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Solid Earth An interactive open-access journal of the European Geosciences Union
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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 11 Nov 2019

Submitted as: research article | 11 Nov 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Solid Earth (SE).

Characteristics of a fracture network surrounding a hydrothermally altered shear zone from geophysical borehole logs

Eva Caspari1,2, Andrew Greenwood1,2, Ludovic Baron1, Daniel Egli3, Enea Toschini1, Kaiyan Hu1,4, and Klaus Holliger1,5 Eva Caspari et al.
  • 1Institute of Earth Sciences, University of Lausanne, Lausanne, Switzerland
  • 2Chair of Applied Geophysics, Montanuniversitaet Leoben, Austria
  • 3Institute of Geological Sciences, University of Bern, Bern, Switzerland
  • 4Department of Geophysics, Peking University, Beijing, China
  • 5School of Earth Sciences, Zhejiang University, Hangzhou, China

Abstract. Hydrothermally active and altered fault/shear zones in crystalline rocks are of practical importance because of their potential similarities with petrothermal reservoirs and exploitable natural hydrothermal systems. The petrophysical and hydraulic characterization of such structures is therefore of significant interest. Here, we report the results of corresponding investigations on a prominent shear zone of this type located in the crystalline Aar massif of the central Swiss Alps. A shallow borehole was drilled, which acutely intersects the core of the shear zone and is entirely situated in its surrounding damage zone. The focus of this study is a detailed characterization of this damage zone based on geophysical borehole measurements. For this purpose, a comprehensive suite of borehole logs, comprising passive and active nuclear, full-waveform sonic, resistivity, self-potential, optical televiewer, and borehole radar data, was collected. The migrated images of the borehole radar reflection data together with the optical televiewer data reveal a complicated network of intersecting fractures in the damage zone. Consequently, the associated petrophysical properties, notably the sonic velocities and porosities, are distinctly different from intact granitic formations. Cluster analyses of the borehole logs in combination with the structural interpretations of the optical televiewer data illustrate that the variations in the petrophysical properties are predominantly governed by the intense brittle deformation. The imaged fracture network and the high-porosity zones associated with brittle deformation represent the main flow pathways. This interpretation is consistent with the available geophysical measurements as well as the analyses of the retrieved core material. Furthermore, the interpretation of the self-potential and fluid resistivity log data suggests a compartmentalized hydraulic behavior, as evidenced by inflows of water into the borehole from different sources, which is likely to be governed by the steeply dipping structures.

Eva Caspari et al.
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Eva Caspari et al.
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Short summary
A shallow borehole was drilled to explore the petrophysical and hydraulic characteristics of a hydrothermally active fault in the crystalline Aar massif of the Alps. A key objective of studying surficial features of this kind is to establish analogies with natural and deep-seated engineered hydrothermal systems. A wide range of geophysical borehole logs was acquired, which revealed a complex fracture network in the damage zone of the fault and a related compartmentalized hydraulic behavior.
A shallow borehole was drilled to explore the petrophysical and hydraulic characteristics of a...