Introduction

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 field will cause current to flow in any conductive (or semiconductive) object that it encounters. The EM field propagates freely through space and most semiconducting materials, as described by the wave equation. When the low-frequency EM field encounters an object with a different conductivity from the host material, then the magnetic field of the EM field 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 field components. The direction of the current flow in the conductive object will tend to oppose the direction of the original field (sometimes called the primary, or inducing field), and will cause its own EM field (sometimes called the secondary field). 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 field, and this process is often referred to as induction. Sensors (coils, or long wires) are used to measure this secondary EM field, and the task of the geophysicist is to determine the cause of the anomalies that are present in the data.

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