Geophysical appraisal of groundwater potential in the crystalline rock of Kishi area, Southwestern Nigeria

Abstract

The need for alternative water supply is inevitable in Kishi area, Southwestern Nigeria due to inadequacies of surface water supply and common occurrences of low yield and abortive boreholes coupled with increase in population and developments in the area. Thus, electrical resistivity investigation was deployed to map fracture pattern and orientation, and obtain the geo-electric parameters of the crystalline rock which were used to evaluate the groundwater potential of the area. The result obtained was compared with the borehole yield as well as the geology that underlain the study area. A total of seven Radial Vertical Electrical Soundings (RVES) and ninety-three Vertical Electrical Soundings (VES) were acquired within Kishi area using Schlumberger array with half-current electrode spacing (AB/2) ranging from 1.0 m to 133 m. The RVES was measured as a function of bearing at 0° 45° 90° and 135° about a fixed central point. The acquired data were interpreted and the layer model obtained from the sounding curves revealed three to four geo-electric layers comprising topsoil, lateritic clay (compacted), weathered basement, fractured formation and basement. Thematic maps for overburden thickness, basement relief, bedrock resistivity, aquifer resistivity, aquifer thickness, and reflection coefficient were generated from the geoelectric parameters. Overburden thickness above 20 m were considered thick, basement relief of less than 355 m above sea level were considered as basement depressions, bedrock resistivity of less than 800 Ωm were interpreted as fractured basement, aquifer resistivity of less than 100 Ωm were considered saturated formation, aquifer thickness of above 15 m were considered thick and the reflection coefficient of less than 0.75 were considered as fractured basement. These maps were integrated to generate groundwater potential map of Kishi area and was used to delineate area of good, medium and poor groundwater yielding zone. The groundwater potential map generated for the study area correlates with basement fracturing with area of intense fracturing corresponding to area of high groundwater yield. The correlation observed between the groundwater potential map and the geological map show that area with high groundwater yield and intense basement fracturing fall on quartzite/quartz schist and granite gneiss.