TITLE
The Effect of Conduction on VHF Radar
Images Shot in Water-Filled Boreholes
AUTHORS
Iain M. Mason, Andrew J. Bray, Tim G. Sindle, Carina M. Simmat & Johannes H. Cloete
ABSTRACT
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/µsec 1st
order transverse magnetic pulses which shuttled axially between the radar
and 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 Laterologcomparable resolution, the conductivity profile of the Platreef.
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I. INTRODUCTION
It is common in the wirelining industry to transfer data from downhole probes to surface
along insulated strands inside a logging cable. It is well-known that electromagnetic
signals can also flow outside the wireline cable. Guided electromagnetic (EM) pulse
propagation has been observed along a wire cable that was slung between a short-pulsed
borehole radar (BHR) transponder and the surface [1], [2]. Polar patterns of borehole
antennas have been deformed by proximity coupling into the metal logging cables to
which the radars were tethered [3], [4].
Links have been established between the transmission of a wire-guided wave down a
borehole and the dielectric properties of the surrounding formation [5]. The amplitude
and the phase of an EM pulse measured at two axially separated borehole receivers can
be translated into the conductivity and permittivity of the proximal formation [6], [7]. It
has been shown [8] that even in the absence of a metallic conductor, saline (conducting)
borehole water can guide an electromagnetic pulse.
Detailed studies have been made [9] [10] [11] of an analogous EM waveguide, developed
for soil moisture profiling. Based on a 1.6mm thick TiO2, coated 10 mm diameter
aluminium rod hammered into soil, the UHF (Ultra High Frequency) moisture profiler
can be regarded as a 1:5 scale model of a VHF (Very High Frequency) water-sheathed
drill string in a borehole. In dry soil at 300 MHz most of the energy in the profiler’s
dominant TM01 mode is confined within a critical radius (rc) of a third of a metre.
Finite-Difference Time-Domain (FDTD) models have been constructed [12] to establish
the EM fields generated by a novel dielectric logging tool. A voltage pulse fed into a
small gap between two relatively thick, long conductors lying in tandem in a water-filled
borehole, has been shown numerically to produce three distinct wave fields:
1. A spherical wavefront radiating into the formation.
2. A guided wave propagating along the conductor.
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3. A non-propagating field locked to the feed point.
These results are mirrored in the experimental observations described in the sub-divisions
of the main section this paper, in which we extend earlier observations [13] in 200 mm
wide boreholes by showing that:
1. A VHF borehole radar transceiver, run downhole on an insulating cord, can map
steep geological faults.
2. If the cord is replaced by a wire or by saline the VHF transceiver will drive guided
EM pulses axially along the borehole.
3. The non-propagating field at a radar’s feedpoint can be translated into a high
resolution resistivity log.
II. BOREHOLE RADAR TRIALS
Two different radar configurations were used in the field trials reported here (Fig. 1.).
The first and most conventional was a fixed-offset bistatic borehole radar, in which 1.6 m
long transmitter and receiver cases were separated by a 2 m non-conductive fibre optic
spacer. The second, less conventional system was a monostatic radar, in which
transmitter and receiver shared the same 1.6m long case and the same antenna. In a
monostatic system, a VHF duplexer protects the receiver during firing. In bistatic mode
the same protection is achieved by physically separating the transmitter and the receiver.
Both radars have a core diameter of 28.575 mm. They are sheathed inside dielectric hulls
of 31.7, 35 or 44 mm diameter, and tested to 2.5 km depth. In both radars the transmitter
fires a train of -400 V steps, with a pulse repetition frequency (PRF) of ~10 kHz, into a
broadband (10-125 MHz) asymmetric dipole. One dipole arm is resistively loaded; the
other, a metal tube, houses electronics and battery. Echoes, once amplified, are fed to a
250 MS/s analog-to-digital converter (ADC) which is controlled by a field programmable
gate array (FPGA). Stacking in the FPGA raises dynamic range to 12 bits (70 dB).
