Published in Australia's Longwalls
GPR was initially developed for glacier research, and subsequently used by the military to map the thickness of ice sheets in the Arctic and Antarctic. And while GPR has long been accepted in the geotechnical and civil engineering sectors for near-surface imaging, it is only relatively recently that the technique has been applied to large-scale mining exploration and operations.
IMC is a world leader in the collection and processing of GPR data, most successfully in the delineation of nickel laterite and bauxite deposits for integrated mining and processing projects, according to Bob Gallagher, principal consultant (mining) with IMC’s Brisbane office.
GPR can continuously image the details of the laterite profile, which can be highly variable between drill holes. In one case, the greater data set made it possible to introduce GPS-guided mining at a bauxite mine -- not possible with sparse drill hole and pitting data. The result has been a significant increase in resource recovery and an accompanying reduction in unit cost per product tonne.
The primary advantages of GPR as a geophysical tool are that surveys are relatively inexpensive, have little or no environmental impact and can provide a higher resolution view of the subsurface than other geophysical techniques.
GPR uses radio waves to penetrate and map the subsurface as well as some of the same principles as the seismic methods commonly used in petroleum exploration. The two methods are often complementary since the radar waves generally do not penetrate to depths greater than 50m (although under particular ground conditions, depths of up to 110m have provided reasonable resolution and definition), and seismic reflection exploration is very difficult at depths of less than 25m. Further, seismic surveys may be inhibited by materials in the stratagraphic column such as basalt.
The short wavelength used for GPR has limited penetration, but provides much higher resolution than other geophysical methods, making it ideally suited for near surface or near underground opening geological exploration and definition.
Gallagher said previous research carried out principally in the early and mid-1990s by the US Bureau of Mines and others in both surface and underground coal generally concluded that the method holds great potential. This research examined a range of applications, from mapping immediate below-surface geologic conditions, to assessing hazards in the roofs of underground mines. Since then, The National Institute for Occupational Safety and Health (NIOSH) has continued to pursue applications of GPR in mining, though at a reduced level due to funding cuts.
Recent GPR work by NIOSH has been in abandoned mine detection and location, and investigations of application to various near opening and near surface geotechnical investigations.
“Since the initial US research work was completed, significant progress has been made in GPR technology and processing. In Australia, NERDDC (National Energy Research Development and Demonstration Council) and ACARP (Australian Coal Association Research Program) research has generally been limited to development of sensory tools for equipment control (longwall shearer, drilling and continuous surface miner guidance). It has not really been considered in the context of US research for direct application to exploration and operations (eg strata control), with the exception of coal pillar stability evaluation at Cooranbong,” Gallagher said.
Both GPR and processing technologies have advanced, significantly enhancing capabilities of the technique. For example IMC has created and applied algorithms designed to detect hyperbolic reflections and to generate radar wave velocity maps of the subsurface, with subsequent use of appropriate multipliers on the two-way travel times on the final processed GPR profiles. This is a significant improvement on calibration using boreholes and trial pits and adoption of average radar wave velocities.
Moreover, borehole GPR equipment has been further developed and applied at depths up to 1500m. Maximum borehole spacings of around 80-120m are likely required for assessments within the coal seam between holes for borehole to borehole applications. However, borehole to surface (for shallow applications) and borehole reflection surveys (to 1500m) present alternatives. Such equipment allows for detailed fault structure investigation within a longwall block from the surface totally independent of underground mining operations if so desired.
There is a wide range of potential applications for the technology in the underground coal industry, particularly in view of the removal of the requirement for approvals for underground use, and replacement with risk managed approaches for portable electrical equipment, Gallagher said.
