identifying target signals
page 40
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Metallic targets will usually give a 'solid' sounding signal when the coil is swept
across the object from any direction. A metallic target generally produces a short,
sharp and mostly symmetrical signal. Ground noises usually give a broad uneven
signal when the coil is swept from different directions and often may only give a
signal from one direction and no signal on the return sweep.
If you are not sure if the sound is ground noise or a target signal, you need to investigate.
Scrape a shallow hole about 70-100mm deep over the suspected target. Sweep the coil over
the hole at the original ground level. Do not dip the coil into the hole. If the signal has
decreased in volume or is less defined, it is probably ground noise.
If the signal remains the same or becomes louder, it is likely a metallic target. If you are still
not sure, make the hole deeper and repeat the process.
A 'halo effect', which may be built up around a buried metal object, makes the object appear
to be larger to the detector than it actually is. This will be reduced once the target is disturbed
from its position in the ground (e.g. a small object, detected at a substantial depth, may be
more difficult to detect once disturbed from the ground and lying in the loose dirt. If the object
is reburied, the 'halo effect' will not be present).
HINT
Do not try to eliminate what might appear to be a faint, isolated ground noise by
balancing the detector on top because you may be 'balancing out' the target response from
a deeply buried metallic target. Better to ground balance around the target without going
across it, then switch to Fixed and try Pinpointing.
identifying target signals
– continued
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GP 3500
www.minelab.com
page 41
Pinpointing
To find an object and reduce the size of the hole required to remove it from the ground, it is
necessary to pinpoint the exact location of the object.
If a target is heard, first confirm it by setting an accurate ground balance and then pinpointing.
To ground balance, if detecting in Fixed position, hold down the Quick-Trak push button and
pass the coil around the area of the target, making sure that the target is not detected (keep
the coil well away from where the target is). Once completed relax the push button.
If detecting in Tracking mode, move the coil slowly around the area of the target, keeping well
away from the target itself, then hold down the Quick-Trak push button to go to Fixed and pin-
point across the target.
When a target is detected, sweep the general area with the coil, taking note of where the
strongest signal is received.
By shortening the length of the sweep it should be possible to draw an imaginary line in the
ground where the strongest signal is located.
Line up the target at 90° from the initial direction and repeat the process. The object is located
where the two imaginary lines cross (see
Figure 5.4
).
NOTE – Pinpointing will not locate a target in the centre of the coil if operating
with the Coil switch set to M (Mono) or Cancel, it will be slightly to one
side of centre
Figure 5.4
Pinpointing technique
Target could be
anywhere in
this Area
Imaginary Line of
Strongest Signal
Sweep Coil 90 degrees
to Previous Sweep
Sweep
Direction
Exact
Location
of Target
