Geo-hydraulic characterization of a coastal aquifer system in South-eastern Nigeria with the inverse slope method

Geophysical surveys were undertaken to determine the geo-hydraulic properties within a major coastal aquifer in Eket, South-eastern Nigeria. Information about geo-hydraulic properties of an aquifer are critical in the management and conservation of groundwater resources, hence it was necessary to characterize this major aquifer’s geo-hydraulic properties. Geo-electrical data was acquired with the Schlumberger configuration and interpreted using the inverse slope method. The interpretation of the geo-electrical data as undertaken by the inverse slope method correlated with lithological information from well logs obtained close to the study area and indicated the presence of three geological layers (motley topsoil, fine sand, and coarse sand). Resistivity values of the overburden were noticeably higher than those of underlying layers, and the resistivity of the subsurface layers ranged from 0.5- 434 Ωm. A performance evaluation of the aquifer was carried out via quantitative characterization of the geo-hydraulic properties which included bulk resistivity, water resistivity, porosity, formation factor, hydraulic conductivity, transmissivity, permeability and tortuosity. Contour plots were generated to show how these geo-hydraulic parameters varied across the study area and empirical formulations were developed to aid in the modeling of aquifer properties in locations with similar geology. This work shows that the inverse slope method is an efficient method for interpreting resistivity data acquired from heterogeneous environments.