HR: 0830h
AN: S11E-0340    [PDF]
TI: Synthesis of Green functions from Coda Correlation: Ultrasonic experiments.
AU: * Larose, E
EM: Eric.Larose@ujf-grenoble.fr
AF: LGIT Universite Joseph Fourier and CNRS, BP 53, Grenoble, 38041 France
AU: Derode, A
EM: arnaud.derode@espci.fr
AF: LOA ESPCI and CNRS, 10 rue Vauquelin, Paris, 75005 France
AU: Campillo, M
EM: Michel.Campillo@ujf-grenoble.fr
AF: LGIT Universite Joseph Fourier and CNRS, BP 53, Grenoble, 38041 France
AU: Margerin, L
EM: Ludovic.Margerin@ujf-grenoble.fr
AF: LGIT Universite Joseph Fourier and CNRS, BP 53, Grenoble, 38041 France
AU: van Tiggelen, B
EM: Bart.Van-Tiggelen@grenoble.cnrs.fr
AF: LPMMC Universite Joseph Fourier and CNRS, BP 166 X, Grenoble, 38042 France
AB: Recent works in seismology show that cross-correlation functions between signals recorded at two points and averaged for several earthquakes contain deterministic arrivals. This is discussed in a companion paper. Weaver and Lobkis (2001) successfully retrieved the Green function between two points from the correlation of diffuse fields in a cavity at ultrasonic frequencies. We present here laboratory experiments with ultrasound that show that the principle remains valid in an open scattering medium for which the normal mode representation of the field does not hold. We designed a set of experiments to test the feasibility of imaging from correlations of coda waves produced by distant sources. The scattering is caused by a random arrangement of steel rods in water. The experiments show that the quality of the reconstruction of the Green function is enhanced in the presence of scattering because scattering tends to spread the energy in the different directions of propagation. We show that stacking cross-correlations allows the precise measure of velocity between two sensors in a medium with velocity changes. The reflection from a strong interface can be detected as well as the diffraction from a single scatterer. In practice the quality of the reconstruction is controlled by the duration of the time windows, by the number of sources but also by the spatial distribution of source and scatterers relative to the two sensor locations. This is made easily understandable by the analogy between the empirical synthesis of Green function from field correlation and time-reversal experiments. This analogy gives also a physical interpretation for the loss of time symmetry of the cross-correlations observed both with seismological data and in the laboratory.
DE: 0935 Seismic methods (3025)
DE: 7260 Theory and modeling
SC: Seismology [S]
MN: 2003 Fall Meeting