Effects of pore collapse and grain crushing on ultrasonic velocities and Vp/Vs

Compressional, shear wave velocities and their ratio, Vp/Vs, were measured along with porosity variations during wet and dry hydrostatic compaction of Bleurswiller sandstone, a 25% porosity Vosgian sandstone. At first, increase in hydrostatic pressure was accompanied by a simultaneous increase of both Vp and Vs as expected. At a critical effective confining pressure P*, a large mechanical decrease of porosity was observed that was due to pore collapse and grain crushing. Theoretically, two different processes are affecting the elastic wave velocities in counteracting ways during cataclastic compaction: cracking and porosity decrease. Our experimental results show that cracking is the dominant effect, so that grain crushing and porosity reduction were accompanied by a large decrease in velocities. The ratio Vp/Vs was also observed to change during our experiments: In the wet specimen, Vp/Vs value increased from 1.72 to 1.84, while in the dry specimen, it increased from 1.59 below P* to 1.67 beyond P*, respectively. To quantitatively interpret these results, an isotropic effective medium model (EM) was used that considered the sandstone as a mixture of spheroidal pores and penny‐shaped cracks. In particular, the increase in Vp/Vs, in the wet case, is well reproduced and shows the important role played by the mechanical coupling of fluid with low aspect ratio cracks (