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Spectra VX
3
User’s Guide
VDI
As the loop sweeps over a target the received target signal
rises, peaks, and then falls. Ideally the VDI
1
of a “well-
behaved
2
” target is independent of signal strength, but in reality
weak signals will show errors in the VDI. For that reason, the
most accurate VDI will occur at the peak of the target signal
response. Therefore, as you sweep over a target (especially with
a slow sweep) you may see the VDI rapidly change before it
locks onto a final value:
In three frequency mode the VDI is a composite number
based on the peak VDI readings of at least the two strongest
responding frequencies. For that reason, you may occasionally
notice that the VDI number does not exactly match the Spectra-
Graph response, especially for a weak frequency. In single fre-
quency mode the VDI is obviously taken off the one active
frequency.
Since VDI accuracy diminishes with depth, deep targets can
produce erroneous VDIs. High ground mineralization can also
shift VDIs; again it gets worse for deep targets. This is where
experience determines whether you pass over a 1914D Lincoln
because the VDI looked like a zinc cent. The best way to learn
your detector is to dig a lot of targets, paying close attention to
exactly what the detector was telling you before you dug it up.
1. Visual Discrimination Indicator; see Chapter 1.
2. Such as a flat round disc-shaped homogeneous metal target lay-
ing horizontally.
No VDI
No VDI
Peak VDI
Changing VDI
