chapter  6
Electromagnetic Induction Methods
Pages 20

Electromagnetic (EM) techniques are the methods widely utilized for locating conductive and metallic objects in the subsurface. The foundation concept of EM induction is that any time-varying EM eld will cause current to ow in any conductive (or semiconductive) object that it encounters. The EM eld propagates freely through space and most semiconducting materials, as described by the wave equation. When the low-frequency EM eld encounters an object with a different conductivity from the host material, then the magnetic eld of the EM eld induces eddy currents (vortex currents) in an object that has a conductivity contrast with the surrounding medium, along with galvanic currents caused by the electric eld components. The direction of the current ow in the conductive object will tend to oppose the direction of the original eld (sometimes called the primary, or inducing eld), and will cause its own EM eld (sometimes called the secondary eld). The presence or absence of the galvanic currents is a function of the size, shape, and electrical properties of the buried object and host material. The eddy and galvanic currents emit a secondary EM eld, and this process is often referred to as induction. Sensors (coils, or long wires) are used to measure this secondary EM eld, and the task of the geophysicist is to determine the cause of the anomalies that are present in the data.