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HOT ROCKS
A hot rock is a rock which causes the metal detector to sound off because the
rock contains iron minerals. They come in two basic types.
Negative hot rocks
(also called cold rocks) are usually magnetite or
contain magnetite, and give a negative response because their ground
balance value is a higher number than the soil they are found in. They
tend to be dark in color, usually black, and usually heavy. In some cases
they will have rust stains. They are usually attracted to a magnet, and for
this reason gold prospectors always carry a magnet -- the ultimate
ferrous/nonferrous discriminator. In All Metal Mode, negative hot rocks
produce a boing sound rather than the zip sound of a metallic target;
recognize the difference and you will learn to ignore them. As the
searchcoil passes over a hot rock, this boing sound, or negative response,
is quite distinctive. To hear this response, you must be properly Ground
Balanced and in All Metal Mode with an audible threshold hum. First, as
the center of the searchcoil passes over the negative hot rock, the
detector will go quite; the threshold hum momentarily silences. Then,
passing beyond the negative hot rock, you hear the boing sound. As you
pass the searchcoil back and forth over a negative hot rock, it will be
impossible to pinpoint and will seem as if it moves around.
Positive hot rocks
are iron-bearing rocks which have been oxidized by
natural weathering processes so that their GND BAL value is a number
lower than the soil they are found in. They are often small, right on the
surface, sound just like a gold nugget, and are common in many gold
prospecting areas. They are usually, but not always, drawn to a magnet.
They are most often reddish in color but are often black, brown, or yellow.
On relic hunting sites, red clay bricks and rocks which have lined a fireplace
or a campfire will often be hot rocks. The discriminator will usually eliminate
them without difficulty if widely scattered, but if there is a large concentration
of them, the discriminator may not quiet them all. In that case, you can
revert to the rule of thumb -- “don’t dig non-repeatable signals.”
remember to always carry a magnet to help discriminate gold from hot rocks
and iron.
• Gold will not be attracted to a magnet.
• Pieces of iron will always be attracted to a magnet.
• Negative hot rocks will almost always be attracted to a magnet.
• Positive hot rocks will usually be attracted to a magnet.
2. Trash
If searching for coins, you want to ignore items like aluminum foil, nails and
pull-tabs. These undesirable items are generally identified toward the lower
end of the 0-100 scale. You can listen to the sounds of all objects detected,
and decide on what you want to dig up. Or you can eliminate unwanted
metals from detection by using the DISCrIMINATION feature.
3. Identifying Buried Objects
Different types of metals are classified along the arc at the top of the screen
on a 0-100 scale from left to right. A 2-digit numerical reading is also
provided in the middle of the display for more precise Target Identification
in Discrimination Mode.
4. Size and Depth of Buried Objects
When using the detector in the motion DISC Mode, the relative depth of an
object is displayed to the left of the display over the SIGNAL strength
indicator. A more accurate depth reading is available using PINPOINT.
Pinpoint displays target depth, in inches. The Pinpoint Mode does not
require the searchcoil to be in motion to detect metals. The ability to hold
the searchcoil motionless over the target also aids in tracing an outline of
the buried object, or in pinpointing the exact location of the object using
techniques described in the pinpointing section of this manual.
5. Electromagnetic Interference (EMI)
electromagnetic interference (eMI), can cause a metal detector to chatter
spontaneously, to lose sensitivity for no apparent reason, or to cause a
periodic wobbly audio sound. Common sources of eMI include power
lines, electronic communication equipment such as cellphones, fluorescent
lamps, military electronics such as radar, other metal detectors and
computer equipment.
Your first line of defense against eMI is to reduce the Gain and/or
Threshold. In areas with heavy eMI, operating at reduced sensitivity levels
will result in the loss of some depth, but at least the metal detector will be
usable.
See the 2-page section on eLeCTrOMAGNeTIC INTerFereNCe (eMI)
for a more thorough explanation of eMI and how to manage it.
THE BASICS OF METAL DETECTING