Seeing coal-seam top ahead of the drill bit through seismic-while-drilling

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Blast damage to the tops of coal seams due to incorrect blast standoff distances is a serious issue, costing the industry in Australia about one open-cut mine for every ten operating mines. The current approach for mapping coal-seam tops is through drilling and pierce-point logging. To provide appropriate depth control with accuracy of ±0.2 m for blast hole drilling, it is typically necessary to drill deep reconnaissance boreholes on a 50 m x 50 m grid well in advance of overburden removal. Pierce-point mapping is expensive and can be inaccurate, particularly when the seam is disturbed by rolls, faults, and other obstacles.Numerical modelling and prototype-field testing are used in this paper to demonstrate the feasibility of two seismic-while-drilling-based approaches for predicting the approach to the top of coal during blast hole drilling: (i) reverse “walk-away” vertical seismic profiling recording, in which the drill bit vibration provides the source signal and the geophones are planted on the surface near the drill rig, and (ii) in-seam seismic recording, in which channel waves, driven by the coupling to the coal of the seismic signal emitted by the approaching drill bit, are guided by the seam to geophones located within the seam in nearby or remote boreholes.

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