A guided wave is induced along a conductive logging cable or a conductive water-filled borehole during borehole radar (BHR) surveying. Such a guided wave is normally considered as unwanted interference to normal BHR waves and needs to be suppressed, either through BHR designs or data processing. In this letter, we advocate that the guided wave can be coupled onto a drill string, which can act as a forward-looking antenna for imaging ahead of the drill bit while drilling. Using data collected by a monostatic BHR with different configurations at an abandoned mine site in Brukunga, South Australia, we demonstrate that the forward-looking capability of the BHR is about 2-6 m in the tested borehole section. Index Terms-Borehole radar (BHR), guided electromagnetic (EM) wave, imaging.1545-598X

Damage to the top of coal seams, caused by incorrect blast stand‐off distances, results in coal losses of up to 10–15% to the Australian open cut coal mining operations. This is a serious issue to be addressed. Here we propose to use a new forward‐looking imaging technique based on the borehole radar technology to predict the coal seam top in real time while drilling blast holes. This is achieved by coupling the conventional borehole radar waves on to a steel drill rod to induce a guided wave along the axial drill rod. The drill rod ahead of the borehole radar behaves as a forward‐looking antenna for the guided waves. Both numerical modelling and field trials simulating a drill rod as an antenna are used to investigate the feasibility of the proposed technique for prediction of the coal top under typical open cut environments. Numerical modelling demonstrated that conductivity of the overburden is the most important factor affecting our ability to see coal seams ahead of the drill bit, the guided borehole radar waves could be used for top coal prediction and a theoretical prediction error less than 10 cm and a forward‐looking capability of 4–6 m can be achieved. Field trials at Australian open cut coal mines also demonstrated that guided borehole radar waves can be observed and used for prediction of coal top ahead of drill bit during blast‐hole drilling in resistive, open cut environments (the average resistivity should be higher than 75 Ωm).

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