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

Compressional, shear wave velocities and their ratio, V-p/V-s, 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 V-p 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 V-p/V-s was also observed to change during our experiments: In the wet specimen, V-p/V-s 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 (< 10(-2)). In the dry case, however, our experimental results highlight an increase of Vp/Vs ratio during cataclastic compaction, in apparent contradiction with the predictions of the EM model. Indeed, increases in V-p/V-s ratio, and hence in Poisson's ratio, are, in general, attributed to fluid saturation. A closer look to the microstructure may provide a possible interpretation: Beyond P*, grains are no longer cemented. Using Digby's granular model as an alternative model, we were able to reach a quantitative agreement with the experimental results. The possible implication is that in both dry and wet conditions, cataclastic compaction due to grain crushing induces an increase in V-p/V-s ratio.