Importance of interface hydraulic condition on the generation of second bulk compressional wave in porous media

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Abstract
The fundamental prediction of Biot’s thoery [J. Acoust. Soc. Am. 28, 168 (1956)] for propagation of acoustic waves in saturated porous media is the existence of a second compressional wave (SCW). This wave has been observed experimentally by Plona [Appl. Phys. Lett. 36, 259 (1980)]. In this work, we modify Plona’s experiments on saturated porous plates by sealing off interface permeability using a thin coat of paint on both sides of the plates. Our work focuses on the influence of this coating on the observed acoustic waves. No SCW generation is observed on coated plates which was predicted by Geertsma and Smit [Geophysics 41, 169 (1961)] using Biot’s theory. Furthermore, due to total energy conservation, the generation of the SCW substantially modifies the energy of the other ordinary waves. A comparison between experimental and synthetic signals using Biot’s theory shows good agreement. These results show the importance of free fluid transfer at the interface between the porous medium and the fluid for the generation of the SCW.