Groundwater exploration in the basement of Lake-Iro District-Chad, using electrical resistivity imaging approach

Abstract

The complex geological setting of groundwater systems in the Lake-Iro District (LID) gives cause for the adoption and application of a systematic methodological approach in groundwater exploration utilizing effective geophysical techniques. Among these geophysical methods, the electrical resistivity imaging (ERI) method has been found to be the most suitable for this kind of environmental study, due to the conductive nature of most waste liquids. The measurements were performed with the Syscal Junior R1-Switch-48 multi-electrode systems (Iris Instruments) of which pole-dipole and pole-pole configurations were used in 25 villages for a total of 30 productive boreholes. Data were processed using a 2D inversion program (Res2Dinv) and by finite element method (FEM) modeling. Interpretation of the processed electrical images has revealed the utility of this technique in the selection of boreholes sites in difficult terrains, such as the case of LID. The ERI method has produced significant pseudo-sections with high quality in terms of structural resolution leading to significantly defined layering of different lithological units, depth to bedrock and weathering profiles and identification of structures such as fractures at the subsurface. The aquifer, whose fractures control the flow, has been found to be heterogeneous, anisotropic and discontinuous. The relationships between the weathered thicknesses/operating flow rate and lithology when compared with hydrodynamic parameters has made it possible to highlight the groundwater potentiality in the basement of LID. In conclusion, the results of borehole drillings have confirmed a success rate of 65.22% and the groundwater is generally exploitable in the granitic formations with reservoirs having dissociated properties. Key words: Lake-Iro District, electrical resistivity imaging, basement, groundwater exploration.