Résumé
Initially as part of the EC-funded ALIANCE project (FP5), a new kind of near-field, high frequency downhole hydrogeophysical observatory was designed and set-up at two different field sites in terms of geological context, hydrological regime and local human impact. The new downhole method was conceived for long-term in situ monitoring and prevention of brine intrusion in coastal aquifers. The principle of the observatory is based on the high frequency (such as daily) probing of the formation electrical resistivity around a borehole, over periods of several years. For so-called "real-time" salt intrusion management, this device is aimed at producing accurate near-field boundary conditions to reduce uncertainties in models, and thereby contribute to the decision-making process for endangered aquifer management. The first prototype was deployed for instrumental purpose at Maguelone (Languedoc, France) in 2004, within unconsolidated sand-shale sequences of the Mediterranean shoreline. It was tested there to 40 m depth in a dipole-dipole mode, using an instrumented PVC liner as downhole support, then calibrated against induction resistivity logs. It has now kept functioning in this hostile environment (including both seawater and H2S gas in the pores) for more than 5 years. An improved prototype was later installed through the reefal platform of SE Mallorca (Baleares, Spain), where a deeply penetrating seawater intrusion enhanced by overdrafting. It is located to the SW of Campos, 6 km inland from the coast. In the consolidated Miocene carbonates, a flute was deployed down to 90 m and across the 20 m thick transition from fresh water to seawater.