Faults and Absorption in Coal Seams

At the Centre for Geophysical Exploration Research at Macquarie University, computer programs have been developed to model the propagation of SH and P–SV waves in coal seams. These computer programs are based on the finite element method and include allowance for irregularities in the coal seam and for absorption in the coal and rock. In the longwall mining method, it is of the greatest economic importance to be able to detect faulting and other serious interruptions of the coal seam well ahead of the mining face. Scientists of the Broken Hill Proprietary Co. Ltd have conducted surveys of the transmission of SH and P–SV coal seam waves both by borehole observations and underground roadway observations. At frequencies below 200 Hz, interruptions of the coal seam are not readily detectable by SH and P–SV seam waves. At frequencies above 200 Hz, it is not easy to distinguish the effects on the transmitted SH and P–SV seam waves of interruptions of the seam from the effect of more than usual absorption in the seam. Thus, at a frequency of 400 Hz, for a coal seam offset one seam width by a vertical fault, finite element modelling shows that the effect of the fault is to reduce the amplitude of the SH fundamental mode by 82 percent. Allowance for a Q value of 25 in the coal seam and of 50 in the rock above and below the seam indicates a further decrease of 11 percent in only 24 m. Similar results have been obtained for the P–SV M23 mode in the same finite element model and for SH and P–SV modes in a model of a coal seam intersected by a vertical hard dyke. Because interruptions of the seam scatter SH and P–SV energy back and above and below the seam, it is desirable to make both reflection measurements and transmission measurements near the central plane of the seam to distinguish more readily the effect of a seam interruption from that of high absorption.