Evaluation of Geophysical Methods for Mapping Aquifer Parameters in Crystalline Basement Terrain of Kwara State

Groundwater exploration in the crystalline basement terrain of Kwara State in North Central Nigeria is complicated by the highly heterogeneous nature of weathered and fractured aquifers, which limits the reliability of conventional exploratory borehole drilling without geophysical guidance. This study evaluates the combined application of electrical resistivity tomography and audio-frequency magnetotelluric sounding for mapping aquifer parameters in basement terrain across Offa, Ilorin South, and Moro local government areas. Schlumberger vertical electrical sounding is conducted at 45 stations, and 2D ERT profiles are acquired using a 48-electrode Wenner-Schlumberger configuration. Apparent resistivity data are inverted using RES2DINV software. Interpreted parameters including weathered layer depth, fractured zone thickness, and resistivity are correlated with driller's logs and pumping test data from 28 existing boreholes. Results demonstrate that a weathered layer resistivity threshold of 100 ohm-metres and thickness exceeding 15 metres predicts borehole yields above 1 litre per second with 80 percent accuracy. ERT profiles effectively delineate bedrock topography and palaeochannel features that represent the highest-yield drilling targets. The study provides a geophysical drilling guidance protocol for the Kwara State Rural Water Supply Agency to improve borehole success rates. Keywords: electrical resistivity tomography, aquifer mapping, basement terrain, Kwara State, groundwater