A borehole radar system for South African gold and platinum mines

Borehole radar is an electromagnetic tool that can be applied to assist in the delineation of orebody geometry, ideally using routinely drilled cover and exploration boreholes. Successful trials of borehole radar for delineating reef horizons on South African gold and platinum mines have led to the development of a borehole radar system specifically designed for routine application in those enyironments. The radar design includes novel elements, including a receiver with instantaneous sampling down the borehole, and it is implemented in probes that can operate in 48 mm boreholes, with development planned for 38 mm boreholes. The radar is known as the Aardwolf BR40. The need for information about dislocations of 3 m to reefs determines the desirable radar resolution while available access geometry determines the range requirement. The electrical properties of typical gold and platinum rocks show that the range/resolution trade-off is feasible for the majority of economically important reef horizons. Boreholes drilled horizontally or upwards are accessed using a borehole crawler. Trials of the radar show that it meets its performance specification. The radar is robust enough for routine work imderground and is easy to use. The borehole radar is a useful addition to the toolbox of the mining geoscientist because it can give information about the reef plane along a line, rather than the single point information about the reef given by a borehole. Introduction Borehole radar is the application of Ground Penetrating Radar (GPR) within a borehole. GPR is a geophysical tool that creates images of the subsurface by using short pulses of radio energy. GPR and borehole radar offer the highest spatial and depth resolution of any geophysical imaging technique for resistive rock environments. The pulses are transmitted into the earth, and reflect off discontinuities in electrical properties, for example the interface between a quartzite and a shale (Annan and Davis, 1977; Daniels, 1997). There are three reqLiirements for successful implementation of GPR or borehole radar reflection imaging: 1. The host rock must be resistive. Radio waves do not travel through conductive rock. 2. There must be a sharp interface in electrical properties between the host and the target horizon. Graded contacts do not produce a good reflector. In addition, the target horizon must have different electrical properties to the host rock. 3. The target must ain parallel or sub-parallel to the survey line. With borehole radar, the borehole is the survey line. Targets that intersect the borehole at right angles will not produce a reflection. In 1996 and 1997, borehole radar trials were conducted on a number of economically significant South African mining targets, using the Swedish Mala RAMAC system. Sui'veys targeting the Ventersdorp Contact Reef (VCR) were particularly successful (Vogt et al, 1997). At the time, the RAMAC system did not function at the temperatures encouintered in deep level gold mines but the success of the technique convinced the DEEPMINE collaborative research programme to ftind further work. The GeoMole borehole radar system developed at the Centre for Mining Technology and Equipment in Sydney, Australia (Turner et al, 2000) was applied at a number of surface and underground sites with success (Trickett et al, 1999; 2000), Late in 2000, the CSIR decided that the local requirement warranted development of a borehole radar system specifically for South African mines. The design of that tool, the Aardwolf BR40, is described here. Probtem description The majority of gold in South Africa occtirs along the edge of the Witwatersrand Basin, which was a vast inland sea more than 2714 million years ago (Armstrong et al, 1992). The gold bearing seams, known as reefs, are typically a metre thick, gently clipping and of very large lateral extent. The reefs are typically conglomerate layers hosted within a sedimentary succession consisting of quartzites, shales and conglomerates. Basin evolution ended with the outpouring of the Ventersdorp Supergroup lavas at 2714 Ma (Schweitzer and Johnson, 1997). Some of the reefs have very strong physical property contrasts from the surrounding rocks, particularly the VCR, because of the difference in density and electrical properties between the lava and the reef footwall rock types. Other reefs are not associated with any physical property changes, for example, the Vaal Reef. Some reefs such as the Carbon Leader that do not themselves have a contrasting physical property to the surrounding rocks are closely associated with marker horizons. For example, the Carbon Leader is overlain by the Green Bar Shale (Schweitzer and Johnson, 1997). SOUTH AFRICAN JOURNAL OF GEOLOGY, 2006, VOLUME 109 PAGB 521-528 522 A BOREHOLE RADAR SYSTEM EOR SOUTH AFRICAN GOLD AND PLATINUM MINES