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Geological Society, London, Engineering Geology Special Publications; 1988; v. 5; p. 423-433;
DOI: 10.1144/GSL.ENG.1988.005.01.47
© 1988 Geological Society of London

Session 10: Induced Seismicity

Microseismic monitoring for the prediction of outbursts at Cynheidre Colliery, Dyfed, S. Wales

P. Styles & S. J. Emsley

Department of Earth Sciences, University College Swansea, Singleton Park, Swansea SA2 8PP, UK

T. Jowitt

British Coal, 16/17 South Parade, Doncaster, South Yorkshire SN1 2DX, UK

During the past three years a network of six permanent surface seismometer stations has been established over the Cynheidre workings. In parallel with this an 8-channel on-line microprocessor-based event detector has been developed in order to provide warnings of high rates of activity.

The main results of this twofold approach are as follows:

  1. Considerable normal microseismic activity is generated from within Cynheidre with an excellent correlation between the patterns of daily extraction and the levels of activity which can exceed 600 events per shift. It is thought these events principally originate from failure of the roof cantilever and the gob. They have impulsive onsets and durations of 0.5-1.0 s.
  2. Observations during outbursts suggest a completely different type of activity. The envelope of the events is very emergent and almost monochromatic with a frequency of about 30 Hz. These events appear to originate from within the coal, to be related to the emission of methane and show clear peaks in activity immediately prior to an outburst. In fact this type of activity is extremely rare except before periods of gas emission or an outburst.

The microprocessor has clearly shown increases in the automatically picked events prior to the outburst of 24 June 1985 particularly on the station adjacent to the event. The microprocessor also gave an alarm prior to the recent heavy gas emission (8,500 m3) associated with ‘pouncing’ (20 February 1986) which was controlled successfully without proceeding to a full outburst.