Geophysical Characterisation of Regolith and Saprolite Profiles for Hard Rock Aquifer Development in Ekiti State, South West Nigeria

Hard rock aquifer systems in Ekiti State's Precambrian basement complex support groundwater supply for over 60 percent of the state's rural population through boreholes that intersect regolith and saprolite horizons of variable productivity. Professional characterisation of the regolith-saprolite profile using geophysical methods is essential for optimising borehole yields and reducing failure rates. This study integrates electrical resistivity sounding, electromagnetic frequency-domain profiling, and borehole geophysical logging at 35 sites across Ekiti Central Senatorial District to characterise the depth, thickness, and hydrogeological properties of the regolith-saprolite system. Schlumberger VES measurements were acquired at all 35 sites to derive layer models, while EM34 profiles were used to map the lateral continuity of the weathered horizon between sounding stations. Downhole gamma-ray and resistivity logging was conducted in 10 boreholes to validate surface geophysical interpretations against direct subsurface measurements. The integrated dataset reveals a bimodal distribution of regolith thickness: a first mode at 10 to 18 metres associated with gentle slopes and a second mode at 25 to 40 metres in valley bottom positions where mechanical downslope transport of weathered material augments in situ chemical weathering. Saprolite transmissivity estimated from calibrated geoelectric parameters ranges from 8 to 65 square metres per day, with the highest values in valley-bottom regolith accumulation zones. The study provides a regolith characterisation protocol with direct application to borehole design optimisation in Ekiti State. Keywords: regolith, saprolite, hard rock aquifer, ERT, Ekiti State