Real-time Rockmass Response from Microseismics

Microseismics is routinely used in hard-rock mines as a hazard-management tool for rockbursts. Its use in coal, however, has been limited by the challenges of seismic propagation in a coal environment. CSIRO, with support from ACARP and the industry, has been working to adapt the technique for use in coal mines. Application of microseismics to providing warnings of roof-weighting, wind blast, and other caving issues has been demonstrated, along with potential for monitoring fault movement, and gas flow though fractures. However, this work has always involved manual processing of the data, resulting in long delays before results were available. Realising the potential of microseismic monitoring will require the automation of processing, so that results can be made available in near-real-time.

A real-time system benefits a mine by enabling a forewarning to be given of face and roadway roof instability, allowing proactive measures to be taken to prevent fall of ground. The economic implications are significant: A longwall stoppage due to geotechnical problems can cost a mine around a million dollars a day, so the detection and prevention of a major fall could save several million dollars.

The primary objective of this project was to develop a prototype real-time microseismic monitoring system for strata control management and forewarning of geotechnical hazards. The major problems preventing the routine application of seismic monitoring were, on the hardware side were:

· the logistical and cost issues of requiring hundreds of metres of cables running from seismic sensors to data acquisition units;

· the requirement for generators to supply power; and

· the problem of communicating results to the mine office.

On the software side, there was a requirement to develop automatic processing algorithms, since previous seismic surveys had been processed manually, resulting in long time lags before results were available.

Power and communications problems have been addressed by developing a wirelessly connected network of solar-powered acquisition nodes, one at the top of each instrumented borehole. The open-source 'earthworm' earthquake acquisition software, which can run on different hardware platforms, and use different acquisition cards, was modified for use in a coal environment by developing special new arrival-picking and event-location procedures. The system was field-trialled at Moranbah North mine. The acquisition software performed well, as did wireless communications and solar power. There were issues with the acquisition hardware selected, including problems with timing synchronisation, which is essential for seismic event location. Although these were fixed during the test, different hardware is likely to be used in future installations.

Most of the problems preventing routine use of seismic monitoring have been successfully addressed.