Protecting the coal-seam top
Damage to the top of coal seams due to incorrect blast stand-off distances is a serious issue for Australian miners and accounts for a loss of up to 15% of coal in Australian open cut coal mining operations.
A critical step in open-cut coal mining is clean removal of the overburden. This is usually achieved by drilling, blasting and digging.
Mine operators need an accurate coal-seam model so that during blast-hole drilling the driller stops at the correct blast stand-off distance, designed to leave a protective cap of rock above the coal seam.
Miscalculations lead to serious damage to the top of the coal, rendering it useless.
It has been estimated that incorrect stand-off distances on blast-holes cost the Australian coal industry the equivalent of one open-cut mine for every 10 producing mines.
This translates into a loss of billions of dollars per year in addition to the permanent loss of millions of tons of the precious resource.
To minimise such damage and loss, the depth of the coal seam must be known accurately, so that the driller stops in time to leave a ~1 m protective layer of harder sedimentary rock above the seam.
Drilling beyond this protective layer causes exposure and dilution of the uncapped, damaged coal seam, leading to significant losses of the coal.
Conversely, drilling short of this protective layer means more work will be required to remove the intact overburden.
Radar gives the driller real-time ‘sight’
We are developing a real-time imaging-while-drilling technology using guided borehole radar waves to detect the coal-seam top ahead of the drill bit.
A borehole-radar (BHR) based technique provides “sight” for the driller to detect the coal seams ahead of the drill-bit in real-time while drilling blast-holes.
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 will be reflected by geological discontinuities, such as the top of coal, and recorded by the BHR.
This provides potential for a conventional BHR to image ahead of the drill-bit by integrating the BHR with the steel drill string.
'Seeing' ahead of the blast zone
With the support of ACARP (the Australian Coal Association Research Program) and its associated mines, we have conducted both numerical modelling and field trials to investigate the feasibility of the proposed techniques for coal-top prediction.
Our 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 BHR waves could be used for coal-top prediction
- 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 demonstrated that guided BHR waves can be used to predict the coal top ahead of the drill bit during blast-hole drilling.
As a result of these findings, ACARP has provided further funding to ready this world-leading technology for commercial use.
Part of this project extension involves addressing the practicalities of integrating the BHR onto the drill-rod for real-time prediction of coal-seam top while drilling.