temperature, pressure and many other factors. As a result, one should not be surprised to see both non-linearity and frequency dependent conductivity in real materials. Generally these are second order effects but they must be considered when advanced use of GPR is contemplated. For this basic GPR overview, they will be treated as secondary issues.

3.3 DISPLACEMENT (POLARIZATION) CURRENTS

Displacement currents are associated with bound charges which are constrained to limited distance of movement. Examples of this are the electron cloud around an atomic nucleus, the electricalcharge in a small metal object imbed-ded in an insulating environment, and the redistribution ofthe molecular dipole moment intrinsic to some molecules. Figure 3-4 depicts the concept. When an electric field is applied, bound charge moves to another static configuration.This transition occurs virtually instantaneously after which the charges no longer move. During the transition, energy is extracted from the electric field and the energy is stored in the material. When the field is removed, the charge moves back to the original equilibrium distribution and energy is released. This type of behavior is typical of what happens in a capacitor in an electric circuit. Energy is stored by the buildup of charge in the capacitor and then energy is extracted by the release ofthat charge from the device.

Figure: 3-4 Conceptual illustration of charge movement associated with displacement currents. Figure 3-5 depicts the characterization of charge separation in a material. When an electrical field is applied, dis-placement of charge in a bulk material gives rise to a dipole moment distribution in the material. The charge separa-tion is described in terms of a dipole moment density, D.In a more formal derivation, D is called the electric displacement field (see chapter 2). In simple materials, the induced dipole moment densityis directly proportional to the applied electric field and the proportionality constant is referred to as the dielectric permittivity of the material and

has units of Farads/m (F/m).