Open Cut » Drilling & Blasting
Damage to the tops of coal seams caused by incorrect blast stand-off distances is a serious issue for the Australian coal industry. It results in coal losses of up to 10-15%, translating to ~40 million tonnes of lost coal per year. To date, there have been no effective and economically-sound techniques developed that map and characterise coal seam structures in the open cut environment to remedy this issue.
In this project, we used a new forward-looking imaging technique based on the borehole radar (BHR) technology to predict the coal seam top in real-time, while drilling blast-holes, to reduce coal top damage and subsequent loss of product. The method uses a conventional 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 when some energy is reflected back and the remainder radiates from the drill bit. The radiated energy is reflected by geological discontinuities such as the top of the coal, and is recorded by the BHR. These reflected guided-waves present themselves as obliquely striped patterns or forward-looking events in BHR profiles. These forward-looking events can be used to predict the coal top ahead of the drill bit. This provides potential for a conventional BHR to image ahead of the drill-bit by integrating the BHR with the steel drill string.
This project investigated the feasibility of the proposed guided BHR wave imaging technique for prediction of the coal top under typical open cut environments. This is achieved through both numerical modelling and field trials.
Numerical modelling investigation is focused on the feasibility of top coal prediction using guided BHR waves as well as various factors such as overburden resistivity, waveguide length and borehole size, which may affect the ability of predicting the coal seam top through the guided BHR wave imaging. The modelling results suggest:
- A conventional BHR can be electrically coupled onto 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 forward-looking antenna;
- Forward-looking events are relatively weak compared with the direct arrivals but can be enhanced by suppressing the direct arrivals and used for estimation of the coal seam tops ahead of a drill-bit;
- The prediction error is less than 10 cm, significantly better than the 30 cm required by the industry;
- The forward-looking capability of the BHR is about 4 - 6 m based on the modelled data;
- Conductivity of the overburden is the most important factor affecting the ability to see the coal seam top ahead of the drill bit;
- Coal seam thickness variation does not affect the prediction of the coal seam top.
The numerical modelling confirms that guided BHR waves could be used for coal top prediction during blast-hole drilling providing the overburden is relatively resistive (the average resistivity should be higher than 80 ohmm).
Field tests clearly demonstrate that guided BHR waves could be observed and used for top of coal prediction during blast-hole drilling even in relatively conductive environments where the prediction ahead of the drill-bit > 2 m required by the industry. However, no useful guided BHR waves are observed at the other two mines due to their very conductive ground. These results are consistent with the numerical modelling results.