Investigation of Seismic Anisotropy in the Rotondo Granite by Crosshole Seismic Surveys

Abstract

In the framework of a geothermal project in the deep underground BedrettoLab, we have tested the hypothesis of stress-induced anisotropy by analyzing seismic wave propagation and considering the ambient state-of-stress. We present the results of comprehensive crosshole seismic surveys in three adjacent boreholes drilled into the undisturbed granitic host rock (Rotondo Granite) and we discuss novel approaches of data acquisition and analysis to fully exploit the information content offered by the recorded wavefields. In particular, we analyzed signals from various seismic sources recorded on different receiver types. Initial results of the measured traveltimes of both P- and S-waves indicate that the host rock can be described to a first order by a tilted transverse isotropic (TTI) model with a peak anisotropy of about 9%. However, there is also evidence that a more complex anisotropy model may be required to fully explain the seismic data, which could be indicative for a combination of structural and stress-induced anisotropy. This conjecture is currently being tested with more controlled laboratory-scale (cm to dm) experiments, as well as with a seismic inversion of crosshole data in a larger, more complex rock volume. Once the relation between seismic anisotropy, fracture orientations and the ambient stress field are better understood, this can potentially help to monitor changes in the stress field during geothermal operations for a better hazard assessment.