Adjusting Sensitivity - Fisher Research Labs F70 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 Autotune 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 F70 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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