GM24k Searching
20 Garrett Metal Detectors
Field Use & Tuning Tips
(cont.)
Negative Hot Rocks:
Negative hot rocks are ones that sit below the
ground balance point of the
GM24k
. They
actually produce a null or no sound when directly
under the coil, making a sound once the coil pass-
es them. This sometimes sounds like a “boing”
when the ground mineral and the rock mineral
are far apart in mineral TYPE, making them easy
to recognize. At any rate, these are usually black or
grey in color and usually magnetic. When hunt-
ing in LOCKED ground balance, they can often
be tuned out by performing a Ground Grab right
over them and then proceeding to search with the
GM24k
“over tuned” to the ground. When using
this procedure, be sure to search slowly and keep
the coil close to the ground and level. This will
eliminate the bother of such “hot rocks.”
In some parts of the world these specific types of
rocks are called “cold rocks,” since they ring up
beneath your current ground balance setting.
They are usually easy to identify thanks to the
“boing” sound and difficulty in pinpointing them.
If the ground is littered with these negative hot
rocks, not just an occasional one, the XGB will
track near the hot rock balance point, lowering
the
GM24k
’s response. You may be forced to dial
in more SAT speed and/or reduce SENSITIVITY
to keep a workable THRESHOLD “hum”.
Positive Hot Rocks and Alkali:
Positive hot rocks are usually red or various
shades of red to almost black. They sit above your
current ground balance point. They are some-
times as small as BB’s and sound just like nuggets.
These positive “hot rocks” are difficult to tune out
and ring up very low on your Target I.D. scale.
They actually give a positive signal like metal and
that is why they are so hard to differentiate. The
XGB will track some of these out automatically.
Using a higher SAT setting can also decrease your
GM24k
’s response to these positive hot rocks.
Using a Double D coil on the
GM24k
will allow
the detector to operate amongst hot rocks better
than a concentric. And as a last resort, the Iron
Cancel feature should all but silence any remain-
ing hot rocks.
Wet alkali washes can be extremely difficult to
work due to the sensitivity of high frequency gold
detectors to conductive dissolved salts and their
similarities with the responses of small gold. But
you can use the same techniques for alkali as you
use for positive hot rocks.
Adjusting SENSITIVITY with XGB:
If while searching with XGB you experience
erratic behavior such as false signals or con-
stant beeping and popping, you may be using a
SENSITIVITY level which is too high for the
ground mineralization. This is often the easiest
thing you can do to increase your ability to hear
gold in bad ground. Simply tap the down but-
ton to lower the sensitivity on your machine. Try
searching again. Continue this until you can hunt
without having erratic signals. Your detector is
cancelling ground mineral as you sweep, so you
may notice a slight fluctuation in the threshold
hum as the various ground minerals are tracked
out.
Even though it may seem counter-intuitive, ad-
justing the sensitivity down in tough ground
conditions can actually increase your chances of
finding a nugget. This is due to the fact that your
ears are an extension of the metal detector. Hear-
ing one faint signal against a smooth threshold is
much easier than hearing a medium signal against
hundreds of smaller ones.
Even though the
GM24k
comes with advanced
features like SAT, Iron Cancel, and XGB, don’t
overlook the first step you should take on any
outing—finding the right sensitivity level for the
ground your are walking over.
