Study on the relationship between coal structure and acoustic characteristics under wetting conditions

The wetting of coal seams during water injection can be effectively reflected by changes in ultrasonic P-wave velocity. To investigate the ultrasonic P-wave velocity of water-bearing coal with different structures, this study examines both raw coal and briquettes. A 3D coal body model was first reconstructed using computed tomography technology. Ultrasonic measurements were then performed on wet raw coal samples to analyze the effects of water on the wave velocity of coal with varying structures. Relationships between P-wave velocity, wet ratio, porosity, and fractal dimension were explored. Additionally, ultrasonic tests were conducted on briquette samples prepared under different conditions to evaluate the influence of macroscopic cracks on ultrasonic characteristics under wetting. The results show that dry Class A samples, characterized by fewer pores, lower porosity, and smaller fractal dimensions, exhibit high P-wave velocities, but their velocities increase slowly after wetting, indicating a limited wetting effect. Conversely, Class B samples, with higher microporosity and larger fractal dimensions, which are more sensitive to water, display lower initial velocities but significant increases after wetting. This sensitivity can be attributed to the higher microporosity and greater tortuosity of micropore walls in Class B samples. The wave velocity increment is positively correlated with porosity and fractal dimension. For briquettes, a distinct bedding effect on P-wave velocity is observed. The influence of crack thickness on wave velocity increases with the degree of wetting, while crack size has the opposite effect. Under dry conditions, crack inclination is positively correlated with wave velocity; however, this correlation diminishes as wetting increases. These findings provide theoretical guidance for evaluating coal seam wetting levels using ultrasonic detection techniques.