HR: 0830h
AN: S11E-0336    [PDF]
TI: Breakdown of wave diffusion in 2D due to loops
AU: Haney, M
EM: mhaney@dix.mines.edu
AF: Dept. of Geophysics and Center for Wave Phenomena, Colorado School of Mines 1500 Illinois Str., Golden, CO 80401 United States
AU: * Snieder, R
EM: rsnieder@mines.edu
AF: Dept. of Geophysics and Center for Wave Phenomena, Colorado School of Mines 1500 Illinois Str., Golden, CO 80401 United States
AB: There is a growing interest in incorporating the multiply-scattered coda into our understanding of the earth's interior using energy transport theory. Using energy transport, the envelopes of seismograms are modeled and interference effects between individual arrivals in the coda are ignored. Also known as radiative transfer, this picture of the coda leads to the so-called diffusive regime at late times. There are several novel applications at this level, among them obtaining the Vp/Vs ratio from the partitioning of P- and S-energies and the ability to separate scattering and intrinsic Q. However, that individual wave arrivals in the coda may interfere constructively, and the underlying wave character of the multiply-scattered seismic energy emerges. Constructive interference in the coda renders the usually diffusive regime "non-diffusive", most notably in the presence of a coherent backscattering peak at the source position. Here, we test the validity of the diffusion approximation for the average intensity (squared envelope) of multiply-scattered waves with numerical simulations in a strongly scattering 2D medium of finite extent. We show that the diffusion equation underestimates the intensity and attribute this to both the neglect of recurrent scattering paths and interference within diffusion theory. We present a theory to quantify this discrepency based on counting all possible scattering paths between point scatterers. Interference phenomena, due to loop paths, are incorporated in a way similar to coherent backscattering. This may ultimately help to bridge the conceptual gap between the regimes of "weak" and "strong" localization. In addition, the work suggests a new way in which the microstructure of a discrete random medium can become imprinted in the average transmitted intensity. This could lead to sample size dependencies as have been previously reported for coherent backscattering. [Haney, M. and R. Snieder, R., Breakdown of diffusion in 2D due to loops, Phys. Rev. Lett., 91, doi:10.1103/PhysRevLett.91.093902, 2003.]
DE: 1734 Seismology
SC: Seismology [S]
MN: 2003 Fall Meeting