Ultrasonic characterization of the anisotropic behavior of air-saturated porous materials

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This paper provides a comprehensive review of the propagation of ultrasonic waves in anisotropic porous materials. The equivalent fluid model (or Allard-Johnson theory) which is relevant for air-saturated porous media is described. It takes into account viscous and thermal losses occurring during the movement of the fluid within the motionless solid frame. When the skeleton is moving as well, the coupled Biot 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 Biot 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 characterize anisotropy in porous media. Both reflection and transmission configurations have been used. Special attention has been devoted to the measurements of the anisotropic tortuosity, but also to the viscous and thermal characteristics lengths. Finally, some inverse problems related to these measurements are solved and others, which are still open, are presented.