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Characterization of geothermally relevant structures at the top of crystalline basement in Switzerland by filters and gravity forward modelling

Abdelfettah, Yassine ; Schill, Eva ; Kuhn, Pascal

In: Geophysical Journal International, 2014, vol. 199, no. 1, p. 226-241

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    Titre
    • Characterization of geothermally relevant structures at the top of crystalline basement in Switzerland by filters and gravity forward modelling
    Auteur
    • Abdelfettah, Yassine. Centre of Hydrogeology and Geothermics - CHYN, University of Neuchâtel, Neuchâtel CH-2000, Switzerland. E-mail: yassine.abdelfettah@unine.ch
    • Schill, Eva. Institute of Nuclear Waste Disposal, Karlsruhe Institute of Technology KIT, D-76021 Karlsruhe, Germany
    • Kuhn, Pascal. Eidgenössisches Departement für Verteidigung, Bevölkerungsschutz und Sport VBS armasuisse Bundesamt für Landestopografie swisstopo Landesgeologie Seftigenstrasse 264 CH-3084 Wabern
    Type de document
    • Postprint
    Langue
    • Inglese
    Publié dans
    • Geophysical Journal International, 2014, vol. 199, no. 1, p. 226-241. Oxford University Press
    Autre version électronique
    Identifiant OAI-PMH
    • oai:doc.rero.ch:293745
    Summary
    Some of the major geothermal anomalies in central Europe are linked to tectonic structures within the top of crystalline basement, which modify strongly the top of this basement. Their assessment is a major challenge in exploration geophysics. Gravity has been proven to be suitable for the detection of mainly large scale lithological and structural inhomogeneities. Indeed, it is well known and proven by different wells that, for example, in northern Switzerland extended negative anomalies are linked to such structures. Due to depth limitation of wells, there vertical extension is often unknown. In this study, we have investigated the potential of gravity for the geometrical characterization of such basement structures. Our approach consists in the combination of the series of Butterworth filters, geological modelling and best-fitting between observed and computed residual anomalies. In this respect, filters of variable wavelength are applied to observed and computed gravity data. The geological model is discretized into a finite element mesh. Near-surface anomalies and the effect of the sedimentary cover were eliminated using cut-off wavelength of 10km and geological and seismic information. We analysed the potential of preferential Butterworth filtering in a sensitivity study and applied the above mentioned approach to part of the Swiss molasses basin. Sensitivity analyses reveal that such sets of residual anomalies represents a pseudo-tomography revealing the distribution of different structures with depth. This finding allows for interpreting negative anomalies in terms of 3-D volumes. Best-fitting then permits determination of the most likely 3-D geometries of such basement structures. Our model fits both, geological observations and gravity: among 10 deep boreholes in the studied area, six reach the respective units and confirm our distribution of the negative (and positive) anomalies