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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
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.
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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
Figure 3. ![]() |
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Figures 4 and 5 compare NMO-corrected
gathers using velocities picked from the conventional and
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
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
semblances. The anomalous region is clearly visible in the attribute
display.
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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
Figure 5. NMO-corrected CMP gathers from Figure 1 using velocity functions picked from ![]() |
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attr
Figure 6. NMO-corrected CMP gathers from Figure 5 colored according to the AVO indication attribute. |
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