<title>Borehole radar imaging from deviating boreholes</title>

Abstract

We have carried out borehole radar (BHR) surveys at gold mines in the Witwatersrand Basin. South Africa in order to map the Ventersdorp Contact Reef (VCR). In one such survey 20 kW transmitter and receiver pairs, 32 mm in diameter, with a bandwidth of 10-125 Mllz were used to profile a ~300 metre long section of the reef from a borehole that intersected it at an angle of 26°. Structures on the VCR were visible to a distance of 8Om, before noise started to dominate the signal. We established that the V.C.R. is sufficiently reflective, and its host rocks are transparent enough to open not only the certainty of high resolution echo sounding along the nadir line, but also the possibility of mapping off-axis back-scatterers by applying modified SAR reconstruction techniques to VHF BHR data. One of the problems facing synthetic aperture borehole radar is that it is difficult to build thin, efficient, directional radar antennas. Thin borehole radars are cylindrically omnidirectional and cannot be used to distinguish left from right. In this paper we show that borehole curvature can be used to address the difficulty of determining on which side ofthe survey line a backscattering object might lie.