Introduction

There is a wide variety of electromagnetic (EM) survey methods. Each method, however, involves the measurement of one or more electric or magnetic field components induced in the subsurface by a primary field produced from a natural (transient) or an artificial alternating current source. The primary field spreads out in space both above and below the ground and induces currents in subsurface conductors (Fig. 7.1), in accordance with the laws of EM induction. These currents give rise to secondary EM fields, which distort the primary field. In general, the resultant field, which may be picked up by a suitable receiving coil, will differ from the primary field in intensity, phase, and direction and reveal the presence of the conductor.

If the primary field is not continuous but transient, the secondary field induced in the subsurface conductor will decay gradually when the primary field is switched off. The decay is slower within a body of higher conductivity. Measurement of the rate of decay of the secondary currents and their field thus provides a means of locating anomalously conducting bodies.

Most EM systems employ an active transmitter so that the source geometry and wave frequency (or the transient pulse duration) can be controlled during the field operations. The main function of the artificial field EM methods is the detection of bodies of high electrical conductivity. Most favorable targets are metallic ores and underground pipes and cables, although the methods have been used in delineating faults, shears, and thin conducting veins, and in groundwater studies.