Evaluation of Groundwater Potentials for Borehole Drilling by Integrated Geophysical Mapping of Auchi-South Western Nigeria Using Very Low Frequency Electromagnetic Profiling (VLF-EM) and Vertical Electrical Sounding (VES)

Abstract

An integrated geophysical survey involving Very Low Frequency Electromagnetic (VLF-EM) and electrical resistivity sounding was conducted at Auchi- Southwestern Nigeria, in order to delineate the fissured zones and associated groundwater- bearing media for borehole drilling in the study area. The area is underlain by the Precambrian Basement Complex rocks and in some places sedimentary rocks. The VLF-EM traverses were established along two traverses using ABEM-WADI equipment, with a station interval of 10 m and with lengths ranging from 200 to 240 m. Linear fissures presumed to be geologic fissures inferred from the filtered real and filtered imaginary assisted in selecting seven VES points that were further probed using ABEM SAS 1000 Resistivity meter. The spreading were conducted using the convectional Schlumberger electrode configuration with half-current electrode separation (AB/2) that ranged from 1 to 225 m,was used for the sounding. The VES data were presented as depth sounding curves and were suitably iterated using Schlumberger O’Neil software. The VLF filtered real profile showed a high peak trend signifying a fracture signature. The delineated weathered and basement column constituted the aquifer units for borehole drilling. Six lithologic formations were delineated which included the top soil, clay/shale, fine grained sand, coarse grained sand, fractured bedrock and fresh bedrock. Four Model field curves were delineated which included HAKQ, HKQQ, KHAA and KHKH. There are two main targets for groundwater exploration in the study area. These included the weathered zone (fine and medium grained sand) and the fractured zone. The thickness of these zones varied between 15.2 to 50.1 m. Based on the geophysical parameters such as thin overburden thickness, clayey weathered layer and low fractured basement characterized by the study area, it is therefore inferred that the groundwater potential of the area is low. Nevertheless, the study has justified the use of integrated geophysical mapping as a better tool in evaluating the groundwater potential in crystalline terrains.