Characterization of the hydrostratigraphic units of the Sodwana area using the electrical resistivity method
This study was aimed at defining the geological facies of the Sodwana area. It formed part of a bigger project designed to investigate the linkages between the hydrological and ecological drivers of the Mgobolezeni Catchment, Sodwana. Monitoring boreholes were drilled by the Department of Water Affairs (DWA) at several locations in the study area for the purpose of monitoring groundwater changes in the Catchment. There was a need to interpolate the geology and groundwater conditions between the monitoring boreholes. This research provided the geological conditions between the monitoring boreholes using the electrical resistivity geophysical method. The Geotron (G41) resistivity meter was used to conduct the survey. Vertical electrical sounding (VES) were conducted initially in close proximity to DWS monitoring boreholes for calibration purposes. The VES data were plotted and interpreted using IPI2win computer software. Hydrostratigraphic models were developed for all VES locations in close proximity to monitoring boreholes. These models were used for interpolation of the geology between the boreholes. A total of 20 VES were conducting in the study area. 16 VES results showed 5-layer models while 4 VES results showed 4-layer models. The top layer showed very high resistivity values (> 2000Ωm) and was interpreted as dry sand (Kwambonambi Formation). The second layer showed a resistivity range of 200Ωm – 2000Ωm. In some locations, the second layer showed a resistivity range of 120Ωm - 200Ωm and was interpreted as the shallow aquifer in the Kwambonambi Formation. The third layer showed a resistivity range of 50Ωm - 120Ωm and was interpreted as the low transmissivity Kosi Bay/Isipingo Formations. The fourth layer had a resistivity range of 10Ωm - 50Ωm and was interpreted as the deeper aquifer unit of the Uloa Formation. The basement Cretaceous rock formed the fifth layer that showed a clear increase and higher resistivity values than that of the overlying fourth layer. Transmissivity values observed from the pumping tests conducted on the calibration boreholes were combined with VES results to define the transmissivity of the inferred deeper aquifer in between boreholes. The thicknesses of the geoelectric layers obtained from VES results showed a good relationship with corresponding geologic units in borehole logs. The varying apparent resistivity values and lithological information enabled the demarcation of the geoelectric units into hydrostratigraphic units. VES results also showed that the surface of the Cretaceous unit is deeper in the eastern side of the study area than in the western side, showing that the Cretaceous unit dips towards the east. The modelled transmissivity also showed a good statistical relationship (R2 = 0.99) v with the observed transmissivity. Therefore the VES method used for this study successfully achieved the aim of this study.
hydrology --ecology --hydrostratigraphic models
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