Study on the relationship between coal structure and ultrasonic wave velocity under wetting conditions

Water injection into coal seams can suppress coal dust and improve coal mass stability,and the wetting state is critical for evaluating the effectiveness of this process. To investigate the coupled effect of coal structure and wetting state on ultrasonic wave velocity,Xray computed tomography (CT) was employed to reconstruct raw coal samples in three dimensions and to quantify structural parameters including porosity and fractal dimension. Ultrasonic Pwave velocity tests were then conducted under different wetting conditions to establish the coupled relationship among velocity,structural parameters,and moisture state. In addition, molded coal samples with prefabricated fractures were prepared to analyze the combined influence of fracture geometry and wetting state on velocity response. The results show that the velocity increment of raw coal from dry to wet states is positively correlated with porosity and fractal dimension. In molded coal samples,the correlation between fracture thickness and velocity variation strengthens with increasing moisture content, whereas the effect of fracture size gradually weakens. These findings elucidate the coupled mechanism between coal structural parameters and moisture state governing ultrasonic propagation characteristics,and provide a theoretical foundation for monitoring the effectiveness of coal seam water injection.