Very High Resolution Hard Rock Seismic Imaging for Excavation Damage Zone Characterisation

Seismic and Radar investigations performed in 2009 at Olkiluoto in a niche excavated on the side of the ONKALO access tunnel. The objectives have been to confirm the existence of the EDZ at Olkiluoto, to evaluate its extent and to determine its physical characteristics. Three crosshole and one singlehole seismic sections have been measured before and after the excavation. The results of these investigations are presented and discussed. As direct consequence of the EDZ along the tunnel wall, a decrease of acoustic P and S wave velocities and an increase in attenuation can be observed both from direct cross-hole measurements done in the immediate vicinity of the excavated wall and from remote sensing by side-scan measurements done in a borehole parallel to the tunnel wall and away from it. The existence of the EDZ has been proven beyond doubt by both transmission and reflection seismic imaging.