EM surveys use an instrument called an electromagnetic induction meter (coils of wire) that induces an electromagnetic signal into the ground and measures how well it is conducted by the underlying rock. The frequency of the signal may vary but there must be no electrical connection between the survey instrument and the ground, unlike induced polarization instruments. The induction meter uses a coil near ground surface to “broadcast” its high frequency signal that is received by another coil, also near ground surface. The transmitted signal causes the conductive material below the meter to generate its own signal that is detected by the receiver coil. Both coils are built into the meter and the spacing between them governs the effective depth to which the meter can measure earth conductivity.

The instrument is not designed to be a metal detector but highly conductive metals also generate a strong signal in response to the meter. The electronics of the meter converts the signal into the measure of conductivity and so the technique is most useful for detecting sulfide mineralization.