Detection of Top Coal by Conductively-Guided Borehole Radar Waves: Results from Numerical Modelling

Damage to the tops of coal seams caused by incorrect blast stand-off distances is a serious issue in the Australian coal industry, costing the equivalent of about one open cut mine for every 10 operating mines in lost coal. To date, no effective and economically-sound techniques have been found to map and characterize coal seam structures in the open cut environment. We propose to use conductively-guided borehole radar (BHR) waves for real-time prediction of coal tops during blast-hole drilling. The method uses a conventional borehole radar (BHR) with a dipole antenna, which can image sideways around the borehole, electrically coupled to a conductive wire or steel drill-rod to induce a guided wave along the axial drill-rod. The drill-rod ahead of the BHR becomes part of the radiating antenna. The guided wave travels to the end of the drill-bit where some of its energy is reflected back, while the remainder radiates from the drill bit. The radiated energy will be reflected by geological discontinuities such as the top of a coal seam, and recorded by the BHR. In this paper, numerical modeling is used to investigate the feasibility and affecting factors of this guided-BHR-wave-based approach for predicting the approach of coal seams during blast-hole drilling. It is demonstrated that conductivity of the overburden is the most important factor affecting our ability to see coal seams ahead of the drill bit; that the guided BHR waves could be used for top coal prediction; and that a theoretical prediction error less than 10 cm and a forward-looking capability of 4-6m can be achieved based on the modelling results.