Outburst danger criteria for acoustic prediction methods at the initial and final stages of preparing sudden coal and gas outburst

Two stages of preparing coal and gas outburst are studied in the article. At the first stage the cracks which create a coal block structure are being developed ahead of the moving workings in a zone of high rock pressure. At the second stage the crashing of a narrow coal layer in the mouth of starting outburst cavity and its outsqueezing into the workings takes place. The control of cracks development is fulfilled by acoustic emission method. For controlling the rock pressure spectral-acoustic method is applied. The outburst danger criterion of spectral-acoustic method is corrected depending on the coal strength and in-situ pressure and they are estimated according to the methane concentration in the workings atmosphere. The results of outburst danger criteria comparison for different variants of spectral-acoustic method in a form of relation between high frequency and low-frequency amplitudes of the operating equipment “noise” spectrum and in the form of current and critical values of amplitude-frequency characteristic medians of noise are introduced. It is demonstrated that a crack development criterion and the criterion of a coal layer outsqueezing coincide if there is no gas in a coal seam. With the increasing of methane concentration at the face the crack development criteria decrease much faster than outsqueezing criterion. It is concluded that the result of the outburst danger prediction made by acoustic emission method is a necessary but not sufficient condition for initiating the outburst. The prediction, made by acoustic emission method supplemented by controlling methane concentration and coal strength allows estimating outburst danger within the whole process of outburst preparation.