1. 
 Karlsson, Anders, et al.
(författare)

Transient wave propagation in gyrotropic media
 1992

Ingår i: Invariant inbedding and inverse problems.  SIAM.  0898713056 ; s. 7789

Bokkapitel (övrigt vetenskapligt)abstract
 In this paper transient electromagnetic wave propagation in an inhomogeneous, cold plasma is considered. It is assumed that a constant magnetic induction is present and that the plasma is spatially inhomogeneous in the direction of the magnetic induction. Losses in the plasma are modeled with a collision frequency ν. The direct problem, which is to calculate the reflected and transmitted responses of the plasma, is considered in this paper. Special attention is paid to the precursor effects in the plasma and several examples of precursor effects in an inhomogeneous plasma are showed.


2. 
 Karlsson, Anders, et al.
(författare)

Wave splitting and imbedding equations for a spherically symmetric dispersive medium
 1992

Ingår i: Invariant inbedding and inverse problems.  SIAM.  0898713056 ; s. 103113

Bokkapitel (övrigt vetenskapligt)abstract
 The direct problem of time dependent electromagnetic scattering in the dispersive sphere is solved by a wave splitting technique. The electric field is expanded in a series involving vector spherical harmonics, leading to a system of wave equations for each term. These systems are reduced to scalar wave equations for each term, which are solved via reflection operators. Some preliminary numerical results are presented.


3. 
 Kristensson, Gerhard, et al.
(författare)

Time domain inversion techniques for electromagnetic scattering problems
 1992

Ingår i: Invariant imbedding and inverse problems.  SIAM.  0898713056 ; s. 129

Bokkapitel (övrigt vetenskapligt)abstract
 This paper presents a review of and a comparison between two different methods to solve an inverse scattering problem in the time domain. The problem is that of propagation of transient electromagnetic waves in spatially inhomogeneous slabs of finite length. The permittivity and conductivity profiles are assumed to vary only with depth and the scattering problem is thus onedimensional. Several algorithms to solve the direct and inverse scattering problems for continuous and discontinuous permittivity profiles are suggested. Some of these algorithms have not been published before. The aim of this paper is to compare and review these methods. More specifically, the numerical performance of the invariant imbedding approach (layerstripping)and the Green functions formulation (downward continuation)is compared. Some new results based upon time reversal techniques for a lossless slab are presented in an appendix.


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5. 
 Björkberg, Jonas, et al.
(författare)

Threedimensional subterranean target identification by use of optimization techniques
 1991

Rapport (övrigt vetenskapligt)abstract
 The identification of a subterranean metallic ore from scattering experiments, conducted on the surface of the ground or in a bore hole, is a classic geophysical problem. In general this problem is not wellposed. However, a priori information about the shape of the target provides enough regularization to make the problem numerically stable. The problem is solved by minimizing the meansquare error between an eleven parameter model, based on the null field approach, and the data. The optimization is done with a Newton technique in which a singular value decomposition of the model Jacobian is employed. The algorithm is very stable to noise and makes good reconstructions from feasible starting guesses, for realistically noise contaminated data.


6. 
 Fridén, Jonas, et al.
(författare)

Transient electromagnetic wave propagation in anisotropic dispersive media
 1993

Ingår i: Journal of the Optical Society of America A..  Optical Society of America. ; 10:12, s. 26182627

Tidskriftsartikel (refereegranskat)abstract
 Transient electromagnetic wave propagation in a stratified, anisotropic, dispersive medium is considered. Specifically, the direct scattering problem is addressed. The dispersive, anisotropic medium is modeled by constitutive relations (a 3 3 matrixvalued susceptibility operator) containing time convolution integrals. In the general case, nine different susceptibility kernels characterize the medium. An incident plane wave impinges obliquely upon a finite slab consisting of a stratified anisotropic medium. The scattered fields are obtained as time convolutions of the incident field with the scattering kernels. The scattering (reflection and transmission) kernels are uniquely determined by the slab and are independent of the incident field. The scattering problem is solved by a wavesplitting technique. Two different methods for determining the scattering kernels are presented: an embedding and a Green's function approach. Explicit analytic expressions of the wave front are given for a special class of media. Some numerical examples illustrate the analysis.


