Craig Hickey
Wayne Prather
James M. Sabatier
Natl. Ctr. for Phys. Acoust., Univ. of Mississippi, University, MS 38677
Probe microphone measurements of air-borne sound penetrating into air-filled sands and soils indicate two absorption coefficients for the frequency range 40--4000 Hz. The probe microphone signal attenuates rapidly with depth in a region near the surface. Below that region the microphone signal attenuates significantly slower with depth. Rigid-capillary-tube porous models, which allow for pore-fluid motion only, correctly describe the rapid attenuation of probe pressure. Using these models, pore properties (tortuosity and air permeability) are typically deduced from the measured complex absorption coefficient. The Biot poro-elastic-capillary tube model describes both attenuation regimes. The two absorption coefficients are associated with the Biot type I and II waves. The large attenuation in the region near the surface is associated with the Biot type II wave. The much smaller attenuation coefficient is a consequence of the elasticity of the matrix and is associated with the Biot type I wave. Biot's model is used to calculate the microphone pressure from both Biot type waves as a function of depth. [Work supported by USDA.]