Characterization of Seismic Attention and Permeability of an Underground Longwall Gob Using Crushed Rock and Coal Samples

In this study, 60 crushed samples comprising four groups of particles of different sizes were used to simulate the porous and crushed conditions of a gob area in an underground mine. Size distribution, porosity range, and the ratio of wavelength to the particle size of the crushed samples were taken into account with reference to the field observations in the actual gob. The ultrasonic wave velocity and attenuation were measured at an expected porosity under a loading pressure. Meanwhile, the permeability of the samples in the same conditions as the wave measurement was measured using the steady-state method. The results showed that the P-wave velocity of the crushed material was in the range of 400-1000 m/s, which was similar to the velocity of the fractured rock in the actual gob. The model for estimating the seismic attenuation of the crushed rock was confirmed to be a function of the porosity (phi), the density of discontinuity (delta), transmission coefficient (zeta), wave frequency (f), and the scatter factor (kappa) proposed in the empirical formula based on the equipment results. The estimated seismic attenuation was consistent with the measurements in the experiment as well as the actual value based on previous study. In particular, the estimation was quite accurate, while the ratio of wavelength (lambda) to particle size (d(m)) was between 35 and 65. The estimated permeability of the crushed rock mass fell into a magnitude range of 10(-12) to 10(-13) m(2). The permeability showed consistency with the Kozney-Carman equation but with a larger tortuosity coefficient. The field gob permeability estimated by the laboratory correlation was from 1225 to 15,866 10(-12) m(2), showing good agreement with the previous research. The relationship between the permeability and the seismic attenuation of the crushed rock was established based on the measurements.