| 21. |
- Boström, Anders E, 1951, et al.
(författare)
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Elastic wave scattering by a three-dimensional inhomogeneity in an elastic half space
- 1980
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Ingår i: Wave Motion. - 0165-2125. ; 2, s. 335-353
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we will consider scattering of elastic waves in a half space. The half space is an isotropic, linear and homogeneous medium except for a finite inhomogeneity. The T-matrix method (also called the "extended boundary condition method" or "null field approach") is extended to derive expressions for the elastic field inside the half space and the surface field on the interface. The assumptions on the source that excites the half space are fairly weak. In the numerical applications found in this paper we assume a Rayleigh surface wave to be the incoming field, and we only compute the surface displacements. We make illustrations on some simple type of scatterers (spheres and spheroids; the latter ones can be arbitrarily oriented).
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| 22. |
- Boström, Anders E, 1951, et al.
(författare)
-
Scattering of a pulsed Rayleigh wave by a spherical cavity in an elastic half space
- 1983
-
Ingår i: Wave Motion. - 0165-2125. ; 5, s. 137-143
-
Tidskriftsartikel (refereegranskat)abstract
- The time-dependant scattering by a spherical cavity in an elastic half space is considered. The incoming wave is a pulsed Rayleigh wave. The stationary part of the problem is solved by the T-matrix method, and an integration in frequency is perforemd with a modified gaussian weight function. The displcsement components at some points on the surface of the half space are computed and shown in a number of plots.
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| 23. |
- Cheney, Margaret, et al.
(författare)
-
Optimal electromagnetic measurements
- 2001
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Ingår i: Journal Electromagnetic Waves and Applications. - 1569-3937. ; 15:10, s. 1323-1336
-
Tidskriftsartikel (refereegranskat)abstract
- We consider the problem of obtaining information about an inaccessible half-space from electromagnetic measurements made in the accessible half-space. If the measurements are of limited precision, some scatterers will be undetectable because their scattered fields are below the precision of the measuring instrument. How can we make optimal measurements? In other words, what incident fields should we apply that will result in the biggest measurements? There are many ways to formulate this question, depending on the measuring instruments. In this paper we consider a formulation involving wave-splitting in the accessible half-space: what downgoing wave will result in an upgoing wave of greatest energy? This formulation is most natural for far-field problems. A closely related question arises in the case when we have a guess about the configuration of the inaccessible half-space. What measurements should we make to determine whether our guess is accurate? In this case we compare the scattered field to the field computed from the guessed configuration. Again we look for the incident field that results in the greatest energy difference. We show that the optimal incident field can be found by an iterative process involving time reversal "mirrors". For band-limited incident fields and compactly supported scatterers, this iterative process converges to a sum of time-harmonic fields.
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| 24. |
- Cheney, Margaret, et al.
(författare)
-
Optimal Electromagnetic Measurements
- 2000
-
Rapport (övrigt vetenskapligt/konstnärligt)abstract
- We consider the problem of obtaining information about an inaccessible halfspace from electromagnetic measurements made in the accessible half-space. If the measurements are of limited precision, some scatterers will be undetectable because their scattered fields are below the precision of the measuring instrument. How can we make optimal measurements? In other words, what incident fields should we apply that will result in the biggest measurements? There are many ways to formulate this question, depending on the measuring instruments. In this paper we consider a formulation involving wavesplitting in the accessible half-space: what downgoing wave will result in an upgoing wave of greatest energy? This formulation is most natural for far-field problems. A closely related question arises in the case when we have a guess about the configuration of the inaccessible half-space. What measurements should we make to determine whether our guess is accurate? In this case we compare the scattered field to the field computed from the guessed configuration. Again we look for the incident field that results in the greatest energy difference. We show that the optimal incident field can be found by an iterative process involving time reversal “mirrors”. For band-limited incident fields and compactly supported scatterers, this iterative process converges to a sum of time-harmonic fields.
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| 25. |
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| 26. |
- Cheney, Margaret, et al.
(författare)
-
Resonance Enhancement in Noise
- 2023
-
Rapport (övrigt vetenskapligt/konstnärligt)abstract
- This document compares two approaches to finding resonance peaks from asingle sensor: 1) averaging multiple measurements of the transfer function orfrequency-domain scattering matrix, versus 2) using the iterative time-reversalprocess, which involves iteratively re-transmitting a time-reversed version ofthe scattered field at the previous iterations. The averaging method has theadvantage of handling arbitrarily much noise if sufficiently many averages areused. On the other hand, up to a certain level of noise, the time-reversalmethod has dramatic advantages over the averaging method; but it also requires more complex equipment. This document discusses the tradeoffs involved, with the goal of providing information that may be useful in the system design process.
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| 27. |
- Cheney, Margaret, et al.
(författare)
-
Three-dimensional inverse scattering: layer-stripping formulas and ill-posedness results
- 1988
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Ingår i: Inverse Problems. - : IOP Publishing. - 0266-5611 .- 1361-6420. ; 4:3, s. 625-642
-
Tidskriftsartikel (refereegranskat)abstract
- The authors consider the three-dimensional direct and inverse scattering problems for the Schrodinger equation and for the reduced wave equation with variable velocity. The scatterer is probed with either point sources or plane waves of fixed frequency. They ask the question, 'How does the wave field change when the scatterer is truncated?' Simple formulae for the derivative of the wave field with respect to the truncation parameter are obtained. Similar formulae are obtained for the scattering amplitudes. These formulae are used to derive ill-posedness results for various inverse scattering problems. The ill-posedness results apply when data are collected over a range of frequencies.
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| 28. |
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| 29. |
- Derneryd, Anders, et al.
(författare)
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Antenna signal correlation and its relation to the impedance matrix
- 2004
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Ingår i: Electronics Letters. - : Institution of Engineering and Technology (IET). - 1350-911X .- 0013-5194. ; 40:7, s. 401-402
-
Tidskriftsartikel (refereegranskat)abstract
- Theory is presented that extends the analysis to signal correlation between any antennas in an array of elements. It is generalised to include non-identical elements and arbitrary load termination of passive antenna ports. The method is related to the power balance concept and based on the antenna impedance matrix.
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| 30. |
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