Vertical Electrical Sounding for Aquifer Characterisation in the Chad Formation of Borno State

This study uses vertical electrical sounding to characterise aquifer layers within the Chad Formation in Borno State, North East Nigeria. The multi-layer Chad Formation represents one of Nigeria's largest freshwater aquifer systems, supporting domestic, agricultural, and pastoral water needs across the Lake Chad Basin. Progressive water table decline attributed to reduced recharge, increased abstraction, and climate variability has created an urgent need for improved aquifer characterisation to guide sustainable groundwater management. This study acquires VES data at 35 sounding stations in Kukawa, Monguno, and Nganzai local government areas using the Schlumberger array with maximum AB/2 spread of 500 metres. Apparent resistivity data are interpreted using partial curve-matching and computer-aided iterative inversion using IX1D software. Interpreted layer models are correlated with available borehole lithology logs from the Lake Chad Basin Authority database. Findings reveal a typical four to five layer earth model comprising surface sand and clay sequences overlying the productive Chad Formation sand aquifer at depths between 40 and 120 metres. Aquifer resistivities range from 28 to 110 ohm-metres, with higher values indicating coarser and more permeable sands. Hydraulic conductivity estimates derived from Dar-Zarrouk parameters suggest productive zones in the northern sector of the survey area. Isopach mapping of the aquifer unit shows greatest thickness in the Kukawa district. The study recommends priority borehole siting in areas identified as having both adequate aquifer thickness and high hydraulic conductivity.

Keywords: vertical electrical sounding, Chad Formation, aquifer characterisation, Borno State, Schlumberger array.