Analysis of strong scattering at the micro-scale

Abstract

Exploiting the fine structure of strongly scattered waves could provide a wealth of new information in seismology, ultrasonics, acoustics, and other fields that study wave propagation in heterogeneous media. Therefore, noncontacting laser-based measurements of ultrasonic surface waves propagating in a strongly disordered medium are performed in which the ratio of the dominant surface wavelength to the size of a scatterer is large, and waves that propagate through many scatterers are recorded. This allows analysis of scattering-induced dispersion and attenuation, as well as the transition from ballistic to diffusive propagation. Despite the relatively small size of the scatterers, multiple scattering strikingly amplifies small perturbations, making changes even in a single scatterer visible in the later-arriving waveforms. To understand the complexity of the measured waveforms, elastic spectral-element numerical simulations are performed. The multiple-scattering sensitivity requires precise gridding of the actual model, but once this has been accomplished, we obtain good agreement between the measured and simulated waveforms. In fact, the simulations are invaluable in analyzing subtle effects in the data such as weak precursory body-wave diffractions. The flexibility of the spectral-element method in handling media with sharp boundaries makes it a powerful tool to study surface-wave propagation in the multiple-scattering regime.