Application of 3D seismics to enhance mapping of potholes in the western Bushveld Complex, South Africa

Abstract

The ~2.06 Ga Bushveld Complex in South Africa is widely known as the world’s largest layered igneous intrusion. The Bushveld Complex hosts the world's largest platinum and chromium deposits. The two of the major economic platinum-bearing ore bodies (known as reefs) are the Merensky and the Upper Group-2 (UG-2) reefs, which are located at depths between 500 m and 1.5 km below surface. Mining operations on these horizons are often complicated by the presence of geological features such as faults, dykes, potholes and iron-rich ultramafic pegmatite (IRUP) bodies. This emphasises the importance of characterising these features and predicting their occurrence.

In this study, the 3D reflection seismic method is employed to delineate these structures (mainly potholes) within the Western Bushveld Complex, South Africa. Potholes are slump structures that disturb mining processes and ultimately decrease the mining productivity. Various state-of-the-art techniques were used to identify and analyse potholes, including (i) horizon-based seismic attributes, (ii) complex-trace attributes, (iii) 3D volumetric attributes, (iv) difference-of-two-surfaces, and (v) geostatistical methods. The seismic techniques complimented each other in detecting and identifying approximately 43 potholes, which were then used for geostatistical analysis. The results suggest that pothole structures are asymmetric and are often associated with faults. This study also reveals that a majority of the potholes imaged in the seismic data have propagated between the Merensky Reef and the UG-2 levels, i.e., MR and UG-2 are affected by the same potholes. These potholes are randomly distributed and exhibit some clustering. A k-mean cluster analysis was implemented and revealed two clusters that are present in the study area. The various hypotheses on the formation of potholes are investigated. This study rules out some of the hypotheses for the formation of potholes and proposed that the potholes might have formed from multiple mechanisms as opposed to just one. Statistical analyses suggest a positive linear relationship between the pothole depth and the pothole diameter, indicating that the pothole may have grown in width and depth at the same time.