Periodic Buried Object Approach For Solving Scattering Problems Related To Periodic Rough Surfaces

Tez (Yüksek Lisans) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 2008Thesis (M.Sc.) -- İstanbul Technical University, Institute of Science and Technology, 2008Bu tez çalışmasında, elektromanyetik dalgaların periyodik engebeli dielektrik yüzeylerden saçılmasına yönelik yeni bir yöntem ele alınmıştır. Ele alınan yöntem, Periyodik Gömülü Cisim Yaklaşımı, sonsuz uzunluktaki engebeli yüzeylerden saçılma problemini cözen Gömülü Cisim Yaklaşımı’nın periyodik yüzeylere uygulanmasıyla elde edilmiştir. Gömülü Cisim Yaklaşımı, engebeli yüzeylerden saçılma problemini düzlemsel arayüz ile ayrılan iki boyutlu uzayda yer alan silinidirik cisimlerden saçılma şeklinde varsayarak modeller. Bu sayede, periyodik Green’s fonkisiyonun yardımıyla problem ikinci tür bir Fredholm integral eşitliği probleminin çözümüne indirgenir. İki boyutlu uzayın periyodik Green’s fonksiyonu efektik yöntemler kullanılarak sayısal hesaplamları yapılan Floquet mod açılımıyla hesaplanır. Tezde alınan Periyodik Gömülü Cisim Yaklaşımı,aynı zamanda sonsuz uzunukta olup bölgesel engebeye sahip yüzeylerden saçılma problemlerinin çözümünde de kullanılabilir. Sayısal simülasyonlar, yeni olan bu yöntemin rasgele dağılıma sahip yüzeyler için efektif ve doğru sonuçlar verdiğini göstermektedir.In this study, a new approach for the scattering of electromagnetic waves from periodic dielectric rough surfaces is addressed. The method is an extension of the buried object approach (BOA), which is developed for rough surfaces of infinite extend, to the present problem. The BOA allows modeling the original problem as the scattering of electromagnetic waves from cylindrical objects located in a two half spaces medium with planar interface. Then, the problem is reduced to the solution of a Fredholm integral equation of second kind through the periodic Green s function of two half space medium. The periodic Green s function of two-half spaces medium is calculated via Floquet mode expansion, whose numerical evaluation can be accelerated by using effective methods. The method can also be used to solve the scattering problems of rough surfaces of infinite extend and having a localized roughness. Numerical simulations show that the method yields effective and accurate results for surfaces of arbitrary variation.Yüksek LisansM.Sc

FURTHER INVESTIGATION ON MAGNETICALLY INDUCED SUBSEQUENT FAULT AND STUDY ON ELECTROMAGNETIC SCATTERING OF OBJECTS BURIED BELOW A RANDOM ROUGH SURFACE

This dissertation contains two subjects: further development of numerical technique for the analysis of magnetically induced subsequent fault (MISFault) in overhead power lines and its implementation into a software upgrade; and first-phase of study on the electromagnetic scattering from objects buried below a random rough surface making use of the multidomain pseudospectral time domain (PSTD) method and Monte-Carlo simulation. An initial electric fault can result in strong magnetic torque on the overhead power line conductors, which will make them swing and may bring them to close proximity or in contact with one another, causing a subsequent fault. In Chapter 2, Computer simulations for the analysis of the subsequent fault in transition spans, which are often required in power line topology, are developed. A dynamic analysis of swing movement of power line conductors subsequent to an initial fault is presented to track the smallest distance between the conductors. In Chapter 3, the simulation is implemented into the upgrade of the MISFault analysis software. Its functions are depicted in details. The MISFault software is being used by Duke Energy Company and is expected to be useful to a utility for eliminating the magnetically induced subsequent faults. The multidomain pseudospectral time domain (PSTD) method has been developed and successfully applied to solve a variety of electromagnetic scattering problems in the past decade. It is a novel algorithm with improvement over traditional FDTD method. In Chapter 4, a multidomain PSTD algorithm is developed to investigate the scattering from a 2-D cylinder in free space. Sample numerical results are presented and validated. Then, the theoretical derivations are extended for the analysis of scattering from 2-D objects buried below a random rough surface

Study on Electromagnetic Scattering of Cylinders Buried in a Half Space with Random Rough Surfaces of Finite/Infinite Length

