Constraining the far-field in situ stress state near a deep south african gold mine

Researchers from The Natural Earthquake Laboratory in South African Mines (NELSAM) project are investigating the physics and mechanics of mining-induced earthquakes using the access to seismogenic depths provided by deep South African gold mines and the large number of seismic events that occur near these mines. To study these events, it is necessary to quantify the far-field stress field around the mine, determine how the presence of the mining excavation perturbs this stress field, and investigate how these mining-induced stress changes affect the pre-existing faults. In this paper, we develop and test a new technique for determining the far-field virgin state of stress near the TauTona gold mine. The technique we used to constrain the far-field stress state follows an iterative forward modeling approach that combines observations of drilling induced borehole failures in borehole images, boundary element modeling of the mining-induced stress perturbations, and forward modeling of borehole failures based on the results of the boundary element modeling. Following this approach, we determined that the state of stress is a normal faulting regime with principal stress orientations that are slightly deviated from vertical and. Modeling of breakout rotations and gaps in breakout occurrence associated with recent fault slip on critically stressed faults further confirmedthis stress state.