ABSTRACT:

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 confirmed this stress state.

1. INTRODUCTION

As mining around the world moves deeper underground, understanding the stress field at depth and how mining activity perturbs it becomes increasingly important for mine safety. As a result of the mining-induced stress perturbations, deep underground mines tend to have appreciable mining-induced seismicity associated with them. In cases when these seismic events cause damage to the excavation, they may result in injuries and fatalities. A more complete characterization of the farfield stress will lead to better modeling of the mining induced stress perturbations around the excavation. In turn, this can guide mining activities in the future and improve overall safety in the mines. Constraining the far-field stress state is an important part of the Natural Earthquake Laboratory in South African Mines (NELSAM) project, which is working to develop a very near-field laboratory to study earthquakes at seismogenic depths [1-3]. The deep gold mines of South Africa are unique locations for near-field studies of earthquake mechanics because of the high rate of mining-induced seismicity and the direct access to faults at seismogenic depths. However, the perturbation of the in situ stresses by mining activities creates a complex stress field that complicates the understanding of the physical mechanisms controlling the induced seismicity. As part of the NELSAM project, we developed a new method to constrain the far-field in situ stress state surrounding the TauTona gold mine in the Western Deep Levels of the Witwatersrand Basin of South Africa. Much of the previously published work on characterizing the far-field in situ stress state near the deep mines in the Witwatersrand Basin relies on borehole strain relief measurements [4-6]. In their review of global stress measurements, McGarr and Gay [7] showed that in South Africa the stress state at depths below 2 km is typically a normal faulting stress regime, in which the vertical stress exceeds the horizontal stresses.

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