Depth of mining
As the magnitude of vertical stress increases with the depth of mining, it seems reasonable to say that these stresses will increase the probability of outburst at greater mining depths. However, this does not exclude the chance of outburst at small depths. While many parts of the world have not experienced outbursts at depths less than 180 metres, the Moura mine in the Bowen Basin, Australia has experienced three gas outbursts at a depth of approximately 130 metres. These outbursts were attributed to shear zones present at that depth and it is accepted that with the existence of certain geological conditions, outbursts may occur at depths as little as 100 metres (Lama & Bodziony, 1996). There is also little record of outbursts occurring when the depth of mining exceeds 500 metres. This is probably due to a change in the rank in coal as depth increases. Therefore, it can be surmised that while mining depth plays a role in the majority of outbursts, there can be no general rule found for the likelihood of outburst at a given depth.