Remotely sensing the thickness of the bushveld complex UG2 platinum reef using borehole rada.

Abstract

The planar, 80 cm thick, lousy dielectric reefs of the Bushveld are embedded in rocks that are almost transparent at ground penetrating radar frequencies of 10–125 MHz. Pothole sensing practices are based largely on using borehole radars to observe departures of the reefs from planarity. Surveys are run in ~200 m near-horizontal boreholes that are drilled into the footwalls of the reef. Careful laboratory measurements of the Jonscher dielectric parameters of the stratigraphic column through the UG2 reef are translated by electro-dynamic modelling into a prediction that platinum reef thinning can be sensed remotely by footwall borehole radars. This proposition sheds light on the results of a recent borehole radar survey that was shot in ~180 m long AXT (48 mm diameter) boreholes. Areas of sub-economical UG2 thickness (typically less than ~50 cm) were mapped by studying the relative amplitudes of echoes from the reef and a pyroxenite–anorthosite interface in its hanging wall, with the radar deployed beneath the UG2 in its footwall.