Garrett Metal Detector GTI 1500 Owner's Manual page 56

Another indication of a reject target is an
audio signal as you scan one way, and no
response when scanning from the other
direction.
If you encounter a large target or scan too
close to it, the detector will give an audible
overload indication, which sounds like a
buzzing alarm. Target identification, size
and depth readings may be incorrect if an
overload occurs.
Setting Sensitivity
Garrett engineers designed your GTI 1500
to achieve maximum detection depth, but
you may not always be able to search
at 100% sensitivity (depth)...just as you
rarely can (or, even want to) operate your
automobile at 100% of its speed capacity.
Outside electromagnetic interference and
irregular ground mineralization may some-
times require that you search at reduced
sensitivity.
Don't worry about losing "deep targets!"
You won't ever be losing anything because
you'll always be able to search as deeply
56 - Garrett Metal Detectors