How to estimate the Green's function of a heterogeneous medium between two passive sensors? Application to acoustic waves

The exact Green's function of a heterogeneous medium can be retrieved from the crosscorrelation of the fields received by two passive sensors. We propose a physical interpretation based on time-reversal symmetry. We address the issue of causality and show the role of multiple scattering for the reconstruction of the Green's function. Ultrasonic experimental results are presented to illustrate the argument. Applications to geophysics and ocean acoustics are discussed. (C) 2003 American Institute of Physics.