Precursory changes in wave velocity for coal and rock samples under cyclic loading

Elastic wave velocity is a critical parameter in evaluating rock stability and integrity. Temporal changes in wave velocity during mine dynamic hazards including rockburst and gas outburst might potentially illuminate physical processes associated with stress concentration, crack propagation and fault slip behaviors. Laboratory and theoretical studies predict changes in wave velocity before rock failure; however, these dynamic hazards occur in a range of modes and little is known about precursors for those modes. Here, we show that precursory changes of wave velocity occur in laboratory tests for coal and sandstone samples under cyclic loading. We survey the ultrasonic wave velocity variation using the method of coda wave interferometry and find fluctuations of wave velocity during the cyclic loading corresponding with the stress-strain change, which may be explained by crack nucleation and propagation processes. Our data show that accumulated micro-cracks cause significant reduction of ultrasonic wave velocity and elastic moduli during the transitional phase preceding failure, which suggests that continuous wave speed monitoring might be a means to detect mine dynamic hazards precursors.