Assessment of Buried Valley Aquifers and Water Supply Potential Using 2D ERT and TDEM in Ogbaru, Anambra State

The lowlands of Ogbaru Local Government Area in Anambra State, located near the Niger-Anambra River confluence, are underlain by alluvial sediments that may host significant buried valley aquifer systems requiring systematic geophysical characterization. This study assesses buried valley aquifer systems and water supply potential in Ogbaru using two-dimensional electrical resistivity tomography and transient electromagnetic methods. Ten ERT profiles were acquired using the Wenner-Schlumberger configuration with 72 electrodes and 5 m spacing, providing depth penetration of approximately 45 m. TDEM soundings were conducted at twenty-five stations on a 1 km grid using a coincident loop configuration to achieve greater depth penetration of up to 150 m. Joint interpretation of both datasets leverages the complementary depth sensitivity ranges of the two methods. ERT tomograms reveal a complex heterogeneous subsurface with alternating high-resistivity (greater than 80 ohm-m) coarse gravel and sand bodies and low-resistivity (less than 15 ohm-m) clay-silt units. TDEM one-dimensional models identify a laterally continuous high-resistivity horizon at depths of 50 to 100 m, interpreted as a regional coarse sand aquifer confined by the overlying clay sequence. The buried valley fill is estimated at 80 to 130 m in thickness along the main palaeochannel axis. Estimated aquifer transmissivity values derived from resistivity-hydraulic conductivity transforms indicate highly productive zones suitable for large-capacity boreholes. Four priority development zones are delineated with recommended drilling depths. These results provide a comprehensive hydrogeophysical framework for groundwater supply planning in Ogbaru.

Keywords: ERT, TDEM, buried valley aquifer, water supply, Ogbaru