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Évaluation de l’implantation géophysique des forages d’eau en zone de socle en milieu tropical (Bénin, Afrique de l’Ouest) : apport de la tomographie de résistivité électrique pour la caractérisation de la cible hydrogéologique

Abstract : In the basement rocks of West Africa and particularly in Benin, which consists of 80% basement rock, despite prior borehole siting studies, 40% of boreholes are considered "negative" (<700 l / h) thus constituting a significant loss in effort. This study aims to improve the geophysical aspects of borehole siting in order to improve the borehole success rate. An evaluation of the so-called "Direct Current" geophysical method (mainly used to date by practitioners) is carried out, after having determined the hydrogeological target to be favored for the borehole sitting. For this evaluation, at 6 experimental sites we evaluated the sensitivity of the parameters of the electrical method (resistivity and chargeability) to (1) the hydrogeological target (by comparing the parameter values measured in boreholes with borehole logs), (2) the hydrogeological parameters (specific yield and hydraulic conductivity obtained by pumping tests), and (3) the clay content (via X-ray diffractometry and methylene blue test). Subsequently, we assessed the capacity of different geophysical measurement techniques (Electrical Profiles - EP, Electrical Soundings - ES, and Electrical Resistivity Tomography - ERT) to characterize the target from the surface by means of (1) numerical modeling and (2) field measurements at 7 sites. The results reveal that the hydrogeological target to be favored in a basement context in tropical environments (where saprolite is generally considerable) is the zone where the thickness of the “saprolite” and the first meters of the “Stratiform Fractured Layer” (SFL) are the most important. Saprolite and SFL must be characterized by resistivities respectively between 150 and 400 ohm.m and between 800 and 2,000 ohm.m. Resistivity is more discriminating than chargeability. These two geophysical parameters increase with specific yield and with hydraulic conductivity, and decrease significantly with swelling clay content (smectites). This work demonstrates that the currently applied traditional geophysical techniques (EP and ES) can largely deceive the practitioners: the EP produces important anomalies in clayey zones and do not characterize the hydrogeological target, while the ES underestimates almost systematically the target thickness (saprolite + SFL). However, the most elaborate technique, the ERT, characterizes well the hydrogeological target by giving reliable information on its geometry. ERT allows the assessment of the hydrogeological quality of the target because it adequately recognizes the clay zones which are to be avoided while drilling. An analysis of the current economic model of geophysical prospecting in Benin shows that the extra cost due to the use of ERT as a matter of routine should be largely compensated by the reduction of the failure rate by at least 5%, a figure which will most probably be exceeded thanks to the gain in very significant information that this technique provides regarding the knowledge of the local hydrogeological conditions around a future borehole. Consequently, we recommend the abandonment of the techniques of electrical profiles and soundings traditionally used for the benefit of the Electricity Resistivity Tomography.
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Iboukoun Christian Alle. Évaluation de l’implantation géophysique des forages d’eau en zone de socle en milieu tropical (Bénin, Afrique de l’Ouest) : apport de la tomographie de résistivité électrique pour la caractérisation de la cible hydrogéologique. Géophysique [physics.geo-ph]. Université d’Abomey-Calavi, Cotonou (Bénin), 2019. Français. ⟨tel-02544949v4⟩

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