Présentations Colloques

    Oral Presentation
    Session 2.02: Groundwater for sustainable water supplies in developing countries
    Allé Iboukoun Christian
    Improvement perspectives for borehole implementation in tropical basement rock aquifers using electrical resistivity- the example of Benin, West Africa.
    40% of the wells drilled in hard rocks of Benin are negatives (yield less than 700 l per hour) although 1D geophysical techniques (Electrical Profiling – EP and Electrical sounding – ES) are routinely used for siting boreholes. Targeting a possible improvement of borehole implementation in a context of deeply weathered hard rock aquifers, our study is conducted to estimate (1) the interest and the limitations of EP and ES techniques, and (2) the advantages of the 2D resistivity techniques (Electrical resistivity tomography – ERT).**First, we define the range of resistivity of the three main geological compartments (i.e. saprolite, fissured zone and fresh bedrock) based on resistivity logging carried out in six sites located in different hard rock groups of Benin. Second, we set up numerical modelling of typical tropical weathered hard rock aquifers and we compare the model responses of EP, ES and ERT. **We found that (1) resistivity can discriminate the different hydrogeological compartments in about 80% of the cases, (2) several geophysical implementation points selected from typical anomaly in EP are clearly uninteresting because they correspond to shallow clayey structures, (3) ES is subject to strong equivalences and suppression phenomena which makes the interpretation results highly uncertain, and (4) interesting targets (large deepening resistive saprolite zones) are clearly identified by ERT interpretation (with the notable exception of deep isolated fracture systems) and uninteresting structures are clearly evidenced and thus can be discarded for drilling. Our numerical modelling approach is then confirmed on several field test sites in Benin, all displaying interesting and uninteresting implementation points.**At last, we conclude that the use of ERT should be highly encouraged by hydrogeologists in Africa, while current use of EP and ES should be abandoned to avoid unappropriated borehole implementation failure and increase success rate.


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