© 2023 Radiodetection Ltd 36
switch from Power mode and use Power Filters to
discriminate individual lines.
Switch the locator to Radio Mode. Increase sensitivity to
maximum and repeat the above grid search procedure
over the area. Pinpoint, mark, and trace out any lines that
are located.
In most, but not all areas, radio mode will locate lines that
do not radiate power signals and a grid search should be
made in both power and radio modes or in Passive
Avoidance mode.
Inductive search
An inductive search procedure is a more certain technique
for locating unknown lines. This type of search requires a
transmitter and locator and two people. This type of
search is referred to as a ‘two person sweep’. Before
starting the sweep, define the area of search and the
probable direction of lines crossing the area. Ensure the
transmitter is switched on in induction mode.
Figure 6.5: Inductive search
The first person operates the transmitter and the second
person operates the locator. The transmitter induces a
signal onto lines as it passes over them and the lines are
then detected with the locator at a suitable distance away
from the transmitter (around 15 meters / 50 feet – although
this will depend upon the level of induction power used.)
Hold the transmitter with its length aligned with the
assumed direction of any lines.
The second person holds the locator at the start of the
area to be searched and with the locator antennas at right
angles to the probable direction of the buried lines. Set the
locator sensitivity level as high as possible without the
locator picking up any airborne signals directly from the
transmitter.
When the transmitter and locator are in line both operators
start to move forward in parallel. The operator with the
locator sweeps it backwards and forwards, keeping the
locator vertical, as they proceed in parallel with the
transmitter. This method allows for misalignment of the
transmitter, locator and buried line.
The transmitter applies the strongest signal to the lines
directly below it, which are then located with the locator.
Move the transmitter from side to side to establish the
highest signal which indicates that the transmitter is also
directly above the line(s).
Mark the ground at the point of each Peak signal detected
with the locator. Repeat the search along any other
possible paths of lines. Once the positions of any lines
have been marked, reverse positions, place the
transmitter over and along each line in turn, and trace the
line out of the search area.
6.8 Nulling Out
Sometimes it is possible for some utilities to be masked
by other utilities and this can happen when one or more
utilities are in close proximity to each other or when
stronger signals may radiate. In certain applications and
congested areas, the ‘Nulling’ technique allows operators
to eliminate the induction signal directly beneath the
transmitter but at the same time induces the transmitter
signal onto other nearby utilities that previously have not
been able to be located
Figure 6.6 Nulling Out effect
Two persons Nulling Out technique:
1. Place the transmitter close to the services you
wish to trace (a parallel sweep may be used for
this) and using the locator at a distance of
approximately 10m/30’ from the transmitter,
pinpoint the strongest signal.
2. The sensitivity on the locator is adjusted so that
the bar graph response is approximately 75%
3. Release the side support arm.
4. With the locator over the centre of the signal the
second operator should hold the transmitter at
waist level, with the transmitter on its side –
release support arm pointing down.
5. The transmitter is then moved from side to side
to find the ‘Null’ spot between two detectable
signals; the transmitter should then be lowered
towards the ground keeping it in the ‘Null’ spot.
6. At ground level we are ideally looking for a ‘Null’
no wider than 50mm/2” (Sensitivity on the locator
may need to be adjusted to achieve this).
7. With the transmitter left on the ground in the
‘Null’ spot. The locator is used to check for
additional signals either side of the ‘Nulled’
signal.
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