The Proton Magnetic Resonance (PMR) or Nuclear Magnetic Resonance (NMR) method, coupled with geometrical aquifer modelling, has been used to create a map of groundwater reserves over a 270 km2 study area in a weathered basement setting. Most of the reserves are contained in a stratiform multi-layer aquifer whose geometry is influenced by the weathering front. The depths to the interfaces determined by PMR are considered and validated by comparison with the geometrical approach. Water contents and decay times of the PMR signal for each weathered layer are compared with the hydrogeological model. The results of the study show a decrease in water content from the top downwards for the three main aquifer layers (respectively : unconsolidated alterite, and an upper and a lower fissured zone). The groundwater reserves (80 % in the fissured zone and 20 % in unconsolidated alterite) represent approximately three years of average infiltration.

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This research aims to quantify the geophysical signature of the laminated layer, one of the main layers constituting the weathering profile of hard rocks. This laminated layer acts as a marker for locating the underlying groundwater productive stratiform fractured layer (SFL). The study is based at two sites on the interpretation of 50 km of electrical resistivity tomography (ERT) profiles, compared with outcrops and boreholes by geophysical modelling. For the first time, the geophysical signature of the laminated layer, located at the base of the saprolite, is characterized within granite formations. Where the stratiform weathered layer is detected by pole-dipole ERT profiles, the laminated layer is identified as a resistant layer on 90% of the SFL length using an appropriate inversion method. In addition, this layer is also revealed for the first time in certain types of metamorphic formations; here it is revealed in micaschists (62% of the SFL length). The location of the laminated layer in the weathering profile is important (1) for water well siting by determining if an underlying SFL exists in the weathering profile and (2) for assessing the residual thickness of the saprolite, and then evaluating water storage and the protection of the SFL aquifer.

Integrated geophysical approach in assessing karst presence and sinkhole susceptibility along flood-protection dykes of the Loire River, Orléans, France.

International audienceThe 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