Evaluation of intrinsic and crack-induced elastic anisotropy in a gneissic rock

The dynamic characteristics of an anisotropic rock (‘Antigorio gneiss’ from the Central Alps) were determined by means of measurements of P- and S-wave velocities. The investigated gneiss was found to be characterized by an orthotropic symmetry, its lower elastic modulus being in the direction of the schistosity pole. On unstressed specimens the elastic moduli have been found to be lower and the anisotropy degree higher than the intrinsic values of the rock matrix, indicating that what prevails is the influence of sets of microcracks. At increasing stresses, the elastic moduli become higher, whereas the anisotropy degree decreases. In order to identify the distributions of the orientation and of the aspect ratio of the cracks, non-interacting crack models were utilized.