Geology Reference
In-Depth Information
8.2.2 Instrumentation
The Geonics EM31 (Figure 8.9) is an example of a co-planar coil instrument
that can be used, at some risk to life and limb on difficult sites, by one operator
to obtain rapid estimates of apparent resistivity. Man-made conductors such
as buried drums and cables may also be detected. The instrument is designed
to be used with the coils horizontal, giving, at low induction numbers, a
penetration of about 6 m and a radius of investigation of about 3 m with the
fixed 3.7 m coil spacing. This compares very favourably with the 20 to 30 m
total length of a Wenner array with similar penetration (see Section 6.1.3).
Measurements can also be made (although not easily), with the coils vertical,
halving the penetration. A shorter, and therefore more manoeuvrable version,
the EM31-SH, is only 2 m long and provides better resolution, but only about
4 m of penetration.
Both versions of the EM31 operate at 9.8 kHz. The more powerful, two-
person, Geonics EM34-3 uses frequencies of 0.4, 1.6 and 6.4 kHz with
spacings of 40, 20 and 10 m respectively. The frequency is quadrupled each
time the coil separation is halved, so the induction number remains the
same. Coil separation is monitored using the in-phase signal, and phase is
monitored using a signal sent along a reference cable. Penetrations are 15, 30
Figure 8.9
EM31 in use in open country.
