Investigating S-wave Anisotropy in the Rotondo Granite by croshole seimic Surveys

Abstract

The Bedretto Laboratory, located within the Rotondo granite, conducts stimulations of the host rock to investigate the possibility of enhanced geothermal energy and tries to better understand the risk of induced seismicity. Seismic anisotropy plays an important role when processing seismic data, making its characterization essential in such a laboratory. In a previous study, through crosshole seismic, the apparent velocity of P-waves was obtained and used to give some first values of the parameters characterising anisotropy assuming a Tilted Transversely Isotropic medium (TTI). Further investigating the obtained data, S-wave splitting could be detected. This phenomenon occurs in anisotropic media, resulting in horizontal S-waves (SH) and vertical S-waves (SV) having different seismic velocities. A sensitivity analysis was performed to assess the significance of S-wave information. The analysis revealed the benefit of incorporating the information provided by the S-waves to improve the current anisotropic model. Several crosshole surveys were carried out using different instruments, such as a S-wave sparker or a 3-component geophone. The aim was to understand the behavior of S-waves better and accurately identify the arrival time of both types of waves. The results reveal that rotating the 3-component geophones data was the best choice to isolate both types of S-waves. Time arrivals were picked, and apparent velocities were calculated knowing the ray path length. They show a change from 2747 m/s to 3216m/s for SH waves, and from 2830m/s to 3105m/s for SV-waves. An attempt was made to obtain the anisotropy parameters that successfully describe the observed velocities in the Rotondo granite.