Adjusting Sensitivity - Fisher Labs F75 User Manual

SEARCH TECHNIQUES (continued)
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 Motion 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.
Positive hot rocks are iron-bearing rocks which have been oxidized by natural
weathering processes so that their Ground Balance number 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".
Using the sensitivity control
When the F75 is first turned on, the Sensitivity is at a medium setting appropriate for most
coinshooting. For relic hunting or gold prospecting, higher Sensitivity settings are usually
preferred.
In the event of detection of electrical interference from electrical power lines, electrical or
electronic appliances, or another metal detector, it is usually necessary to reduce the
Sensitivity setting to achieve quiet operation. Alternatively, use the Frequency Shift feature
described earlier in this manual.
If, while searching, you are constantly getting signals from which you cannot recover metal
targets, you may be detecting small or deep targets which are not recoverable using the
methods at hand. So, you may do better if you reduce the Sensitivity setting.
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