The Chaîne des Puys (French Massif Central) is a North-South alignment of approximatively 80 volcanoes, including domes, cones and maars. The edifices and their products lay on a Variscan granite-gneissic weathered basement. Volcanic products are globally highly permeable, while the weathered basement is comparatively impermeable, so water flows in the volcanic cover above the interface. The volcanic aquifers supply drinking water for about 350,000 people. For a better resource management, especially in a context of climate changes, the geometry definition of the volcanic aquifers (i.e., volcanic-basement interface) and watersheds is a key step to define water resources availability.In 2020, the BRGM (French Geological Survey) conducted an airborne geophysical survey including electromagnetism and magnetometry to investigate electric and magnetic properties in the firsts 350 meters below the surface and especially to delineate the interface between the magnetic resistive volcanic products and the amagnetic conductive weathered basement. Survey covers about ~275km² with EM sounding every 40m and flight lines 4000m apart.After a smooth spatially constrained inversion using Aarhus Workbench, and the computation of a layered earth model that produced equivalent data, we manually identify the interface from both results. From the resistivity cross-section in volcanic domain and the DEM in granitoidic domain, we interpolate a new DEM of the top of the basement that allows us to derive hydrogeological watersheds and observe paleo thalweg. Moreover, using a layer-earth model let us compute sensitivity to the depth of the limit and resistivities on its both sides. These results revise the commonly accepted watershed boundaries, which will influence future qualitative and quantitative management strategies for water resources. Airborne geophysics has proven to be suitable to provide needful structural informations for hydrogeological analysis at large and locale scale.