Locating buried pipes and cables
GLOBAL INFRASTRUCTURE X PROCESS EQUIPMENT X DIAGNOSTIC TOOLS
COMPANY CONFIDENTIAL
April 8, 2015
1
Radiodetection
ABC to XYZ of locating buried pipes and cables - For
the beginner to the specialist
Radiodetection Corp.
154 Portland Rd
Bridgton
ME, 04009
877 247 3797
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Cable locators do not find cables.....
?
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...they find magnetic fields
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Why does it matter?
Because fields do things that pipes and cables don’t do
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Buried conductors don’t move, but the fields we’re tracing are
subject to…
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...Distortion
Affected by:
1. Method of signal application
2. Grounding
3. Peak or Null
4. Congestion
5. Frequency applied
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What is a Magnetic Field ?
A magnetic field is radiated by a current carrying conductor
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Aerial
Volts
…Which can be detected by a receiving coil which is excited
by the expansion and contraction of the field.
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What is Electromagnetic Induction?
S
N
0
+
volts
If a bar magnet is inserted into a coil of wire, a voltmeter will show a deflection
but only while the magnet is moving. When the magnet stops, the meter
reads Zero. When the movement of the magnet is reversed, the meter
deflection is reversed.
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What is an Alternating Magnetic Field?
one a.c. cycle
Alternating current creates a moving and reversing magnetic
field
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What is Electromagnetic Induction ?
Bulb will not light
Direct current does not produce an electro - magnetic field
detectable with a locator. DC Current builds to its voltage and
produces a constant level of magnetism. There is no change in this
voltage to excite an adjacent receiving coil.
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
Passive
Active
There are two methods of signal detection
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Passive
Passive signals are a by-product of the 20th century. The
proliferation of power and radio technology in this century has
caused almost every underground metallic pipe or cable to
emit detectable magnetic fields.
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Passive
Detectable Radio signals come from high power, low
frequency international communication and navigation
systems.
Such signals are present nearly everywhere on earth.
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Passive
Fast
Easy
Does not identify
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Passive
However tracking lines to their source can also aid
identification
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Passive
Ensure a full grid search is done to detect conductors laid in
different directions.
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Passive

Active
There are two methods of signal detection
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Active
Increases Locator Versatility
Depth Measurement
Positive Identification
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…Distortion
Affected by:
1. Method of signal application
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Applying a signal
Connection
"Clip"
Signal Clamp
"Clamp"
Induction "Spill"
There are three main methods of Active Signal Application
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Connection
m
A
m
A
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A successful connection to the
metallic pipe or cable will be
indicated by a change in the
transmitter audio or visual indicator.
Always make the best possible
connection for reliable signal.
Direct Connection Locating
Requires three components:
1. A transmitter or signal source
2. A metallic conductor
3. A return path. Ideally, the earth.
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…Distortion
Affected by:
1. Method of signal application
2. Grounding
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Connection
The ground return point is the "back door" that the outgoing signal returns
to. What you ground to and where you place the ground can significantly
affect your results.
Remember: Every milliamp of signal that is going out the metal to metal
connection on the red lead has to come back in through a metal to dirt
connection on the ground lead.
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Connection
Ground stake too close to target conductor: Less range,
some signal transfer.
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Connection
Badly positioned remote ground causes more signal transfer
Where possible, place ground rod away from known adjacent
utilities which may act as return paths.
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Connection
Remote ground = better range, less signal transfer.
As a general rule, try to position the ground point at right
angles to and 5-10' from the connection point and direction of
conductor.
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Connection
Grounding to a structure which is also grounded can produce
multiple signals.
The signal returns on every conductor that shares the same
ground
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Connection
Positioning the ground stake in the direction you wish to
locate will often encourage the signal to flow in that
direction
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Connection
When locating Tees, position ground stake in the direction
you wish to locate.
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Connection
For greater signal travel, an extension cable can be used to
place the ground stake further away.
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The Cable Reel
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Connection
A double-ended connection does not use ground as part of
the circuit and is more dependable. This is particularly true
when trying to locate metallic pipes buried close together.
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Connection
Metal fences may seem a convenient ground point, but might produce
interfering signals.
Always use an independent ground such as a ground stake, large
screwdriver or other isolated metallic object buried in the ground.
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Connection
After making a connection, sweep a complete circle around a
signal source; measure and mark all occurrences of signal.
Repeat to assure accuracy.
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Connection
At older buildings where the services are all metallic and may
share a common ground, connecting close to multiple service
bondings may produce multiple signals.
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Connection
Energizing in the street and tracing toward the bonding point
can be more reliable
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Clamp
The Signal clamp is a current transformer which induces or
spills a locate signal onto a target line
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Transmitter Clamps
A clamp safely applies a signal to a live cable without
interrupting the supply
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Clamp
The Signal Clamp is a means of applying a transmitter signal to an
insulated pipe or cable, without physical connection. It's most useful
for live electric cables.
