As interest in shear waves (S-waves) grows for prestack depth migration of ocean-bottom node (OBN) data, it is imperative to obtain a good initial elastic model of the shallow seabed, for optimal wavefield separation and imaging. Compressional wave (P-wave) methods that use mirror migration, velocity analyses and tomography are not available for shallow converted P- to S-wave (PS-wave) data because of sparse node acquisition and low S-wave velocities. Conventional techniques use reflections from just below the seabed to invert for the elastic properties VP, VS and density ρ. We estimate these properties by inverting the amplitude variations with ray parameter (horizontal slowness) of the direct arrival. Synthetic data shows exact results compared with analytic expressions for reflection and transmission coefficients of the seabed interface. Inverting field data from the deep-water Jubarte ocean-bottom cable (OBC) survey offshore Brazil also shows good results. However, some challenges remain related to calibrating particle velocity data with the pressure data for acoustic wavefield separation.

This content is only available via PDF.
You can access this article if you purchase or spend a download.