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Spectra VX
3
User’s Guide
Ideally all targets would have tight responses that are
repeatable. The closest you can get to that is to air test targets 3-
4” from the coil, and that is not a bad start to learn how different
targets respond. However, just like we discussed with VDI
numbers, as a given target moves farther from the coil, its
response gets weaker and exhibits shifts and spreading in the
graph response, and is not as repeatable. This gets even worse
in mineralized ground.
Even with three frequencies and SpectraGraph, interpreting
target responses can be tricky business. Experience is the best
teacher (learn by digging lots of targets!), but there are a few
tricks we can discuss.
Smearing
A “well-behaved” target will have a response that closely
follows a constant VDI as the loop is swept over it. The vector
response for such a target would start at zero, follow a straight
line on a constant VDI up to the signal peak (where the loop is
centered on the target), and return to zero as the loop is moved
away. It will graphically look like
3
:
This might represent a somewhat shallow quarter, and would
have a SpectraGraph response with nice tight bars. Note the
black dot, which indicates the peak signal strength; this is also
the point where the VDI number is calculated. The same quarter
buried deeper begins to exhibit a small amount of curvature in
the vector response:
3. See Ch. 1 for background info on vector responses
.
Quarter
-95
+95
0
+83
