Chapter 5 GMT Searching
Backlight and Battery Life
With a short tap of the power button, the
GM24k’s backlight will turn on, giving in-
creased visibility in low light conditions. In
bright sunlight it will not be obvious this back-
light is enabled, so there is a light bulb icon to
indicate the status as well (icon on = light on).
This light consumes a great deal of power so it is
best used only when necessary, as it may lower
the operating time by 50%.
This backlight is on for a few seconds when
powering on the GM24k, but only to allow for
those situations when the unit is turned on in
total darkness. It will turn itself off after a few
seconds and the icon will disappear until the
backlight is enabled or the detector is powered
back on again.
Operating the GM24k in Extreme Minerali-
zation:
Thanks to the combination of XGB ground bal-
ance, SAT, and Iron Cancel, the GM24k is able
to operate in a wider variety of conditions than
many other VLF detectors. However, it takes
some knowledge of how to use each setting to
combat bad ground, and the differences between
types of ground.
In very strong mineralization, such as heavy
iron stone or serpentine, it will be necessary to
lower the sensitivity. There is enough voltage
at the search coil to cause the GM24k to over-
load in bad ground, so take some time to find a
good setting for sensitivity before making other
adjustments. To check the ground strength you
can enable Ground Scan by holding the LOCK
button.
Another issue you may run into is highly vari-
able ground mineralization. In these conditions
you will not get an overload response, but will
have to contend with lots of false signals with
your coil sweep. This is a result of the ground
phase shifting rapidly under the coil. The first
step you should take is to increase your SAT set-
ting by holding the Music Note button, and then
tapping the Up button to set the SAT to a higher
setting. This will allow the detector to adjust
more quickly to the rapid ground changes.
As a last resort, enable the Iron Cancel feature
(the crossed-out nail button). This will not allow
those low-range signals to break through the
threshold in either audio mode. Only in the most
extreme conditions should you adjust the Iron
Cancel (by holding the crossed-out nail button)
to higher settings.
Problems from Low Ground Mineralization:
The GM24k is designed to work in an environ-
ment of heavy iron mineralization. In some
instances there may be so little mineralization
that the search-coil will never go quiet as it ap-
proaches the ground. In other words, there never
seems to be any change or only a faint increase
in hum as the coil is pumped. In this instance,
lock the ground balance (lock button) and per-
form a ground grab before detecting.
Hot Rocks
The ever-present “hot rock” has caused frus-
tration with almost every electronic prospector.
Most large ones are on the surface and can be
kicked aside when identified. You will soon
learn what they sound like. The “overshoot” or
“boing” sound will soon become familiar. XGB
is really a good option when they are plenti-
ful. We find it best to tune to the majority of
the ground matrix and learn what a “hot rock”
sounds like. The XGB of the GM24k will re-
cover quickly upon leaving the vicinity of a “hot
rock” and will remain sensitive to gold nuggets.
Since there are different kinds of “hot rocks”,
it is almost impossible to eliminate all of them
with one setting. Some will be identified by the
Target I.D. system as if they were iron. In every
instance, practice makes perfect.
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Summary of Contents for GM24k
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