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7 Correlation Concepts

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Chapter 7 Correlation Concepts The most important concept to keep in mind when correlating reflection seismic data is that the reflection seismic method provides indirect timedomain measurements of the 3D space of subsurface geology. The character and position of the reflections you correlate, which are the seismic responses to impedance contrasts across real geologic boundaries, depend on the geometry and properties of the subsurface velocity field — critical factors in stacking quality and in
  83 Chapter 7 Correlation Concepts The most important concept to keep in mind when correlating reectionseismic data is that the reection seismic method provides indirect time-domain measurements o the 3D space o subsurace geology. The characterand position o the reections you correlate, which are the seismic responsesto impedance contrasts across real geologic boundaries, depend on thegeometry and properties o the subsurace velocity feld — critical actorsin stacking quality and in the accuracy o positioning your data (migration),controlling what you see and where you see it on a record section.The undamentals o acquisition and processing o reection seismicdata and the nature o the earth imply that there will always be some dis-tortion in every seismic image, so you should never consider any seismicsection — no matter how careully processed — to be a true geologic crosssection. In other words, all seismic sections require interpretation. As youinterpret, you must remember that there is no such thing as a noise-reeseismic record, that all seismic data have limits to their temporal and spatialresolving powers, that virtually every 2D seismic line suers rom its inabil-ity to image a 3D earth accurately, and that even the most sophisticated 3DPSDM volume will not be an exact replica o the subsurace. Again, theseelements o data quality aect the interpretability o reection seismic data. First look When taking your frst look at a seismic section, you should see thewhole section and not ocus or concentrate on any particular portion o the image. Scan the section rom top to bottom and rom side to side,taking in as much o the image as you can to orm initial concepts aboutthe geologic setting and overall quality o your data. In the workstation Downloaded 17 Feb 2012 to Redistribution subject to SEG license or copyright; Terms of Use:   84  First Steps in Seismic Interpretation environment, use the computer’s capability to modiy display size andscale to change the magnifcation (zoom and unzoom) and/or the aspectratio (stretch or compress) to acilitate viewing the data. You might alsotry dierent color tables to see i any eatures in the data are more clearlyvisible (to your eyes, at least) using a particular color scheme. It’s a goodidea to inspect all o your data using movie or animation unctionalityon your workstation beore beginning your actual interpretation. Thisallows you to more ully assess data quality and provides an additionalcheck that you have all o your data. Beyond knowing the acquisitionand processing history o your data, at frst you should assume as little asnecessary about your project area; you’ll have plenty o time and oppor-tunity throughout your interpretation to ocus eort and careully inte-grate observations and correlations into a consistent story. You are notwell served by running o with premature speculation or unwarrantedassumptions early in your project.Your initial assessment o data quality — noise content, resolvingpower, and fdelity o imaging — will aect three important interpretivedecisions:1) Are the data o sufcient quality to deal with the interpretive issues athand, or is there need or reprocessing or additional acquisition?2) What raction o the data can be correlated using automatic versusmanual picking (in the workstation environment)?3) What is the best way to record variations in data quality across theproject area?Answers to these questions determine how and with what confdence hori-zons and aults (i any) can be correlated; the geologic setting and com-plexity o the project area, together with any prescribed objectives or theinterpretation, determine how many horizons (and aults) need to be cor-related to describe the geology accurately and to meet business objectives. Horizons versus aults The essence o correlation is recognizing patterns in seismic data, ol-lowed by associating these patterns with known analogs or modeled rep-resentations o real geology. At its most basic level, seismic interpretationinvolves correlating two primary types o geologic suraces: horizons andaults.A horizon is the surace separating two dierent rock layers (Sheri,2002), which gives rise to a seismic reection according to the acoustic Downloaded 17 Feb 2012 to Redistribution subject to SEG license or copyright; Terms of Use:   Chapter 7: Correlation Concepts 85 impedance contrast between the two layers (recall Figure 1 in Chapter 1).According to Vail et al. (1977), two types o physical suraces are present insediments at the time o deposition: stratal suraces and unconormities. Eacho these can cause seismic reections i there is a sufcient impedance con-trast across it. Stratal suraces have chronostratigraphic implications. Manyare geologic-time suraces because they are ormer depositional beddingsuraces that were synchronous over their areas o occurrence; unconormi-ties have chronostratigraphic signifcance because, by defnition, all o therocks below the unconormity are older than those above the unconormity.A ault is a racture or racture zone along which there has been displace-ment o the two sides relative to one another parallel to the racture. Sheri (2002) defnes a ault as a displacement o rocks along a shear surace.The undamental dierence between correlating seismic horizons ver-sus aults is that the ormer is based on recognizing and tracking continu-ous or predictably changing patterns o reections, whereas the latter isbased on recognizing discontinuities or osets o patterns that are other-wise continuous or predictably changing (see Figure 1). O course, aultsthemselves oten can be tracked as predictable patterns o discontinuities, Figure 1. Image o a seismic line, illustrating the dierence between correlatingseismic horizons as continuous or predictably changing patterns o seismicreections versus aults as discontinuities or osets o patterns that are otherwisecontinuous or predictably changing (courtesy WesternGeco). t  Downloaded 17 Feb 2012 to Redistribution subject to SEG license or copyright; Terms of Use:   86  First Steps in Seismic Interpretation depending on the structural setting and quality o imaging. Occasionally,reections rom ault planes are imaged clearly (see Figure 4 in Chap-ter 5) and can be used to assist ault picking. In a strictly geometric sense,you can think o correlating horizons and aults as marking the boundariesthat defne common dip amilies (packages o reections with internallyconsistent character and orientation), ater which you explain the geologicnature o the boundaries and then reconstruct the geologic history o theinterpreted data.You have two primary concerns in correlating aults; both depend onthe quality o your data — specifcally, noise content and imaging fdelity:1) Tracking ault suraces on individual lines (or on 3D horizontal slices)and rom line to line (2D) or across a volume (3D).2) Accurately correlating horizons across aults.Addressing the frst concern, you should naturally pick aults on imagesthat most clearly show the discontinuities, osets, or reection terminationsthat are your evidence or aulting. Whether 2D or 3D, a seismic line thatis true dip to a ault, assuming that migration and other processes are donesatisactorily, should aord the clearest and sharpest view o the reectionterminations that you would pick as the ault (the block diagram in Figure 2illustrates the defnitions o strike and dip). In working with 2D data, youare limited to the orientations o lines in your grid o data, which oten arenot true dip to the aults o interest; so when correlating aults, you will havedeal with the problems inherent in 2D imaging (see Chapter 8). In the 3Dworld, however, i neither the inline nor the crossline direction is in true diporientation to aults, then you can create arbitrary lines that are in true diporientation to aults, on which you can very accurately pick the aults, dataquality permitting.Figures 3 and 4 are an inline and crossline, respectively, rom a depth-migrated 3D survey on which a normal ault has been picked. In this exam-ple, the ault in question is more clearly imaged and easily picked on the linethat is more nearly true dip to the ault (in this case, the inline) rather than onthe orthogonal line (the crossline), on which there is no obvious terminationevidence or it. Here, you would pick the ault frst on the inline, and thenyou would use that pick as a tie or reerence point to fnd evidence or theault on the crossline. Figures 5 and 6 illustrate the dierence between dipand strike views o normal and reverse (thrust) aults, respectively. As youcan envision, termination evidence or a ault will be hard to see on linesthat are parallel or subparallel to the ault trend, no matter whether the aultis a normal or a reverse ault. Note also that regardless o the apparent dip o  Downloaded 17 Feb 2012 to Redistribution subject to SEG license or copyright; Terms of Use: 
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