Geogas transport in fractured hard rock - correlations with mining seismicity at 3.54 km depth, TauTona gold mine, South Africa

An on-site gas monitoring study has been conducted in the framework of an earthquake laboratory (the international NELSAM-DAFGAS projects) at the TauTona Gold mine, South Africa. Extensive underground activities began in 2004 with the establishment of a 25 m² cubby and the drilling of five boreholes up to 60 m apart within the Pretorius Fault zone at 3.54 km depth. Instruments for chemical and seismic monitoring were then installed within the cubby and boreholes. Since 2007 sensitive gas monitoring devices have been continuously improved to enable the direct observation of geogas concentration variations in the DAFGAS borehole. The major gas concentrations are constant and air-like with about 78% N₂, 21% O₂, 1% Ar, while the geogas components CO₂, CH₄, He and H₂ show most interesting trends and variations on the minute-by-minute basis. Time series and cross correlation analysis with meteorologic and seismic data allow the identification of two different gas components (geogas and tunnel air) and the identification of the two most significant processes influencing the borehole gas composition: 1) pumping-induced tunnel air breakthrough events through networks of initially water-saturated fault fractures; and 2) blasting induced permeability enhancement of the fault fractures to above ~5*10^-10 m². The current set-up of the gas monitoring system is sensitive enough to quantify the resulting geogas transport during periods of intense blasting activities (including recorded blasts with seismic moment ≤ 1*10⁹ Nm, located within 1000 m of the cubby) – and, we suggest, also during induced earthquakes – a final goal of the project.