Présentations Colloques

Oral Presentation
Session 8.07: Hydrogeophysics: innovative non-invasive technologies for groundwater resources exploitation and management
Belle Pierre
Interest and limits of electrical tomography for groundwater survey in metamorphic hard rock aquifer context
In hard rocks, and particularly in the metamorphic context, borewell siting for groundwater abstraction is complex. Consequently the failure rate is currently higher than in other hard rock contexts. In this study, we valorize the experience of more than 10 years of surveys aiming at prospecting for new natural mineral water resources in the Southern part of France, on the basis of a multidisciplinary approach (geology, hydrogeology, 39 km of ERT profiles, 70 drillings).**The calibration of pole-dipole electrical profiles on exploratory boreholes highlights several important results. First, electrical profiling is not accurate to identify the lithology. Second, the weathering profile (saprolite and the underlying fissured layer) is clearly identified by the electrical geophysics in most cases. In contrast to granites, and in accordance with lithological contrasts and folding, in such metamorphic rocks the depth of the interface between the weathering profile and the underlying fresh rocks is strongly varying spatially. The majority of the water inflows is located at the transition area between the upper conductive and the lower resistant bodies, attributed to the top of the fissured layer. Third, vertical conductive anomalies, intersecting the resistant substratum at depth (>50 m), are observed. These anomalies are generated by two main types of structures- (i) a saturated fractured medium (of variable permeability) (ii) a strongly weathered low permeability medium. Saturation by highly mineralized also generates conductive anomalies.**This case study defines more precisely the capacity of hydrogeophysical interpretation for surveys in the context of metamorphic rocks. The fissured layer is well imaged by geoelectrical methods. More than 90% of exploration wells show there an interesting productivity (Q>2 m3 h).**As regards the deep conductive anomalies, the success rate is lower (≈+70% of wells with a Q < 1 m3 h), due to the equivalent geophysical signatures of high and low permeability zones.