Elastic properties of coal measure rocks from the Sydney Basin, New South Wales

Abstract

This laboratory study determined the densities and ultrasonic velocities (compressional and shear) of a large number of samples of NSW Permian bituminous coal, and enclosing roof and floor rocks. The tabulated results were obtained for three important Sydney Basin coalfield seams: the Wongawilli (coking), Katoomba (steaming) and Lithgow (steaming). The dry bulk densities range from 1.34 to 1.45 t m-3 which is relatively high for bituminous coal (1.1?1.5 t m-3 usual range), but low in comparison to adjacent sandstones, mudstones and shales (2.3 t m-3). Compressional wave velocities of the coal samples perpendicular and parallel to the bedding were approximately 2000 and 2300 m s-1 respectively, except for the Wongawilli which averages 1740 and 1850 m s-1. Shear wave velocities perpendicular and parallel to the bedding average, about 960 and 1080 m s-1 respectively, with the Wongawilli again giving lower values of the order of 800 m s-2 The ultrasonic coal velocities are low compared to the sedimentary rocks (Vp~3500 m s-1). Velocity anisotropy for coal was marked with velocity differences up to 36%. The dynamic elastic moduli ranged from 1000 to 5000 MPa and show considerable variability. Poisson's ratio for coal spans a narrow range of 0.34 to 0.37 which is high compared to most rocks (0.25). The application of this laboratory data to in situ conditions needs cognizance of the following factors: degree of water saturation, coal friability, local jointing and sediment partings, and confining pressure.