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    45

    Introduction

    Coherence is a measure o similarity between wave-

    orms or traces. When seen on a processed section, the seis-

    mic waveorm is a response o the seismic wavelet con-

    volved with the geology o the subsurace. That response

    changes in terms o amplitude, requency, and phase, de-

    pending on the acoustic-impedance contrast and thicknesso the layers above and below the relecting boundary. In

    turn, acoustic impedance is aected by the lithology, porosi-

    ty, density, and luid type o the subsurace layers. Conse-

    quently, the seismic waveorms that we see on a processed

    section dier in lateral character that is, strong lateral

    changes in impedance contrasts give rise to strong lateral

    changes in waveorm character.

    Figure 1a shows a laterally stable waveorm indicating

    a coherent event. Figure 1b shows a synclinal but laterally

    invariant waveorm. In contrast, Figure 1c and 1d shows vari-

    ations in waveorm that are the result o channels. Geologi-

    cally, highly coherent seismic waveorms indicate laterallycontinuous lithologies. Abrupt changes in waveorm can

    indicate aults and ractures in the sediments. In this chap-

    ter, we will demonstrate that lateral changes in coherence

    provide interpretation insights.

    Figure 2a shows a segment o a seismic section, and

    Figure 2b shows its equivalent coherence section. Notice

    that there are no sharp breaks within the highlighting hex-

    agon; however, a close examination does reveal changes in

    the seismic waveorms. The coherence section shows these

    changes as low-coherence eatures. It is that sensitivity to

    waveorm changes that makes coherence a useul tool or

    extracting subtle inormation rom seismic data. Similarly,

    Figure 3b demonstrates the ease with which aults can be

    seen on the vertical coherence section and easily can be put

    on a seismic section (Figure 3a).

    In Figure 4, we redisplay one o the earliest published

    applications o coherence: application o the original three-

    trace crosscorrelation coherence algorithm to a large 3D

    survey acquired over South Marsh Island, Louisiana, U.S.A.

    In Figure 4a we see a time slice through seismic data, and

    in Figure 4b we see the crosscorrelation-based coherence

    volume. The extremes o the high and low values o coher-

    ence were indicated by yellow and red at the time. Unless

    otherwise stated, other examples in the book use a more

    consistent black-gray-white color scheme.

    3D seismic interpretation

    A 3D volume o seismic data allows us to visualize thespatial evolution o structural or stratigraphic eatures. Such

    a continuous evolution is basic to our understanding o sed-

    imentary deposition and o the eventual tectonic olding or

    aulting, and to our study o the coniguration and luid

    content o a reservoir. The irst step toward 3D interpreta-

    tion is to use time slices along with the vertical sections

    pulled out o the volume. Whereas vertical seismic sections

    describe inline dips, the crossline dip can be investigated

    either by animating through successive inline seismic sec-

    Chapter 3

    Coherence

    Chapter Objectives

    After reading this chapter, you will be able to

    summarize the physical and mathematical basis o currently available seismic coherence algorithms

    evaluate the impact o spatial and temporal analysis window size on the resolution o geologic eatures

    recognize artiacts that result rom structural leakage and seismic zero crossings

    apply best practices or structural and stratigraphic interpretation

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    46 Seismic Attributes for Prospect Identification and Reservoir Characterization

    must be interpreted. Most o the channels and aults (barring

    the obvious ones) could go undetected i only vertical sec-

    tions are interpreted. Time slices are a great help or such an

    interpretation. Obviously, the interpretation depends on the

    objective at hand and the quality o the data. Interpretation

    o subtle details could be a nightmare i the data are o poor

    quality.

    Interactive workstations

    Interactive seismic-interpretation workstations are valu-

    able tools or interpreting large volumes o 3D seismic data

    eiciently. Inlines, crosslines, time slices, and arbitrary

    proiles can be accessed readily rom a single data volume,

    and that acilitates convenient review and ast editing o the

    data. Folded displays such as that shown in Figure 5 are a

    great help in our understanding o subsurace geology.

