Application of Self-Potential and VLF-EM Methods in Groundwater Prospecting in Onicha, Ebonyi State

Onicha Local Government Area in Ebonyi State faces persistent groundwater scarcity exacerbated by the hydrogeological complexity of the local basement geology. This study applies self-potential (SP) and very low frequency electromagnetic (VLF-EM) methods to prospect for groundwater and delineate permeable fracture zones in Onicha. SP measurements were collected at 2 m intervals along six traverses, and VLF-EM data were recorded using a WADI receiver configured to receive signals from the NAA transmitter. SP anomaly maps were constructed and qualitatively interpreted for fluid movement signatures. VLF-EM in-phase and quadrature data were processed using the Fraser filter and Karous-Hjelt filter to generate current density pseudosections highlighting conductive subsurface zones. Negative SP anomalies of up to negative 95 mV are interpreted as zones of upward groundwater seepage from fractures, while positive anomalies exceeding 35 mV reflect lateral groundwater movement. VLF-EM conductive anomalies identify three principal subsurface fracture zones trending northwest-southeast, which are interpreted as potential aquifer conduits. Intersection zones between different fracture orientations are prioritized as optimal borehole sites, with three such zones identified from the combined dataset. A follow-up VES survey at the recommended sites confirms productive aquifer conditions at depths of 40 m to 72 m. The integration of SP and VLF-EM methods provides a cost-effective reconnaissance approach for groundwater prospecting in basement complex terrains. This study contributes to improved water supply planning in Onicha and comparable areas of Ebonyi State.

Keywords: self-potential, VLF-EM, groundwater prospecting, fracture zone, Onicha