Geophysical Prospecting, 2015, 63, 166–182 doi: 10.1111/1365-2478.12172 Seeing coal-seam top ahead of the drill bit through seismic-while-drilling Binzhong Zhou 1 ∗ , Iain Mason 2 , Stewart Greenhalgh 3 and Shankar Subramaniyan 3 1 CSIRO Energy Flagship, Kenmore, QLD 4069, Australia, 2 ARCOLAB, School of Geosciences, The University of Sydney, NSW 2006, Australia, and 3 Institute of Geophysics, Swiss Federal Institute of Technology (ETH), CH-8092 Zurich, Switzerland Received January 2014, revision accepted April 2014 ABSTRACT 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. INTRODUCTION In open-cut coal mining, the overburden must be first loos- ened and removed through blasting. Blast holes are typically drilled on a regular grid such as 10 m x 10 m with a blast hole depth varying from 30 m to 50 m. It takes approximately 20 minutes to drill a 30-m-deep blast hole to just above the seam to be mined. The depth of the holes needs to be accurately determined and controlled relative to the top of coal; otherwise, blasting will either damage the coal seam or leave a waste-rock skin too thick to be excavated with- out secondary blasting. Both situations seriously compromise production and profitability. Reducing such coal damage and loss requires the drilling of blast holes with sufficient accuracy to leave an approxi- ∗ E-mail: Binzhong.Zhou@csiro.au mate meter-thick protective layer of harder sedimentary rock above the seam, as illustrated in Fig. 1. Drilling beyond this 1-m margin causes exposure and dilution of the uncapped damaged coal seam, with losses of 10%–15% of the coal. Drilling short of this 1-m margin results in ineffective blasting and non-removal of the overburden, with consequent steril- ization of the resource. To date, the usual method of determining the top of coal is to drill exploration boreholes penetrating the target seam on a 50 m x 50 m grid. This pierce-point mapping technique yields spot depth accuracy of <0.5 m, but it is expensive and inaccurate when the seam is not flat and affected by rolls, washouts, and faults. Interpolation of depths between the widely spaced holes can result in serious errors. Drilling opti- mization and damage reduction requires that the blast holes be stopped short of the coal with accuracy of 0.2 m, not in depth but in stratigraphic position, because coal seams often 166 C  2014 European Association of Geoscientists & Engineers