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3 PHYSICAL PROPERTIES I

3.1 WHY ARE PHYSICAL PROPERTIES IMPORTANT?

GPR investigates the subsurface by makinguse of electromagnetic fields which propagate into the subsurface. EM fields which are time varying consist of coupled electric (E) and magnetic (H) fields. As discussed in section 2 the fields interact with the surrounding media. This interaction is macroscopically described by the constitutive equations 2.5 to 2.7. The manner in which the electromagnetic fields interact with natural materials controls how electro-magnetic fields spread into the medium and are attenuated in the medium. In addition, the variation in physical properties gives rise to the observed subsurface reflections obtained with a GPR system.

In most geological and NDT (non-destructive testing) applications of GPR, electrical properties tend to be the domi-nant factor controlling GPR responses. Magnetic variations are usually weak. Occasionally magnetic properties can affect radar responses and GPR users should be cognizant of magnetic effects.

An electric field in a material gives riseto the movement of electric charge, (i.e., electric current). The current flow depends on the nature of the material. There are two types of charge in a material, namely bound and free, which give rise to two types of current flow, namely displacement and conduction. In the following, we will provide a simple overview of the two types of current flow. An in-depth discussion of electrical properties can be found in the text by

Von Hippel,(1954).

Magnetic properties are controlled by the electric charge circulation character at the atomic and molecular level. Macroscopic magnetic properties are addressed briefly in these notes. Von Hippel (1954) addresses some of the basic concepts.

3.2 CONDUCTION CURRENTS

Most people are very familiar with electrical conduction currents. Conduction currents are created when unbound(free) charges move in a material. The electrons which flowin a metal wire are an example of conduction current. In a metal, electrons move through the metallic matrix to transfer charge from one point to another. Another common conduction mechanism is the movement of ions in a water solution. The later is much more important in most GPR applications.

Conduction currents arise whenfree charge accelerates to a terminal velocity (basically instantaneously) when an electric field (E) is applied. As long as the electric field is applied, the charge moves; when the electric field is removed, the charge