Analysis of electromagnetic scattering of buried objects is a subject of great interest due to its practical importance in both military and civil applications, such as subsurface investigation and target detection. In reality, the earth is of layered structure of random rough interfaces, which leads to a greatly increased complexity of the analysis. However, it is necessary to incorporate the nature of random rough surface and the layered structure because they both have substantial impact on the scattered signature and hence affect the study of inverse scattering and detection of buried objects. In this dissertation, a Monte-Carlo multidomain pseudospectral time domain (MPSTD) method is developed for investigating the scattering from cylinders buried below a random rough surface separating two half spaces under various conditions. As a prelude, the formulation of multidomain PSTD algorithm is presented. Then, this formulation is extended and combined with the Monte-Carlo approach to analyze the scattering of an object buried below a random rough surface of finite length. In the analysis, special attention is paid to the treatments of the random rough surface including its profile generation, matching with CGL points, and subdomain patching. Next, the scattering of a cylinder buried below a random rough surface of infinite length is studied and a two-step computation model based on the Monte-Carlo MPSTD method is developed. Further, in order to better simulate the real situation, the analysis is then extended to study the scattering from one or more cylinders embedded in a layered half space with random rough surfaces. Finally, a near-zone field to far-zone field transformation technique is developed and presented. Sample numerical results under different conditions, involving random rough surface of various roughness, lower half space with different permittivities, and cylinders of circular and rectangular shapes are presented, validated, and analyzed. Throughout this research, a numerical technique based on Monte-Carlo method and MPSTD approach has been developed and validated for investigating cylinders buried in a half space with random rough surfaces. It is observed that the roughness of the random rough surface and the electromagnetic properties of the lower half space can significantly affect the scattered signature of the buried object

Full Wave 2D Modeling of Scattering and Inverse Scattering for Layered Rough Surfaces with Buried Objects.

Efficient and accurate modeling of electromagnetic scattering from layered rough surfaces with buried objects finds applications ranging from detection of landmines to remote sensing of subsurface soil moisture. In this dissertation, the formulation of a hybrid numerical/analytical solution to electromagnetic scattering from layered rough surfaces is first developed. The solution to scattering from each rough interface is sought independently based on the extended boundary condition method (EBCM), where the scattered fields of each rough interface are expressed as a summation of plane waves and then cast into reflection/transmission matrices. To account for interactions between multiple rough boundaries, the scattering matrix method (SMM) is applied to recursively cascade reflection and transmission matrices of each rough interface and obtain the composite reflection matrix from the overall scattering medium. The validation of this method against the Method of Moments (MoM) and Small Perturbation Method (SPM) will be addressed and the numerical results which investigate the potential of low frequency radar systems in estimating deep soil moisture will be presented. Computational efficiency of the proposed method is also addressed. In order to demonstrate the capability of this method in modeling coherent multiple scattering phenomena, the proposed method has been employed to analyze backscattering enhancement and satellite peaks due to surface plasmon waves from layered rough surfaces. Numerical results which show the appearance of enhanced backscattered peaks and satellite peaks are presented. Following the development of the EBCM/SMM technique, a technique which incorporates a buried object in layered rough surfaces is proposed by employing the T-matrix method and the cylindrical-to-spatial harmonics transformation. Validation and numerical results are provided. Finally, a multi-frequency polarimetric inversion algorithm for the retrieval of subsurface soil properties using VHF/UHF band radar measurements is developed. The top soil dielectric constant is first determined using an L-band inversion algorithm. For the retrieval of subsurface properties, a time-domain inversion technique is employed together with a parameter optimization for the pulse shape of time delay echoes from VHF/UHF band radar observations. Some numerical studies to investigate the accuracy of the proposed inversion technique in presence of errors are shown.Ph.D.Electrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/58459/1/kuoch_1.pd

Numerical methods for electromagnetic wave propagation and scattering in complex media

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2004.Vita.Includes bibliographical references (p. 227-242).Numerical methods are developed to study various applications in electromagnetic wave propagation and scattering. Analytical methods are used where possible to enhance the efficiency, accuracy, and applicability of the numerical methods. Electromagnetic induction (EMI) sensing is a popular technique to detect and discriminate buried unexploded ordnance (UXO). Time domain EMI sensing uses a transient primary magnetic field to induce currents within the UXO. These currents induce a secondary field that is measured and used to determine characteristics of the UXO. It is shown that the EMI response is difficult to calculate in early time when the skin depth is small. A new numerical method is developed to obtain an accurate and fast solution of the early time EMI response. The method is combined with the finite element method to provide the entire time domain response. The results are compared with analytical solutions and experimental data, and excellent agreement is obtained. A fast Method of Moments is presented to calculate electromagnetic wave scattering from layered one dimensional rough surfaces. To facilitate the solution, the Forward Backward method with Spectral Acceleration is applied. As an example, a dielectric layer on a perfect electric conductor surface is studied. First, the numerical results are compared with the analytical solution for layered flat surfaces to partly validate the formulation. Second, the accuracy, efficiency, and convergence of the method are studied for various rough surfaces and layer permittivities. The Finite Difference Time Domain (FDTD) method is used to study metamaterials exhibiting both negative permittivity and permeability in certain frequency bands.(cont.) The structure under study is the well-known periodic arrangement of rods and split-ring resonators, previously used in experimental setups. For the first time, the numerical results of this work show that fields propagating inside the metamaterial with a forward power direction exhibit a backward phase velocity and negative index of refraction. A new metamaterial design is presented that is less lossy than previous designs. The effects of numerical dispersion in the FDTD method are investigated for layered, anisotropic media. The numerical dispersion relation is derived for diagonally anisotropic media. The analysis is applied to minimize the numerical dispersion error of Huygens' plane wave sources in layered, uniaxial media. For usual discretization sizes, a typical reduction of the scattered field error on the order of 30 dB is demonstrated. The new FDTD method is then used to study the Angular Correlation Function (ACF) of the scattered fields from continuous random media with and without a target object present. The ACF is shown to be as much as 10 dB greater when a target object is present for situations where the target is undetectable by examination of the radar cross section only.by Christopher D. Moss.Ph.D

