Electromagnetic method is a geophysical technique based on physical principles of the inducing and detecting electrical current flow within geological strata. A primary alternating electric current of known frequency and magnitude is passed through a sending coil creating a primary magnetic field in the space surrounding the coil, including underground. The eddy currents generated in the ground in turn induce a secondary current in underground conductors which results in an alternating secondary magnetic field, that is sensed by the receiving coil. The secondary field is distinguished from the primary field by a phase lag. The ratio of the magnitudes of the primary and secondary currents is proportional to the terrain conductivity.
The readings are commonly expressed in the conductivity units of mS/m. The depth of penetration is governed by the coil separation and orientation. The EM method should not be confused with the electrical resistivity method. The difference between the two techniques is in the way that the electrical currents are forced to flow in the subsurface. In the electromagnetic method, currents are induced in the subsurface without any direct contact with the ground surface.
EM surveys are categorised as frequency domain and time domain. Frequency domain instruments measure the amplitude and phase of the induced electromagnetic field while time domain instruments measure the decay time of the induced field. The applications of this method include:
The common applications of these methods are to: