Deep electrical resistivity tomography for the prospection of low- to medium-enthalpy geothermal resources

Abstract

SUMMARY The growth of the geothermal industry sector requires innovative methods to reduce exploration costs whilst minimizing uncertainty during subsurface exploration. Until now geoelectrical prospection had to trade between logistically complex cabled technologies reaching a few hundreds meters deep versus shallow-reaching prospecting methods commonly used in hydro-geophysical studies. We present a recent technology for geoelectrical prospection, and show how geoelectrical methods may allow the investigation of medium-enthalpy geothermal resources until about 1 km depth. The use of the new acquisition system, which is made of a distributed set of independent electrical potential recorders, enabled us to tackle logistics and noise data issues typical of urbanized areas. We acquired a 4.5-km-long 2-D geoelectrical survey in an industrial area to investigate the subsurface structure of a sedimentary sequence that was the target of a ∼700 m geothermal exploration well (Geo-01, Satigny) in the Greater Geneva Basin, Western Switzerland. To show the reliability of this new method we compared the acquired resistivity data against reflection seismic and gravimetric data and well logs. The processed resistivity model is consistent with the interpretation of the active-seismic data and density variations computed from the inversion of the residual Bouguer anomaly. The combination of the resistivity and gravity models suggest the presence of a low resistivity and low density body crossing Mesozoic geological units up to Palaeogene–Neogene units that can be used for medium-enthalpy geothermal exploitation. Our work points out how new geoelectrical methods may be used to identify thermal groundwater at depth. This new cost-efficient technology may become an effective and reliable exploration method for the imaging of shallow geothermal resources.

3‐D Deep Electrical Resistivity Tomography of the Major Basin Related to the 2016 Mw6.5 Central Italy Earthquake Fault

“…Following 3‐D survey strategies, this system can acquire a large number of subsurface data in a relatively short time (Gance et al., 2018). Thanks to its flexibility that allows it to tackle challenging logistics, this cost‐effective technology has been successfully applied to map 3‐D aquifer geometry and shallow geothermal fields in an urbanized setting (Carrier et al., 2019). It has also been used to survey large‐scale hydrogeological and volcanic structures in areas characterized by slope instability (Lajaunie et al., 2019), and dense urbanization (Troiano et al., 2019).…”

3D Basin‐Scale Groundwater Flow Modeling as a Tool for Geothermal Exploration: Application to the Geneva Basin, Switzerland‐France

“…The thermal evolution observed in Model 0 corresponds to a re‐equilibrium between heat flux and geothermal gradient. Only the Satigny control point (blue square) shows about 20 °C more than what was measured at about 600 m depth (Carrier et al., 2019). The cross sections in Figure 6b suggest that the isotherms (initially following the SE‐dipping of the geological units) bulge in the center of the Geneva Basin after about 200 kyrs.…”

“…An example in the Geneva Basin is the recently drilled Geo‐01 Satigny well (Figure 10). The fluid in the permeable damaged zone is warmer than the surroundings, and artesian flow and temperature anomalies were recorded (Carrier et al., 2019). Below the Keuper, the flow velocity decreases (Figure 10f) and both the topography and fault influence are drastically reduced.…”

“…Bottom hole temperatures have been corrected in the framework of ongoing geothermal exploration (Chelle-Michou et al, 2017). New gravity and geoelectric data (Carrier et al, 2019(Carrier et al, , 2020Guglielmetti et al, 2020), as well as passive seismic methods (Antunes et al, 2020;Planès et al, 2020), provided further constrains for the modeling part. These wells and geophysical measurements allowed the definition of the local geothermal gradient after correction of temperatures measured at numerous wells drilled in the GGB (Chelle-Michou et al, 2017).…”

“…In this framework, the Canton of Geneva is exploring geothermal opportunities in the Greater Geneva Basin (GGB, Figure 1) (Faessler et al., 2015) thanks to the GEothermie2020 program (https://www.geothermie2020.ch). Preliminary results have been encouraging (Carrier et al., 2019) and numerical models have already been used to evaluate the feasibility of heat storage in the Molasse and Malm formations of the GGB (Collignon et al., 2020). However, basin‐scale fluid flow processes remain poorly documented in the region.…”