Studies into the detection of buried objects (particularly optical fibres) in saturated sediment. Part 2: design and commissioning of test tank

This report is the second in a series of five, designed to investigate the detection oftargets buried in saturated sediment, primarily through acoustical or acoustics-relatedmethods. Although steel targets are included for comparison, the major interest is intargets (polyethylene cylinders and optical fibres) which have a poor acousticimpedance mismatch with the host sediment. This particular report details theconstruction of a laboratory-scale test facility. This consisted of three maincomponents. Budget constraints were an over-riding consideration in the design.First, there is the design and production of a tank containing saturated sediment. Itwas the intention that the physical and acoustical properties of the laboratory systemshould be similar to those found in a real seafloor environment. Particularconsideration is given to those features of the test system which might affect theacoustic performance, such as reverberation, the presence of gas bubbles in thesediment, or a suspension of particles above it. Sound speed and attenuation wereidentified as being critical parameters, requiring particular attention. Hence, thesewere investigated separately for each component of the acoustic path.Second, there is the design and production of a transducer system. It was the intentionthat this would be suitable for an investigation into the non-invasive acousticdetection of buried objects. A focused reflector is considered to be the most costeffectiveway of achieving a high acoustic power and narrow beamwidth. Acomparison of different reflector sizes suggested that a larger aperture would result inless spherical aberration, thus producing a more uniform sound field. Diffractioneffects are reduced by specifying a tolerance of much less than an acousticwavelength over the reflector surface. The free-field performance of the transducerswas found to be in agreement with the model prediction. Several parameters havebeen determined in this report that pertain to the acoustical characteristics of the waterand sediment in the laboratory tank in the 10 – 100 kHz frequency range.Third, there is the design and production of an automated control system wasdeveloped to simplify the data acquisition process. This was, primarily, a motordrivenposition control system which allowed the transducers to be accuratelypositioned in the two-dimensional plane above the sediment. Thus, it was possible forthe combined signal generation, data acquisition and position control process to be coordinatedfrom a central computer.This series of reports is written in support of the article “The detection by sonar ofxdifficult targets (including centimetre-scale plastic objects and optical fibres) buriedin saturated sediment” by T G Leighton and R C P Evans, written for a Special Issueof Applied Acoustics which contains articles on the topic of the detection of objectsburied in marine sediment. Further support material can be found athttp://www.isvr.soton.ac.uk/FDAG/uaua/target_in_sand.HTM

High-Resolution Crystal Truncation Rod Scattering: Application to Ultrathin Layers and Buried Interfaces

In crystalline materials, the presence of surfaces or interfaces gives rise to crystal truncation rods (CTRs) in their X‐ray diffraction patterns. While structural properties related to the bulk of a crystal are contained in the intensity and position of Bragg peaks in X‐ray diffraction, CTRs carry detailed information about the atomic structure at the interface. Developments in synchrotron X‐ray sources, instrumentation, and analysis procedures have made CTR measurements into extremely powerful tools to study atomic reconstructions and relaxations occurring in a wide variety of interfacial systems, with relevance to chemical and electronic functionalities. In this review, an overview of the use of CTRs in the study of atomic structure at interfaces is provided. The basic theory, measurement, and analysis of CTRs are covered and applications from the literature are highlighted. Illustrative examples include studies of complex oxide thin films and multilayers

Linear chemically sensitive electron tomography using DualEELS and dictionary-based compressed sensing

We have investigated the use of DualEELS in elementally sensitive tilt series tomography in the scanning transmission electron microscope. A procedure is implemented using deconvolution to remove the effects of multiple scattering, followed by normalisation by the zero loss peak intensity. This is performed to produce a signal that is linearly dependent on the projected density of the element in each pixel. This method is compared with one that does not include deconvolution (although normalisation by the zero loss peak intensity is still performed). Additionaly, we compare the 3D reconstruction using a new compressed sensing algorithm, DLET, with the well-established SIRT algorithm. VC precipitates, which are extracted from a steel on a carbon replica, are used in this study. It is found that the use of this linear signal results in a very even density throughout the precipitates. However, when deconvolution is omitted, a slight density reduction is observed in the cores of the precipitates (a so-called cupping artefact). Additionally, it is clearly demonstrated that the 3D morphology is much better reproduced using the DLET algorithm, with very little elongation in the missing wedge direction. It is therefore concluded that reliable elementally sensitive tilt tomography using EELS requires the appropriate use of DualEELS together with a suitable reconstruction algorithm, such as the compressed sensing based reconstruction algorithm used here, to make the best use of the limited data volume and signal to noise inherent in core-loss EELS