survey using the Wenner array and the ABEM Terrameter
EM and resistivity methods are used in two different modes: profiling
and sounding. Profiling is used to detect lateral variations
across a site by taking a series of readings along a line using a fixed configuration
of coils or electrodes. (EM is typically used in the profile mode).
Soundings are used to estimate vertical variations in electrical conductivity
or resistivity. A resistivity sounding is made by taking many readings
with increasing electrode separations at a single location. An EM sounding
is obtained by taking readings at a single location with several coil spacings
and coil orientations. The data are then inverted to produce a model of
conductivity (resistivity) variations with depth. Due to the greater number
of readings possible, resisitivity soundings provide better vertical resolution
than EM soundings.
Profiles and soundings may
be obtained simultaneously to yield a 3-dimensional model.
While both EM and resistivity measure apparent ground conductivity, their
response to certain kinds of anomalies differs markedly. EM is very
sensitive to highly conductive media, so a thin, high conductivity layer may
dominate over much thicker, low conductivity layers. Also, if conductivities
are very high, the measurements become non-linear and eventually turn negative.
The resistivity method is less sensitive to thin, high conductivity layers
and can measure even the lowest and highest apparent conductivities.
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