Integrated 3D geological modelling of the northern Upper Rhine Graben by joint inversion of gravimetry and magnetic data

Abstract

The crystalline basement of the Upper Rhine Graben is a major target for deep geothermal exploration due to the generally high reservoir temperatures and the increased radiogenic heat production. The geothermal potential is strongly dependent on the lithology because mainly the thermal but also the hydraulic properties are affected by the rock type. For this reason, the so far most detailed 3D model of the basement in the northern Upper Rhine Graben was developed based on existing structural models, in particular the Hesse 3D 2.0 and GeORG models. Since only a few boreholes fully penetrate the thick sediment cover, additional magnetic and gravity data provided valuable information on the geometry of the deep horizons. To interpret the Bouguer anomalies reasonably with respect to the crystalline basement, the regional gravity field and sedimentary effect were subtracted from the observed data. In comparison to the commonly applied deterministic modelling approaches, a stochastic joint inversion of the gravity and magnetic anomalies was performed that utilizes the principles of a Monte-Carlo-Markov-Chain simulation. For an appropriate attribution of the model units, existing petrophysical databases of the region have been used and in addition, the magnetic susceptibility of more than 430 rock samples was measured. High-resolution voxel models of the density and susceptibility distribution were generated as a result of the inversion, which allow conclusions about the crustal composition under the sedimentary formations. An interpretative map of the basement geology, derived from the inversion results, is presented.