Using highly accurate land gravity and 3D geologic modeling to discriminate potential geothermal areas: Application to the Upper Rhine Graben, France

Abstract

New land gravity data results acquired in northern Alsace were presented. Compared to the available old Bouguer anomaly, we recovered an accurate Bouguer anomaly field showing data uncertainties [Formula: see text]. A qualitative data analysis using pseudotomographies reveals several negative anomalies suggesting a decrease of the bulk density at the depth of geothermal interest. We have performed a quantitative study on the basis of the existing 3D geologic model derived from a reinterpretation of the vintage seismics. The theoretical gravity response indicates a great mismatch with the observed Bouguer anomaly. The stripping approach was applied, and the stripped Bouguer anomaly indicates that the density values of the Jurassic, but especially for the Triassic, the Buntsandstein, and the upper part of the basement, were overestimated even using the density values measured in the deep geothermal borehole. This suggests that the borehole density values do not reflect the density variations occurring at larger scale. To reduce the Bouguer anomaly during stripping, a negative density contrast should be affected to the Buntsandstein layer overlaying the basement, suggesting that the part located between the Buntsandstein and the upper part of the basement presents a low-density value compared to the reference density, which is not necessarily expected and is not observed in the densities measured in the borehole. Interestingly, a correlation is found between the gravity analyses and the thermal gradient boreholes in the northern part of the study area. For two boreholes, the gravity interpretation suggests a huge density decrease in the Buntsandstein, which may arise from a combination of high-density fracturing and the important quantity of geothermal fluid significantly affecting the bulk density. Analysis of the thermal borehole data suggests that these two boreholes indicate higher geothermal potential compared with the other boreholes.

Simultaneous Modelling of Gravity and Magnetic Data in a Measured Heat Flux Area to Characterize Geothermal Heat Sources: A Case for Eburru Geothermal Complex, Kenya

“…Forward modelling of gravity and magnetic data was done simultaneously using Gm-sys in oasis montaj geosoft software. It entailed an iteration process where the anomaly generated by the constructed computer model was compared with the observed residual anomaly (Abdelfettah et al, 2020). The starting model was postulated from Euler deconvolution depths and models constrained based on the structural geology and stratigraphy data from existing wells within the study area.…”

“…Forward modelling of gravity and magnetic data entails determination of size, shape and physical parameters of the field anomaly source from the potential field measurements (Telford, Geldart, & Sheriff, 1990). It entails an iteration process where the anomaly generated by the constructed computer model is compared with the measured residual anomaly (Abdelfettah et al, 2020). It is based on a starting model that is postulated from the geology of the study area (Aziz, Miller, Giraldo, & Carigali, 2019).…”

Assessment of deep geothermal research and development in the Upper Rhine Graben

“…The potential field data are often applied to map crustal-scale structures. However, in the case of high-resolution surveys, also near-surface features, for example fault networks (e.g., Deckert et al 2017;Abdelfettah et al 2020) or even petrophysical properties can be explored (Frey et al 2021b). By applying frequency filters to the observed potential fields, information about specific depth ranges can be obtained (pseudo-tomography, Baillieux et al (2014)).…”

Short‐wavelength Bouguer gravity anomaly and active fault distribution in the northeastern Japan arc

“…In particular, this can be seen at active faults around the Ou Backbone Range. The negative anomalies and grooves associated with active faults potentially arise from the decrease in crustal density due to volumetric strain and cracks by faulting (Abdelfettah et al, 2014(Abdelfettah et al, , 2020Hagiwara, 1991;Yamasaki & Nagahama, 1999). However, the fracture porosity (crack) effects common in fault damage zones are likely not visible in the SWBA map with a 1-km grid data unless the width of fault damage zones is over 1 km, even if the porosity is large enough to induce the appropriate change of mass density there (e.g.…”