WHAT DOES CONTROL V-P V-S IN GRANULAR ROCKS

The compressionnal V-p and shear V-s velocities of rocks are known to depend on composition and porosity. In the case of high porosity, poor ly consolidated dry rocks, [Winkler, 1983] showed that V-p/V-s is esse ntially constant and close to 1.56, in disagreement with theoretical p redictions. Winkler suggested that this discrepancy could be attribute d to either packing models or contact stiffnesses models. We have inve stigated first the influence of packing models using results derived d irectly for disordered packings [Digby, 1981] and for ordered ones. Id entical Foisson's ratio values, lower than the experimental values, ar e obtained for close packings and random ones. This shows that the rol e of packing is a minor one. In agreement with the conclusion of [Palc iauskas, 1992], the role of contact stiffnesses appears to be the main one. We show that experimental results can be explained by a simple a nd robust roughness model. Averaging over a distribution of microconta cts implies that only half of these contribute to shear so that the ra tio of both stiffnesses has a value close to that previously derived b y Palciauskas on the basis of a more complex oblique contact model. Th e observed average Poisson's ratio value of 0.14-0.15 and V-p/V-s valu e of 1.56 is a consequence of a symmetric asperity distribution and of the asymmetry of shear.