| 1. |
- Fuks, Peter, et al.
(författare)
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Permittivity profile reconstructions using transient electromagnetic reflection data
- 1990
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Rapport (övrigt vetenskapligt/konstnärligt)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 one-dimensional 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 set-up 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.
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| 2. |
- Karlsson, Anders, et al.
(författare)
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Transient wave propagation in gyrotropic media
- 1990
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Rapport (övrigt vetenskapligt/konstnärligt)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.
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| 3. |
- Karlsson, Anders, et al.
(författare)
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Wave splitting in the time domain for a radially symmetric geometry
- 1990
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Ingår i: Wave Motion. - : Elsevier BV. - 0165-2125. ; 12:3, s. 197-211
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Tidskriftsartikel (refereegranskat)abstract
- A decomposition of the field of the wave equation in three dimensions is suggested. This wave splitting decomposes the field into two components such that in free space simple propagation properties are obtained. The inhomogeneous region is assumed to have a phase velocity variation that varies only in the radial coordinate r from the origin. The suggested wave splitting is a generalization of the wave splitting concept in one spatial dimension. Specifically, the propagation properties in free space in the three-dimensional wave splitting are very analogous to the corresponding propagation properties in free space in one dimension. The three-dimensional decomposition is illustrated in an example of scattering by a sound-soft sphere of radius a.
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| 4. |
- Kristensson, Gerhard
(författare)
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Direct and inverse scattering problems in dispersive media-Green's functions and invariant imbedding techniques
- 1990
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Transient electromagnetic wave propagation in a dispersive medium is reviewed. The medium is assumed to be 1) linear, 2) invariant to time translations, 3) causal, 4) continuous, and 5) isotropic. The constitutive relations are then uniquelyrepresen ted bya Riemann-Stieltjes integral in the time variable. The kernel in this convolution is the susceptibilityk ernel. Two explicit examples of mathematical models of the susceptibilityk ernel are given. The medium treated in this paper is assumed to varyonlywith depth. In the direct problem the reflection and transmission data are computed. The inverse scattering problem is to find the susceptibilityk ernel from known reflexion data. It is, thus, a problem of finding a function depending on the time variable. In the spatiallyhomogeneous case the inverse scattering problem is solved from reflexion data bysolving a Volterra integral equation of the second kind. This inverse problem is therefore well-posed and easyto solve.
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| 5. |
- Kristensson, Gerhard
(författare)
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Time domain inversion techniques for electromagnetic scattering problems
- 1990
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Rapport (övrigt vetenskapligt/konstnärligt)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 (layer-stripping)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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| 6. |
- Malmqvist, Lennart, et al.
(författare)
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Electromagnetic fields in boreholes-measurements and theory in three dimensions
- 1990
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- A comparison between experimental measurements of the electromagnetic field in boreholes and a mathematical model is made. The transmitting antenna is a rectangular wire loop on the ground and the three orthogonal components of the magnetic field are measured below the ground in the borehole. In the mathematical model the ground is assumed to be a stratified half space with a rectangular wire antenna on the ground. Excellent agreement was found between the experimental results and the theoretical ones in most cases and where deviations were found, the assumption of a stratified ground was inaccurate due to present conductive inhomogeneities. The method presented in this paper is capable of determining an ore body within a radius of 100 m from the borehole. The position (direction and distance) of the target can be estimated from the measurements in the borehole. The impact on the economical effectiveness of a drilling programme is considerable when a prediction of the location of additional boreholes can be made. In the case of reconnaissance drilling of a geological structure or trend the distance between adjacent boreholes can be considerably enlarged.
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