Traces accumulate in an on-board flash memory, which is downloaded via Bluetooth
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when the transceiver returns to surface. These self-supporting capabilities lead to an
umbilical-free radar which can be deployed on the end of light weight high tensile cord. It
follows that:
•
Winch design simplifies. A light (20 kg) winch can hold 1000+ m of cord with a
breaking strength measured to be 300 kg.
•
Radars simplify mechanically. Reducing ports strengthens pressure hulls.
•
Unleashing borehole radar eases the task of training drillers to run surveys.
•
Greater depths and lengths can be profiled. Radars log continuously for four
hours.
During the surveys conducted in these studies the radars were deployed downhole at 10
m/min. The depth of the radar was digitally tracked via a shaft encoder located on the
winch and related to the trace number via synchronous clocks positioned at the surface
and on board the probe.
The data presented in this paper were acquired from three boreholes. Two 60 mm vertical
holes (A and B), drilled in the Northern Limb of the Bushveld Complex, passed first
through the granite dominated Main Zone and into the underlying Platreef. The third
borehole (C) studied was drilled in greenstone volcanic rocks near Kalgoorlie, Western
Australia. It was plastic cased, 700 m long, water-filled and it dipped at ~30 degrees.
A: Conventional radar operation
The first 1.25 km deep BHR profile was shot with a bistatic radar on an insulating cord.
As the radar descended into the borehole, broadband EM pulses were radiated into the
formation [14]. Echoes from dielectric discontinuities in the rock mass were recorded
near the point of excitation. Recordings were stacked to form traces spaced by ~30 cm.
The traces were gathered together to form time sections.
Borehole radar time-sections are frequently decorated by hyperbolic and flat events.
These are shaped both by the geometry of the object, and perhaps more significantly by
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changes in the offset between an object and a radar transceiver as it travels downhole.
[15]. Many such patterns can be seen in the time section shown in Fig. 2a. That some are
sharp and impulsive confirms both that the radar is non-resonant, and that the dielectric
host offers little attenuation.
The time section is dominated by a steeply dipping planar feature, in this case a large
fault, which intersects the borehole at a depth of approximately 240 m. The borehole’s
geological log, which is plotted on the left of each time section in Fig. 2, indicates
fracturing over a 4.5 m interval at this depth. Sub vertical faults are difficult to pick up in
a surface based survey. The perpendicular relation between the discontinuity and the
radiation pattern of the radar leads to specular reflections being scattered down and away
from the receiver. Profiling surface pilot holes offers significant geometrical advantages
in steep fault detection.
B1: Guided wave operation
Replacing the insulating cord in a water-filled borehole by a thin conductive wire creates
a waveguide. The guiding characteristics of a conductor coated with a dielectric sheath,
in this case water, are well known [5], [9], [16] [17], [18].
The dominant mode is a 1st order transverse magnetic wave (TM01), the energy within
which is tightly bound by the high dielectric constant of the water cladding (εr water ~ 81).
The characteristic radius within which most of the energy is confined varies with
frequency and geometry. In the case of the 60 mm and 47 mm water-filled boreholes
studied here, the characteristic radius is of the order of metres, and the guided waves are
only weakly dispersive at the principal frequencies (~10-40 MHz).
A grid of near parallel diagonal reflection events dominates the time section in Fig 2b.
Axially guided EM pulses travel up the wire, reflect at lithological discontinuities, and
then travel downward to be sensed by the borehole radar’s receiving antenna, and
recorded.
Some echoes evidently make two, even three, round trips between radar
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transceiver and reflector. Based on the moveout of the parallel reflection events, the
velocity of the guided wave is approximately 75 m/µsec. This velocity is considerably
slower than the free wave velocity of EM waves through Bushveld granites which was
experimentally determined to be 112.5 m/µsec.
B2: Energy splitting.
