Fluid-Solid Coupled Elastic Full Waveform Inversion in the Curvilinear Coordinates for OBC Data

The marine seismic exploration with the ocean bottom cable (OBC) technology could record the compressional (P-) wave and shear (S-) wave information simultaneously. Elastic full waveform inversion (EFWI) uses both P- and S-waves of OBC data to invert multi-parameters with adequate amplitude information and complete illumination of the subsurface. We use different formats of wave equation in fluid and solid mediums, and the appropriate boundary condition applied enables the energy exchange on the interface, which is more stable and efficient than the traditional integrated simulation scheme. However, if the fluid-solid coupled medium has an extremely irregular seabed interface, the conventional rectangular-grid-based finite difference (RFD) method cannot obtain accurate source and receiver wavefields. We introduce the curvilinear coordinates to overcome the problem of the RFD method on handling the irregular fluid-solid interface. To reduce the crosstalk of inverted P- (Vp) and S-velocities (Vs), we derive the direction formulas of Vp and Vs based on P- and S-wave modes separation in the curvilinear coordinates, and finally propose a Curvilinear-grid-based Fluid-Solid Separated- wavefield EFWI (CFS-SEFWI) method. In the numerical examples, a modified Marmousi II model with irregular fluid-solid interfaces is used to test the proposed CFS-SEFWI method.