We generalized Snell's law and the resulting ray parameter from the isotropic to the anisotropic case.
Mathematically, this is a consequence of translational symmetry, and hence follows from Noether's theorem.
It facilitates both conceptually and computationally raytracing and inversion in laterally homogeneous media but anisotropic.
Slawinski, M.A., Webster, P.S. (1999) On generalized ray parameters for vertically inhomogeneous and anisotropic media. Canadian Journal of Exploration Geophysics 35(1/2), 28–31
Epstein, M., Slawinski, M.A. (1999) On rays and ray parameters in inhomogeneous isotropic media. Canadian Journal of Exploration Geophysics 35(1/2), 7–19