HR: 0800h
AN: S31B-1053    [Abstracts]
TI: Estimating Scattering Strength from the Transition to Equipartitioning
AU: Scales, J A
EM: jscales@ruelle.mines.edu
AF: Physical Acoustics Laboratory and Department of Geophysics, Colorado School of Mines 1500 Illinois St, Golden, CO 80401 United States
AU: * Malcolm, A E
EM: amalcolm@dix.mines.edu
AF: Center for Wave Phenomena and Department of Geophysics, Colorado School of Mines 1500 Illinois St, Golden, CO 80401 United States
AU: van Tiggelen, B A
EM: Bart.Van-Tiggelen@grenoble.cnrs.fr
AF: CNRS/Laboratoire de Physique et Mod\'{e}lisation des Milieux Condens\'{e}s, Universit\'{e} Joseph Fourier Maison des Magist\`{e}res BP 166, Grenoble, F-38042 France
AB: A pulse launched in a strongly heterogeneous medium first travels as a coherent packet of energy. Suppose the packet travels from one receiver to another. Then cross-correlating the signal at the two receivers will show a (causal) correlation peaked at the time associated with the group velocity of the packet. As time progresses (i.e., after a few mean free times) scattering causes the pulse to spread out and its energy to propagate diffusively. This means that there will be some energy propagating from the second receiver back to the first, resulting in a build-up of acausal correlations. Eventually, after many mean free times, there is as much energy propagating from one receiver to the next as there is back. In this "equipartitioning" regime, after suitable spatial averaging, the cross correlation of the diffuse wave speckle, leads to a completely symmetric result, corresponding the the sum of the causal and the anti-causal Green Function of the average medium. We will show results from a laboratory experiment tracking this transition in a granular rock sample. The amount of time required for this transition depends on the scattering properties of the medium. Through the symmetry of the two peaks it is possible to extract the mean-free path, which is a measure of the scattering strength of the sample.
DE: 7260 Theory and modeling
DE: 5102 Acoustic properties
DE: 5112 Microstructure
DE: 5139 Transport properties
DE: 3220 Nonlinear dynamics
SC: Seismology [S]
MN: 2004 AGU Fall Meeting