Garrett AT Gold Owner's Manual page 17

The examples shown below illustrate how an iron object can
often "mask" out the signal of an adjacent good target when
too much iron discrimination has been applied. using the High-
res Iron Discrimination, apply just enough iron discrimination
to reject the undesired iron nail shown in this example. By
using only a minimal amount of iron discrimination, the
detector will detect the combined conductivity of the gold
nugget and nail together thus overcoming the potential
"masking" problem.
Illustration 1
Iron targets, such as the nail shown in Illustration 1, can sometimes
mask a good target's signal. If too much iron discrimination is
applied, the good target (seen in Illustration 2) can be missed.
read page 29 to learn how to apply the proper amount of iron
discrimination to eliminate the nail shown in Illustration 1 and still
detect the gold nugget shown in Illustration 2.
30 Garrett Metal Detectors
Illustration 2
Example: Detecting Targets in Trash with High-Res Iron Discrimination
In the illustration above, the AT Gold is operating in the DISC 1 Mode with
an IrON DISC setting of 20. The nail seen in Illustration 1 (on page 28)
registers from 18 to 24 on the Digital Target ID scale. To eliminate the nail
from detection, increase the iron discrimination level to 24 using the IrON
DISC (+) pushbutton.
In Illustration 2, the same iron nail is laying above a gold nugget. Since the
Iron Discrimination level is now set to 24, the nail by itself would not be
detected; however, the two objects (nail and gold nugget) have a combined
conductivity of more than 24.
Therefore, the gold nugget is detected due to the combined conductivity
being higher than that of the discriminated target (nail) alone.
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AT Gold ™