    Rapid displays o horizon amplitude maps are possible that

    inject detail into our understanding o the ield. The work-stations increased dynamic range o color displays and color

    enhancement allows interpreters to readily detect subtle ea-

    tures or stratigraphic interpretation. Because stratigraphy

    is best studied and displayed with vertical exaggeration, zones

    o interest can be conveniently zoomed and color can be

    enhanced interactively.

    In areas with high dip or with stratigraphic eatures that

    cut through dierent stratigraphic horizons, lattening con-

    sistent stratigraphic horizons or suraces helps us obtain a

    greater understanding o the stratigraphy. Investigation o

    amplitudes along a seismic horizon on which sequences

    prograde can give the direction o progradation, when the seis-mic volume is lattened on the horizon and sliced through.

    We may conduct similar analysis when we study an uncon-

    ormity surace and the layers subcropping below it, to

    image a channel system or to determine the areal extent o

    a reeal buildup. An important shortcoming exists, however

    interpretive bias usually enters the data set when we use

    horizon slices in tracing stratigraphic eatures, because the

    Figure 1. Examples o lateral variations in seismic wave-

    orms: (a) a at, laterally invariant, or coherent, waveorm,

    (b) a synclinal, but otherwise laterally invariant, or coherent,

    waveorm, (c) a laterally variable waveorm indicative o

    lateral changes in impedance or thickness, and (d) a rapidly

    varying waveorm associated with three channels.

    a)

    b)

    c)

    d)

    Figure 2. (a) Lateral changes seen on a vertical slice through

    a seismic data volume. (b) Such lateral changes show up as

    low-coherence eatures in the corresponding coherence slice.

    a) b)

    Figure 3. Vertical slices through (a) a seismic data volume

    and (b) the corresponding coherence volume, indicating the

    clarity with which the aults appear on coherence displays.

    a) b)

    tions or by taking a perpendicular crossline slice through

    the seismic data volume. In contrast, time slices show relec-

    tor strike. Changes in strike can be tracked by animating

    through successive time slices.

    Depositional systems oten show up better on time

    slices than on vertical sections. For example, river channels

    usually cut their neighboring geologic strata in characteris-tically meandering patterns. Such patterns may be obvious

    on a time slice. The areal disposition o such channels or

    ault planes is not apparent on one vertical section, so to get

    a eel or such patterns, several vertical inlines or crosslines

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    Coherence 47

    diicult, time-consuming and subjective process o picking

    the horizon has to be ollowed.

    3D data for reservoir description

    In addition to providing structural and stratigraphic de-

    tails, 3D seismic surace measurements have come into

    prominence as an essential element or reservoir descrip-

    tion. Their use is particularly widespread in those parts o

    the world where new reserves are generated by inill and

    extension drilling that is based on detailed knowledge o

    the reservoir characteristics.Reservoirs exhibit a range o physical properties that

    can be detected as changes in response over appropriate

    time intervals (e.g., by reservoir monitoring during en-

    hanced oil recovery). Time-lapse seismic analysis is evolv-

    ing rapidly to identiy bypassed hydrocarbons in a produc-

    ing reservoir. Three-dimensional seismic surveys are carried

    out over the reservoir area at dierent times during the lie

    o the ield. The irst survey generally is done beore produc-

    tion is begun, and the second and subsequent surveys are

    conducted ater signiicant production has started. Changes

    Figure 4. One o the frst applications o

    coherence, showing a time slice at t= 1.200

    s through (a) the seismic data volume and (b)

    the crosscorrelation-based coherence volume.

    The color scale is red (or lowest coherence),

    black, gray, white, and yellow (or high-

    est coherence). Ater Bahorich and Farmer(1995).

    a)

    b)

    between preproduction and postproduction surveys are com-

    puted and attributed to changes in luid properties, such as

    saturation, density, and the like that have taken place be-

    tween the times o the surveys. Thus, 3D seismic data are

    used to identiy portions o the reservoir that have been de-

    pleted and other regions that still have commercially viable

    hydrocarbon

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