Time-Domain Electromagnetic Survey for Subsurface Characterization and Borehole Siting in Ezza North, Ebonyi State

Access to potable water in Ezza North Local Government Area of Ebonyi State remains inadequate, largely due to poorly sited boreholes drilled without prior geophysical investigation. This study employs the time-domain electromagnetic (TDEM) method to characterize subsurface resistivity distribution and recommend optimal borehole locations in Ezza North. A total of thirty TDEM soundings were conducted using a coincident loop configuration with a 40 m by 40 m transmitter loop. Data were acquired with a Geonics EM47 system and processed using IX1D inversion software to generate one-dimensional resistivity-depth profiles. Geoelectric sections constructed from adjacent soundings reveal a laterally variable subsurface comprising clayey topsoil, weathered and fractured zones, and relatively fresh basement rocks. Resistivity values within the productive weathered-fractured basement range from 18 to 75 ohm-m, while values exceeding 400 ohm-m characterize dry competent basement rock. The depth to the resistive basement varies from 30 m in the eastern sector to more than 80 m in the central graben structure identified in the study area. Conductive anomalies at intermediate depths are interpreted as saturated fractured zones constituting productive aquifer targets. Six optimal borehole sites are recommended based on the depth extent and resistivity of productive fractured zones and the thickness of overlying protective layers. Validation against a previously drilled borehole confirms the accuracy of the electromagnetic interpretation. This study demonstrates the value of TDEM as a rapid and non-invasive method for groundwater prospecting in crystalline basement terrains.

Keywords: time-domain electromagnetic, TDEM, aquifer, borehole siting, Ezza North