Authors: Schaub, C; Smith, R

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Schaub, C & Smith, R 2014, 'Determining the feasibility of a real time geophysical magnetic and electric measurement system for monitoring strain underground – laboratory magnetic test', in M Hudyma & Y Potvin (eds), Proceedings of the Seventh International Conference on Deep and High Stress Mining, Australian Centre for Geomechanics, Perth, pp. 443-456.

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High stress areas, which can lead to rockbursts, are a reality in deep mining environments around the world. The unpredictable nature, and sometimes fatal consequences of rockbursts, makes working towards identifying increases in strain that occur prior to failure events, of principal importance for mine safety. The piezoelectric, electrokinetic, and seismoelectric effects result in electric and magnetic fields that can potentially be measured in rock mass under stress. The implementation of real time monitoring of these signals has the potential to significantly improve deep mine safety by mapping the evolution of strain underground and indicating potential areas susceptible to failure. Previous laboratory stress tests on core show measurable currents of nano-Ampere magnitude. These currents have associated magnetic responses which can be very small; however, with proper filtering and noise removal algorithms, magnetic signal identification from rock under stain may be just as feasible as measuring electric signals. A multi-part experiment aimed at identifying strain related signals in a mine environment has been proposed. The first part of the experiment was completed 26 April 2014 in a parking lot located on Laurentian University campus, Sudbury, Ontario. In the experiment a total of 32 rocks, consisting of eleven different kinds, were stressed until failure using a load frame. Magnetic coils located around the load frame were used to measure any potential magnetic fields induced when the core was stressed. The thirty-two rock samples were stressed with periodic pressure increases to failure at approximately 620 kPa/s with 10 s pauses between every 3.5 MPa increase in pressure. All the samples used for the experiment were insulated from the load frame to isolate any currents induced through strain to the sample being stressed. Base station magnetic measurements, taken 19.5 m from the load frame will be used for subtracting out slow varying ambient magnetic noise. Preliminary analysis shows measurable magnetic signals associated with audible cracking sounds for most rock samples, as well as consistent signals across all samples prior to failure. The laboratory test should be followed up with an above ground test to become familiar with, and optimise, the data acquisition equipment, in an electromagnetic-noise-reduced environment. This will also allow a brief study on the effects of seismic motion on the measurement equipment. Finally, a carefully controlled, passive acquisition, underground test should be carried out in a deep mine location. Successful identification of signals due to strain through these experiments could potentially prove the feasibility of a real-time underground geophysical monitoring system to improve safety.

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