Application of the electrical resistivity method in groundwater exploration in some communities in the Northern Region of Ghana

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The electrical resistivity method is widely used as an aid to siting boreholes for water supply in a number of terrains. In this report the electrical resistvity method has been used to determine hydrogeologically suitable places to site boreholes to supply drinking water to a number of rural communities in the Gushiegu-Karaga and Bole districts in the Northern Ghana. The hydrogeological conditions in the middle Voltain sedimentary basin is alternating grey, green and brown shale’s, siltstones, sandstones, conglomerates and limestone. Here rock decomposition and primary porosity are generally low and groundwater availability is controlled by the extent of fractures and similar weak zones in the rocks. Boreholes which have intercepted deeply weathered schist and clay-rich metasediments have yielded little water. The process of siting boreholes began with terrain evaluation which involved field reconnaissance visits to study the lithology, structure, topography and soils to identify target areas for geophysical survey. Horizontal resistivity profiling using the Schiumberger electrode array was conducted along traverses demarcated in the target zones in the communities. Two current electrode separations, namely 80 m and 38 m were used to probe two different depths respectively. The anomalous zones were identified as areas where the resistivity profile curves indicated resistivity values within the range of 20 Ωm to 150 Ωm. Vertical electrical sounding (VES) were then conducted within the identified anomalous zones. The VES data gave the resistivities of the layers within the overburden. The presence of an aquifer formation within the subsurface structure is inferred from the VES curves as an intermediate layer of lower resisitivity located at depths beneath 20 m from the surface. These aquifer signatures were depressions found in the sounding curves. Ten (10) sites were drilled after the study. One site produced a marginal yield of 8 litres/minute. The maximum yield was 270 litres/minute and the general success ratio was 90%. The geological logs of the drilled cores indicated that the factor of success in terms of borehole yield was the presence of extensive fractures in the regolith/saporolite at depths exceeding 20 m from the surface.
A thesis submitted to the Board of Postgraduate Studies, Kwame Nkrumah University of Science and Technology, Kumasi, in partial fulfilment of the requirement for the award of the Degree of Master of Science in Physics, 1999