The EM technique measures the electrical properties of materials contained in the subsurface including soil, rock, groundwater, and any buried objects.  An alternating current in the EM transmitter coil creates a magnetic field which induces electrical current loops within the ground; the current loops, in turn, create a secondary magnetic field.  Both the primary magnetic field (produced by the transmitter coil) and the secondary field induce a corresponding alternating current in the EM receiver coil.  After compensating for the primary field (which can be computed from the relative positions and orientations of the coils), both the magnitude and relative phase of the secondary field can be measured.

These can be converted to components in-phase and 90° out of phase with the transmitted field.  The out-of-phase (or quadrature-phase ) component, using certain simplifying assumptions, can be converted to a measure of apparent ground conductivity.  The in-phase component, while generally not responsive to changes in bulk conductivity, is especially responsive to discrete, highly-conductive bodies such as metal objects.  The apparent conductivity measurement is the average conductivity of one or more layers in the ground in the proximity of the instrument, to a depth of investigation dependent on the coil spacing, orientation, operating frequency of the instrument, and the individual conductivity of each ground layer.

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