Passive Line Tracing
In passive mode, the SR-60 is looking for electromagnetic "noise"
that has found its way onto a buried utility line by any available
means. Electromagnetic signals can get onto buried utility lines
in a variety of ways.
The most common way is by means of direct connection to some
signal source. All operating electronic devices that are connect-
ed to AC power will radiate a certain amount of electronic "noise"
back onto the power lines they are connected to. Examples of
such devices include computers, copy machines, refrigerators,
anything with an electric motor, TV sets, air conditioning units,
etc.
Another common way electromagnetic noise can get onto the
line is by way of induction that can operate without any direct
physical connection to the buried line. In some areas for example,
buried utilities act as antennas for high powered, low frequency
radio transmissions (submarine navigational and communica-
tion signals in the UK for example) and will reradiate these sig-
nals. These reradiated signals can be very useful for locating.
Similarly, buried lines that run side by side near each other, par-
ticularly for longer distances will tend to bleed signals onto each
other. This effect is more pronounced for higher frequencies.
Due to coupling, all metallic lines in an area may be energized.
Because of this, it is possible to locate lines passively, but it is dif-
ficult to identify which line the locator is tracing.
Pipes can also have 60 Hz signal randomly induced into them
by nearby power-line fields, and other frequencies can be picked
up on phone lines, for example, from the energy of radio-broad-
cast towers in the vicinity. In short, frequencies can show up on
buried conductors in numerous ways, and these can be picked
up passively, if the fields are strong enough.
1.
Select a Passive Line Trace Frequency (
2.
Choose an orderly pattern of search that will cover the area
you are interested in.
3.
Use the Tracing Line, Depth, and Signal Strength to steer you
to the lines which have that frequency energizing them.
4.
If possible, once you have found a target of interest, find an
accessible point and do an Active Trace on it to confirm your
results.
The SR-60 has multiple Passive Line Trace frequency settings.
Power frequencies (identified with the power icon ) are used to
locate signals generated as the result of power transmissions,
usually 50 or 60 Hz. To reduce the effects of inherent noise from
line-load or neighboring devices the SR-60 can be set to locate
various multiples (or harmonics) of the base 50/60 Hz frequency
up to 4,000 Hz. (<4 kHz setting.)
The 50/60 Hz 9x multiple is the setting most commonly used to
locate 50/60 Hz signal. In well-balanced high voltage electric dis-
tribution systems, the 5x multiple may work better. The 100 Hz
(in 50 Hz countries) and 120 Hz (in 60 Hz countries) frequency
settings are particularly useful for pipelines that have been
equipped with cathodic protection using rectifiers.
As in Active Line Tracing, the Tracing Line will reflect distortion in
the detected field by appearing unfocused or cloudy in propor-
tion to the distortion. This "distortion response" is useful in rec-
ognizing when the field being traced is being distorted by other
fields of metallic objects in the vicinity.
or
icon).
There are also two higher radio frequency bands
locate lines passively. They are:
The Radio Frequency and <4 kHz bands can be useful in discrimi-
nating when tracing in a noisy environment. They are also very
helpful in finding lines on blind searches. When searching over
a wide area where the location of targets is unknown, one use-
ful approach is to have multiple frequencies selected for use and
to check the area at a number of frequencies in sequence look-
ing for meaningful signals. Even more convenient is to use the
OmniSeek setting described below.
Ridge Tool Company
SeekTech SR-60
Figure 29: 60
9th
Hz Passive Trace Frequency
4 kHz to 15 kHz (LF)
15 kHz to 35 kHz (HF)
to help
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