Attenuating pseudo S-waves in acoustic anisotropic wave propagation

The importance of anisotropy in seismic imaging has been recognized for several decades. In recent years, a growing number of anisotropic applications of reverse time migration (RTM) and full waveform inversion (FWI) have attempted to account for anisotropic effects of the real-world subsurface physics. In anisotropic media the P-wave, SV wave and SH wave are intrinsically coupled, therefore the media are elastic in nature. But the elastic anisotropic wave equation is computationally very demanding and requires S-wave velocity models, for which in practice we often have little information. Consequently, the acoustic assumption is widely-used in seeking anisotropic wave equations. Stable acoustic anisotropic wave equations may create apparent pseudo S-waves during wave simulation. These S-waves then show up as artifacts in migrated images from RTM and in velocity perturbations from FWI. We propose the model taper method to eliminate the source generated S-wave, and derive a series of approximate formulae that allow accurate and efficient attenuation of pseudo S-waves after acoustic anisotropic wave propagation, as a supplementary routine. Synthetic data and real data examples are shown to verify the proposed prescription in complex media.