A shallow near-vertical hydrothermally active fault zone embedded in fractured crystalline rocks of the central Alps has been drilled and geophysically explored in view of its potential analogies to planned deep petrothermal reservoirs in the Alpine foreland. Hydrophone-based zero-offset vertical seismic profiling (VSP) data were found to be highly effective for detecting open fractures and determining seismic velocity changes due to deformation. Walk-away hydrophone VSP data were acquired with a 45-degree crooked-line survey geometry with respect to the borehole plane, which requires 3D processing methods for seismic imaging. For imaging purposes, a laterally changing velocity cube was generated from the ZVSP velocities and projected along the strike of the fault. The subsequent, pre-stack-depth-migration imaging has been successful in delineating vertical structures, the target fault core, and an as of yet unknown, and correspondingly enigmatic, sub-horizontal structural feature.


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