P- and S-wave separated elastic reverse time migration for OBC data from fluid-solid coupled media with irregular seabed interfaces

Abstract

In marine seismic exploration, elastic reverse time migration (ERTM) uses both compressional (P-) and shear (S-) waves of ocean-bottom cable (OBC) data to produce multi-component images with adequate amplitude information and illumination of the subsurface. If the fluid-solid medium has an irregular seabed interface, the conventional rectangular-grid-based finite difference (FD) method cannot obtain accurate source and receiver wavefields. In addition, the conventional ERTM applies Helmholtz decomposition to separate P- and S-wavefields, suffering from a change of the wavefield dynamics properties and polarity reversals in PS- and SP-image components. To overcome these problems, we propose a curvilinear-coordinate marine separated-wavefield elastic reverse time migration (CM-SERTM). In this method, the fluid-solid velocity models are meshed into orthogonal grids and mapped to the curvilinear coordinates. A curvilinear-coordinate acoustic-elastic coupled forward modelling algorithm is then developed by introducing boundary functions to combine the acoustic and elastic equations together. The decoupled P- and S-wave equations in the curvilinear coordinates and vector imaging conditions are introduced to implement the CM-SERTM. In numerical examples, we use a simple fluid-solid model and a modified Marmousi2 model to verify that our CM-SERTM method has great superiority over three conventional ERTMs.