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Event Identification

In theory, the maximum transmission of locally converted S-wave energy across a HVL occurs at P-wave critical angle. The reflected P-wave also exhibits an amplitude ``bloom'' at its critical angle. While this P-wave bloom should be readily visible on ADCIGs, Ogilvie and Purnell (1996) show that it is not visible as a function of offset. We observe this amplitude bloom on depth slices of a constant ph migration cube, and use it as an indicator of critical angle in our search for converted waves. We also believe that overturned, or diving waves may be present on the ADCIGs as events with a high apparent velocity. Fliedner and White (2001) show the utility of diving waves in estimating the thickness and vertical velocity gradient of the basalt.

When identifying converted waves on ADCIGs, we expect to see events with a lower apparent velocity than primary reflections, and which occur only at larger ph. Multiples will be seen for all ph (and similarly have a lower apparent velocity than primaries), while primaries will appear below basalt for small ph only. If the multiple suppression is incomplete at far offsets, we lose the ability to discriminate between multiples and converted waves via their respective amplitude-versus-ph dependence. As mentioned earlier, this 2-D survey suffered from strong cable feathering, and we have reason to expect that the SRME demultiple method may not fully attenuate multiples at far offsets. For these reasons, we feel that the raw data should be migrated at some point in the future.


next up previous print clean
Next: Results Up: Brown et al.: Converted Previous: Migration Algorithm
Stanford Exploration Project
9/18/2001