=∈ （3-5）

In addition, another term called the loss tangent is defined. The loss tangent is the ratio of conduction to displace-ment currents in a material. =

=∈ （3-6） The term loss target tends to be most common in electrical engineering contexts.

Conductivity and permittivity are not independent of the excitation frequency. There is always some variation. This topic is beyond the scope of this chapter of the notes but there is considerable literature (i.e., Olhoeft, 1975) on fre-quency dependent electric

3.5 MAGNETICPERMEABILITY

Magnetic permeability is seldom of major importance for GPR applications. For completeness and to address those exceptional situations where permeabilitymay become important, we review some of the basic aspects of magnetic permeability.

Magnetic permeability is actually related to the intrinsic electrical characteristics of the basic building blocks of phys-ical materials. In simple terms, charged particles, which form atoms , which in turn make molecules, have a quantum mechanical property referred toas spin. When combined with the charge on the particle, spin results in the particle having a magnetic dipole moment. When an electron moves around an atomic nucleus, the charge motion can also create a magnetic moment.

The simple analogy is to have electrical charge uniformly distributed on a spherical ball and then spin ball. The resulting rotating charge appears to be a circular loop of current, which in turn gives on rise to a magnetic dipole.

Magnetic properties are essentially the properties of an electrical change moving around a closed path.al properties which can be referred to.

a）