Application of 2-D resistivity survey to groundwater aquifer delineation in a sedimentary terrain: A case study of south-western Nigeria

Abstract

This study demonstrates an application of two-dimensional resistivity surveys to groundwater exploration in a problematic sedimentary terrain where thick clay layers impedes groundwater aquifer recharge. The objectives of the study are to (i) select locations for citing new boreholes and (ii) investigate causes of the failure of the two boreholes previously drilled in the area. Six profiles of 830 m length each were surveyed to probe the subsurface lithologies and their groundwater potentials. Data acquisition system comprised a Super Sting Resistivity Meter, 84 metallic electrodes, and their accessories. Data acquired from the survey were forward modeled and tomographically inverted, using finite difference techniques. Results of the study revealed the different rock layers beneath the survey lines, their spatial distribution and their resistivities. Two new boreholes, BH3 and BH4, drilled based on the inverted 2-D resistivity images are very productive and have respective yields of 46 l/s and 48 l/s in the dry season. The choice of the locations for the new boreholes was informed by the presence of low resistivity structures interpreted as saturated (wet) sand, good vertical and lateral extents of the saturated sands, the depth of the aquifer in relation to the water table, and the absence of impermeable (sandy clay) cover that could retard groundwater recharge and discharge. The resistivity images of the 2-D survey also show that the failed boreholes – BH1 and BH2 were located on low resistivity structures interpreted as aquifers. But the aquifers have limited vertical and lateral extents, and are adjacent to thick impervious sandy-clay layer. Overall, the study demonstrates the suitability and the superiority of 2D resistivity survey to the traditional 1D – four electrodes survey.