Integration of 3D seismic data analysis techniques and mining-induced seismic data to elucidate the mechanical behaviour of the rock mass in the gold mines of the Witwatersrand Basin
In many hard rock underground mines worldwide, seismicity is observed as stress increases due to increasing mining depth. The rock mass in the hard rock underground mines is generally composed of high strength brittle rock types, so the risk of violent failures increases with increasing mining depth due to increasing stress levels. Seismicity poses a significant geotechnical challenge to the mines as it may damage excavations mining equipment and infrastructure, and cause injuries or even deaths of mining personnel. Mining operations within the Witwatersrand area have mostly taken place at depths of 1.5–3.5 km, but are now migrating to ultra-depth as mines mature. This transition is met with different complexities in the local geology, rock mass conditions, thickness and dip of the reef (ore body). Mining at these greater depths will result in higher mining-induced stresses, induced seismicity and rockbursts. All these complexities affect mining standards and procedures. This study attempts to mitigate the risk of rockburst by integrating 3D reflection seismic images of geological structures (especially dykes and faults) with the source mechanisms of mininginduced earthquakes to better understand the main drivers of seismicity. The seismicity data covering the Kloof Extension Area (KEA) was investigated in this study. Moderate to large mining-induced seismic events (ML≥1.5) within the defined seismic volume were analysed in an area where mining was taking place. The effects that these complexities (high stress, complex structural geology, mining elements, etc.) have on current mining operations was studied to understand the impact on future mine planning and ore reserve evaluation. High-resolution images of the gold-bearing orebody, the Ventersdorp Contact Reef (VCR), were extracted using seismic attributes. The images are the result of an interpretation of the seismic reflection data. The investigation includes the formulation of a workflow for the computation and interpretation of seismic attributes. The 3D reflection seismic data is undersampling a series of minor structures (e.g., small faults with a throw less than 5 m and mainly the dykes) that have been identified by the in-stope underground geological mapping. Comparison indicates that the faults determined by the underground mapping generally correlate with those mapped in the 3D reflection seismic data. Mining-induced seismic events frequently occur close to the in-stope identified mapped structures. Numerous faults not detected or visible on the VCR depth structural map and seismic sections were successfully imaged using horizon-based seismic attributes, viz. edge detection and dip-azimuth. The dip azimuth attribute provides a better delineation of faults. The source mechanisms of seventy-five ML≥1.5 seismic events were studied in detail. Focal plane solutions were calculated, interpreted, and compared with the geological model derived from underground mine mapping and the interpretation of the 3D seismic reflection data and elements of the mining layout such as abutments, back areas, and pillars. When comparing mining-induced seismic events occurring on mining elements and on geological structures, it was found that they release approximately equal amounts of seismic energy. It was found that moment tensor inversion generally yields fault plane solutions that align with the mining geometry and geological structures. The Es/Ep ratio, routinely used to classify seismic events, was not consistent with the source mechanisms produced by the moment tensor calculations. This study indicated that the parameter is not sufficiently stable to be used for classification. The data analysis indicates that mining activities may lead to a rockburst (a mining-induced seismic event that causes damages underground) or a slip on pre-existing geological planar structures. However, the geological structures are not the only driving factors of these large seismic events within the 3D reflection seismic volume. Mining elements, such as pillars, remnants and abutments, are sometimes implicated. All seismic data recorded since the extraction of the Thuthukani shaft pillar commenced in January 2007 has been analysed. We compared seismicity in two five-year periods (1 January 2007 to 30 November 2012 and 1 December 2012 to 31 July 2017) and found an increase in the rate of seismicity and the maximum magnitude (Mmax) from 2.7 to 3.2. An assessment of seismic hazard using intermediate term probability of occurrence indicates an increase in seismic risk for M≥1.0 seismic events in period 2 as compared to period 1. Strategies to mitigate the seismic risk during shaft extraction are discussed.
A thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy to the Faculty of Science, University of the Witwatersrand, 2022
3D seismic data analysis techniques, Mining-induced seismic data, Rock mass