We present the elastic anisotropy of two rock samples from two underground research laboratories (URL) that are focused mainly on the investigation of functionality of deep repositories: (1) BUK sample – migmatized paragneiss, typical for the test site of the URL Bukov, Czech Republic; (2) GRM sample - Central Aare granite, a predominant host rock of the URL Grimsel, Switzerland. We used a detail ultrasonic measurement of P and S wave velocity to obtain a general form of stiffness tensor with 21 independent elastic constants for selected pressure levels from 0.1 to 100 MPa. The transformation of tensor to its principal coordinate systems shows: the Bukov migmatized gneiss is orthorhombic, whereas the Grimsel granite is transversely isotropic under atmospheric pressure. The anisotropy strength of both rocks decreases with applied confining pressure due to closing of preferentially oriented cracks. Grimsel granite becomes almost isotropic at high pressures. A great part of anisotropy in the Bukov paragneiss remains even under high pressures due to its texture. Both rocks are anisotropic under the equivalent of the overburden pressure acting at the in-situ URL conditions.
Rocks, in general, display elastic anisotropy, as a combined effect of preferential orientations of their constituents: crystals, mineral grains and microcracks (eg. Babuska and Cara, 1991). This anisotropy depends on acting stresses, as was demonstrated by the pressure dependence of seismic velocities (eg. Pros et al. 1998). At lower pressures (<50 MPa), the elasticity and its anisotropy is controlled by the cracks with characteristic exponential increase in seismic velocity. Above the crack closing pressure, the velocity/pressure relation becomes linear depending on rock matrix (crack free) properties.
The ultrasonic sounding is a laboratory technique to determine the anisotropic elastic constants for a different symmetry types, most commonly the transverse isotropy (Sarout et al., 2007) or orthorhombic symmetry (Sano et al., 1992). With decreasing level of symmetry, the number of independent elastic constants is increasing up to 21, for the most general case of triclinic symmetry. Lokajicek and Svitek (2015) presented an experimental approach to estimate such a stiffness tensor estimated from the ultrasonic sounding performed on the spherical specimen in 132 independent directions for three waves with different wave polarizations (P, SH, SV). In this experiment, the symmetry type or its orientation does not have to be known/supposed before the experiment, or may even change with the applied pressure (Lokajicek et al., 2020). The obtained general stiffness tensor can be simplified for the higher symmetries by rotation into its principal coordinate system (Aminzadeh et al., 2022).
