Field, electromagnetic

Electromagnetic geophysical methods arc based on the study of the propagation of electric currents and electromagnetic fields in the earth. There arc two major modifications of electromagnetic methods one is based on direct current (DC), and another employs a transient electromagnetic field. 

DC methods, or resistivity methods, involve injecting electric current in the earth by a system of current electrodes and measuring the electrical potential with receiver electrodes. In practice, it is more convenient technically to use a low frequency (below 10 Hx) current, which propagates inside the earth practically like a direct current. DC surveys are used to determine the resistivity of rock formations. 

Resistivity is a very important physical parameter that provides information about the mineral content and physical structure of rocks, and also about fluids in the rocks. 

In both cases the electric potential distribution recorded by the receiver electrodes is used to map the spatial resistivity distribution of the rock formation. The main limitation of the resistivity method is that direct current cannot penetrate through resistive formations. Electromagnetic induction methods, based on transient electromagnetic fields, overcome this difficulty because a transient field can easily propagate through resistors like a radiowave propagates in the air. At the same time, 

Another typical electromagnetic survey is used in an airborne system. In this case the transmitter loop is located on the plane while the receiver loop is positioned 

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Writing the electromagnetic field in terms of transverse electric (TE) and transverse magnetic (TM) components, the electric field has the form ... 

Calculation of the Electromagnetic Field around an Exciter Coil... 

Maxwell s equation are the basis for the calculation of electromagnetic fields. An exact solution of these equations can be given only in special cases, so that numerical approximations are used. If the problem is two-dimensional, a considerable reduction of the computation expenditure can be obtained by the introduction of the magnetic vector potential A =VxB. With the assumption that all field variables are sinusoidal, the time dependence... 

The development of Remote Field Eddy Current probes requires experience and expensive experiments. The numerical simulation of electromagnetic fields can be used not only for a better understanding of the Remote Field effect but also for the probe lay out. Geometrical parameters of the prohe can be derived from calculation results as well as inspection parameters. An important requirement for a realistic prediction of the probe performance is the consideration of material properties of the tube for which the probe is designed. The experimental determination of magnetization curves is necessary and can be satisfactory done with a simple experimental setup. 

The measurement and evaluation methods of chapter 3.1. and 3.2. work with inductive sensors in an absolute circuit. The results on two different formed coils, a pot core coil and a cylinder core coil are selected. For presentation in this paper the third method, described in chapter 3.3., uses a Hall-effect device to detect the information and a coil system in a differential circuit to excite the electromagnetic field. 

Both coils are constructed in the same way, geometry and number of windings are equal. A permanent sinus current flows through these coils and excites an electromagnetic field around each coils. 

The electromagnetic field created by the transducer propagates through the examined material as a wave with the length... 

In the previous sections we have described the interaction of the electromagnetic field with matter, that is, tlie way the material is affected by the presence of the field. But there is a second, reciprocal perspective the excitation of the material by the electromagnetic field generates a dipole (polarization) where none existed previously. Over a sample of finite size this dipole is macroscopic, and serves as a new source tenu in Maxwell s equations. For weak fields, the source tenu, P, is linear in the field strength. Thus,... 

This poses a special problem because the source of the electromagnetic field may lie outside the defined boundaries of the system. A detailed discussion of this is outside the scope of this section, but the basic features can be briefly sunnnarized. 

Both infrared and Raman spectroscopy provide infonnation on the vibrational motion of molecules. The teclmiques employed differ, but the underlying molecular motion is the same. A qualitative description of IR and Raman spectroscopies is first presented. Then a slightly more rigorous development will be described. For both IR and Raman spectroscopy, the fiindamental interaction is between a dipole moment and an electromagnetic field. Ultimately, the two... 

Raman scattering has been discussed by many authors. As in the case of IR vibrational spectroscopy, the interaction is between the electromagnetic field and a dipole moment, however in this case the dipole moment is induced by the field itself The induced dipole is pj j = a E, where a is the polarizability. It can be expressed in a Taylor series expansion in coordinate isplacement... 

