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This seismic line was acquired over a major thrust fault in the Utah-Wyoming Overthrust belt. See the index map for location of the Overthrust belt. Older, high-velocity material has been thrust up over younger, slower material, causing a subthrust velocity anomaly (a pull-up) near the center of the unmigrated section, Figure 1. This pull-up is labeled on Figure 2, which is an interpreted version of Figure 1.

On the interpreted Figure 2, notice also the classic bow-tie pattern in the upper right-hand corner. This bow-tie is indicative of a syncline out in front of the major west-to-east thrust fault.

Figure 3 shows a depth section obtained using the stacking velocities for seismic migration. It has not done a very believable job, since the fairly wild undulations on the left-hand side are not compatible with what we see on the unmigrated data.

A better way is to create a depth model using all available data (sonic logs or generalized interval velocity information, depths at known borehole locations) and to generate synthetic seismic data using this model. The synthetic data should match the real data at the borehole; the trick is to make it match away from the borehole. If the match is good, the depth model can be used as a velocity guide for seismic migration.

In an attempt to improve the understanding (and migration) of this line, forward modeling is used. The depth model is shown in Figure 4. Notice the absence of a subthrust anticline, and notice also the syncline in the upper right-hand corner.

The synthetic seismic traces, shown in Figure 5, are a good match with the real data in Figure 1. In general, such modeling allows seismically compatible depth interpretations to be made, and velocity pull-ups and pull-downs can be quantitatively accounted for in the interpretation process. If there are enough constraints to allow confidence in the model, it can be used as a velocity distribution for seismic migration.

An example of this is shown in Figure 6. The migration obtained using the depth model's velocity distribution is far superior to the migration performed using the stacking velocities alone.

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