1、1 外文文献 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 th
2、e 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 m
3、edium. 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 v
4、ariations 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 materia
5、l. 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
6、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 M
7、ost 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 fro
8、m 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 a
9、pplied. As long as the electric field is applied, the charge moves； when the electric field is removed, the charge 2 decelerates and stops moving Figure 3-1 illustrates these concepts. Figure: 3-1 Conceptual illustration of charge movement for conduction currents. a) Charge velocity versus time afte
10、r E field applied. b) Energy is extractedfrom the applied electric field versus time. Figure: 3-2 When an electric field is applied, unbound electrical charges accelerate to a terminal velocity. After initial acceleration, velocity becomes constant and a continual transfer of energy to the surroundi
11、ng material in the form of heat occurs All the time that charge is moving, the moving charge is working against its 3 surroundings dissipating energy in the form of heat. The moving charge bumps into non-movingobjects and transfers mechanical energy which appears in the form of heat in the medium. C
12、onduction currents represent an energy dissipating mechanism for an electromag-netic field. Energy is extracted from the electromagnetic field and transferred irreversibly into the medium as heat. Mathematically one describes the relationship between conduction current and the applied electric field
13、 as indicated in Equation 3-1. = （ 3-1） In simple materials, the relationship is linear and the proportionality constant is referred to as the electrical conductiv-ity. Electrical conductivity has units ofSiemens per meter (S/m). For many applications, however, it is more useful to work with units o
14、f milliSiemens per meter (mS/m). Conductivity is dependent on the charge density and the inter-nal statistical mechanical interaction ofthe charge with its surroundings. These details are beyond the scope of this discussion. It should be noted that electrical conductivity and resistivity are directl
15、y related. Refer toFigure 3-3 for the relation-ship and the expression of Ohms law. Electrical resistivity is the inverse of electrical conductivity. Figure: 3-3 Relationship between current and applied field as well as the relationship will Ohms law and resis-tively. It is important to note that there are simplifications in the above discussion from the general form shown in Chapter 2. The conductivity is shown as being a constant. In fact it can be a function of the rate of change of the electric field,the amplitude of electric field itself, as well as
