Elastic anisotropy and pore space geometry of schlieren granite: direct 3-D measurements at high confining pressure combined with microfabric analysis

Abstract

Pore space geometry of granitic rocks and its evolution with depth are key factors in large-scale seismics or in projects of enhanced geothermal systems or of deep hazardous waste repositories. In this study, we studied macroscopically anisotropic schlieren-bearing granite by experimental P-wave velocity (V-P) measurements on spherical sample in 132 directions at seven different confining pressures in the range 0.1-400 MPa. In order to discriminate the phenomena affecting the rock elastic properties we analysed the orientation of microcracks and of grain boundaries and we measured the anisotropy of magnetic susceptibility of the rock. Three sets of microcracks were defined, with two of them linked to the massif exfoliation process and one to cooling contraction of the massif. During pressurization the measured mean V-P and V-P anisotropy degree at ambient pressure and at highest confinement (400 MPa) yielded 3.3 km s(-1) and 24 per cent, and 6.2 km s(-1) and 3 per cent, respectively. The associated V-P anisotropy pattern was transversely isotropic and governed by the schlieren, with a minimum V-P direction perpendicular to them and a girdle of high V-P directions parallel to them. The highest change in V-P was observed between 0.1 and 10 MPa, suggesting a significant closure of porosity below depths of 500 m. Change of the V-P anisotropy pattern to orthorhombic together with increase of mean V-P and V-P anisotropy degree during depressurization was attributed to inelastic response of one of the sets of microcracks to the loading-unloading cycle.