Characterising Roof Fall Signatures from Underground Mines

A sequence of seismic events caused by multiple impacts of rocks of varying size hitting the mine floor and small rock rupturing events, associated with three roof falls, are analysed in terms of seismic moment, radiated seismic energy and apparent stress. The study site is an underground limestone quarry and the data were obtained from a multi-channel, in-mine seismic monitoring system. Estimates of seismic moment and radiated seismic energy are calibrated using rock drop tests and local quarry blasts. Experiments show that falling rocks are very inefficient sources of high frequency seismic energy, and that the condition of the mine floor (whether clear or debris-covered) plays a substantial role in determining the amount of radiated seismic energy. Massive (2,400 metric ton) roof falls differ from rock shear/rupture type events in that their signatures are emergent and long in duration, and they have a low apparent stress value. The low apparent stresses are thought to be caused by the inefficient source of roof rocks striking the mine floor. The observed moment magnitudes of four roof falls ranged from 0.1 to 1.4, with apparent stress less than other local mining induced seismic events. Prior to these massive roof failures, an increased level of seismic events associated with fracturing of the roof strata and ranging in magnitude from -1.4 to 0.3, are observed. The characteristic low apparent stress of the massive roof falls and precursory low-magnitude rock fracture events may hold promise for successful seismic monitoring of hazardous mine roof conditions.