Surface wave ray tracing and azimuthal anisotropy: a generalized spherical harmonic approach

We explain in detail how azimuthally anisotropic maps of surface wave phase velocity can be parametrized in terms of generalized spherical harmonic functions, and why this approach is preferable to others; most importantly, generalized spherical harmonics are the only basis functions adequate to describe a tensor field everywhere on the unit sphere, including the poles of the reference frame. We introduce here a new algorithm, designed specifically for the generalized harmonic parametrization, to trace surface wave ray paths in the presence of laterally varying azimuthal anisotropy. We describe the algorithm, and prove its reliability in view of future applications.