Electrical resistivity survey for groundwater investigations and shallow subsurface evaluation of the basaltic-greenstone formation of the urban Bulawayo aquifer

Abstract

Electrical resistivity surveying methods have been widely used to determine the thickness and resistivity of layered media for the purpose of assessing groundwater potential and siting boreholes in fractured unconfined aquifers. Traditionally, this has been done using one-dimensional (1D) vertical electrical sounding (VES) surveys. However, 1D VES surveys only model layered structures of the subsurface and do not provide comprehensive information for interpreting the structure and extent of subsurface hydro-geological features. As such the incorporation of two-dimensional (2D) geophysical techniques for groundwater prospecting has often been used to provide a more detailed interpretation of the subsurface hydro-geological features from which potential sites for successful borehole location are identified. In this study, 2D electrical resistivity tomography was combined with 1D VES to produce a subsurface resistivity model for assessing the availability of groundwater in the basaltic-greenstone formation of the Matsheumhlope well field in Bulawayo, Zimbabwe. Low resistivity readings (<50Ωm) towards the central region of the study area suggest a high groundwater potential, while high resistivities (>500Ωm) around the western margin of the study area suggests a low groundwater potential. 2D electrical resistivity surveys provide a more detailed subsurface structure and may assist in identifying the configuration of possible fractures which could conduct groundwater into the shallow subsurface of study area. It is concluded that 2D electrical resistivity methods is an effective tool for assessing the availability of groundwater in the highly weathered and fractured basaltic greenstone rocks. The methods provided a more precise hydro-geophysical model for the study area compared to the traditional VES. Results from this study are useful for technical groundwater management as they clearly identified suitable borehole locations for long term groundwater prospecting.