Electromagnetic Methods
Electromagnetic (EM) methods detect the electrical properties of the subsurface by inducing EM energy within the subsurface and measuring the response of earth materials. Generally, EM geophysical instruments output a time-varying electric current into its transmitter coil, or loop. As the current travels in the transmitter loop, it generates a magnetic field that has the same frequency and phase as the current. This induced field propagates lines of force that penetrate the earth.
If a conductive body or conductive material exist in the vicinity of the instrument, an electromotive force (i.e., voltage) forms within it (i.e., Faraday’s Law). Electrical current flow is then initiated within the subsurface conductors, and a secondary magnetic field is generated. The secondary field is detected by the receiver coil of the EM geophysical instrument, which compares this received energy to the transmitted energy and records the data. These data are then processed and interpreted to characterize the subsurface EM properties.
Subsurface EM energy is propagated due to the three electromagnetic properties of matter: electrical conductivity, dielectric permittivity, and magnetic susceptibility. Electrical conductivity and magnetic susceptibility govern the magnitude of the received EM signal and, therefore, are used to understand the electrical properties of subsurface materials. Though there are many surface EM methods, the select few that are further discussed are as follows: