Electrical Exploration, Inductive Methods of

(also inductive electromagnetic methods), a group of electrical exploration methods that are based on electromagnetic induction. Inductive methods were first developed in the early 20th century in Sweden and the USA. In the USSR, they were developed between 1928 and 1930.

In the inductive methods, the source of the primary magnetic field is an ungrounded circuit that is located at the surface and through which an alternating current is introduced into the ground. The currents that are induced by the primary magnetic field in sections of the earth’s crust that are good conductors—for example, in ore deposits—produce a secondary magnetic field. The total magnetic field is measured at the surface by means of electromagnetic detectors. The location, in the earth’s crust, of magnetic objects or of objects that are good conductors is determined from plots of the measured vertical or horizontal components of the magnetic field intensity.

According to the time dependence of the applied field, a distinction is made between low-frequency inductive methods and transient methods. The low-frequency methods are based on harmonic oscillations of the field intensity. In the transient methods, the primary field is varied in steps and the transient response after the dissipation of the primary field is analyzed.

Depending on the type of field source used, the following inductive methods are distinguished: the ungrounded-loop method, the long-cable method, and dipole induction profiling.

In the ungrounded-loop method, the field source is a rectangular loop with sides ranging from several hundred meters to a few kilometers in length. The magnetic field is measured in profiles at the center of the loop and perpendicular to its long side. The method is used to search for deposits of ores that are good conductors.

In the long cable method, the primary-field source is a rectangular cable up to several kilometers long. The magnetic field of the cable is studied along profiles perpendicular to the cable. The method is used to solve problems of geologic cartography and problems of tracing ore controls.

In dipole induction profiling, the field source is a magnetic dipole, that is, a coil with many turns and a diameter of about 1 m. The method is characterized by a reduced depth of exploration. It is used in prospecting for ores that are good conductors and in geologic cartography.