An EM survey relies on the response of the ground to variations in electric and magnetic forces due to the presence of conducting materials. A primary field is induced by passing an alternating current through a coil placed over the ground. The primary field spreads and changes as it encounters conductors in the ground. These changes are detected by receivers as data points which can then be processed to provide information about the conducting material in terms of geometry and intensity.
EM instruments are commonly used during environmental investigations to measure the effect of human activity on the earth since inorganic contaminants introduced into the sub-surface through human activity can dramatically effect ground conductivity. Thus EM surveys are excellent tools for mapping the lateral extent of inorganic impacts.
EM surveys are widely used by oil/gas facilities to complement regulatory Environmental Site Assessments. The extent of environmental impacts can be determined in a non-intrusive manner and reduce the number of soil sampling points required. In many cases, surveys are carried out over regular intervals to track changes with time.