ULTRASONIC CHARACTERIZATION OF THE ANISOTROPIC BEHAVIOR OF AIR-SATURATED POROUS MATERIALS

This paper provides a comprehensive review of the propagation of ultra sonic waves in anisotropic porous materials. The equivalent fluid mode l (or Allard-Johnson theory) which is relevant for air-saturated porou s media is described. It takes into account viscous and thermal losses occurring during the movement of the fluid within the motionless soli d frame. When the skeleton is moving as well, the coupled Blot theory should instead be used. This theory becomes intricate when anisotropy is considered due to a very large number of physical parameters to be determined. A strong formal correspondence between the anisotropic Blo t wave and the thermal wave of dynamic thermoelasticity in non-porous media is outlined. Standard ultrasonic methods, generally used at low frequency (i.e. 20-500 kHz) are very effective in order to characteriz e anisotropy in porous media. Both reflection and transmission configu rations have been used. Special attention has been devoted to the meas urements of the anisotropic tortuosity, but also to the viscous and th ermal characteristics lengths. Finally, some inverse problems related to these measurements are solved and others, which are still open, are presented. (C) 1998 Elsevier Science B.V.