Présentations Colloques

Oral Presentation
Session 8.03: Geometrical structure and hydrogeological properties of Hard-Rock aquifers.
Belle Pierre
Granitic hard rock aquifers deep geometry from a 3D geological model based on an exceptionally high density geophysical survey
Survey of groundwater resources, especially in hard rock context, requires the use of geophysical methods. In this context, electrical resistivity tomography (ERT) is commonly used to image the structures associated to the weathering- the saprolite and the stratiform fissured layer, and deeper discontinuities. However, the geological and hydrogeological interpretation of the results is often complex, and boreholes siting has often a uncertainly success rate. We present the results of a high-resolution geophysical survey performed in France (50 km of ERT profiles, dipole-dipole and pole-dipole arrays, on 10 sq km area) and the results of 4 deep vertical and inclined boreholes.**The results show significant vertical resistivity contrasts (superficial subhorizontal layers with variable resistivity, and deep resistants related to the weathering profile) confirmed by the boreholes. The base of the conductor corresponds to the base of the connected fissured layer. Moreover, deep subvertical conductive structures are highlighted, and interpreted from boreholes as the deepening of the weathering along fault zones related to a regional graben. These low permeability (<10-6 m s) structures have an exceptional extension- depth> 200 m and width> 400 m, kilometric length.**On the basis of these data, we map the base of the fissured layer all over the studied area, the extension of deep weathered zones, and the one of some faults that offset the weathering profile. The resulting model provides a precise description of the weathering profile geometry. Especially, it highlights the absence of correlation between the orientation of the main tectonic structures and topography (valleys) but often a topographic control on the depth of the weathering profile. For groundwater surveys in such a context, we recommend an optimum profiles mesh of 300 x 400 m for a precise location of the main weathered structures edges, and the identification of artefacts induced by lateral 3D effects.**