Forward modelling for subsurface measurements in open-cut coal mine using a drill bit source
Characterization of the overburden rock mass and identification of the top surface of the coal are of critical importance in any open cut coal mining operation, because they play a major role in achieving better coal recovery and better performance in drilling and blasting and in the design of highwall slopes. Among various techniques for identifying the top of the coal seams, the seismic while drilling offers cost effective solutions in terms of identifying the top of the coal seam in advance of drilling and in developing a real-time subsurface measuring system. In this system, the bit noise generated during rotary drilling can be monitored using three-component vibration sensor placed either on the surface or in the drilled borehole, and subsurface measurements can be carried out. In this paper, a full waveform forward modeling was carried out using finite difference scheme to determine the feasibility of identification of coal seams using an optimized wavelet as a drill-bit source. As an example, a hypothetical case is considered wherein the stratigraphic layers were assumed to be flat. Two source-receiver distances of 10m and 50m were considered. For an assumed wave-let source and stratigraphic sedimentary layers, the interpreted results have clearly revealed that the top and bottom of the coal layers at difference depths can be identified. For a frequency range of 1000Hz, 30-35° incident angles was found to be critical for all the coal layers. Forward modeling play an important role in understanding structurally complex issues, and in particularly important in designing the optimal program for seismic while drilling before actual field experiments are carried out. © 2011 ARMA, American Rock Mechanics Association.