Application of geophysical techniques in the delineation of aquifer systems in the Beaufort West area, Western Karoo, South Africa
Sekiba, Fhatuwani Matome Adolph
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A geoscientific research project of which the current study forms part is underway in the Karoo Basin, near Beaufort West in the Western Cape, South Africa. The aim of the research is to better understand the potential impact of the geo-resource exploration activities on the Western Karoo Basin. The current study strongly emphasis the use of airborne and deep-probing magnetotelluric (MT) geophysical techniques to better understand geological structures and deep aquifer systems. The need to understand the deeper aquifer systems arose from the possibility of shale gas exploration in the Main Karoo Basin. The high resolution airborne magnetic survey was undertaken in order to upgrade the geology map and reveal the presence of hidden geological features such as dolerite (dykes or sills), faults or other magnetic lineaments. The results showed that magnetic method can play a significant role in generating groundwater targets. The total magnetic field showed positive anomalies that exceed the local magnetic field of the area as recognised by a steep gradient, while negative anomalies are below the local magnetic fields. The high-resolution data clearly mapped dykes, sills, faults, geological contacts and intrusive bodies. These lineaments could be an indication of foliation in the area in response to shearing. The development of the foliation within the Beaufort Group sediments is restricted to the finer grained sandstone, mudrock and siltstone. Magnetotelluric results suggested that the very high resistive layers that appear to have been intruded within the slightly conductive zone might be associated with dolerites. The displacement of a conductive zone around distance 2200 m and 5000 m might be an indication of faulting. The resistive substratum reaching a depth of around 2000 m to the east and the inclined resistive layer might be an indication of dolerite sill and dyke respectively. Based on this results it may be fair to note that magnetotelluric method is a non-intrusive tool that played a major role in obtaining information of the subsurface at great depth. The 2D magnetic modelling confirms the resistive layer observed in MT inversion model might be related to dolerite. The results shows the magmatic feature with susceptibility value of around 0.012981 SI might represent a slight vertically inclined dolerite to the center at distance ranging between 2 km and 4 km, while to the west and east of the profile appears to be horizontal. These structures may have been connected during their previous formations, as can be seen that it might have been disrupted due to present day erosion. The Poortjie Member might have been heavily intruded, whereas the Ecca and Dwyka Groups appear to be suppressed. These intrusive bodies have been covered by younger alluvial. These results enhance the knowledge that aquifers within the Karoo may be occurring beneath the well-studied depth. Two monitoring boreholes (R01-BW and R02-BW) were drilled to a depth 1402 m and 516 m respectively. The boreholes were drilled at a larger distance (± 20 km) from the magnetotelluric traverse, due to the electromagnetic highly influenced by DC powered railway which is near the boreholes site. Magnetotelluric results may not be constrained with confidence with the borehole information due to the larger distance separation. Drilling of the boreholes was aimed on determining the lithology and stratigraphy of the Beaufort Group and underlying Ecca Group, and the properties of shallow groundwater. Borehole R01-BW drilled to a depth of 1402 m intersected from surface downwards shale and sandstone, 57 m thick, of the Poortjie Member (Middleton Formation), siltstone, shale and sandstone, 677 m thick, of the Abrahamskraal Formation, sandstone, siltstone and shale, 169 m thick of the Waterford Formation, and siltstone and shale, 483 m thick, of the Tierberg Formation. Dolerites 16 m and 2 m thick, intrude the Abrahamskraal Formation. Borehole R02-BW drilled to a depth of 516 m intersected from surface downwards sandstone, siltstone and subordinate shale, 64 m thick, of the Poortjie Member (Middleton Formation), and sandstone, siltstone and shale, 434 m thick, of the Abrahamskraal Formation. A dolerite 17 m thick, intrudes the Abrahamskraal Formation. Due to the challenge of interpreting rock chips during geological logging as may be mixed during drilling, the downhole geophysical was conducted on the two monitoring boreholes in order to obtain in-situ geotechnical and structural information. Borehole R01-BW gave a good correlation with the identification of dolerite at depth between 489 and 508 m. a great assistance can be observed at depth between 690 and 692, where the geophysical identified dolerite which was missed by geological logging. Borehole R02-BW, the identification of dolerite at depth between 159.5 and 176.5 m gave a good correlation with the geological log. Geophysical log may have identified sandstone below depth 186 m, but without good correlation from the geological. The positions of the two monitoring boreholes were removed by a large distance from the positions of the magnetotelluric survey. Although the combined use of airborne and ground geophysical surveys was of a great assistance in understanding the subsurface geological and geohydrological conditions, the information obtained from the boreholes cannot be used with confidence to constrain the geophysical interpretation. Considering this and also the ambiguity and none uniqueness in geophysical interpretation, it is therefore highly recommended that geophysical results be constrained through drilling.