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

Abstract

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/µs 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.

Coupling ground penetrating radar and fluid flow modeling for oilfield monitoring applications

“…Pioneering borehole radar tools fixed in the subsurface have been proposed for monitoring the movement of the water level at considerable depths (Ebihara et al, 2006;Sato and Takayama, 2007), and EM logging tools have already proved the GPR potential in imaging the neighborhood of a well (Liu et al, 2004;Mason et al, 2008). We believe our study can stimulate research in order to allow an innovative and promising application for GPR technology.…”

Conductively Guided Borehole Radar Wave for Imaging Ahead of a Drill Bit

“…In addition to the side reflections observed in Configuration 1, many strong near-parallel diagonal reflection events dominate the time section. These events are caused by the axially guided EM waves travelling up the wire, reflecting at lithological discontinuities, then travelling downward and being recorded by the BHR [26]. These events are marked by the solid red lines in Fig.…”

“…We used a monostatic slimline 10-to 125-MHz pulsed BHR from Geomole for our experiments, as described by Mason et al [26]. The transmitter (Tx) and receiver (Rx) electronics, including batteries, onboard Flash memory for data recording, and antenna are housed in a 32-mm polyvinyl chloride tube terminated with brass ends, as shown in Fig.…”

“…However, such a radiation pattern can be changed if the BHR is attached to a conductive wire or is used in a conductive saline water-filled borehole. Guided BHR EM wave propagation can be then observed along the wire [1], [22]- [26] and the conductive water column [25]- [28]. The EM wave guiding characteristics of a conductor in a dielectric layer or water for the case of BHR are well investigated [ [29]- [31].…”

Interference Criterion for Coaxial-Fed Circular Dipole Array Antenna in a Borehole

“…However, because of the costs to drill a borehole, measurement with only one borehole is sometimes preferable. In singlehole borehole radar measurement, the transmitting and receiving antennas are set in a single borehole, and some objects such as fractures and faults are measured with a reflected wave from the objects [7]- [9]. Recently, researchers tried to use the singlehole borehole radar to detect metal ore [58].…”