Predictions of Outbursts in Underground Coal Mines Based on Radon-222 Anomalies

The frequency and magnitude of instantaneous outbursts increase with depth and rising advance rate. These conditions are increasingly common in coal mining. Effective outburst management strategies will have to respond to this challenge. Unfortunately, most current methods of evaluation of the outburst hazard require extensive exploration drilling and gas content sampling. Hence, there is a need for new, automated and non-disruptive outburst prediction methods to be developed and deployed in modern mechanised mines working outburst prone strata, if commercial viability of operations is to be maintained. They will provide reliable, real-time information on potential outburst hazards ahead of development roadways.

The developed method relies on an indirect monitoring of the change of radon-222 flux from an area of the monitored workings. This is done by measuring radon concentration in ventilation air, which passes along a roadway towards the face and then back to the main headings. An advanced radon-222 measurement technology developed at ANSTO is utilised in the project. Radon-222 is a naturally occurring radionuclide which is present in trace quantities in coal seams and rock. Its rate of release into underground ventilation air strongly depends on changes in the formation, prior to an instantaneous outburst.

This is a local, continuous face monitoring, fully automated method, real-time changes are recorded and can be analysed on-line with a possibility of full integration with the mine alarm system. Other features include:

• the analysed signal comes from a large, user defined area, hence the signal is more informative than spot measurement, bearing in mind that outbursts tend to involve the seam or a large part of the seam
• Safety considerations are simplified as the detection unit can be placed at the non-hazardous zone, far away from the advancing face.

Because of the novelty of the method, the report provides a minimum background required for the understanding of underlying principles, describes principle of detection, elaborates on specific requirements imposed on detectors by underground conditions, which include an adequate detector sensitivity and stability, and safety aspects of detector construction.

Description of the results gained in the first deployment at South Bulli Colliery are given. Detectors proved to work well underground and produced a first set of test results. The results have verified all relevant assumptions concerning expected absolute and incremental radon levels in underground air. Radon concentrations have been determined with the designed precision on the required time scale. The sensitivity proved to be adequate, even for the gateroad development with a relatively small face area. The detectors' dynamic range accommodates environmental changes in radon concentration and the instrumental performance has fully validated the choice of construction materials required for the underground environment.

This is a novel method. To be successfully used as an outburst management tool, impending outburst criteria need to be developed based on statistically significant measurements accompanying outbursts in carefully targeted workings in different mines.

The method can be applied without any modification, for prediction of rock failure leading to methane emission. Such an application seems to be an attractive proposition since methane emissions in longwall mining are more frequent than outbursts, and a much bigger face area of the longwall would probably lead to correspondingly higher net radon signals.