Chapter 4: Reverse Time Migration of Prism Waves for Salt Flank Delineation

Vertical structures such as salt flanks are usually not illuminated by primary reflections and so cannot be well imaged by conventional migration methods (Hale et al., 1992). If on the other hand strong diving waves are present, they can be reflected from the salt flank, recorded on the surface, and migrated by a two-way migration method, such as Kirchhoff migration (Ratcliff et al., 1991, 1992) or reverse time migration (RTM) (Baysal et al., 1983; McMechan, 1983; Whitmore, 1983). Even a one-way migration method can be modified (Hale et al., 1992) to incorporate diving waves for salt flank imaging.

If the diving wave is not extant due to the absence of a strong velocity gradient or a limited recording aperture, prism waves can be migrated to illuminate vertical reflectors. With reverse time migration, the migration of the prism waves can be accommodated in the process by embedding the subhorizontal reflection boundaries in the velocity model (Jones et al., 2007). However, incorporating the sharp boundaries into the velocity model is not trivial, and the complex migration velocity will excite complex wavefields that lead to artifacts in the RTM images (Liu et al., 2011). Another problem is that prism waves are doubly scattered waves, which are usually weaker than primaries, so that the contribution from the prism waves might be weak. In this chapter, I propose a new RTM method for migrating the prism waves separately from the other reflectors by utilizing the migration image from conventional RTM. The advantages of this approach over conventional RTM are as follows: (1) It does not require modifying the migration velocity as conventional RTM does; (2) It separately images different structures at different steps and reduces the artifacts from crosstalk of different phases. The disadvantage of the proposed method is that its computational cost is twice that of conventional RTM.