The pressure dependent elastic anisotropy of Grimsel granite and Bukov paragneiss

Abstract

Abstract A methodology for obtaining the stiffness tensor of an elastic material in its most general form (with 21 independent parameters) using ultrasonic measurements is presented. The methodology is used to define the anisotropic properties of two spherical rock samples: a migmatized gneiss from Bukov (Czech Republic) and a granite from Grimsel (Switzerland) underground research laboratories. The general stiffness matrix for both rocks is determinded for different confining pressures suggesting that the Bukov sample is orthotropic at atmospheric pressure while the Grimsel sample is transversely isotropic. However, the Bukov sample remains orthotropic even at a high pressure of 100 MPa but the Grimsel sample becomes almost isotropic. Both rocks are anisotropic under the overburden pressure acting at the in-situ URL conditions. A significant amount of cracks remains open at these pressures and contribute to the overall anisotropy of rocks.