Numerical simulations and wavefield analysis of in-seam wave advance detection in viscoelastic media

In-seam waves are commonly applied to the advance detection of hidden faults in front of mining faces. However, coal seams, which have viscoelasticity actually, imposing absorptive and attenuation effects on in-seam waves. To investigate the wave field characteristics and propagation patterns of in-seam waves in viscoelastic coal seam media, this study built a three-dimensional geological model of coal measures with faults based on Kelvin-Voigt’s first-order velocity-stress equation. Using this equation, three-dimensional finite-difference numerical simulation was carried out. The results indicate that (1) for reflected in-seam waves in viscoelastic media, which exhibit high energy attenuation, their propagation and attenuation characteristics are more consistent with the actual situation of coal seams than those of reflected in-seam waves in completely elastic media; (2) High Q values of coal seams correspond to better advance detection effects of in-seam waves. By contrast, too low Q values are associated with poor advance detection effects. In this case, the x component of reflected shear waves and the z component of the P-S waves can be used for advance detection; (3) When the fault throw is less than the coal thickness, the y and z components of in-seam waves can yield great advance detection effects. When the fault throw is greater than the coal thickness and above, the y component of in-seam waves can be employed for advance detection; (4) When the angle between a fault plane and the tunnel is greater than 60°, the y and z components of in-seam waves can be employed for advance detection. When the angle is less than 60°, the x and y components of in-seam waves can be used for advance detection. The results of this study can provide theoretical support for the in-seam wave advance detection of coal mines.