Full-Wave Inversion of Shear-Wave Splitting for 3D Azimuthal Anisotropy

Seismic anisotropy in the mantle plays an important role in our understanding of the Earth's internal dynamics, and shear-wave splitting has always been a powerful observable in the investigation of seismic anisotropy. So far the interpretation of shear-wave splitting in terms of anisotropy has been largely based on the ray-theory modeling of a single vertically incident plane SKS or SKKS wave, which is a strong assumption and results in the rejection of measurements in many cases, thus severely limits our ability to make full use of shear-wave splitting data to resolve the spatial variations in anisotropy. In this study, we develop an approach to the inversion of 3D anisotropy structure using the sensitivity (Fréchet) kernels calculated by an efficient and flexible full-wave algorithm based on the normal-mode theory. Predictions of SKS splitting by these full-wave sensitivity kernels demonstrate a significant bias in ray-theory treatment caused by the unaccounted interference between the SKS wave and other contaminating phases with similar arrival times. The full-wave sensitivity kernels accurately account for all the interactions of multiple phases for a wide spectrum of source-receiver geometry. We combine the full-wave kernels with a wavelet-based model parameterization, which provides us a multi-scale approach to dealing with the spatially variable path coverage commonly encountered in tomographic inverse problems.