A fast pseudo-3D model for inversion of full SH waves in layered half-spaces

SH and Love waves have certain advantages over P-SV and Rayleigh waves when inverting shear-wave velocity (V_S) profiles. They can also supplement Rayleigh waves to provide better model constraints on characterizing anisotropic media. Since the true 3D forward solution of full SH waves requires intensive computation, we propose, in this paper, a simple 2D wavefield solution and further modify it as an efficient pseudo-3D solution. This is achieved by applying an empirical amplitude correction function for geometric attenuation. In the transformed domain, this newly proposed pseudo-3D solution yields a dispersion image (frequency-velocity spectrum, FVS) practically equivalent to that of the true 3D solution which is also developed by the authors. Furthermore, this pseudo-3D solution is significantly more accurate than the modal summation method. Thanks to its fast computation speed and accuracy, the pseudo-3D algorithm is demonstrated to be an excellent surrogate for full wavefield inversion based on FVS. The FVS of SH waves is shown to be much more sensitive to embedded thin weak layers than that of P-SV waves. The superiority of FVS inversion over fundamental mode inversion is further demonstrated by examining their corresponding error functions, showing that FVS provides a much stronger constraint on the possible V_S models. Examples demonstrate that the FVS inversion based on the pseudo-3D model outperforms the modal summation approach in both accuracy and generalization. The proposed pseudo-3D solution is quite general. It takes into account the source-to-receiver geometry and deals with special layer profiles where Love-type wave does not exist.