Integrated geophysical survey as applied to groundwater exploration

Mamre is a small agricultural town north of Cape Town on the semi-arid west coast of South Africa. The town is completely dependant on groundwater that is supplied by the Atlantis and Witzand water wellfields some 10 km to the south of Mamre. These water boreholes have been drilled into the unconsolidated sediments of the Springfontyn Member of the Bredasdorp Formation. The borehole depths vary from about 20 to 40 m. The groundwater is chlorinated and softened and it is thus an expensive water source. The local municipality decided to investigate alternative water sources to supplement the town water supply. The only feasible option was to develop the groundwater occuring in the Mamre area. The groundwater will be used for irrigation purposes if the water quality is within allowable limits. The objective of the thesis project was to delineate target zones for groundwater development in the Mamre area, using three geophysical methods. The two possible targets identified were areas of deeper weathering in the local shale and granite aquifers and the baked contact between the shale and the intruded granite. The shallow seismic reflection, electromagnetic and resistivity techniques were applied in the study area during this thesis project. The three geophysical methods were successfully used to delineate zones of deeper weathering of the Malmesbury Shales. The seismic data were quantitatively interpreted to ascertain accurate estimates of depth to bedrock in anomalous zones. The electromagnetic and resistivity data could however only be interpreted in a qualitative sense to delineate zones of deeper weathering. The contact between the Malmesbury Shale and the Cape Granite could not be delineated with either of the three geophysical methods used. This is probably because the contact is not present in the subsurface along the profile investigated. Potential drilling positions can be located using the information gathered during this geophysical survey --Abstract, pages iii-iv