Multifrequency analysis of underwater acoustic waves reflected by complex media: principles and water tank experiments

Acoustic imaging of geological media is frequency-dependent. For a better understanding, we have developed a multiscale method, the wavelet response, based on the wavelet transform properties. We show that the ridge functions allow defining the regularity of the reflector: we have tested and validated the method by designing acoustic experiments in a water tank where the reflector consists in a planar interface with finite thickness. Covering a large frequency range, we distinguish the global and the internal structures of the discontinuity. We have extended the multiscale analysis to granular medium interfaces where the discontinuity consists in monodisperse glass beads immersed in water. From the experimental ridge functions, we identify five characteristic ranges, from the low to the high frequencies. In particular, we put in evidence a strong transition between the propagation to the diffusion regimes: multiple scattering occurs at high frequencies, identified from the coda part of the reflected wavelets strongly scattered by the interface heterogeneities.