White’s Electronics Spectra VX3 - Manual (Page 71)

Page 6-8
Spectra VX 3 User’s Guide
You can also use the Ground Probe to analyze target signals,
and can see exactly how signal phase and strength vary with
frequency. This may be of limited use in the field, however,
because a buried target also has ground above it so the Ground
Probe will give you the total response of the two. If the coil is
placed on the ground (target-free area) when the Ground Probe
is zeroed, this will largely leave you with only the target signal,
but it is subject to errors due to variations in ground as you
move the coil around.