Constraining the Far-Field Stress State near a Deep South African Gold Mine

Abstract

The deep gold mines in the Witwatersrand Basin of South Africa are some of the deepest underground mines in the world. For over a century, numerous, often deadly, mining induced earthquakes have been observed. In this work, we develop and test a new technique for determining the virgin state of stress near the TauTona gold mine. This work was completed as part of the Natural Earthquake Laboratory in South African Mines (NELSAM) project. The technique we use to constrain the far-field stress state follows an iterative forward modelling approach that combines observations of drilling induced borehole failures in borehole images, boundary element modelling of the mining-induced stress perturbations, and forward modelling of borehole failures based on the results of the boundary element modelling. The final result is a well constrained stress state consistent with all the observed stress indicators. We find that the stress state is a normal faulting regime with principal stress orientations that are slightly deviated from vertical and horizontal (denoted with a *). The maximum principal stress, Sv*, is deviated 0-20o from vertical towards the NNW and has a magnitude gradient of 27 ± 0.3 MPa/km. The intermediate principal stress, SHmax*, is inclined 0-20o from horizontal with an azimuth of 145o to 168o and has a magnitude gradient of 21.5 to 26 MPa/km. The least principal stress, Shmin*, is inclined 0-10o from horizontal with an azimuth of 235o to 258o and has a magnitude gradient of 13 to 15.5 MPa/km. This stress state indicates that the crust is in a state of frictional faulting equilibrium, such that normal faulting is likely to occur on pre-existing fault planes that are optimally oriented to the stress field. The stress concentrations caused by the mining activities can dramatically alter the range of fault orientations upon which fault slip could occur.