Identification of structures inside basement at Soultz-sous-Foret (France) by triaxial drill-bit vertical seismic profiling

Seismic signals produced while drilling a Hot Dry Rock (HDR) geothermal well in Soultz- sous-Foreà t (France) were detected using a 4-component seismic detector and two single- component detectors in the granite basement. The recorded signals had a signal-to-noise ratio of around 8 dB with respect to the background noise and show relatively high spectral whiteness. Results from three-dimensional hodogram analysis were successfully interpreted using a model in which the drill bit generates predominantly continuous shear waves. Re¯ected waves were detected using both the spatial and temporal cross-correlation of hodograms. The distribution of re¯ectivity from the tri-axial drill-bit vertical seismic pro®ling (TAD±VSP) suggests that the bottom of the arti®cial geothermal reservoir, created by hydraulic stimulation, has a high re¯ection coecient. The pre-existing permeable zone inside the reservoir was also detected by the TAD±VSP. The re¯ection image of the TAD±VSP agrees well with that found by the AE (Acoustic Emission) re¯ection method. The TAD±VSP was successfully used to detect the internal structure of the deep basement. # 1999 CNR. Published by Elsevier Science Ltd. All rights reserved.