Page 5-7
Spectra VX
3
User’s Guide
As the loop is swept over the bottle cap, the phase response var-
ies wildly. This will not only result in smearing of the Spectra-
Graph plot, but will also cause it to jump around significantly
before it locks on to a final reading. Such a final reading might
look like:
Notice the peak point from which the composite VDI is cal-
culated; it is clearly in the non-ferrous (good) region, even
though bottle caps are typically steel. The point of all this is to
pay close attention to what
VX
3
is telling you
all the time
, not
just the locked-in response. When you initially detect a target,
go back over it with deliberately slow sweeps and watch the
dynamic responses.
Unaligned bars
Because
VX
3
normalizes the VDI responses of each fre-
quency, the SpectraGraph bars tend to be vertically aligned. In
some cases they are skewed, indicating a possible “problem”
with the target. For example, in the prior SignaGraph response
of the bottle cap, the bars have a very poor correlation, caused
by the ferrous properties of the bottle cap.
However, like everything else, there is no hard-and-fast rule
at work here. Mineralized soil can also affect frequencies differ-
ently such that deeper targets have unequally shifted phase
responses. In the “deep quarter” SpectraGraph on the previous
page, there is some skewing of the bars. Again, look at the
dynamic responses, and also pay attention to the repeatability of
the target responses. If every sweep across the target produces a
different response, then it is questionable (but if it is very deep,
consider digging it!).
