The elastic wave dispersion mechanism of different coal structures under room-dry condition

Abstract The elastic characteristics of different coal structures (primary, cataclastic, granulated and mylonitized) are important for coalbed problems such as methane exploration and mining. However, due to various factors influencing coal elastic parameters, there is still little understanding about it. In this study, we selected four samples with different coal structures and measured their elastic parameters in seismic frequency band (4–300 Hz) under varying confining pressure (room pressure up to 20 MPa). The results show that coal elastic wave velocities are frequency dependent under room-dry condition. These results contrast with the previous understanding that rock dispersion phenomena are caused by fluids. The dispersion of elastic wave velocities decreases with increasing confining pressure and increases as the coal structure becomes more fragmented. Under room pressure, the P-wave velocity in primary coal and mylonitized coal exhibited 5.91% and 12.04% increases from 4 to 300 Hz, respectively. However, at 20 MPa, these increases were reduced to 2.78% and 4.75%. The fractal viscoelastic model, which is grounded in the staggered interlayer structure of coal matrix, effectively describes the dispersion characteristics of coal. That reveals the fractal structure of the coal may be used to describe the coal viscoelastic characteristics. We found that frictional dissipation and the viscoelastic components, such as clay and organic matter, caused the dispersion. The difference in compliant pore content is the primary reason for the different dispersion in various coal structures in this study. This research may provide a new basis for studying the elastic wave dispersion of different coal structures.