Présentations Colloques

    Session 8.11: Hydrogeodesy: new surface tools to characterize and monitor active and managed groundwater systems
    Hinderer (orateur)
    Basement aquifer tests monitored by gravimetry - a sensitivity study

    Basement aquifers are able to provide limited and localized water resources well-suited for a distributed water service in small cities and villages. Drilling for wells without precise knowledge of aquifer properties leads generally to unsuccessful flow rates. Various exploration geophysical methods have been proposed to image aquifer properties before drilling, electrical tomography or magnetic resonance sounding, for example. Combined piezometric and gravity monitoring were proved recently to allow a better definition of specific yield of sedimentary aquifers. Up to recent years, the limited precision of portable gravimeters was limiting the extension of this method. However the increasing precision and portability of gravimeters allow us to reconsider it for preliminary exploration of aquifer before drilling. We propose here a sensitivity study based on a 7 day long theoretical aquifer test experiment monitored by gravimetry and leveling. The aquifer includes a superficial saprolitic layer of known properties, and three different hypotheses were considered for the underlying medium- 1) fresh unpervious basement, 2) a high transmissivity fissured medium typically observed on granitic basement aquifers overlying fresh basement 3) the same fissured layer overlying a vertical tectonic fracture. Subsidence is observed during pumping due to compression of clays in the saprolites. Monitoring is achieved daily during pumping thanks to a network of 6 gravity and 12 leveling stations. For the simplified medium adopted here, the present precision of 2µ+gal for gravimetry and 2 mm for leveling allows us to retrieve the order of magnitude of both the Young modulus of clays and the transmissivity of the fissured medium. With the increased precision of 0.2 µ+gal which can be expected for portable iGrav superconducting gravimeters, permeability heterogeneities of the fissured medium could be accessed. It is therefore suggested that simultaneous gravity and leveling monitoring should be tested on some well-known basement aquifer experimental sites.< p>



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