RS (or BS) theory is driven by the magnetic component of the electromagnetic field, not the electric (as discussed exclusively in the present chapter). 

In order to describe the second-order nonlinear response from the interface of two centrosynnnetric media, the material system may be divided into tlnee regions the interface and the two bulk media. The interface is defined to be the transitional zone where the material properties—such as the electronic structure or molecular orientation of adsorbates—or the electromagnetic fields differ appreciably from the two bulk media. For most systems, this region occurs over a length scale of only a few Angstroms. With respect to the optical radiation, we can thus treat the nonlinearity of the interface as localized to a sheet of polarization. Fonnally, we can describe this sheet by a nonlinear dipole moment per unit area, -P ", which is related to a second-order bulk polarization by hy P - lx, y,r) = y. Flere z is the surface nonnal direction, and the... 

The nonlinear response of the interface may then be characterized in tenns of a surface (or interface) nonlmear susceptibility tensor. This quantity relates the applied electromagnetic fields to the induced... 

Electronic structure theory describes the motions of the electrons and produces energy surfaces and wavefiinctions. The shapes and geometries of molecules, their electronic, vibrational and rotational energy levels, as well as the interactions of these states with electromagnetic fields lie within the realm of quantum stnicture theory. 

Not only can electronic wavefiinctions tell us about the average values of all the physical properties for any particular state (i.e. above), but they also allow us to tell us how a specific perturbation (e.g. an electric field in the Stark effect, a magnetic field in the Zeeman effect and light s electromagnetic fields in spectroscopy) can alter the specific state of interest. For example, the perturbation arising from the electric field of a photon interacting with the electrons in a molecule is given within die so-called electric dipole approximation [12] by ... 

Between any two atoms or molecules, van der Waals (or dispersion) forces act because of interactions between the fluctuating electromagnetic fields resulting from their polarizabilities (see section Al. 5, and, for instance. 

In order to rmderstand how light can be controlled, we must first review some of tire basic properties of tire electromagnetic field [8], The electromagnetic tlieory of light is governed by tire equations of James Clerk Maxwell. The field phenomena in free space with no sources are described by tire basic set of relationships below ... 

The necessary boundary conditions required for E and //to satisfy Maxwell s equations give rise to tire well known wave equation for tire electromagnetic field ... 

Altliough a complete treatment of optical phenomena generally requires a full quantum mechanical description of tire light field, many of tire devices of interest tliroughout optoelectronics can be described using tire wave properties of tire optical field. Several excellent treatments on tire quantum mechanical tlieory of tire electromagnetic field are listed in [9]. 

For tire electromagnetic fields E and H tire fonn of tire waves of interest is... 

The last attribute of tire electromagnetic field we need to discuss is wave polarization. The nature of tire transverse field is such tliat tire oscillating field disturbance (which is perjDendicular to tire propagation direction) has a particular orientation in space. The polarization of light is detennined by tire time evolution of tire direction of tire electric field... 

Given tire general description of tire electromagnetic field, let us explore the sources available for optoelectronics. The one primary light source for optoelectronic device and system architectures is tire laser. The laser [10] is tire source of choice simply because if we want to control light fields tliey need to be well defined at tire start and tire laser is tire most... 

The acronym LASER (Light Amplification via tire Stimulated Emission of Radiation) defines the process of amplification. For all intents and purjDoses tliis metliod was elegantly outlined by Einstein in 1917 [H] wherein he derived a treatment of the dynamic equilibrium of a material in a electromagnetic field absorbing and emitting photons. Key here is tire insight tliat, in addition to absorjDtion and spontaneous emission processes, in an excited system one can stimulate tire emission of a photon by interaction witli tire electromagnetic field. It is tliis stimulated emission process which lays tire conceptual foundation of tire laser. 

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