Geology Reference
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targets. Most modern metal detectors, including 'super metal detectors' such
as the Geonics EM63, which was designed specifically for UXO detection
at depths of a few metres, use TEM principles.
8.4.4 TEM and IP
TEM superficially resembles the time-domain IP methods discussed in
Chapter 7. The most obvious difference is that currents in most IP surveys
are injected directly into the ground and not induced by magnetic fields.
However, at least one IP method does use induction and a more fundamental
difference lies in the time scales.
Time-domain IP systems usually sample only at delays of between 100 ms
and 2 s, and so avoid most of the EM effects, which occur mainly at delays
of less than 200 ms. There is thus a small region of overlap between the
two methods, and some frequency-domain or phase IP units are designed
to work over the whole range of frequencies from DC to tens of kilohertz
to obtain conductivity spectra (see Section 7.4.2). However, it is usually
possible to regard the EM and IP phenomena as completely separate and to
avoid working in regions, either of frequency or time delay, where both are
significant.
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