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.