Laser characterization of ultrasonic wave propagation in random media

TLDR

Lasers can excite and detect ultrasonic waves, enabling absolute particle motion measurement without mechanical disturbances, while wave fields are typically accessible only at sample boundaries. The study aims to demonstrate noncontact laser-based measurements of ultrasonic wave propagation in randomly heterogeneous rock samples, comparing samples with similar elastic moduli but differing grain sizes to assess scattering strength. The authors employ noncontact laser source and detector techniques to measure the surface wave field within the scattering region of randomly heterogeneous rock samples. Scanning the sample surface reveals complex dynamics of diffraction, multiple scattering, mode conversion, and whispering gallery modes, and the intensity data fit a radiative transfer model that allows inference of the scattering mean free path.

Abstract

Lasers can be used to excite and detect ultrasonic waves in a wide variety of materials. This allows the measurement of absolute particle motion without the mechanical disturbances of contacting transducers. In an ultrasound transmission experiment, the wave field is usually accessible only on the boundaries of a sample. Using optical methods, one can measure the surface wave field, in effect, within the scattering region. Here, we describe noncontacting (laser source and detector) measurements of ultrasonic wave propagation in randomly heterogeneous rock samples. By scanning the surface of the sample, we can directly visualize the complex dynamics of diffraction, multiple scattering, mode conversion, and whispering gallery modes. We will show measurements on rock samples that have similar elastic moduli and intrinsic attenuation, but different grain sizes, and hence, different scattering strengths. The intensity data are well fit by a radiative transfer model, and we use this fact to infer the scattering mean free path.