Geonics Limited EM31-SH, Operating Manual

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EM31-SH MANUAL Page
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6.1 Introduction (cont’d)
effective depth penetration of the system is determined by
the intercoil spacing and is independent of skin depth and
thus of ground conductivity. This feature greatly simplifies
interpretation of survey results. In order to vary the depth
to which the resistivity is sensed one simply varies the
intercoil spacing in analogy with conventional resistivity
surveys. Conversely each survey carries out at a fixed
intercoil spacing is essentially analogous to a survey
carried out with conventional resistivity equipment at a
fixed interprobe spacing.
The Geonics EM31 is a one-man portable instrument designed
for engineering geophysical applications down to depths of
the order of six meters. The intercoil spacing is fixed at
twelve feet (3.66 meters). The
effective penetration
referred to above is an average value; in those regions where
a conductive medium is to be located beneath a resistive
layer the penetration depth is substantially larger.
The EM34 is operated at 3 intercoil spacings viz 10, 20 and
40m, resulting in effective depth penetrations of the order
of 7.5 to 60 m depending on the intercoil spacing employed
for the particular survey. Operation of the EM34 requires two
men; however, measurements are still taken virtually as fast
as the team walks.
Most of the case histories in the technical note have been
taken with the EM31; however, in some cases data from EM34
surveys are presented in order to further elucidate the
features of inductive electromagnetic terrain resistivity
mapping.