Relationship between wave speed variation and microstructure of coal under wet conditions

Abstract To explore the influence of coal microstructure on the wave speed in different wetting conditions, we first reconstructed a 3-D coal body model with computed tomography technology. We then performed ultrasonic measurements on wet coal samples and analyzed the effects of water on the wave speeds of coal samples with different structures. At last, we examined the relationships of the longitudinal wave speed to water-bearing saturation, porosity and fractal dimension of coal. The results showed that 1) longitudinal wave speed is relatively high in type I sample at dry condition, and slowly increases after wetting, presenting a poor wetting effect; 2) longitudinal wave speed rapidly increases in type I samples with water-saturation >80%, and in type II samples with water-saturation > 75%. These results indicate that the wave speed in type II samples is more sensitive to water than that in type I samples, which can be explained by that type II samples have more micropores and greater micropore wall tortuosity; 3) wave speed increment is positively correlated with porosity and fractal dimension, obeying an exponential function relationship; 4) wave speed is smaller at nature water saturation than at vacuum water saturation due to the distribution of pores and cracks and their connectivity; and 5) using wave speeds in different water saturation states can predict coal's cementation, connectivity as well as the number of isolated pores.