Acoustic wave propagation through porous media: Theory and experiments

A new lossless poroelastic wave propagation theory is verified by means of ultrasonic transmission measurements on artificial rock samples in a water-filled tank. The experiments involved are similar to those performed by Plona [Appl. Phys. Lett. 36, 259-261 (1980)]. In this physically and mathematically mutual consistent new theory the coupling terms between the fluid and solid phase of the porous medium are completely determined by the measured wave speeds and the mass densities and constitutive parameters of both constituting phases. Verification of the amplitudes of the received bulk waves in both the time domain and frequency domain provide information on the validity of the combined effect of propagation characteristics and new macroscopic fluid/fluid-saturated-rock boundary conditions resulting from this theory. The comparison technique between theory and experiments is based on the Fraunhofer diffraction theory, and is first tested in a perfectly elastic medium transmission configuration. Subsequently, this comparison technique is used for the poroelastic medium. It is shown that this technique is very accurate and reliable. The experimental results for the compressional wave and the shear wave in the perfectly elastic medium are in excellent agreement with the theoretical predictions. For the fluid-saturated porous samples, good agreement is found only for the fast compressional wave. For both the shear wave and the slow compressional, it is obvious that there is some kind of loss mechanism involved, which cannot be explained by the current theory. Despite the fact that the bulk losses in the porous medium can be explained qualitatively by the full frequency range Blot theory, it is conjectured that even a quantitative fit is feasible if Johnson's loss model [D. L. Johnson ct al., J. Fluid Mech. 176, 379-402 (1987)] is applied in the lossy counterpart of the current theory [T. W. Geerits, J. Acoust. Sec. Am. 100, 2949-2959 (1996)]. (C) 1997 Acoustical Society of America.