The Effect of Conduction on VHF Radar Images Shot in Water-Filled Boreholes

A downhole digital memory-logging pulsed borehole radar transceiver operating in the 10-125-MHz band was run repeatedly down two 1.25-km-deep, uncased, water-filled 60-mm boreholes in the Northern Limb of the Bushveld Complex, South Africa. Suspended on an insulating cord, it mapped a steep fault in the Main Zone from a range of 75 m down through its intersection with the borehole. Lowered on a wire rope, the transceiver launched guided ~75 m/mus first-order transverse magnetic pulses which shuttled axially between the radar and the bedding planes. Decoupled from the wire by 2 m of insulating cord, it yielded a profile in which radar reflections and guided bedding plane echoes superimposed. As the radar descended through the mineralized, stratified Platreef, traces were found to be imprinted with voltage level shifts that showed, with Laterolog-comparable resolution, the conductivity profile of the Platreef.

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