Tips On Ground Balancing - Fisher Labs F70 User Manual

Search Techniques (continued)

TIPS ON GROUND BALANCING

When the F70 first turns on, the ground balance setting is preset to 90. This
will give a positive response on nearly all soils. If you search
in the Discrimination mode, you will probably not have to
balance to the ground. If you switch to Autotune mode,
ground balancing will probably be necessary.
You must find a spot of ground which is free of metal to
accurately balance to the ground. Before
you attempt to Ground Balance, sweep
back and forth to see if any metal target is
present. Locate what seems to be a clear
area and then Ground Balance using the
GroundGrab procedure. After you have ground balanced,
sweep back and forth to see if there is little or no audible response
to the soil. This is best done either in Autotune mode, or in
Discrimination mode with Disc Level set to zero. Alternatively, use
Pinpoint to check the spot. If there is little or no response, ground
balancing was successful. If there is still substantial response, there may have been metal
present where you attempted to ground balance, so find another promising spot and try again.
If you cannot find a spot to successfully ground balance, it is time to give up.
In most areas, once you have ground balanced, the ground balance setting will remain
satisfactory for a long time. However, if the soil has been disturbed by digging or the addition
of fill dirt, or if you are in a geologically complex setting such as is commonly encountered in
gold prospecting areas, you may have to frequently perform the ground balancing procedure
to accommodate changing soil conditions.
When you ground balance, the numerical Ground Phase will momentarily appear on the LCD
screen. In general, sandy or gravelly soils will tend to read in the 75-95 range, light colored
loams and clays will tend to read in the 50-80 range, and red clays will tend to read in the 35-
55 range. To express it in other terms, the more highly weathered, oxidized, or finely grained
the soil is, the lower the numeric reading will tend to be.
The DIRT bar graph indicates how much iron mineralization is present. For it to work, the
search coil must remain in motion. The most accurate readings will be achieved by pumping
the coil as you do when ground balancing. The higher the mineralization, the greater the
necessity to ground balance the detector for the best depth performance.
If you are searching for relics, you can make a map of the soil of the site. Make a grid of the
site. Then collect data. Ground Balance to document mineral type; view mineral amount on
the DIRT bar graph. Then plot the data on the site map and draw isolines. In this way you
may be able to locate areas which have been dug, backfilled, or subjected to fire. This
information in turn helps to reveal the history of the site.
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