Acoustic head‐wave arrival times in anisotropic media

The reflection travel times, caused by a flat interface separating two media, are calculated for incident acoustic waves emanating from a point source when both media have anisotropic acoustic velocities. Special attention is given to the effects of anisotropy on the head‐wave arrival time and it is found that the critical angle for production of heads, as well as the head‐wave travel time and the specularly reflected wave’s travel time, are significantly altered for anisotropies of order 10%–20% as observed in real subsurface situations. In particular, in the case of reflection by an interface separating an upper isotropic medium from a lower anisotropic medium with propagation speed parallel to bedding planes larger than perpendicular, it was found that the critical angle is decreased, and that head‐wave arrival prior to specular arrival occurs at smaller offsets than in the isotropic case since the head wave, but not the specular wave, has its travel time decreased by the anisotropic medium. These calculations suggest that head waves arriving prior to specularly reflected waves should be a sharp diagnostic of conditions at the interface. The computations also suggest that mode conversion effects, curvature and roughness effects, and thin bedding effects should be considered together in order to delineate the precise symptoms of the subsurface being carried by the observed head waves.