Effect of the rock properties on mining-induced seismicity around the Ventersdorp Contact Reef, Witwatersrand basin, south Africa

The Ventersdorp Contact Reef is characterised by a variety of different rock types that are found above and below the reef in different mining areas. These variations in the host rock type have been classified into six main geotechnical areas. Knowledge of the difference in seismicity related to these geotechnical areas could influence the design of deep level stopes and optimise the support systems. In this study, the total seismicity was evaluated using the relationship between the cumulative seismic moment and volume of convergence. In total 300,000 seismic events, occurred at Deelkraal, Elandsrand, East Driefontein, Kloof and Leeudoorn mines. Kloof and Leeudoorn gold mines were divisions of Kloof Gold Mine Co. Ltd. Mponeng was previously called Western Deep Levels Ltd.-South Mine. The seismicity generated in areas with different mining conditions and geology has been compared. Special attention was paid to evaluate the difference in seismicity associated with different geotechnical classifications. The following variations were found: (i) geotechnical areas with soft (Westonaria) lava hangingwall and quartzite-conglomerate footwall are less active than the geotechnical areas with hard (Alberton) lava hangingwall and quartzite-conglomerate footwall at East Driefontein, Kloof and Leeudoorn Mines, (ii) geotechnical areas with soft lava hangingwall and Jeppestown Shale footwall are less active than geotechnical areas with soft lava hangingwall and quartzite-conglomerate footwall at East Driefontein Mine, (iii) geotechnical areas with hard lava hangingwall and quartzite-conglomerate footwall and hard lava hangingwall and Booysen's Shale footwall at Western Deep Levels-South Mine do not indicate differences in the associated seismicity, (iv) the variations in the properties of the different types of quartzite forming the footwall at Deelkraal, Elandsrand, Mponeng, East Driefontein and Kloof Gold Mines do not appear to influence the level of seismicity.