7. 
 Fridén, Jonas, et al.
(författare)

Transient Electromagnetic Wave Propagation in Anisotropic Dispersive Media
 1992

Rapport (övrigt vetenskapligt)abstract
 In this paper transient electromagnetic wave propagation in a stratified, anisotropic, dispersive medium is considered. Specifically, the direct scattering problem is addressed. The dispersive, anisotropic medium is modeled by constitutive relations (a 3 × 3 matrixvalued susceptibility operator)con taining time convolution integrals. In the general case, nine different susceptibility kernels characterize the medium. An incident plane wave impinges obliquely on a finite slab consisting of a stratified anisotropic medium. The scattered fields are obtained as time convolutions of the incident field with the scattering kernels. The scattering (reflection and transmission)k ernels are uniquely determined by the slab and are independent of the incident field. The scattering problem is solved by a wave splitting technique. Two different methods to determine the scattering kernels are presented; an imbedding and a Green functions approach. Explicit analytic expressions of the wave front are given for a special class of media. Some numerical examples illustrate the analysis.


8. 
 Fuks, Peter, et al.
(författare)

Permittivity profile reconstructions using transient electromagnetic reflection data
 1990

Rapport (övrigt vetenskapligt)abstract
 This paper is concerned with the permittivity reconstruction of inhomogeneous dielectric media. The method applies to profiles that vary with depth only, i.e. it provides a onedimensional profile reconstruction. The data are collected and analyzed in the time domain. In the first part of the paper the theory of the method is reviewed. It is showed that a finite time trace of reflection data suffices to uniquely reconstruct the permittivity profile of the medium. The latter part of the paper presents the experimental setup and contains also a thorough discussion of the errors that affect the measurements. The inverse scattering algorithm that is used is either based upon an imbedding procedure or on a Green functions approach. The input to either of these algorithms is the reflection kernel or the impulse response of the medium, i.e. the delta function response of the medium. Therefore, a deconvolution of the the measured reflected field and the incident field must be performed. This deconvolution problem is also addressed briefly in this paper.


9. 
 Karlsson, Anders, et al.
(författare)

Constitutive relations, dissipation and reciprocity for the Maxwell equations in the time domain
 1992

Ingår i: Journal Electromagnetic Waves and Applications.  VSP BV.  15693937 . 09205071. ; 6:56, s. 537551

Tidskriftsartikel (refereegranskat)abstract
 The main goal of this paper is to establish general constitutive relations for the electromagnetic fields EBAR, DBAR, BBAR, and HBAR in a time domain setting. The four basic assumptions of the medium are linearity, invariance to time translations, causality, and continuity. These four assumptions imply that the constitutive relations are convolutions of the RiemannStieltjes type. A review of the classification of media in bianisotropic, biisotropic, anisotropic, and isotropic media, respectively, is made. Dissipation and reciprocity are defined and the constraints these concepts make on the constitutive relations are analyzed. Furthermore, an appropriate form of time reversal and functions of positive type are introduced and some consequences of these concepts are showed.


10. 
 Karlsson, Anders, et al.
(författare)

Transient wave propagation in gyrotropic media
 1990

Rapport (övrigt vetenskapligt)abstract
 In this paper transient electromagnetic wave propagation in an inhomogeneous, cold plasma is considered. It is assumed that a constant magnetic induction is present and that the plasma is spatially inhomogeneous in the direction of the magnetic induction. Losses in the plasma are modeled with a collision frequency ν. The direct problem, which is to calculate the reflected and transmitted responses of the plasma, is considered in this paper. Special attention is paid to the precursor effects in the plasma and several examples of precursor effects in an inhomogeneous plasma are showed.

