Borehole Radar Delineation of the Ventersdorp Contact Reef in Three Dimensions

A large portion of South Africa‘s gold reserve comes from the Ventersdorp Contact Reef (VCR). The VCR is a thin, tabular ore body, generally less than 1.2 m thick. The geometry of the VCR is mainly controlled by slopes, terraces, and faults. The gold is concentrated in palaeo-river channels situated on river terraces and not on the confining terrace risers or slopes, so there is a good correlation between topography and grade. The geophysical problem is to determine the reef geometry before mining begins, in order to plan optimal ore extraction.

Borehole radar has already been shown to be a suitable technique for mapping topography on the VCR because of the favourable electrical property contrast between the footwall rocks and the overlying lavas. Here, a forward model is used to resolve the directional ambiguity inherent in low-frequency borehole radar data and to accurately position the target reflection in space. The data from five boreholes within a 200 m square block of ground are combined in a single model to estimate the topography of the ore body within the block. The illumination lines from the radar forward model are positioned in 3D. They are incorporated with other ore-body elevation data from mapping and drilling to improve the geological model of the ore body within the block.

The spatial resolution of the geological model is substantially improved with borehole radar. Small faults and other changes in topography are accurately positioned, giving warning of oncoming changes in mining conditions. Terraces and slopes are mapped, facilitating planning of the optimal extraction of the gold within the block. In areas where exploration drilling is primarily undertaken to determine the structure of the ore body rather than its grade, borehole radar provides an effective alternative to conventional drilling.