Mapping and Delineation of Hard Rock Aquifers in Parts of the South-western Nigeria Basement Complex Using Integrated Geophysical Techniques

Abstract: An integrated geophysical survey involving three different techniques; namely, VLF-EM, Seismic Refraction and Vertical Electrical Sounding (VES) has been carried out in parts of Eruwa, Ibarapa East LGA, Oyo State with a view of determining the structural characteristics of the subsurface geology and identifying suitable sites where viable borehole could be sited to reduce the problem associated with acute shortage of groundwater in the area. The area lies within the crystalline basement rock terrain of South-Western Nigeria and bounded by Latitude 7° 32' 59"N and Longitude 3° 25' 59"E. The VES results reveal that the area is made up of four major lithologic layers with the topmost layer being majorly laterites, the second layer being sandy clay, the third is weathered basement, while the fourth layer is fractured basement. The aquifer in the study area is mostly confined with curve responses of HA- and HKH-types. The 2-D pseudosection also showed that the fresh basement is very extensive laterally with undulating topography. The result of VLF-EM revealed high-amplitude anomalies. These anomalies are indicative of high conductive zones within the basement bedrock of the study area. The results of the seismic inversion reveals three lithologic layers; namely, the topmost layer / topsoil which is sandy (moderately loose) with an average velocity of 500 m/s, the middle layer can be sandy clay/laterite/caked clay with an average velocity of 1,149.33 m/s and the third layer can be said to be the beginning of the weathered/fresh basement with a velocity of 2,719 m/s. In conclusion, the three geophysical techniques have shown that the area is a hard-rock terrain with complicated geology with aquifers confined and the water bearing formation with low total longitudinal conductance. Keywords: Basement terrain, Granitic masses, Aquifer, Topography, Curve types.