HR: 0830h
AN: S11E-0342    [PDF]
TI: Acoustic Wave Attenuation and Scattering in Anisotropic Random Media
AU: * Margerin, L
EM: Ludovic.Margerin@ujf-grenoble.fr
AF: Laboratoire de Geophysique Interne et Tectonophysique, Maison des Geosciences BP 53, Grenoble, 38041 France
AB: Most theoretical investigations of seismic wave scattering rely on the assumption that the underlying medium is statistically isotropic. However, deep seismic soundings of the crust as well as geological observations often reveal the existence of elongated or preferentially oriented scattering structures. In this paper, we develop mean-field and radiative transfer theories to describe the attenuation and multiple scattering of the acoustic wavefield in an anisotropic random medium. The scattering attenuation length is found to depend strongly on the propagation direction, while the phase velocity develops a very weak anisotropy. We derive the anisotropic radiative transfer equation from the exact Bethe-Salpeter formalism and propose a Monte-Carlo method to solve the transport equation numerically. At late times, the acoustic energy is shown to obey a tensorial diffusion equation. The components of the diffusion tensor are obtained in closed form and excellent agreement is found between Monte-Carlo simulations and analytical solutions of the diffusion equation. The theory has important potential implications for crustal models where scatterers are (e.g.) flat structures preferentially aligned along the surface. In this simple geometry, analytical expressions of the Coda $Q$ parameter will be given explicitly. It will be further argued that pulse broadening and Coda decay are controlled by different parameters -the eigenvalues of the diffusion tensor- that can differ by more than one order of magnitude.
DE: 7260 Theory and modeling
SC: Seismology [S]
MN: 2003 Fall Meeting