A bistatic radar was run downhole on a thin conductive wire. The radar was electrically
and physically decoupled from the wire by a short length of insulative cord introduced
close to the radar’s suspension point. A geological log of a 300 m length of the core is
plotted on the left of radar data presented in Fig. 3. A number of features are apparent in
the time section:
•
Bedding planes are pin-pointed by prominent events in the time section that move
out as did those linked to wire-guided-waves in the time section shown in Fig. 2b
•
Classical flat and quasi-hyperbolic radar echoes from fault planes and small
inhomogeneities are also evident in the profile.
Setting the length of the insulative gap at 2 m appeared to split the transceived VHF field
nearly equally between the radiated and wire-guided modes. Experimental evidence
suggests that a gap of approximately 6 m is sufficient to completely electrically decouple
the radar from the conductive wire.
One can expect radar echoes to weaken and wire-guided full-waveform EM (FWEM)
reflections to strengthen as the angle between borehole and bedding plane normal
diminishes, since a dipole antenna in a wire free borehole has an axial null.
C: Non-propagating fields & conductivity
The PlatReef, which lies beneath the Main Zone in the Northern Limb of the Bushveld
Complex, is highly mineralized and stratified. Borehole radars are rarely run successfully
6
through mineralized strata because body-wave attenuation usually is very high. The wiretethered monostatic was nevertheless run experimentally through the Platreef.
The geological core log is plotted on the far left of Fig. 4. Wire-guided waves appear
almost as expected on the adjacent BHR time section, though these are truncated abruptly
by certain horizons at 125, 148 and 195 m. An unexpected feature on the time section is
the appearance of a ‘barcode-like’ pattern. This correlates with fine structure in the core.
The link between the horizontal grey-level banding and the geology is explored in the
adjacent BHR log – a trace derived from the grayscale’s amplitude variation with depth
along the vertical line AA’ in Fig. 4. The adjacent trace, the Electric Array Log or “EAL
resistivity”, was logged independently. EALs simulate the use of bucking electrodes to
focus resistance measurements into a two-dimensional plane perpendicular to a borehole.
The close correlation between EAL resistivity and the BHR non-propagating field is
revealed by the 180 m long butterfly plot on the far right of Fig. 4. Electrons flow in two
dimensions in a finely layered mineralized formation. They are repelled radially when the
radar fires, and drawn back as its non-propagating field collapses. The close correlation
drops off above the Platreef where BHR-excited electrons flow in 3D, while, in contrast,
the EAL is still focused on recording 2D electron flow.
D: Saline-supported waveguide modes
An axial conductor is a prerequisite of TM01 wave guiding by a borehole. It seems both
from the work of Vogt (2004) [8] and from the BHR profile shown in Fig. 5 that the
conductor need not be metallic. The profile in Fig. 5 was shot in a high-impact-plastic–
cased borehole that had been flushed with fresh water. Unknown to the field crew, the
casing parted at a depth of 250 m. Though water returning to the collar was fresh the
lower 200 m was filled with saline, of conductivity greater than 20 mS/cm.
7
The conventional BHR profile shown on the left-hand side of Fig. 5 built up as a bistatic
BHR descended the borehole on an insulating cord. Reflections from geological targets
up to 40 m from the borehole were recorded by the receiver. But guided-echo stripes
appeared on the profile when the radar sank into the saline-filled lower section of the
borehole. Unlike the stripes observed in the case of a wire-tethered radar eg. in Fig. 2b,
guided pulses traveled in both directions along the waveguide, both up towards the
fresh/saline water interface and down towards the end of hole.
III. CONCLUSIONS
Borehole radars extend the radius of investigation of surface boreholes in the Bushveld
Complex. They image reflections from steeply dipping faults that are nearly invisible
from surface and easily able to either flood a sinking shaft or destabilize a shaft pillar.
Borehole radars work conventionally and well when they are run on insulating cords in
clean water-filled boreholes, drilled through good dielectrics.
If they are either tethered by conducting wires or run in saline water, then borehole radars
will launch and receive axially guided modes. These modes can be used to assist in depth
control, because they mark the radar’s passage through bedding interfaces. Records of
non-propagating evanescent fields that exist in the radiation zone of a borehole radar’s
antenna can also be used in depth control, because they tie borehole radar profiles to
conventional wireline logs.
