Alison E Malcolm | Search PubMed

PubMed2004

Extracting the Green function from diffuse, equipartitioned waves.

A ballistic pulse launched in a strongly scattering random medium becomes diffusive after a few mean-free times. In this regime of diffusive propagation there is a net flux of energy away from the source. Eventually the flux goes to zero, in the equipartitioned regime, in which the signal consists of equal amounts of energy propagating in all directions. In this regime the two-point, two-time correlation of the wave-field should equal the sum of the advanced and retarded Green functions associated with the average medium. We observe the emergence of the Green function from this correlation at about 9 mean-free times in a highly heterogeneous rock sample.

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PubMed2003

Laser characterization of ultrasonic wave propagation in random media.

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.

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