White’s Electronics Spectra VX3 - Manual (Page 18)

Page 1-12
Spectra VX 3 User’s Guide
composite VDI that can land anywhere between pure ferrite
(VDI = -95) and pure salt. Other areas, like fertilized fields with
residual salt ions, can also have a composite ground response.
Many deserts have a layer of surface salts that have been
leached from the soil; this is generally not a problem as long as
it is dry.
Frequency
Metal detectors are produced using a wide range of trans-
mitted frequencies, from 1kHz up to around 100kHz, though
the vast majority fall in the VLF range of 3kHz - 30kHz. Low
frequencies usually favor thicker targets and metals of higher
conductivity, while high frequencies favor thin and low-con-
ductive targets.
Low frequencies also do a better job penetrating ground
mineralization, including salt. High frequencies tend to gener-
ate stronger ground and salt signals which can limit the ability
to distinguish weak targets. Obviously, when trying to detect
thin low-conductive targets (like nuggets and jewelry) in harsh
ground (like wet salt sand or black sand) there are competing
frequency requirements, so a compromise is necessary. With the
ability to run 3 simultaneous frequencies or any one of them
individually, VX
3 has the ability to deal with a wide variety of
conditions.
Interesting Experiment: Thickness matters because of a
phenomenon known as skin effect. To demonstrate this,
cut several identical flat squares (say, 1”x1”) of aluminum
foil. Test the VDI response of a single square, and see how
the VDI varies as you stack more squares (tightly)
together.