Quantifying the interaction of seismicity and gas transport in fractured hard rock at earthquake focal depth (DAFGAS-II)
Final Report Abstract
For the first time a seismic network and geochemical borehole gas monitoring were simultaneously operating at the Pretorius Fault Zone (PFZ) at 3.54 km depth, TauTona gold mine, Gauteng Province RSA. As observed earlier, geophones and accelerometers detected two different types of earthquakes in the TauTona Mine. They were classified into type A events associated with blasting, excavation and stope closures and type B events, analogous to tectonic earthquakes entailing slip on pre-existing faults or fractures. Very obvious is the lack of seismicity during interruptions of work which indicates, that the vast majority of detected type B events are also induced by the mining process. In contrast to the years before (2007-2010), there was no mining activity in the direct vicinity of the cubby. The observed slight under pressure in the DAFGAS bore hole is the result of continuous longterm gas withdrawal from the borehole to the analytical devices and the mine ventilation. It causes gallery air to pass through the fractured PFZ into the borehole. Mid-daily (surface meteorology) and multi-weekly occurring pressure changes (e.g. during Christmas holidays) are negatively correlated with the CO2 concentration and Rn activity, both heavy gases. Hydrogen does not show such mid-day (surface meteorology driven) variations, but sharp peaks which are typical - but not restricted - to blasting times. The peak size and form is defined by the magnitude and temporal occurrence of simultaneously detected seismic (non-blasting) events. We suggest that the H2 observed in theses sharp peaks is a result of the Si-O reaction with water during earthquake activity. Most of the aims and scopes of this project have been answered so far. Time series and cross correlation analysis allow the identification of different gas components and the identification of their origin. The reaction of Si-O radicals with water and radiolysis of water explains the generation of H2 which can serve as nutrient for the microbiological activity in fault-zones.
