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

Abstract

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. Copyright © 1989, Wiley Blackwell. All rights reserved