Non-geometrical waves-are there any? An asymptotic description of some ‘non-geometrical’ phenomena in seismic wave propagation

A review of a number of asymptotic studies of the so called ‘non-geometrical’ phenomena in seismic wave propagation is presented. The results concerning high frequency asymptotics fall into two groups. Some phenomena are described by a slightly modified ray method or by considering higher order approximations. They are the much spoken of S*-waves, the effect of wave tunnelling through a higher velocity layer and the low frequency depolarization of P- , S-, Love and Rayleigh waves described by the first-order asymptotic approximation. Description of phenomena of the other group requires boundary layer techniques. These phenomena are: generation of S-waves by a pressure centre due to velocity and density gradients or due to weak anisotropy as well as depolarization at caustics and in penumbra.