Maximum allowable shift

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Seismic Data Analysis
Series Investigations in Geophysics
Author Öz Yilmaz
ISBN ISBN 978-1-56080-094-1
Store SEG Online Store

Consider the CMP-stacked section in Figure 3.3-19 associated with a synthetic data set. This data set was created by using the field geometry of a real seismic line. The CMP traces were derived from the first trace of the first CMP location from that real line. This trace first was zeroed out within selected time gates, then treated with the shot and receiver static shifts in Figure 3.3-19 in a surface-consistent manner and with a structure term that only depended on midpoint location (subsurface-consistent). The shot and receiver static shifts were varied from +32 to −32 ms. Finally, the synthetic traces were blended with a band-limited random noise, whose strength varied spatially. (The noise level was set to zero at both ends of the profile and to maximum at the center.) The stacked section constructed from data before treatment with synthetic shot and receiver static shifts is shown in Figure 3.3-20. Once residual statics corrections are made, the stacked section in Figure 3.3-19 should resemble that in Figure 3.3-20. Note how degrading the effect of the synthetic shot and receiver static shifts is on the continuity of reflections in Figure 3.3-19. The sole effect of random noise is seen in Figure 3.3-20.

Consider three different tests of residual statics corrections: one with a small maximum allowable shift (24 ms in Figure 3.3-21), one with a moderately sized shift (80 ms in Figure 3.3-22), and one with a fairly large shift (192 ms in Figure 3.3-23). All three tests had the same input CMP gathers. All three were run using the same set of parameters except for the maximum allowable shift.

The maximum value of combined shot and receiver static shifts for any given trace implied by the model in Figure 3.3-19 is ∓64 ms. When the maximum allowable shift is insufficient (a value of less than 64 ms), then the derived static shifts (Figure 3.3-21) are significantly smaller than the actual shifts (Figure 3.3-19). Thus, stack quality, although significantly improved when compared to that in Figure 3.3-19, is far from the quality of the section shown in Figure 3.3-20.

When the maximum allowable shift is sufficient, then the derived static shifts (Figure 3.3-22) are like the actual shifts imposed on the input model shown on the graph in Figure 3.3-19. Also, stacking quality (Figure 3.3-22) improved so that it now is comparable to the no-static model (Figure 3.3-20).

Reasonable results (Figure 3.3-23) also were obtained for the case allowing excessively large maximum allowable shift, up to 192 ms. However, this result does not imply that we can be liberal on the upper bound of maximum shift in real data situations. In the presence of short-period multiple or reverberation energy, or data with a narrow bandwidth or high noise level, crosscorrelation can yield a multiple number of peaks and cause uncertainty in the estimated time shifts (cycle skipping). In this case, a large maximum allowable shift could cause anomalously large time shifts to be picked.

Based on the tests shown in Figures 3.3-21 through 3.3-23, the maximum allowable shift used in the picking phase should be greater than all possible combined shot and receiver static shifts at any given location along the profile. On the other hand, jumping a leg in correlating events from trace to trace in a CMP gather, commonly known as cycle skip, especially in poor signal-to-noise ratio conditions, also is more likely to occur if the maximum allowable shift is greater than the dominant period of the data.

We may argue that the result of cascading a number of small-shift residual statics solutions is as good as a single-step large-shift solution. This approach might have the same effectiveness as the large-shift solution, while avoiding the possibility of cycle skipping. Unfortunately, cascading small-shift solutions does not work. Starting with the CMP gathers associated with the stack in Figure 3.3-19, we get the CMP gathers corrected for shot and receiver statics based on a 24-ms shift (first pass). The stack is shown in Figure 3.3-21. Using these gathers, a new statics solution was derived and applied to the data (second pass). This process was repeated for the third and fourth times. The result of this last iteration (Figure 3.3-24) does not have the quality of the solution derived with the 80-ms shift (Figure 3.3-22).

Now consider maximum allowable shift tests on the field data as in Figure 3.3-2. Figure 3.3-25 shows CMP gathers from the problem zone of the profile in Figure 3.3-5. Refer to the panels for 24- and 40-ms shifts in Figure 3.3-25, and note that an insufficient maximum allowable shift does not completely correct for all static shifts. Excessively large shifts, however, such as 120- or 160-ms, do not seem to harm this particular data set. While common-shot-point (CSP) stacks (Figure 3.3-26) indicate small shot-static shifts, common-receiver-point (CRP) stacks (Figure 3.3-27) indicate a zone of significant receiver-static shifts. Again, small maximum allowable shifts have not corrected completely for these statics anomalies.

The ultimate judgment is made by examining the stack response and plots of the estimated statics themselves. From Figure 3.3-28 (ungained stack responses), it is clear that the maximum allowable shift must be adequate to accommodate the combined shot and receiver statics present in the data at any location along the profile.

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Maximum allowable shift
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