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Departement of international trade:
GROUND BALANCE
Balancing the device to the ground is the single most important part of
the device settings for accurate search results. Improper ground balanc-
ing can result in decreased responses to targets and may result in insta-
bility as well as an increase in false ground signals.
To explain the basic physics behind the ground balance procedure.
Every object has its own shape, conductivity and magnetic component,
which together are called the VDI (Visual Digital Index). Remember
that the VDI is the angular response that is numerically equal to the an-
gle of deflection of the signal vector time curve (hodograph display) on
the vertical axis. The soil, or ground, depending on the degree of miner-
alization (presence of iron oxides) imposes its signal on the target signal
thus changing the VDI upscale. This is especially true on small or deep
targets, as well as large highly conductive targets (copper, silver, alumi-
num). This can actually push these high VDI targets up and over into
the high negative VDI range known as “VDI wrap around”. To minimize
this effect, it is crucial to perform a proper and accurate ground balance
procedure.
The basic objectives of balancing:
1. Make the soil 'invisible' to the instrument, i.e. exclude false signals from
the ground where there is no target.
2. Install the correct (standardized) reference system number for the VDI.
When receiving a signal from both the ground and a detected object the
detector must subtract mathematically the signal from the ground in order
to display a correct VDI indication of the detected target.
Keep in mind that when performing “air tests” inside after the detector
has been previously balanced in soil may result in incorrect VDI read-
ings. This is due to the correction factor for the VDI previously set in the
last ground balance. This may result in the loss or “invisibility” of large
aluminum, silver and copper targets. If the ground setting is too posi-
tive the VDI will be offset positive and vice versa if the ground balance
is negative. This may also lead to targets air testing as iron, as well as a
false increase in sensitivity when air testing.
Remember
that for every 1 degree of ground balance offset from the 0 or
neutral point we get a corresponding increase or decrease in the
