Hydrogeophysical characterization of bedrock aquifers: the case of the White Bandama upstream watershed (northern Ivory Coast) Caractérisation hydrogéophysique des aquifères de socle : exemple du bassin amont du Bandama Blanc (nord de la Côte d'Ivoire)

The use of hard rock aquifers as the only reliable source of drinking water for people, especially those from rural Africa, has grown considerably. But because of the intrinsic low hydraulic conductivity and low porosity of the basement, hard rock aquifers are complex in many levels, difficult for hydrogeological exploration, different from other types of aquifers, and require technical knowledge to detect and extract significant quantities of water. The lack of sufficient knowledge on this type of aquifers, the flow and the recharge of the groundwater base environment has greatly contributed to irrational use of this resource (Chilton and Foster, 1995). Significant progress has been made recently in the geological and hydrogeological characterization of these complex aquifers (Lachassagne et al., 2001, 2011; Wyns et al., 2004; Dewandel et al., 2006; Durand et al., 2006; and Vouillamoz et al., 2014). Independent or related factors such as lithology and tectonics play an important role in the mode of occurrence of underground water in the base medium, because together they control (1) the nature and thickness of the weathering, (2) the development of fractures, fault zones of vein structures and geological contacts, (3) the presence of highly porous medium (Holland and Witthüser, 2011). The search for these characteristics under the rock weathering appears therefore very important. The importance of these aquifers for water supply makes imperative the need to characterize, to identify these reservoirs hydrogeologically able of high productivity. Indeed, they can be targets, well fields for drinking water to supply the regions of high population densities with little or no alternative water sources. Geophysical methods are commonly used for the installation of boreholes in rock aquifers, and most of hydraulic campaigns are always based on common methods of electromagnetic (EM) and / or direct current resistivity (DC) (Yadav and Singh, 2007; Dutta et al., 2006). Among geophysical methods, electromagnetic methods are ideally suited for hydrogeological investigations (Mcneill, 1990). Excellent resolution of the conductive targets make them a very attractive geophysical tool, with large depth of investigation, rapid deployment at the sounding sites while yielding a complete set of parameters useful in determining the electrical resistivity distribution in the ground. The objective of this study is to develop a new methodology to describe the hard-rock aquifers geometry in sub-saharian context. Extensive electromagnetic soundings were combined with the local knowledge of borehole logs to characterize the aquifer in presence.