In drylands of Africa, a refined knowledge of aquifers characteristics is increasingly needed for a sustainable and balanced use of fresh groundwater resources. In these regions, large sedimentary basins hold most of the resources, but the aquifer limits, embedded within their geological framework, are often poorly known. The aquifer characteristics, spatial limits, and relationships of the upper plateaus of the Iullemmeden Basin (Maggia Valley, similar to 3000 km(2), south-central Niger) were determined by a combined approach of wells and borehole data analysis supplemented by a non-invasive geophysical survey comprising similar to 80 Magnetic Resonance and Time-Domain Electromagnetic soundings. Results show an upstream-downstream pattern where the Quaternary alluvium aquifer is hydraulically fully connected with i) the Upper Cretaceous sandy clayey aquifer upstream, ii) the Paleocene limestone aquifer at mid-slope, and iii) the Oligocene sandstone aquifer downstream. The eastern limits of the Oligocene sandstone and Paleocene limestone aquifers extend up to similar to 20 km beyond that assumed in previous studies. The unconfined aquifer exhibits effective porosity values in the range from 4 to 37% and from similar to 10(-4) m(2)/s to similar to 2 x 10(-3) m(2)/s in transmissivity from upstream to downstream. The non-invasive nature of the survey makes it cost-effective for improving the scope of groundwater-based development policies at fine scale at the edges of large, multilayered sedimentary basins where aquifers limits are often complex to delineate.