Modeling of seismic wave propagation around coal mine roadway with presence of excavation-damaged zone

In-seam seismic methods have been widely used in underground coal mine exploitation since early 80’s. They are helpful for identification of stress concentration zones or to locate geological disturbances within the coal seam. Usually, such surveys are optimized to perform seismic tomography. Therefore, sources and receivers are located on the opposite sides of the longwall. Results are produced in form of velocity maps of body-waves for rock-coal-rock medium or maps of group velocity and frequency of Airy-phase of dispersive waves trapped inside the coal seam, so-called channel waves. However, with the above geometry, the high-resolution imaging of the rock mass close to the roadway, including excavation-damaged zone (EDZ), is hampered by the available ray coverage.  In order to overcome this limitation, sources and receivers should be mounted in the same roadway. There is also a fundamental problem contributing to the lack of a robust method to image such area, which is the complexity of the seismic wavefield in the vicinity of the EDZ in a coal seam, where both surface tunnel waves and Rayleigh and Love-type channel waves overlap. We address this problem using numerical simulations. We use finite-difference method and viscoelastic model with petrophysical parameters for coal and host rock layers representative for the Upper Silesia mining district. First, we analyze seismic waves propagation within simple rock-coal-rock model, particularly channel waves dispersion properties. Then, we add a roadway with 3-meter thick EDZ to the model. Velocity and density within the EDZ linearly decrease up to 70% close to the free surface of excavation. By analyzing particle motion close to the free-surface, we observe that for very short wavelengths, the main energy is traveling as a fundamental mode of Rayleigh surface tunnel wave (for horizontal components). However, for longer wavelengths, the main energy is focused around frequency of Airy-phase of fundamental mode of Love-type channel wave. Eventually, we insert 10% Gaussian-shape velocity anomaly with 20 m width in the middle of the roadway to the model and investigate changes in frequency and group velocity of Airy-phase of Love-type channel waves for different offsets. We notice that the group velocity and frequency of maximum energy correspond to the velocity anomaly. For longer offsets, these parameters are approaching theoretical values for undisturbed medium. We conclude that because the group velocity of the Airy-phase is close to the coal S-wave velocity, it can be possible to image the velocity of such wave in the vicinity of the roadway, especially when the thickness of the coal seam is known.  

 

This research is supported by Polish National Science Centre grant no UMO-2018/30/Q/ST10/00680.