Adjusting Sensitivity - Fisher Labs F 75 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.
Comprehensive Operating Manual & Guide to Metal Detecting
F75
35