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1、1外文文献3 PHYSICAL PROPERTIES I3.1 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 fi
2、elds 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 mediu
3、m. 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 varia
4、tions 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. Th
5、ere 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 th
6、e text byVon 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 CURRENTSMost peopl
7、e 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 poi
8、nt 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. A
9、s long as the electric field is applied, the charge moves; when the electric field is removed, the charge 2decelerates 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 after E field ap
10、plied.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 surrounding material in
11、 the form of heat occursAll the time that charge is moving, the moving charge is working against its 3surroundings 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. Conduction curren
12、ts 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 as indicatedin
13、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 of milliSiemens per me
14、ter (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 directly related. Refer toFig
15、ure 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
16、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 4temperature, 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
