Groundwater exploration in a granite aquifer using the telluric electric frequency selection method (TEFSM) in Eswatini, Southern Africa

Abstract

AbstractRecent technology development has enabled the use of electric potential difference (EPD) in the telluric electric frequency selection method (TEFSM) geophysical approach for groundwater exploration. As a new approach, there is a need to build knowledge and experience in the application of the TEFSM geophysical approach in groundwater exploration in different hydrogeological settings. Hard rock granite aquifers are one of those aquifers where the TEFSM geophysical approach to exploring groundwater is yet to be investigated. In this study, a geophysical survey to identify four drilling sites for community boreholes was first conducted using the TEFSM approach. Vertical electrical profiles (VEP) of EPD up to 120 m per station were analyzed and interpreted to identify the depths of potential aquifers prior to drilling. The EPD VEP was then corroborated with borehole lithology data collected from the drilling to provide hydrogeophysical meaning to the data. The results show that groundwater occurs in the weathered granite layers. However, the water strikes appear to occur at the contact plane between the overlying weathered granite and the underlying fresh amphibolite. This suggests that the groundwater is stored in the weathered granite while the contact plane at fresh amphibolite is a preferential flow path. The granite aquifer at the study site is characterized by the EPD ranging from 0.018 to 0.068 mV. However, not all geological materials in this EPD range had water, some were just a reflection of weathering. The TEFSM geophysical approach was able to delineate layers of weathered granite aquifers and impermeable amphibolite based on low and high EPD contrasts, respectively. The findings assist in improving the practical understanding of the application of TEFSM to delineate aquifers and site boreholes in granite aquifers.