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For ray tracing purposes, the difference between the phase and group velocities is clarified in order to derive numerically the change in ray velocity due to anisotropy. The simplest anisotropy case of broad geophysical applicability is the transverse isotropy or hexagonal symmetry. The main notations introduced by Auld, 1990 to describe the transverse isotropy of vertical symmetry axis (TIV) have been used.