FREQUENCY-DEPENDENCE OF SOUND-PROPAGATION IN SUPERFLUID-FILLED POROUS-MEDIA

We have studied the acoustics of fluid-filled porous media by measuring the velocity and attenuation of ultrasonic (4 to 31 MHz) shear waves in several different ceramic materials. We used liquid helium as the pore fluid and made measurements down to low temperatures where the helium was superfluid. This allowed us to completely eliminate the effects of viscosity and thus to unambiguously determine the velocity changes and attenuation due to the Biot mechanism (fluid sloshing in the pores). By using ceramics with different pore sizes and a corresponding wide range of permeabilities (from 2.6×10-14 to 3.5×10-11 cm2) we were able to make measurements in both the low-frequency regime (where the fluid is viscously locked to the porous frame) and in the high-frequency regime (where most of the fluid is decoupled from the frame). One of the samples had an extremely high porosity (92%), allowing us to study the fluid motion in a very open geometry. In all cases, we found that the Biot model could quantitatively describe the temperature and frequency dependence of our results. This allowed us to determine the structural parameters of the porous media (pore tortuosity c, permeability a and effective pore size ), something which has previously required measurements of the Biot slow wave (fourth sound and second sound in superfluid helium). The acoustically determined parameters were compared to the independently measured static permeability, 0, and to previous experimental and theoretical work on model porous media. Our results indicate that, even in materials with irregular pores and a range of length scales, acoustic measurements made in either the low- or high-frequency regime can be used to estimate the permeability. © 1994 The American Physical Society.