Discrimination Pitfalls - White's MX5 Owner's Manual

Discrimination Pitfalls

To avoid missing good targets it is important to understand
the limitations of target ID and (because it is based on target ID)
discrimination. As explained in the section on VDI, target ID is
based on the phase shift of a target's received signal. We've
already seen that many different types of targets can share VDI
responses so that rejecting certain kinds of trash will also reject
potentially good targets.
Depth also plays a role. A strong target signal (either from a
shallow target or a large target) provides for a strong and repeat-
able phase response from which we can extract a fairly reliable
VDI. As the target signal weakens, its VDI becomes less reli-
able. This can show up as an inconsistent target response as the
coil is repeatedly swept over the target. It is not unusual for
deep targets to produce an inaccurate target ID, and perhaps
even cross over into an adjacent (and perhaps rejected) target
zone.
Furthermore, ground mineralization can cause errors in tar-
get phase. Even targets of moderate depth can produce an inac-
curate target ID if ground mineralization is severe enough. This
can also push the VDI into an adjacent discrimination zone.
Therefore it is important to be aware that when you notch out a
particular disc segment it is possible to lose targets that nor-
mally don't fall in that segment. For example, suppose you
notch out the zones for "5" and "10" to get rid of foil trash;
nickels that normally might respond with a VDI of "20" could
get pushed into the "10" zone due to severe ground, at which
point they would be rejected.
Reliance on target ID should be tempered, and discrimina-
tion should be used with care. A "test garden" with multiple
types of targets buried at various depths is a good way to see
how target responses behave and to hone your skills in reading
these responses.
MX5
Owner's Manual
Page 15