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# Common-image gathers for shot-profile migration

In order to estimate elastic parameters of the subsurface,
geophysicists need reliable information about angle dependent
reflectivity.
In this Appendix, I discuss how imaging non-zero
offsets amounts to estimating off-diagonal elements of Berkhout's
reflectivity matrix. The information in these offset domain
common-image gathers (CIGs) can then be transformed to the angle
domain, and potentially used for lithologic inversion.
CIGs also contain information about how well focused
events are at depth, and so provide a natural domain for
migration-focusing velocity analysis.

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Stanford Exploration Project

5/27/2001