HR: 0800h
AN: S31B-1047    [Abstracts]
TI: Envelope Synthesis In Random Media - Radiative Transfer Versus Finite Difference Modeling
AU: * Przybilla, J
EM: jprzybill@web.de
AF: Leipzig University, Talstr.35, Leipzig, 04103 Germany
AU: Korn, M
EM: mikorn@rz.uni-leipzig.de
AF: Leipzig University, Talstr.35, Leipzig, 04103 Germany
AU: Wegler, U
EM: uli@rz.uni-leipzig.de
AF: Leipzig University, Talstr.35, Leipzig, 04103 Germany
AB: The analysis of the coda portion of seismograms is an effective strategy to investigate the heterogeneous structure of the Earth at small scales. Usually the shape of seismogram envelopes at high frequencies are studied. A powerful method to synthesize envelopes is based on the radiative transfer theory, which describes energy transport through a scattering medium. The radiative transfer equations can conveniently be solved by a Monte Carlo simulation of random walks of energy particles through such a medium. Between single scattering events each particle moves through the background medium along ray paths. The probability of a scattering event is determined by the mean free path length depending on the total scattering coefficient of the medium. Monte Carlo simulations have so far mostly assumed isotropic scattering and acoustic approximations, as well as isotropic source radiation. Here we present an extension of this method to the full elastic case including P and S waves, and for angular dependent scattering coefficients according to the Born approximation. In order to validate this procedure, the results of the simulations are compared to envelopes obtained from full wave field modeling in 2D employing a finite difference method. Envelope shapes agree remarkably well for both short and long lapse times and for a broad range of scattering parameters. This leads to the conclusion that the use of Born scattering coefficients does not pose severe limits to the validity range of Monte Carlo method. From the comparison between elastic and acoustic simulations it becomes apparent that wave type conversions should not be neglected, especially when both P and S coda are interpreted simultaneously. Additionally, the influence of density fluctuations on envelope shapes has also been studied. It appears that the amount of density variations has a large effect on the level of the late coda only, thus showing a possibility to discriminate between velocity and density fluctuations.
DE: 7260 Theory and modeling
DE: 7200 SEISMOLOGY
DE: 7203 Body wave propagation
DE: 7218 Lithosphere and upper mantle
SC: Seismology [S]
MN: 2004 AGU Fall Meeting