Showing papers on "Computational electromagnetics published in 1986"

Electromagnetic Modeling for Microwave Imaging of Cylindrical Buried Inhomogeneities

Abstract: Many diagnostic techniques in geophysics and civil engineering are based on the interaction of electromagnetic waves with objects buried in homogeneous or stratified media. Most of the investigations are concerned with the detection of buried objects, but a few papers have dealt with the problem of identifying the objects. The proposed method is based on the integral representation for a plane wave incident on a lossy half-space containing a cylindrical object of arbitrary cross section and electrical properties. The induced current distribution in the object is obtained from the backscattered field measurement in amplitude and phase. In order to improve the spatial resolution of the image, the scattered field is measured for different plane wave incidence and frequencies. Results of numerical simulations concerning the shape and size of the object for different values of soil electromagnetic parameters are presented in this paper.

Induced-polarization and electromagnetic modeling of a three-dimensional body buried in a two-layer anisotropic earth

Abstract: The integral equation method is used for induced‐polarization (IP) and electromagnetic (EM) modeling of a finite inhomogeneity in a two‐layer anisotropic earth. An integral equation relates the exciting electric field and the scattering currents in the homogeneity through the electric tensor Green’s function deduced from the vector potentials in the lower layer of the earth. Digital linear filtering and three‐point parabolic Lagrangian interpolation with two variables speed up the numerical evaluation of the Hankel transforms in the tensor Green’s function. The results of this integral equation method for isotropic media are checked by direct comparisons with results by other workers. The results for anisotropic media are indirectly verified, mainly by checking the tensor Green’s function. The calculated results show that the effects of anisotropy on apparent resistivity and percent frequency effect are to reduce the size of the anomalies, shift the anomaly region downward toward the lower centers of the ...

Electromagnetic modeling of complex railgun geometries

Abstract: A model is developed to simulate the electro-magnetic behavior of arbitrarily shaped long parallel conductors in the high frequency limit. Specifically, the model solves the Laplace equation in a two-dimensional domain using discrete current sheets and a least square minimization solution technique. The model calculates the inductive current distribution on the surfaces of the conductors and the vector potential and B-fields in the space external to the conductors. This model (LAPLAC) is used to analyze a number of different conductor geometries typically found in railgun systems. A system of rails within a conducting coaxial cylinder is modeled and compared with a similar system without the conducting cylinder. A similar comparison is made between a simple rail system and one which contains an augumenting turn connected in series with the main rails. Performance of these configurations is compared based on the inductive and resistive characteristics associated with the high-frequency current distribution predicted by the LAPLAC model.

Correlation functions for homogeneous, isotropic random classical electromagnetic radiation and the electromagnetic fields of a fluctuating classical electric dipole.

Abstract: The two-point field correlation functions for homogeneous and isotropic random (Gaussian) classical electromagnetic radiation are shown to be related to the electromagnetic fields of a fluctuating electric dipole. The relationships derived between these two quantities are useful in calculations involving classical electric dipole oscillators bathed in classical electromagnetic radiation. Using these relationships, the van der Waals force is evaluated for a harmonic dipole oscillator that is a member of an arbitrary configuration of N oscillators, all of which are bathed in thermal plus zero-point classical electromagnetic radiation. Also, the expectation value of the Poynting vector is shown to be unchanged from its null value when a classical harmonic dipole oscillator is included within a classical electromagnetic isotropic and homogeneous random radiation field. A sketch is given as to how these calculations may be carried over to the case of a system of harmonic dipole oscillators uniformly accelerating through classical electromagnetic zero-point radiation. What enables these calculations to be extended to the situation of acceleration through zero-point radiation is a recent finding that the two-point field correlation functions, evaluated along trajectories described by uniform acceleration through classical electromagnetic zero-point radiation, are related to the electromagnetic fields of a uniformly accelerating electric dipole.

Numerical electromagnetic modeling including studies of characteristic dimensions: A review

Abstract: Recent activity in important approximate methods used in numerical electromagnetic (EM) modeling is reviewed. Comparisons between the results obtained by different numerical methods and between analytical and numerical solutions are presented. The importance of 3D modeling and thin sheet approximations are pointed out. This review also considers and summarizes studies of characteristic dimensions in three topics: source fields, numerical modeling and physical phenomena in the earth and interpretation. The skin depth (i.e., generally the attenuation) of the EM energy is considered to be the most important and fundamental characteristic dimension.

An optimized digital filter for the fourier transform

Abstract: Short filters for calculating Hankel-transformations, with special attention to the d.c.-sounding problem, have been published in recent years. These filters, with a typical length of less than 25 coefficients, have made it possible to implement, e.g., VES-interpretation programs on microcomputers and 3-D electric and electromagnetic modeling programs on minicomputers. Initially the performance of the short filters was rather poor, but with the introduction of short optimized filters there has been a considerable improvement in the accuracy. An optimization procedure is applied to design a 20-point filter for the Fourier sine-transformation. This filter may be useful in electromagnetic prospecting theory, e.g., in the calculation of the electric and magnetic field from a line source.

