The stress and strength at hypocenters at Tau Tona and Mponeng gold mines, South Africa

Abstract:

We report on the stress and strength at hypocenters at Tau Tona and Mponeng gold mines, South Africa. In the previous published reports on 3D stress tensor measurements, the ranges of depth and maximum principal stress (Sigma 1) were limited down to 2.7 km and less than 90 MPa, respectively (gray range in the attached figure; e.g. Stacey and Wesseloo 1998 SIMRAC, Handley 2013 J SAIMM). That has been because no cost-effective methods have been available suitable for routine measurements of local 3-dimeensional stress at sites where borehole breakout or core discing prevails and only small pneumatic machines for AX or BX drilling are available. Ogasawara et al. (2012 J SAIMM, 2014 ARMA) enabled in-situ measurements of local 3D stress beyond the limit, who have used a downsized Compact Conical-ended Borehole Overcoring technique (CCBO; Sugawara and Obara, 1999; ISRM suggested) in 60mm diameter holes. 3.4 km and 146 MPa were the largest depth and maximum principal stress that they succeeded in-situ stress measurements, respectively. The assumption of far-field stress vertical-gradient (Lucier et al. 2009 or Reches et al. this meeting 2014) was then well calibrated by the increased number of in-situ measurements. Hofmann et al. (RS2013) constrained the range of Sigma 1 of about 100 to 180 MPa for seven mining-induced events (ML = 2.1 to 4.0) located by the in-mine seismic network and inferred based upon the geological observation. Japanese German Underground Acoustic Emission Research in South Africa (JAGUARS; Nakatani et al. 2008) at Mponeng mine offered us a rare opportunity because the AE finely delineated the aftershock plane of a ML2.1 event that ran obliquely through a dyke on December 27, 2007 (Yabe et al. 2009 and Naoi et al. 2011). Hofmann et al. (SHIRMS 2012) found that Excess Shear Stress (or Coulomb Stress) was largest on the aftershock plane and successfully constrained the strength with which they successfully reproduced the fault ride equivalent to the seismic moment observed by the in-mine seismic system just after the completion of mining in December 2007. Yabe et al. (RS2013) drilled a hole intersecting the hypocenter, analyzing borehole breakout and core discing and successfully constraining the stress. The constrained stress was qualitatively in good agreement with the stress Hofmann et al. calculated.