The advent of borehole shear slowness measurements in sonically slow formations has lead to breakthroughs in the subsurface profiling of geological bodies. In sand bodies, compressional and shear velocities depend predictably on porosity, mineralogy, grain contacts, and fluid saturation. An interpretation is best performed by decomposing the velocities into moduli that are intrinsic measures of the rock frame and pore fluid compressibilities. Careful experiments on pure materials (i.e., pure quartz sandstones) demonstrate two simplifying constitutive relationships. First, the bulk and shear frame moduli are simple functions of the porosity. A comparison of the measured shear frame modulus to the prediction for the pure material INTRODUCTION