As interest in shear waves (S-waves) grows for prestack depth migration (PSDM) of ocean-bottom node (OBN) data, challenges are emerging to obtain good initial elastic models of the shallow overburden. Full-waveform inversion (FWI) can readily provide initial compressional wave (P-wave) velocities, and mirror migration plus tomographic inversion accurately and robustly updates these velocities. However, these methods are not available for converted P- to S-wave (PS-wave) imaging because S-wave velocities are extremely slow, resulting in near vertical propagation, and sparse node acquisition typically does not provide focusing of PS-wave data for shallow image points.
In this study, we demonstrate the value of conventional PSwave processing techniques applied to common-receiver gathers. These techniques include the utilization of moveout (hyperbolic and ray based) velocities of P- and PS-wavefields, transformation to the plane-wave or ๐-p domain (p is horizontal slowness or ray parameter), and registration of these wavefields to estimate a VP/VS model in P-wave time. We investigate these techniques with field data from the deep-water Jubarte ocean-bottom cable (OBC) survey offshore Brazil. The VP/VS model along with P-wave velocities determined from NMO analyses or PSDM of the P-wave data provides a valuable initial S-wave model of the shallow overburden for PS-wave PSDM.
