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All that noise… can I find meaning? Passive seismic monitoring in an active mine, 2km down.

Mines are a pretty noisy place: manmade sources such as drills, jackhammers, trucks, blasting, etc. and the subsequent response to all this activity, unexpected microseismic events through to collapse. An environment that has been relatively stable since its formation 1.8 billion years is now being carved out, changing the stress-field possibly causing previously stable faults, joints and cracks to reactivate, slip resulting in catastrophic consequences. To be able to predict, or at least catch the warning signs of this risk, has both economic and life-safety incentives.

Rather than identifying the hazard by location, strength and frequency of microseismic events, a reactive method requiring actual rock failure, can passive earthquake site assessment methods be utilised? Sure we no longer have surface waves to give us a velocity structure as a function of depth, but we do have tunnel waves. And really, they are just a more complicated form of a Rayeigh surface wave.

One very simple such passive method, Horizontal to Vertical Spectral Ratio (HVSR) has been applied underground and, surprisingly, actually generates stable estimates that evolve slowly over years. Is this an indication of changing stress over the mine’s life? Or is it just a coincidence…