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Synthetic example

cmp
cmp
Figure 1.
Synthetic CMP gathers. a: no AVO, b: AVO trend with polarity reversal in the middle section.
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Figure 1 shows two synthetic CMP gathers generated by applying inverse normal moveout with a variable moveout velocity and adding a modest amount of random noise. The first gather contains no amplitude variations, while the second gather contains a region of polarity reversal. Figure 2 shows velocity analysis panels using conventional semblance scans and the corresponding automatic velocity picks. The picking algorithm is explained in Appendix B. We can observe that the AVO anomaly causes a signal loss in the semblance measure, which in turn leads to inaccurate velocity picking. The $ AB$ semblance, on the other hand, is not affected much by the amplitude variations and allows for accurate velocity picking in both cases, as shown in Figure 3.

scn
scn
Figure 2.
Conventional semblance scans for CMP gathers from Figure 1. Black curves indicate automatic velocity picks. The loss of signal in the right plot is caused by the AVO anomaly. a: no AVO, b: AVO trend with polarity reversal in the middle section.
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avoscn
avoscn
Figure 3.
$ AB$ semblance scans for CMP gathers from Figure 1. Black curves indicate automatic velocity picks. Semblance remains strong despite the AVO anomaly. a: no AVO, b: AVO trend with polarity reversal in the middle section. Compare with Figure 2.
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Figures 4 and 5 compare NMO-corrected gathers using velocities picked from the conventional and $ AB$ semblance scans respectively. While Figures 4a and 5a are virtually identical, the residual curvature evident in the anomalous region in Figure 4b disappears in Figure 5b, which clearly demonstrates the advantage of the $ AB$ approach. Figure 6 shows NMO-corrected gathers colored according to the AVO-indicator attribute, which I define as a ratio between the conventional and the $ AB$ semblances. The anomalous region is clearly visible in the attribute display.

nmo
nmo
Figure 4.
NMO-corrected CMP gathers from Figure 1 using velocity functions picked from conventional semblance in Figure 2.
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avonmo
avonmo
Figure 5.
NMO-corrected CMP gathers from Figure 1 using velocity functions picked from $ AB$ semblance in Figure 3.
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attr
attr
Figure 6.
NMO-corrected CMP gathers from Figure 5 colored according to the AVO indication attribute.
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Next: Field data example Up: Examples Previous: Examples

2013-03-02