Seismic monitoring is widely used in deep hardrock mines in Canada and around the world. In the province of Ontario, in Canada, more than 70% of underground, hardrock mines have a seismic monitoring system, and all mines deeper than 1000 metres have a seismic monitoring system. Mine seismic systems may record tens of thousands to hundreds of thousands of mining-induced seismic events each year. Each detected seismic event gives information about the relative state of stress and the relative amount of deformation at the time of the event. This potentially allows seismic monitoring systems to be real-time monitors of stress and deformation changes in mines.
Past research has found that the seismic source parameter apparent stress is strongly correlated to increases in mining-induced stress. However, past research has not investigated how apparent stress varies over time, particularly in reference to mining-induced stress changes caused by mine blasting. An empirical technique, Apparent Stress Time History, has been developed that identifies temporal increases in apparent stress, or a relative measure of stress increase in a rock mass.
This paper explains the methodology and the underlying fundamentals. It will be demonstrated that Apparent Stress Time History is strongly correlated with stress change associated with mine blasting.
In underground mines, stress changes occur as the orebody is extracted. In bulk mining, the blasts may be large, resulting in significant excavation changes, often occurring in a few seconds. These blasts induce localized stress changes that may take several hours to reach a state of equilibrium. A geotechnical monitoring program undertaken at Brunswick Mine (Hudyma et al. 1994), allowed for near continuous monitoring of stress redistribution into the surrounding rock mass following a 5000 tonne mine blast. Fig. 1 shows minute-by-minute strain cell measurements at a distance of 50 m from the blast location. Approximately 70% of the total strain change recorded occurred within one minute of the blast. The remaining 30% of the strain change occurred at a much slower rate over the next several hours.