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

Abstract

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 ∼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 ∼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. Copyright 2009 by the American Geophysical Union.