Interpreting stacking velocity

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Problem 9.23

In Figure 9.23a, the hump in the 8000 to 12 000 ft/s stacking-velocity contours might cause some concern. If we know that this is the same section as shown in Figure 9.23b, what conclusions would we draw? How would you modify the velocity data to do a better job of stacking?

Background

Velocity analyses, which determine the stacking velocities that maximize the coherence produced by the NMO correction, are often run only occasionally along seismic lines and stacking velocity (problem 5.12) at other locations is simply interpolated. To reduce the noise in stacking velocity values, data from several (often three to five) adjacent CMP gathers are often averaged. (A gather is a side-by-side display of seismic traces that have some element in common, such as a common-midpoint gather.)

Figure 9.23a.  Stacking velocity along a line.
Figure 9.23b.  Migrated section across a salt dome.
Figure 9.23c.  Suggested modification of velocity to stack data better.

Solution

If we had only Figure 9.23a, we might suspect that the data from the analysis at the hump in the contours (S.P. 74) are erroneous. Having Figure 9.23b radically changes this conclusion because it shows an uplift caused by a small high-velocity salt dome. Additional velocity analyses should be run in this region. A velocity analysis is usually based on data over a range of offsets, and the gather data on which the analysis is based should be examined to make sure that only appropriate data are contributing to the analysis. Narrow bodies of anomalous velocity often affect the data at adjacent points, as is shown in problem 9.24.

To modify the velocity data without getting additional analyses (Figure 9.23c), we note that there is very little structure above 2.0 s in Figure 9.23b to the left of the salt dome and the data dip gently to the right, so we simply smooth the 7000 and 8000 contours. We would use an outline of the salt dome to modify the velocity contours in its vicinity.

The top of the salt dome appears to be at about 3 s, so the higher velocities around SP 74 are probably real. There appears to be some structural relief above 3 s, probably uplift because of the underlying dome, so some of the increase in the 8000 and 9000 ft/s contours may also be real.

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