Magnetic resonance sounding (MRS) and resistivity characterisation of a mountain hard rock aquifer: the Ringelbach Catchment, Vosges Massif, France
Résumé
The Ringelbach catchment, which has been studied since 1975, is highly representative of the crystalline Vosges massif, where the water supply is mainly derived from small aquifers in heterogeneous superficial formations and in the weathered and fissured bedrock. Vertical electrical soundings (VES), resistivity imaging profiles and magnetic resonance soundings (MRS) were applied inorder to investigate the 3D structure and estimate the hydrodynamic characteristics of the subsurface, which is composed of granites partly covered by Triassic sandstone. Despite poor MRS signal-to-noise conditions, the general structure of the catchment is defined using geophysics (electrical methods and MRS combined) and geology. MRS also makes it possible to map the water volume per unit surface within the different geological formations and structural blocks. The corresponding maps are proposed as a basis for evaluating the water storage within the catchment and improving the understanding of its hydrological functioning. The initially proposed geological model is refined and a four-block structure intersected by faults is defined. The schema of a gradual stratified weathering of the granite from surface to depth is not clearly reflected in the resistivity images, partly because of insufficient investigation depth. It is completed with highly weathered, deeply rooted fractured zones associated with faults in order to explain deeply sited low-resistivity zones.The MRS water content in the weathered granite of Ringelbach appears abnormally low in comparison with the higher values observed in similar resistivity settings in other granite regions. Because of the rough catchment topography, these low values may be attributed to unsaturated weathered formations on hillsides or saturated, fine material (colluvial deposits, highly weathered granite) in the valley bottom. The control-wells and laboratory measurements on core samples proposed in the next step of this research will attempt to calibrate the MRS results and validate the proposed structural and weathering schema. Re-interpreted and validated geophysical results will then be used as effective input for improved hydrological modelling.