Poisson's ratios of crystalline rocks as a function of hydrostatic confining pressure

The hydrostatic pressure (P) dependence of dynamic Poisson's ratios (u) has been investigated for 54 samples of the crystalline rocks from the Sulu-Dabie orogenic belt (China) using pulse transmission techniques. The Poisson's ratio of each sample was calculated from its mean P and S wave velocities from three orthogonal directions corresponding to the tectonic framework (X-Y-Z) defined by foliation and lineation. The experimental results display two typical categories of u - P relationship in the range of 40-800 MPa: (1) with increasing pressure, u increases rapidly below similar to 200 MPa and then becomes quasi-constant at higher pressures, and (2) u shows little variation with P. Types 1 and 2 are observed in 32 and 22 samples, respectively. The origin of type 1 can be reasonably interpreted by a small volume fraction (0.1-0.5%) of randomly distributed and randomly oriented thin disk-shaped microcracks that are progressively closed during pressurization. Type 2 is originated from the combined effects of microcrack orientation, crystallographic preferred orientations, and compositional layering. The present study confirms that the crystalline rocks at pressures above similar to 200 MPa show no significant changes in Poisson's ratio with increasing pressure. Below 200 MPa, however, both modal composition and confining pressure play a critical role in influencing the Poisson's ratio.