Gravity disturbance maps and updated gravity anomaly maps in French-Belgian Hainaut from recent gravity acquisitions and legacy data

Abstract. In Wallonia, the centre of Hainaut (SW Belgium) is considered the main area where deep geothermal resources are proven. In this region bordering France, the productive geothermal resource is located within karstified levels resulting from the dissolution of interbedded massive anhydrites in the Carboniferous limestones. To improve current knowledge of deep geological structures and to better delineate the geothermal targets, new detailed regional gravity anomaly and disturbance maps have been produced. These include maps of the observed gravity, free-air anomaly, refined Bouguer anomaly, gravity disturbance, topography-free gravity disturbance.

This has involved the compilation and harmonisation of cross-border legacy and recent gravity data, combined with an homogeneous reprocessing and an interpolation using the equivalent source method. The legacy data come from the Belgian and French databases published by the ROB (Belgium) and BRGM (France) respectively, to which unpublished data from the Battaille 1967 campaign have been added after their retro-processing from Bouguer anomaly data. The new gravity data come from the recent MoreGeo 2019-2022 gravity acquisition campaign. This campaign covers an area of 820 km² with 3,784 gravity stations distributed along densely sampled lines and displays a RMSE range of 41.9 to 67.5 µgal after absolute gravity referencing. The gravimetric processing works presented in this paper rely on open-source Python libraries. These include an advanced total correction budget evaluation and topographic correction up to 167 km² in extent. The resulting maps are consistent with previous ones, in particular the strong correlation between the anomaly and the thickness of the Meso-Cenozoic deposits. It also shows new anomalies that may be due to deep-seated structures. It will be used as a basis for modelling the geothermal reservoir, provided that the effect of the Meso-Cenozoic deposits is accounted for.