A versatile electromagnetic modeling program for 2-D structures

Abstract: A new integral-equation program for calculation of the E-polarization response allows modeling of VLF for plane wave or line source input, e.g., magnetotellurics and Turam responses. The anomalous conducting body is modeled by a number of square cells, each of individual size and resistivity, and with arbitrary position in the host medium. This provides a high degree of flexibility and allows for simulation of rather complex conductivity structures. The computation time has been drastically reduced by using techniques such as digital filtering and fast Fourier transformation. The interpretation of a measured Turam profile and the influence of galvanically channelled currents in the conducting body is discussed.

Transient analysis of beam interaction with the antisymmetric mode in a truncated periodic structure using the three-dimensional computer code "SOS"

Abstract: Transient-analysis plasma-physics/electromagnetic codes were used to model microwave tube circuits excited by the sudden or rapid buildup of an unmodulated electron beam. The objective was to assess the value of Mission Research Corporation's codes "SOS" and "MAGIC" (for three- and two-dimensional analysis, respectively) for analysis of oscillation in microwave tubes. For the 2-D case the resonance frequency and the beam-loading Q of a cavity were obtained and corroborated by a steady-state electromagnetic code. For the 3-D cases the computed resonance frequencies in a resonated section of coupled-cavity TWT were corroborated by cold-test measurement. Despite the coarse frequency resolution inherent in the relatively short-duration transient analysis perfomed for a higher order antisymmetric mode, it was possible to isolate several of the resonances sufficiently to determine the negative Q due to beam loading. Thus the capability of these programs to perform quantitative stability analysis on microwave tube circuits with beam was demonstrated. To be of practical use in routine tube design and analysis, however, a more efficient and user-friendly code must be developed.

The Magnetotelluric Field for a Two-Dimensional Earth Modeled by a Nonuniform Buried Thin Sheet

Abstract: Magnetotellurics (MT) is an electromagnetic sounding technique used in electrical geophysics to obtain information about the subsurface resistivity structure of the Earth. This information is then used to infer the presence of various natural resources such as oil accumulations and sources of geothermal energy. Extensive research has been done in the area of modeling and interpretation of the Earth in terms of one-dimensional Earth models. Difficulties in both modeling and interpretation occur when the conductivity structure of the Earth is allowed to vary laterally as well as vertically. This paper extends the analysis of a particular class of two-dimensional Earth models characterized by the "thin sheet" approximation. Integral equations are developed for the forward going problem of a plane wave normally incident on the surface of an Earth represented by a uniform half-space containing a thin sheet buried at an arbitrary depth and having an integrated conductivity that varies in one lateral direction. Both E perpendicular and E parallel polarization of the incident electric field are analyzed. The integral equations are evaluated numerically, and several example calculations are presented.

Spectral theory of electromagnetic induction in a radially and laterally inhomogeneous earth

Abstract: The partial differential equations of electromagnetic induction in a 3-D Earth of inhomogeneous conductivity are reduced to a system of ordinary differential equations of the 2nd order for the spectral coefficients of the field.

Finite element analysis of electromagnetic propagation in an absorbing wave guide

Abstract: Wave guides play a significant role in microwave space communication systems The attenuation per unit length of the guide depends on its construction and design frequency range A finite element Galerkin formulation has been developed to study TM electromagnetic propagation in complex two-dimensional absorbing wave guides The analysis models the electromagnetic absorptive characteristics of a general wave guide which could be used to determine wall losses or simulate resistive terminations fitted into the ends of a guide It is believed that the general conclusions drawn by using this simpler two-dimensional geometry will be fundamentally the same for other geometries

A Novel technique to the solution of transient electromagnetic scattering from thin wires

Abstract: Previous approaches to the problem of transient scattering by conducting bodies have utilized the well-known marching-on-in-time solution procedures. However, these procedures are very dependent on discretization techniques and in many cases lead to instabilities as time progresses. Moreover, the accuracy of the solution procedure cannot be verified easily and usually there is no error estimation. Recently an alternate approach to the solution of transient scattering by thin wires was presented based on the conjugate gradient (CG) method. In this procedure, space and time are discretized independently into subintervals and the error is minimized iteratively. Unfortunately, this procedure is very slow, not easily extendable to other geometries, and moreover, some of the advantages of marching-on-in-time are lost. In this paper, again the conjugate gradient method is applied to solve the above problem, but this time, reducing the error to a desired value at each time step. Since the error is reduced at each time step, marching-on-in-time can still be done without error accumulation as time progresses. Computationally, this procedure is as fast as conventional marching-on-in-time. Thus, this new method retains all the advantages of marching-on-in-time and yet does not introduce instabilities in the late time. It is also possible to apply this procedure to other geometries. Details of the solution procedure along with numerical results are also presented.

Accuracy of a physical optics approximation to the backscatter of a complex structure

Abstract: The flat-plate physical optics approximation to the calculation of the electromagnetic backscatter of a cylindrical body constructed of curved surfaces is evaluated and compared to the exact diffraction theory. Curves showing the magnitude of the error as a function of frequency are included.

Three-dimensional state space model for electromagnetic field calculations

Abstract: The analytical and numerical solutions to the problems of electromagnetic scattering, penetration and coupling are very complex in nature because of the geometries of the object involved. The current paper presents a three-dimensional state space model for solution to such problems. The paper derives a general response formula and suggests the procedure for its solution.