Ridge Tool Company
28
formers, buried and overhead power lines are a common
source of strong distortion. It may be impossible to get an
accurate locate close to a large power transformer.
Notes on Accuracy
Measured Depth, Proximity Signal and Signal Strength
measurements rely on a strong signal being received by
the NaviTrack II. Remember that the NaviTrack II is used
above ground to sense electromagnetic fields emitted
from underground lines (electrical conductors like metal
wires and pipes) or Sondes (actively transmitting bea-
cons). When the fields are simple and undistorted, then
the information from sensed fields is representative of the
buried object.
If those fields are distorted and there are multiple inter-
acting fields, it will cause the NaviTrack II to locate inac-
curately. Locating is not an exact science. It does require
the operator to use judgment and look for all the infor-
mation available beyond what the instrument readings
may be. The NaviTrack II will give the user more infor-
mation but it is up to the operator to interpret that infor-
mation correctly. No locator manufacturer will claim that an
operator should follow the information from their instrument
exclusively. A wise operator treats the information gained
as a partial solution to the problem of locating and com-
bines it with knowledge of the environment, utilities prac-
tices, visual observation and familiarity with the instrument
to arrive at an informed conclusion.
Locating accuracy should not be assumed under certain
conditions:
•
When other lines or utilities are present.
“Bleed
over” causes distorted fields and will illuminate lines
other than the target line. Use lower frequencies when
possible and eliminate all connections between the two
lines.
Figure 51 – Bleed-over
•
When tees, turns or splits are present in the line.
When following a clear signal that suddenly becomes
ambiguous, try seeking in a circle of about 20' around
the last known point to see whether the signal picks up
again. This may reveal a branch, joint or some other
change in the line. Be alert to “split opportunities” or
sudden changes of direction in the utility being traced.
•
When Signal Strength is low.
A strong signal is nec-
essary for accurate locating. A weak signal can be im-
proved by changing the grounding of the circuit,
frequency or transmitter connection. Worn or dam-
aged insulation, bare-concentric cables and iron pipes
exposed to ground will compromise Signal Strength
through leakage to ground.
•
Far-end grounding
will change Signal Strength sig-
nificantly. Where far-end grounding cannot be estab-
lished, a higher frequency will provide a stronger
signal. Improving ground conditions for the locating cir-
cuit is a primary remedy to a poor signal.
•
When soil conditions vary.
Extremes in moisture, ei-
ther very dry or water-saturated, may affect measure-
ments. For example, ground that is saturated with
salty water will shield the signal severely and be very
difficult to locate in, especially at high frequencies. In
contrast, adding water to very dry soil around a ground
stake can make a major improvement in signal.
•
In the presence of large metal objects.
Simply walk-
ing past a parked car during a trace, for example,
can cause an unexpected increase or decrease in
Signal Strength, which will revert to normal when past
the distorting object. This effect is stronger at high
frequencies, which “couple” more readily onto other ob-
jects. Manhole covers and trench plates can strongly
distort signals.
The NaviTrack II cannot change the underlying conditions
of a difficult locate, but changing frequency, ground con-
ditions, transmitter location or isolating the target line
from a common ground can change the results by making
a better ground connection, avoiding signal splits or re-
ducing distortion. Other locating receivers will indicate
that they may be over a line, but they have less ability to
determine the
quality of the locate.
The NaviTrack II provides
more information. If all of the in-
dicators are aligned and in agreement, mark-outs can
be made with more confidence. If the field is distorted, it
shows immediately. This allows the operator to do some-
thing to isolate the target line, change the grounding,
connection point, move the transmitter or change the
frequency to get better reception with less distortion. For
extra certainty, take steps to inspect the situation such as
by requesting potholing.
NaviTrack
®
II