Signal Clamp must be completely around line with jaws closed.
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Clamp
When using a clamp, the applied signal moves away from
the nearest ground to a distant ground. For this reason,
it's a useful way to put a locate signal on a specific line.
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Clamp
At higher frequencies - or with longer lines - capacitive leakage
completes the circuit.
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Connection
6
12
14
6
Direct connection does not give reliable identification on multiple bonded
lines
As an exception, connection to the bonding ribbon in a manhole will send
tone in one direction, allowing the conduit run to be located.
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Clamp
6
22
10
6
Generally, clamping produces more reliable signal identification.
Excellent on headers. Especially with using current measurement
and direction.
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Clamp
Place the clamp below the earth bond when locating cables. Do not
place above the earth bond.
If there is no bonding strap, the clamp may not transmit a signal.
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Clamp
If using the clamp around a metallic pipe, any insulating flange
must be bridged.
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Induction
Induction allows the user to quickly and easily apply a locate
signal, by placing the transmitter in the vicinity of a known
conductor or conductors
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Induction
A minimum distance of 10 paces between receiver and
transmitter is necessary to avoid the receiver detecting the
signal directly from the transmitter. This is known as "Air
coupling".
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Induction
Often induction will energize every metallic conductor close to
the transmitter. This is useful for checking an area for buried
conductors, but not useful for finding specific conductors in
congested areas.
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Induction
The induced transmitter signal is usually strongest on the
line directly below and in line with the transmitter
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Aerial
The orientation of the aerial is important as seen with an AM
radio
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Aerial
When the aerial is not in line with the magnetic field, no signal
is detected. This is known as "Nulling".
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Induction
When the transmitter is turned onto its side so the coil axis is
vertical, the induction signal is spread over a wider area; but this
results in zero signal directly below the transmitter. Which can be
useful...
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Induction
Using transmitter on its side to 'Null out''
When the transmitter is over the target cable, minimum signal will
be found by the receiver and the adjacent conductors will be
energized strongly.
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Induction
100
20
Estimated induction signal ratios
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5.8
2.7
Induction
0
50
30
Estimated induction signal ratios
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19
14
Induction
Induction Search (two person).
Maintain minimum Rx/Tx separation.
Sweep 3 times ! +/- 30-40 degrees
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Induction
Induction search, radial.
Maintain constant Rx/Tx separation
- Lazy Susan idea
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...Distortion
Affected by:
1. Method of signal application
2. Grounding
3. Peak or null
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Aerial Responses
Signal
response
1
2
3
Signal
response
1
2
Different aerial orientations can be used for different
responses
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3
Receiver Response
In Null, minimum signal is encountered exactly over the center of the buried
conductor field
A maximum signal response will be detected on each side of the minimum.
If both of the "shoulders" are symmetrical, the Null point is accurate.
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Receiver Response
100%
5%
Null antenna gives a sharp response which gives the
impression of good accuracy, but can be less accurate than
Peak.
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Receiver Response
100%
95%
Peak coil response is less sharp, but actually more accurate.
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Receiver Response
This is a typical peak signal response over a buried conductor. Peak
response provides position and direction.
Note that the maximum signal response occurs directly over the center of
the underground conductor
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Receiver Response
Peak
Peak response is more reliable. It enables you to measure depth.
Peak response is more accurate in congested areas
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Receiver Response
Null
d
Null is useful for deep lines and rapid line tracing , but...
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Receiver Response
Null
Null response in an area with congested plant is often
useless and misleading
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Receiver Response
Null
Peak
Peak and Null coils produce opposite types of error when
tracing bends
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...Distortion
Affected by:
1. Method of signal application
2. Grounding
3. Peak or Null
4. Congestion
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Distortion
Current on one line may induce 'stray' signals onto nearby
conductors
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Distortion
When the conductors are close together, the magnetic fields can
interfere with each other, causing a distorted field which is no
longer cylindrical. This can lead to poor locate results and
inaccurate measurements.
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Distortion and Depth
Peak
Null
d
Peak
Null
d
When there is a discrepancy between the aerial responses the following must
be observed.
1. The Peak response will always be more accurate.
2. Push button depth estimation should not be used until the two responses
agree
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Distortion
1
3
Peak
Null
This distortion varies depending upon the direction and magnitude of the
current flow, causing a discrepancy between the peak and null aerial
responses.
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Depth
d
d
Because we know the field is likely distorted,
it's not a good idea to rely on depth readings
taken near a change of direction...
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d
Depth
d
d
or near a tee
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d
Depth
d
or near another line
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d
Depth
d
or near a depth change
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d
d
Depth
d
d
or near an inducing transmitter
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Depth Triangulation Peak Aerials
100%
70%
d
d
Depth estimation can be verified using a triangulation technique. Move
the receiver from 100% to 70% signal strength, on each side of conductor
and measure the distance. This distance will be approximately equal to
the depth.
