Seismic Resonance Modes for Mine Roof Stability Monitoring

Abstract

This work aims at the detection of instabilities in underground mine roadway roof, with the goal of predicting and preventing roof failure and collapse. Openings in the rock have their own resonances, due to the propagation of seismic waves in the rock around the opening. If the surrounding rock is damaged or fractured, this would result in the resonant frequencies decreasing. An experiment was set up in an underground mine to detect these resonances and see how they change in the process of rock degradation leading up to collapse. Accelerometers were grouted into a mine roadway roof, along with displacement and stress sensors. Waveforms from mining-induced microseismic events were recorded. The spectra of the coda of these events were used to search for resonances. Strong resonance modes were indeed seen, which were stable over time. The resonance frequencies did decrease in the days prior to roof collapse, in parallel with measured stress changes. At the time when significant movement was detected in the roof rocks, the resonance modes changed completely, probably due to delamination of the rock causing seismic decoupling. This means that resonance modes could be used for roof stability monitoring.