Intergrated Geophysical Methods to Delineate R21 Sinkholes Near Olifantsfontein, Gauteng, South Africa

Abstract

The Gauteng province in South Africa is known for having a significant portion, at least 25%, of its land is underlain by dolomite rocks. An integrated approach of non-invasive geophysical methods was utilised to map the geometry (shape, size and extent) of the R21 highway sinkhole that formed near Olifantsfontein. These methods include seismic (reflection and refraction), multi-channel analyses of surface waves (MASW), electrical resistivity tomography (ERT), and ground penetrating radar (GPR). The objectives of the investigation were three-fold: (1) to understand the geology and formation of sinkholes in Gauteng; (2) to use geophysical techniques to map and characterize sinkholes in the study area, including determining their geometry, lateral and vertical extents; and (3) to determine the effectiveness of each method in mapping sinkholes. Despite the high level of noise along the highway, the geophysical surveys were conducted successfully and provided a good basis for the integrated interpretations. This study showcases the importance of utilising multiple geophysical methods to obtain a comprehensive understanding of sinkholes and their subsurface characteristics. It also demonstrates the practical application of these methods in real-world scenarios for improved hazard assessment and risk mitigation. The GPR results suggest that a sinkhole extends by ~2.5 m further into the R21 highway surface with a depth of ~8-10 m (top to bottom). The refraction seismic method suggests that the sinkhole is ~20 m wide, while the ERT results suggest that the sinkhole starts at 10 m and extends to 15 m depth. The results from reflection seismics indicate that the R21 sinkhole is structurally controlled and it is characterised by fracturing and faulting that manifest as diffractions on the seismic sections. Understanding the extent and characteristics of sinkholes is crucial for infrastructure planning and hazard mitigation, especially in areas prone to subsidence and sinkhole formation like in Gauteng. These findings can inform decision-making processes related to road maintenance, construction, and land use planning in sinkhole-prone regions. The success of the integrated geophysical approach in this study highlights its potential for similar investigations in other areas with karst geology and sinkhole risks.