ACKNOWLEDGMENT
Valuable contributions were made by K. Palmer, W. Croukamp, W. van Brakel, P. van
der Merwe, B. Woods, by G. Turner, L. Spies, G. van der Walt, H. Lombard, G.
Campbell and S. Comline.
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42, no. 5, pp. 306-7, 2006.
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Geophys., vol. 70, no.1, pp. K1-K11, 2005.
[15] Simmat, C.M., Le R Herselman, P., Rutschlin, M., Mason, I.M. & Cloete, J.H.,
“Remotely sensing the thickness of the Bushveld Complex UG2 platinum reef using
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BIOGRAPHIES
Iain M. Mason graduated B.Sc (Eng) from the University of Cape Town in 1964, Ph.D.
and M.A. from the Universities of Edinburgh and Oxford in 1968 and 1977. He is a
Fellow of St Johns College Oxford, and Director of the ARCO British Geophysical
Imaging Research Laboratory. He worked in the Microwave Research Unit at University
College London 1969-77; taught Engineering Science at Oxford from 1977 until 1995
when he took up the Chair of Geophysics in the University of Sydney.
Andrew James Bray was born in Dubbo, New South Wales, Australia. In 2006 he
graduated with First Class Honours in Geophysics from the University of Sydney. He is
currently enrolled in a Doctoral Program in the School of Geosciences at The University
of Sydney
Tim G. Sindle was born in Johannesburg, South Africa. In 2002 he graduated cum laude
with a degree in Electronic and Electrical Engineering from the University of
Stellenbosch. He gained his M.Sc in Electronics by research, again at Stellenbosch in
2005. Following graduation, he joined the ARCO British Geophysical Imaging Research
Laboratory in the School of Geosciences at The University of Sydney, Australia.
Carina Mai Simmat was born in Sydney. She took her first degree in Geophysics at the
University of New South Wales. She joined the School of Geosciences at Sydney
University, where she graduated B.Sc (Hons) with First Class Honours in 2001, and
Ph.D. in 2006. After a post-doctoral year with the ARCO British Geophysical Imaging
Research Laboratory in the School of Geosciences, she joined GeoForce Pty Ltd in Perth,
W.A.
Johannes H. Cloete was born in Clocolan, Orange Free State, South Africa. He received
electrical engineering B.Sc, B.Eng and Ph.D. degrees from the University of
Stellenbosch, and the M.Sc.Eng degree from the University of California, Berkeley. He
consults in antenna engineering, remote sensing, and propagation in matter. He was an
officer in the South African Navy, and worked at Scientific Atlanta in the USA, the
National Institute for Defence Research of the CSIR, and the University of Pretoria
before joining the Department of Electrical and Electronic Engineering at the University
of Stellenbosch as a professor in 1984.
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CAPTION LIST
Fig. 1. Monostatic and bistatic slimline 10-125 MHz pulsed borehole radars
Fig. 2 (a) Borehole radar profile shot with a 10-125 MHz radar, suspended on a dielectric
cord (survey 1). (b) Full waveform log, shot with a 10-125 MHz radar, suspended on a
wire rope (survey 2). The section is dominated by guided arrivals. Faint profile radar
echoes appear in the background of the time section.
Fig. 3. Axial FWEM and radial borehole radar reflections, shot with a radar partdecoupled from the wire (survey 3).
Fig. 4. Logs from a 300 m column in the Platreef: from left the core log; the FWEM log
(survey 4); the BHR ‘pseudo conductivity’ log; a calibrated EAL focused resistivity log
and a butterfly plot correlating BHR pseudo-conductivity & 2D EAL resistivity.
Fig. 5. Borehole radar profile, shot with the radar suspended on a dielectric chord in a
~500 m deep plastic cased borehole (survey 5). Energy is trapped in the waveguide that is
formed in the un-flushed lower half of the borehole by conducting saline.
FIGURES
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Figure 1
Figure 2
13
Figure 3
14
Figure 4
15
Figure 5
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