Characterizing extended changes in multiple scattering media using coda wave decorrelation: numerical simulations

In multiple scattering media, the coda wave decorrelation relates linearly to the scattering cross-section of structural change when the change is small compared to the wavelength. In practical applications, we assume that the total decorrelation induced by changes in the medium is the sum of the decorrelation induced by each elementary change. In this article, we investigate the validity of this linear approximation for extended changes larger than the wavelength, and the possible signature of the change orientation. Coda waves are simulated using a 2-D finite-difference model in multiple scattering media. We perform a parametric analysis of the decorrelation induced by extended structural changes of various length and orientation, as well as the mutual influence between two identical changes separated by a varying distance. Our findings are: (1) we underestimate the length of the change when it exceeds one wavelength. (2) the decorrelation value is sensitive to the orientation of extended changes at distances smaller than four mean free paths between the source and the receiver. (3) two simultaneous changes are interacting within a distance of the order of the mean free path, but can be considered independent at a separation distance larger than a few mean free paths.