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...Distortion
Affected by:
1. Method of signal application
2. Grounding
3. Peak or null
4. Congestion
5. Frequency applied
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Which frequency should I use ?
Very Low
Less than 1 kHz
Very Long Range
No Induction
Little "Spillage"
Better IDent
High
10-50 kHz.
Easy Induction
Shorter Range
More "Spillage"
Low
1-10 kHz
Long Range
Poor Induction
Less "Spillage"
Very High
50kHz+
Short Range
Excellent Induction
Severe "Spillage"
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Characteristics of different
frequencies.
All of these characteristics get
better or worse linearly with
frequency. There is no signal that
is magically better for one than
another.
Which Frequency ?
High frequency signal quickly escapes from target line and may
return on an adjacent line
Lines in close proximity will readily accept signal. This can lead
to field distortion and poor locate information.
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Which Frequency ?
Higher frequency = shorter range (greater capacitive signal loss
to ground)
This frequency is better for induction onto small or short length
conductors, such as telephone drops, CATV cables or street
light cables.
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Which Frequency?
Low frequency = long range (minimum capacitance signal loss
to ground)
This frequency is better for connection and locating longer
metal pipes or cables
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Which Frequency?
High frequency may locate service drops which are not grounded at the
termination point but range on main line is reduced and may induce
onto crossing services
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Which Frequency ?
Low frequency tends to keep to main line, does not induce onto
crossing lines and bypasses service drops which are not
grounded.
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Which Frequency ?
High frequency can use a capacitive ground connection through a
metal plate
This provides a ground in areas where direct earth ground cannot
be achieved
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General Locating
When pinpointing the signal, keep the bottom of the receiver
blade close to and parallel to the ground.
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General Locating
Always pivot receiver to ensure maximum signal is received
This is particularly true prior to taking a depth reading
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General Locating
Searching for laterals
Use Peak and search 2 paces away from and parallel to the
main line
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Problems
If multiple signals are received at uniform distances apart, you
are probably trying to locate through reinforcing bars.
Raise the receiver up to chest height. This should allow the
main signal to be detected.
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Problems
Lost signal must be either: End of line
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Problems
OR: a T lateral
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Problems
OR: a bend
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Problems
OR: a depth increase
The field has simply become weaker at the coils because of the
change of depth
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Transmitting Sondes
Small Waterproof self contained transmitters enabling non
metallic ducts, drains or sewers to be located and traced
with a Receiver.
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Sondes
Ghost
Ghost
Receiver response to a Sonde signal. Small ghost signals may be
detected either side of main signal
Note: Receiver is held in line with Sonde
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Sondes
Location procedure for pinpointing Sondes
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Sondes
Location procedure for pinpointing Sondes
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Sondes
Using a Sonde to find a collapsed sewer
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Sondes
Tracking an inspection camera with a Sonde
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Sondes
Tracking a boring tool with built-in Sonde
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Sondes
A sonde in a vertical position gives a very precise "null" signal
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Sondes
This can be used for very accurate positioning through a wall
prior to drilling etc
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Sondes
Locating a leak in a non metallic water pipe using a Sonde inside a
"Pig“
Need VLF for metal pipes
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Sondes
d=0.7AB
B
A
d
Triangulation depth method for verifying "push button" depth
or estimating depth which is beyond the receiver's depth
range
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Levels of Confirmation
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Automatic Gain Control - AGC
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RD400 Digital Series
Signal strength
Current reading
Depth
%
%
Signal strength
mA
mA
Current reading
m
m
Depth
Change of depth effects signal strength but not current
reading
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RD7/8000 Digital Series
mA
mA
Current losses returning to signal source
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mA
RD7/8000 Digital Series
mA
Current loss due to poor insulation
There is very specialized equipment for this
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mA
RD7/8000 Digital Series
mA
mA
mA
Current loss due to a Tee connection
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RD7/8000 Digital Series
Target line
always
identifiable
by current
direction
Expected current direction indications and return signal path
For locators equipped with current direction indication only
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RD7/8000 CD Series
Possible current direction indication on conductors.
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RD7/8000 Digital Series
100%
60%
40%
Modern equipment has many “Levels of Confirmation”
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FaultFind
Current flow through an earth fault
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FaultFind
Meter readings before, at and after the fault.
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FaultFind
Distant
fault – no
indication
Line
fault
ahead
Meter readings along a faulted cable
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Line
fault
behind
FaultFind
Fault
Position
Locate
FaultFind
Fault
line
Locate line
It is possible to detect a fault a reasonable distance away, and
to the side of the cable
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FaultFind
Fault
position
Fault line 1
Fault line 2
Pinpointing a fault under a roadway with two or more bearings
(triangulation)
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