Negative Hot Rocks and the CZ
by Allen Morgan
By now you’ve probably got a good handle on how your Fisher CZ works.
Once you think you know what it’s telling you, it’s time to experiment
a bit and learn WHY it does what it does.
One thing that I’ve learned is that not all metallic and/or magnetic
targets will sound off in Discriminate mode on a CZ. “Hot rocks”
are rocks that contain enough metallic or conductive elements to induce
a signal when your detector’s EM field passes over it. Coal cinders,
a.k.a. clinkers, will do this, as well as naturally-occurring rocks.
How your detector reacts to this depends on the type of material in the
hot rock, its orientation in the ground, the mineralization present in the
ground, what kind of detector you are using, and what mode it’s in.
PI machines don’t get affected by mineralization much, due to the way they
If you’ve been using a CZ for awhile, undoubtedly, you know how to ground
balance. When you use the “bobbing” method, and you receive an increase
in sound/tone as you approach the ground, the detector is tuned “positively”.
This sound is identical to the sound a “positive” target, i.e., a coin,
nail, piece of jewelry, lead, aluminum, etc., makes when it’s detected in
Auto-tune. However, when you’re ground balancing, and you receive
an increase in sound/tone on the up-stroke, as you’re moving the coil AWAY
from the ground, then your detector is tuned “negatively”. The sound
made is exactly opposite of a “positive” target, that is, instead of starting
soft and low, then building in volume and pitch, the sound hits hard, loud,
and high, then falls in volume and pitch.
How does this affect your detecting? Well, knowing what sounds
are made when your CZ encounters positive and negative targets will help
you to realize what’s in the ground. It can also help you in determining
why your detector is falsing so much.
Now, the sounds I’ve been describing are all made in Auto-tune.
This is my main search mode for cleaner areas. When I get a positive
target, I’ll flip to 0 Discrimination to ID the target. When I encounter
a negative target, I don’t do this, as a CZ WILL NOT give an audible tone
for smaller negative hot rocks. They are designed to do this, to make
searching easier in Discrimination.
I have noticed, however, on REALLY big negative hot rocks, that I do
get audible signals in 0 Discrimination, and they happen in an odd sequence.
I may be a High Coin signal that seems to move around, several inches at
a time, as much as 6” away from the previous hit. A REALLY big one
might even give me a Bell-tone (large target alert), then the High Coin hit,
interspersed with some mid- and low-tones. These also move around.
With this knowledge, when I get some really odd signals, or signals that
appear to occur out-of-sequence, I also double-check them in Auto-tune
to see if I have a large negative hot rock. If the signal(s) I receive
are kind of a soft, fluttering, sound, then I attribute them to small nails,
wire, and/or rust deposits in the ground. But, if I get a strong
negative hot rock response, I just move on.
If you’re hunting an area that has a lot of these type off signals, you
might want to retrieve a few of them. If you encounter a layer of coal
cinders, as is common in old homes that had a coal-fired furnace, then you
may want to think about using a detector that can work in that environment.
From what I understand, a Fisher Coin$trike is good for this.
Ideas? Suggestions? Complaints?
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