Westerly Granite Anisotropy Study

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ABSTRACT: Westerly granite (WG) is well known rock, believed to be isotropic. We studied four samples of WG heated between 100°C and 600°C, by ultrasonic sounding on spherical samples under hydrostatic pressure up to 400 MPa, neutron diffraction on identical samples and scanning electron microscopy (SEM). Thermal treatment studies are important for localities like nuclear waste storages, geothermal projects, rock and earthquake mechanics. All measurements were done at room temperature. The 3D distribution of P-wave velocities at high pressures reflects intrinsic structure and even though the anisotropy is low, the orientation of the minimum velocity corresponds to the highly preferred orientation of plagioclase (010) and biotite (001). Image analyses showed that there is also preferred orientation of microcracks regardless of their size and thermal treatment level. Neutron diffraction measurements of the samples heated to 100°C and 600°C confirm weak intrinsic elastic anisotropy, which remain unchanged due to the thermal treatment. We can assume that in Westerly granite there are two types of anisotropy: crystal preferred orientation which was formed during igneous crystallization and second one is due to the oriented microcracks which have been formed during tectonic exhumation or during sample excavation in the quarry. Both seems to be unrelated. 1. INTRODUCTION Westerly granite has been studied for decades and its properties are very well known. There were studied mechanical properties, elastic properties, development of cracks introduced by uniaxial or triaxial loading, thermal heating, study of permeability, study of fracturing process by acoustic emission, modelling of crack systems and plenty of others. Westerly granite is considered as fine grained, homogeneous material, isotropic and therefore it is often discussed or even used as a standard for comparison with other granitic rocks. Quantification of elastic properties of granites is important to determine crustal seismic velocities and stress orientation. Generally, it is assumed that granitic rocks are elastically isotropic. In this paper, we study influence of thermal cracks and crack induced anisotropy on P-wave propagation in spherical samples of Westerly granite at different confining pressures. Experimental elastic wave velocity distributions in Westerly granite are compared to the model based on neutron diffraction data on mineral composition and mineral preferred orientations. Due to high penetration depth of thermal neutrons, information on a large representative volume of geomaterial is obtained; and the method of neutron diffraction allows to investigate same bulk samples that were used for elastic wave propagation study. Thus, ultrasonic sounding (US) and neutron diffraction form a pair of complementary methods suitable for in-depth analysis of elastic anisotropy of rocks.