Resistivity survey using the Wenner array and the ABEM Terrameter
The 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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