Determination of Aquifer Depth and Geoelectric Layering Using Vertical Electrical Sounding (VES) In Afikpo Area, Southeastern Nigeria

Abstract

Determination of groundwater depth within the undulating ridge-and-valley topography of the Afikpo sub-basin, southeastern Nigeria, is critical to groundwater development. This study utilizes a high-density matrix of fifty (50) Schlumberger Vertical Electrical Soundings (VES) to delineate regional geoelectric horizons, structural framework variations, and determine aquifer depths. Total spread AB = 400 m (i.e., AB/2 = 200 m) was traversed to probe into the depth of linear structures using an Allied OhmMega resistivity meter (2115A model). The data were interpreted using computerized iterative inversion modeling (IPI2Win software), yielding low RMS fitting errors below 5%. Subsurface configurations across the 50 mapped communities reveal three-layer to six-layer geoelectric systems with dominant HK, KH, and QH curve configurations. The interpreted regional depth to the principal aquifer horizons ranges from 3.6 m to 113.0 m, with an overall average regional aquifer depth of 33.5 m and a mean localized aquifer layer thickness of 24.7 m. Delineated aquifer horizons showcase a pronounced structural partitioning controlled by local formations. In the southern and southwestern sectors underlain by tight argillaceous units (Oso, Ekoli Edda, Nguzu), aquifer zones are localized within fractured shales and mudstones at shallow depths ranging between 10.67 m and 19.32 m, showcasing low resistivities (ρ < 50 Ω·m). Conversely, central and eastern zones (Mgbom, Amachara, Ugwuegu, Ozizza) tap into the highly prolific Afikpo Sandstone member, where true sub-confined regional water tables descend deeper to horizons between 16.00 m and 45.25 m (locally exceeding 74.70 m) with higher matrix resistivities. The results map out groundwater targets, providing engineers with precise target thresholds to minimize dry-well risks and improve borehole yields across the Afikpo sub-basin.