HR: 08:00h
AN: S21D-01 INVITED     [Abstracts]
TI: Phase and Correlation in Random Seismic Fields and the Reconstruction of Green Function
AU: * Campillo, M
EM: Michel.Campillo@ujf-grenoble.fr
AF: Universit‚ Joseph Fourier and CNRS, BP 53X, Grenoble, 38041 France
AU: Derode, A
EM: Arnaud.Derode@espci.fr
AF: Universit‚ Paris 7 and CNRS, ESPCI 10 rue Vauquelin, Paris, 75005 France
AU: Larose, E
EM: Eric.Larose@ujf-grenoble.fr
AF: Universit‚ Joseph Fourier and CNRS, BP 53X, Grenoble, 38041 France
AU: Margerin, L
EM: Ludovic.Margerin@ujf-grenoble.fr
AF: Universit‚ Joseph Fourier and CNRS, BP 53X, Grenoble, 38041 France
AU: Paul, A
EM: Anne.Paul@ujf-grenoble.fr
AF: Universit‚ Joseph Fourier and CNRS, BP 53X, Grenoble, 38041 France
AU: Shapiro, N
EM: nshapiro@fignon.colorado.edu
AF: University of Colorado, Box 390, Boulder, CO 80309 United States
AU: Stehly, L
EM: Laurent.Stehly@ujf-grenoble.fr
AF: Universit‚ Joseph Fourier and CNRS, BP 53X, Grenoble, 38041 France
AU: van Tiggelen, B
EM: Bart.Van-Tiggelen@grenoble.cnrs.fr
AF: Universit‚ Joseph Fourier and CNRS, BP 53X, Grenoble, 38041 France
AB: This paper was strongly influenced by Aki's works on the interpretation of the energy decay of coda waves and on the array processing of seismic noise. We first present a summary of recent results on coda interpretation. We emphasize the observation of the stabilization of P to S energy ratio indicating the modal equipartition of the wave field. This property attests that the coda waves are in the regime of multiple scattering. We use numerical solutions of the elastic radiative transfer equation to illustrate the evolution of the wave-field towards P to S energy stabilization, and asymptotically to complete isotropy. The energy properties of the coda have been widely studied but the phase properties have often been neglected. The recently observed coherent backscattering spot, an expression of the so-called `weak localization', demonstrates that phase effects, as interferences, still persist for multiply diffracted waves. Another manifestation of the persistence of the phase is the possibility to reconstruct the Green function between two stations by averaging the cross-correlation of coda waves produced by distant earthquakes and recorded at those two stations. This reconstruction is directly related to the properties of reciprocity and time reversal of any wave-field. Using broad-band coda waves, we show that the dominant phases of the Green function in the band 10sec-2sec, namely Rayleigh and Love waves, are reconstructed. At higher frequencies, absorption reduces the length of the records available for processing and it is difficult to detect the body waves. However, their amplitude is expected to be small with respect to surface waves for source and receiver at the free surface. We analyse the time symmetry of the cross correlation and shows how the level of symmetry evolves with the isotropization of the diffuse field with lapse time. Similarly we investigate the correlation in continuous ambient noise records. Whereas the randomness of the coda results from multiple scattering on randomly distributed scatterers, we assume that the seismic noise is random mostly because of the distribution of sources at the surface of the Earth. We show that surface waves can be extracted from long time series., We computed the dispersion curves of Rayleigh waves from the correlations. On paths where direct measurements are available, we show that they are in good agreement with those deduced from noise correlation. The measurement of correlation along paths crossing different crustal structures allows to differentiate them, opening the way for a `passive imaging' of the Earth structure.
DE: 7255 Surface waves and free oscillations
DE: 7260 Theory and modeling
DE: 7203 Body wave propagation
SC: Seismology [S]
MN: 2004 AGU Fall Meeting