Dispersive seismic waves in a coal seam around the roadway in the presence of excavation damaged zone
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Excavation damaged zone (EDZ) is a fractured rock mass in the vicinity of the roadway. Knowing its elastic properties, such as seismic velocity, is crucial for the safety of underground mining. However, the seismic wavefield's complexity due to the low-velocity layer (e.g., a coal seam) makes it difficult to obtain reliable velocity estimates. Here, we study the influence of a roadway and EDZ on the seismic wavefield propagation inside the coal seam along the roadway close to the sidewall based on numerical simulation and in-seam seismic measurements. We focus on dispersive waves due to their dominant energetic contribution and, hence, application potential. We use the finite-difference method and viscoelastic model. First, we analyze seismic wave propagation within a simple rock-coal-rock model. Then, we add a cylindrically shaped roadway with 3-meter thick EDZ to the model. We observe a strong so-called roadway mode. Polarization analysis shows that such a mode in a horizontal plane in the center of the coal seam is the mix of Love-type channel and Rayleigh surface tunnel waves. Moreover, the EDZ causes a significant decrease of the group velocity, which turns out to be even lower than the lowest group velocity of the surface tunnel waves which travels solely in a coal seam. We also observe that the roadway mode with the presence of the EDZ is less dispersive, which can be useful with imaging the EDZ by those modes. Those roadway mode features were confirmed by the real seismic data from an underground experiment in the Rydułtowy coal mine in Poland. The rock mass deformation in the vicinity of the roadway strongly correlates with the roadway mode dispersion characteristic. Our study shows the great potential of using roadway mode in imaging and monitoring of underground excavations in the presence of a low-velocity layer.