Characterization of a Hydrothermal Fracture Network Embedded in Crystalline Rock Utilizing Borehole Radar and Geophysics
A. Greenwood, E. Caspari, D. Egli, L. Baron and K. Holliger
Event name: 24th European Meeting of Environmental and Engineering Geophysics
Session: Studies for Geothermal Energy and CO2 Storage I
Publication date: 09 September 2018
Info: Extended abstract, PDF ( 956.57Kb )
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A near-vertical hydrothermally active fault zone, embedded in sheared and fractured crystalline rocks of the Central Swiss Alps, has been drilled and geophysically explored in view of its potential similarities with planned petrothermal reservoirs in the Northern Alpine foreland. The GDP1 Grimsel Pass borehole acutely intersects the core of Grimsel Breccia Fault and is situated entirely within its surrounding deformation zone. Pervasive brittle deformation, which overprints Alpine shear zones is characterized by fractures of varying aperture and is the dominant response in all of the borehole data. In this study, we utilize borehole radar data to image fluid-filled fractures outside of the immediate vicinity of the borehole. This is possible due to their strong permittivity contrast with respect to the granitic host rock. In combination with optical televiewer data analysis, the borehole radar image indicates a complicated network of intersecting fractures. Additionally, to shed more light on the characteristics of the system, we use tube wave data from a hydrophone VSP survey and self-potential data. The former is responsive to mechanically compliant and hydraulically transmissive areas, whereas the latter may indicate zones of in- and outflow.