Multiscale heterogeneity controls ultrasonic wave propagation in tectonically deformed coal: Insights from CT imaging and numerical wavefield modelling

Abstract To elucidate how multiscale heterogeneous structures control ultrasonic wave propagation in tectonically deformed coal, primary coal and cataclastic coal collected from the No. 15–17 coal seam of the Shoushan No. 1 Mine in Pingdingshan, China, were investigated using a combination of ultrasonic testing, X-ray computed tomography (CT), digital core reconstruction, and finite-element wavefield simulation. The pore-fracture characteristics and acoustic responses of different coal structures were systematically analyzed. The results show that the average P-wave and S-wave velocities of primary coal are 2038.33 and 1123.50 m/s, respectively, whereas those of cataclastic coal decrease to 1571.67 and 879.33 m/s, representing reductions of 22.89% and 21.73%, respectively. CT-based reconstruction reveals that the mineral content of both coal types is comparable, accounting for approximately 6%–8% of the total volume. In contrast, the porosity of cataclastic coal (2%–3%) is markedly higher than that of primary coal (1%–2%), accompanied by a broader pore-size distribution and a higher proportion of large-scale pores and fractures (> 150 µm). Acoustic analysis indicates that the dominant frequency of primary coal is concentrated at 75–85 kHz, with the main time–frequency energy distributed between 100 and 250 µs. In comparison, cataclastic coal exhibits more pronounced low-frequency and multi-peak spectral characteristics, with the main energy zone shifting to 150–350 µs, indicating enhanced scattering attenuation and prolonged coda waves. Finite-element simulations further demonstrate that mineral inclusions induce only localized wavefield perturbations with negligible influence on the first-arrival time, whereas pore-fracture networks significantly alter wave propagation paths. In fractured models, the first-arrival time is delayed from 55 to 58 µs (approximately 5.45%), accompanied by the formation of multiple discrete energy clusters within 80–200 µs. These findings indicate that the pore-fracture system is the dominant factor controlling ultrasonic wave propagation, wave velocity attenuation, spectral broadening, and coda-wave complexity in tectonically deformed coal, whereas mineral inclusions mainly exert localized perturbation effects. This study provides a theoretical basis for the acoustic characterization of tectonically deformed coal and the evaluation of coal damage.