Seismic anisotropy analysis based on synthetic and measured data for crystalline rock characterization

The prior characterization of a reservoir volume as well as the monitoring of potential changes during heat extraction is highly important to ensure an economic and safe engineering and operation of a geothermal reservoir. While the use of active seismic measurements from the surface and in boreholes is commonly used to describe the seismic velocity field in the subsurface, its potential to analyze the seismic anisotropy is often neglected. We used active seismic crosshole measurements to analyze seismic anisotropy at the field scale to provide important information about the elastic properties and take advantage of this information for the analysis of the in-situ rock-physical conditions.

For the analysis of seismic anisotropy, a reliable data set is essential. We studied the sensitivity of the resulting anisotropy model to inaccuracies in the survey geometry by computing synthetic data and showed that inaccuracies of only a few degrees in dip and azimuth of the borehole orientation can significantly change the resulting anisotropy model of the volume. Especially the orientation of the symmetry axis used to describe a transverse isotropic model (TI) is highly sensible to source and receiver positions.

Beyond the analysis of the synthetic data, the focus of our study was on the exploitation of real measurement data. These data were recorded in a campaign in the Bedretto Underground Laboratory for Geosciences and Geoenergies (BULGG) in Ticino, Switzerland. The laboratory provides the opportunity to record crosshole data with high quality in a controlled environment. The geometry of the boreholes that we used allows a high coverage of ray path orientations which is crucial for the analysis of anisotropic wave propagation. The recorded data were used to create an anisotropy model of the subsurface based on a grid search and optimization algorithm, searching for both the optimized Thomsen parameters and the ideal orientation of the symmetry axis of the tilted TI model. The results give evidence of an ambient elastic anisotropy, while the host rock (Rotondo Granite) has proven negligible intrinsic anisotropy in previous laboratory measurements at sample scale. Further investigation will analyze the effect of the fracture inventory and ambient state of stress as potential controlling factors of the observed anisotropy. This relation can potentially help to monitor changes in the stress field during geothermal operations for a better hazard assessment.