3-D mapping of permeable structures affecting a deep granite basement using isotropic 3C VSP data
Résumé
This paper illustrates the efficiency of vertical seismic profiling (VSP) for the investigation of dipping and hydraulically conductive structures affecting a granitic basement covered by sediments. A three-component (3C) VSP data set has been acquired in the GPK1 and EPS1 wells of the Soultz-sous-Foreˆts enhanced geothermal system (EGS) located within the Upper Rhine Graben (URG). Our study focuses on the isotropic processing of profiles acquired with vertical vibrator P and their subsequent interpretation. Mainly P-S converted reflections are identified from the analysis of the 3C records. These P-S conversions occur on steep permeable faults that are positioned in space by traveltime modelling. These faults cut the granite basement in several hectometric-scale blocks, and represent the main fluid paths between the boreholes. These faults are thought to be inherited from late Variscan and Alpine deformation periods, reactivated by the current stress field. When properly processed and interpreted, VSP allow the scale gap between surface and borehole data to be bridged.
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