Underground » Geology
In-seam drilling is an integral part of mining in many underground mines and it is undertaken for gas drainage and to prove coal seam conditions. Current drilling practice allows precise survey data to be obtained as to the plan position of the hole. While this means that drills can be steered to chosen targets, it is, however, not possible to guide the holes so that they maintain their position with respect to an undulating roof or floor.
Sub-surface-radar technology has been recognised as having the potential to address this problem and early research in this area concentrated on the development of directional borehole probes based on conventional, electric (E) field radar systems. This work indicated that conventional electric (E) field radar systems have substantial limitations. When used in high quality coal seams with good dielectric contrast between seam, roof and floor, E field radar performs well. However, the radar performance degrades significantly when used in lower quality seams exhibiting electrical losses and/or clutter due to material inhomogeneity.
This report presents the development of a directional borehole probe based on magnetic (H) field radar as an alternative technology with the potential to overcome some of these problems.
The report provides an introduction to borehole radar characteristics, discusses the advantages and disadvantages of various probe configurations and the properties of Australian coal seams that relate to radar performance. Particular note is made of the important factors associated with utilizing borehole radar probes for monitoring-while-drilling applications in coal seams. Since wideband, directional H-field antennas for borehole applications do not feature in the GPR literature, the initial probe development was based on a substantial body of antenna modelling carried out at CTIP using purpose-written finite difference time domain (FDTD) software. Using this background, various borehole probes have been constructed and tested in a range of laboratory and field conditions. The report provides a discussion of the probe components and their influence on performance characteristics. Representative field trial data is presented and discussed.
The H-field probes have been developed to the stage where they can be used as stand-alone, post-drilling, survey tools and have shown considerable performance improvement over E-field probes. Trial results have shown that the wide range of coal dielectric properties present in Australian coal mines and the need for dielectric contrast between roof, seam and floor are the major influencing factors determining borehole radar performance. While the H-field probes are considerably more robust than E-field probes, their potential as monitoring-while-drilling tools has yet to be realised, primarily because there are still major problems associated with building a probe having the mechanical strength to withstand the drilling forces. In this regard, water-jet drilling applications may present less demanding requirements than conventional down-hole drilling.