Innovative Approach to Monitoring Underground Stresses

By Jean-Philippe Mercier and Jim Tod, Montréal, Québec, Canada

E-mail

Sophisticated tools can be employed to model and monitor how rocks are responding to activity at mining sites - leading to much higher confidence in modelling for simulating subsequent mining stages.

Since 2009, Golder has been closely involved in developing tomography software to monitor rock mass response to mining, using data from microseismic systems. There appears to be a promising future for this technique, both in mining and civil engineering applications.

Microseismic systems record and analyse the seismic energy (the cracking and popping) released by the rock mass when mining excavation changes the natural stress field in the rock. Seismic tomography allows Golder to look at how the stresses and strains in the rock mass are changing as mining progresses, using data collected by microseismic monitoring systems already installed in mines. Consequently, use of the technique does not require installation of additional equipment.

Mining activity disturbs in situ stresses in the rock, causing them to reorganise. Image courtesy of Petra Diamonds.

When a mining extraction sequence is back-analysed using numerical stress models, we can use the microseismic tomography information for calibration. This allows us to see if the stress and strain areas observed by the passive tomography match the high and low stress areas predicted by the numerical models at the same stage of mining. When the two match, this gives confidence to use the calibrated model parameters to evaluate possible extraction scenarios. Being better able to understand rock mass behavior helps us design mitigation measures and extraction strategies that can prevent injuries as well as ore and equipment losses.

One application of seismic tomography has been to the last stage of a significant block cave mine in Australia. In the this type of operation, a large excavation is initially created at the bottom of the ore zone, allowing the ore and surrounding rock to fail, or "cave," into the opening. The extensive data provided on the distribution of stresses helped produce more accurate mapping of the extent of the cave.

Other seismic tomography results have imaged anomalous stress conditions that prevail before large seismic events. This suggests that passive source tomography could be important in paving the way to more innovative methods of identifying and mitigating seismic hazard.