Shear-wave velocity update using PS reflection FWI for imaging beneath complex gas clouds

Shear wave velocity model building (S-wave VMB) is a critical and difficult processing step for converted wave imaging. Conventional S-wave VMB depends on PP-PS joint interpretation-based image registration and PP-PS joint tomography-based residual moveout flattening which have certain advantages and drawbacks. We introduce PS reflection FWI (PS-RFWI) for S-wave VMB in the presence of a complex, heterogeneous subsurface, which can address some of the concerns with conventional approaches. This method assumes that we have previously obtained sufficiently accurate pressure wave (P-wave) velocities and reflectivity. PS-RFWI solely updates S-wave velocity by minimizing the kinematic difference between the modeled and recorded PS reflections, while leaving the P-wave parameters unchanged. In conjunction with conventional methods, PS-RFWI can provide a superior PS image, as we will demonstrate with a field data example from offshore Malaysia.