Experimental data indicate that although the compressional vp and shear vs wave velocities of unsaturated, monomineralic granular sedimentary rocks vary with grain mineralogy, degree of compaction (porosity), and confining pressure, their ratio vp/vs is to various degrees insensitive to these factors. A rough contact model for the elastic properties of a granular aggregate is presented which is consistent with these observations. The relevant assumptions of the model are: (1) the medium is a random aggregate of monomineralic, elastically isotropic, equal size grains, and (2) the intergrain contacts are rough and their elastic properties are described by the results of Yoshioka and Scholz [1989>.

The model predicts that vp/vs, the bulk to shear frame modulus ratio K¿/&mgr;¿, and the Poisson's ratio, v¿, of the aggregate are independent of porosity and confining pressure, and only weakly dependent on mineral Poisson's ratio. As the mineral Poisson's ratio varies between 0.08 and 0.40, vp/vs and K¿/&mgr;¿ are essentially constant at 1.55 and 1.07 respectively, and the Poisson's ratio, v¿, varies between 0.141 and 0.144. These values are in good agreement with experimental data on dry, monomineralic granular rocks. ¿ American Geophysical Union 1992