodeling,because hat is the most appropri-ate modeling orm for structuraldefinition. Part I reviews the basic methodsofmodeling. Chapter 2 describes he varieties of ray-tracing approachesand thecircumstances nder which each is applied. Chapter 3 reviews major issues he

interpreter faces in building a model. Chapter 4 discusses he interpretation,mapping, nd migrationof reflection tructure.Chapter5 reviews he limitations,errors, and pitfalls of modeling.

The use of modeling can only be effectively demonstratedby convincing

examples.Seismicstructuralmodeling tudiesoften are presentedn associationwith schematic structures to demonstrate their seismic characteristics [Hron and

May (1978)andWithjack et al. (1987)]. n addition,modelings commonlyused ocreate syntheticdata setswhich in turn are used to investigate he effectsof data

processing echniques. [See Taner et al. (1970) and Miller (1974) for earlyexamples of what is now a quite routine applicationof seismic modeling].Although hese applications an be quite useful, the need still exists o showhow



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modeling can be used to define specific structures. Skeen and Ray (1983) and

Yancey and McClellan (1983) are two such examples, but there is a strong need

for many more. Part II contains ten case histories rom contributing authors, most

of whom are directly involved in petroleum exploration. The purpose of these

case histories is to demonstrate the variety of ways in which modeling can be

employed to define structure.

1. the need for seismic modeling of geologic structures

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