Abstract:

Sound pulses penetrating from water into a sandy silt were found to be dispersive in the 1- to 10-kHz band. Pulses were projected from a parametric source into a sandy silt sediment and received by a buried array of hydrophones. The signals were coherently processed for direction and speed of the sound pulse as a function of frequency. The sound speed was found to increase with frequency at a rate greater than that expected from simple attenuation. Acoustic propagation was modeled with Biot's theory using constant coefficients, and model predictions were unable to match the observed dispersion. One possible explanation is a frequency-dependent frame bulk modulus, due to relaxation effects associated with the interaction between the pore fluid and the grain-grain contacts. [Work supported by ONR 321 OA.]