Seismic coda wave envelope in depth-dependent S wave velocity structure

Abstract

Coda waves are considered to be composed of scattered waves. For simplicity, previous models to generate coda waves have often assumed spatial uniformity of seismic wave velocity. To account for a depth-dependent S wave velocity structure, energy densities of coda waves are synthesized using a Monte Carlo simulation method, based on the multiple isotropic scattering assumption for media consisting of several layers. The wave energy propagation and the scattering process are simulated by random walks of many particles. Reflection/transmission is determined by Snell's law and the ray parameter is preserved at the interface of layers. On the other hand, scattering changes the ray parameter. The energy densities are determined by the overall effect of reflection/transmission, scattering, absorption and geometrical spreading along the ray paths. The results of the simulation show that the energy density depends on the focal depth of the sources even for the late part of the coda. This suggests that the conventional coda normalization method may not work well in case of a strong depth-dependent velocity structure.