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.