Multi-helical Lamb Wave Imaging for Pipe-like Structures Based on a Probabilistic Reconstruction Approach

The special form of pipe-like structure provides the helical route for ultrasonic guided wave. Considering the pipe as a flattened plate but with periodical replications, the helical wave becomes intuitional and a corresponding imaging algorithm can be constructed. This work proposes the multihelical Lamb wave imaging method by utilizing the multiple arrival wavepackets which are denoted as different orders. The helical wave signal model is presented and the constant group velocity point is illustrated. The probabilistic reconstruction algorithm is combined with the separation and fusion of different helical routes. To verify the proposed scheme, finite element simulations and corresponding experiments are conducted. The cases of single-defect simulation and two-defect simulation indicate the successful and robust implementation of the imaging algorithm. The test on actual pipe damage is also investigated to show its capability in imaging an irregular defect. The comparison with imaging results from only first arrival demonstrates the advantage of multihelical wave imaging, including the better imaging resolution and higher localization accuracy

Towards next generation ultrasonic imaging

Recently the use of ultrasonic arrays for imaging defects in metal components has become economically attractive in Non-Destructive Testing. Given a certain array, the image quality strongly depends on how the measurements are process into an image. The current state-of-the-art imaging algorithm in actual use is delay-and-sum beamforming, which has a resolution capability that is fundamentally limited by the physical approximation used to describe how waves interact with matter. This thesis explores the practical use of alternative non-linear “super-resolution” imaging algorithms that use more accurate physical models, and can theoretically achieve unlimited resolution. This is made possible by utilising additional sources of information contained within the measurements, in particular the small amplitude multiply scattered signals. The distribution of information contained in the measurements, and utilised by the imaging algorithms is studied in the context of information capacity of signals. We discover some insights into the limits of imaging which depend on the signal-to-noise ratio. The accuracy of non-linear imaging algorithms can be strongly dependent on the accuracy of the measurements. Therefore several experiments are performed to assess their performance in practice. The experimental implementation of these methods poses a number of challenges, including removal of the incident field, and compensating for array element directivity. Super-resolution capability is demonstrated in a highly attenuative medium for the first time. To further improve the image quality we explore the possibility of using mirror reflections. This gives an increase in the effective aperture. We perform simulated and experimental reconstructions of a complex scatterer and find that the completeness of the image is improved. The mirror interface also allows quantitative speed-of-sound imaging of penetrable scatterers using the HARBUT algorithm. This is tested experimentally for the first time

Novel Inverse-Scattering Methods in Banach Spaces

The scientific community is presently strongly interested in the research of new microwave imaging methods, in order to develop reliable, safe, portable, and cost-effective tools for the non-invasive/non-destructive diagnostic in many fields (such as medicine, civil and industrial engineering, \u2026). In this framework, microwave imaging techniques addressing the full three-dimensional nature of the inspected bodies are still very challenging, since they need to cope with significant computational complexity. Moreover, non-linearity and ill-posedness issues, which usually affects the related inverse scattering problems, need to be faced, too. Another promising topic is the development of phaseless methods, in which only the amplitude of the electric field is assumed to be measurable. This leads to a significant complexity reduction and lower cost for the experimental apparatuses, but the missing information on the phase of the electric field samples exacerbates the ill-posedness problems. In the present Thesis, a novel inexact-Newton inversion algorithm is proposed, in which the iteratively linearized problems are solved in a regularized sense by using a truncated Landweber or a conjugate gradient method developed in the framework of the l^p Banach spaces. This is an improvement that allows to generalize the classic framework of the l^2 Hilbert spaces in which the inexact-Newton approaches are usually defined. The applicability of the proposed imaging method in both the 3D full-vector and 2D phaseless scenarios at microwave frequencies is assessed in this Thesis, and an extensive validation of the proposed imaging method against both synthetic and experimental data is presented, highlighting the advantages over the inexact-Newton scheme developed in the classic framework of the l^2 Hilbert spaces

Integration of ground-penetrating radar and gamma-ray detectors for non-intrusive localisation of buried radioactive sources

This thesis reports on the integration of ground-penetrating radar (GPR) and gamma ray detectors to improve the non-intrusive localisation of radioactive wastes buried in porous materials such as soil and concrete. The research was undertaken in two phases. In the first phase, a new non-intrusive technique for retrieving the depth of a buried radioactive source from two-dimensional raster radiation images was developed. The images were obtained by moving a gamma-ray detector in discrete steps on the surface of the material volume in which the source is buried and measuring the gamma spectrum at each step. The depth of the source was then estimated by fitting the intensity values from the measured spectra to an approximate three-dimensional gamma-ray attenuation model. This procedure was first optimised using Monte Carlo simulations and then validated using experiments. The results showed that this method is able to estimate the depth of a 658 kBq caesium-137 point source buried up to 18 cm in each of sand, soil and gravel. However, the use of only gamma-ray data to estimate the depth of the sources requires foreknowledge of the density of the embedding material. This is usually III IV difficult without having recourse to intrusive density estimation methods or historical density values. Therefore, the second phase of the research employed integrated GPR and gamma ray detection to solve this density requirement problem. Firstly, four density models were investigated using a suite of materials and the best model was then used to develop the integration method. Results from numerical simulations showed that the developed integration method can simultaneously retrieve the soil density and the depth and radius of disk-shaped radioactive objects buried up to 20 cm in soil of varying conditions with a elative error of less than 10%. Therefore, the integration method eliminates the need for prior knowledge of the density of the embedding material. This work represents the first time data from these two systems i.e., GPR and gamma-ray detector, will be integrated for the detection and localisation of radioactive sources. Furthermore, the results from the developed methods confirm that an integrated GPR and gamma-ray detector system is a viable tool for non-intrusive localisation of buried radioactive sources. This will enable improved characterisation of buried radioactive wastes encountered during the decommissioning of nuclear sites and facilities

Microwave Imaging of The Neck by Means of Inverse-Scattering Techniques

In recent decades, in the field of applied electromagnetism, there has been a significant interest in the development of non-invasive diagnostic methods through the use of electromagnetic waves, especially at microwave frequencies [1]. Microwave imaging (MWI) - considered for a long period an emerging technique - has potential- ities in numerous, and constantly increasing, applications in different areas, ranging from civil and industrial engineering, with non-destructive testing and evaluations (example e.g., monitoring contamination in food, sub-surface imaging based on both terrestrial and space platforms; detection of cracks and defects in structures and equipments of various kinds; antennas diagnostics, etc. ), up to the biomedical field [2], [3], [4], [5], [6], [7]. One of the first applications of microwave imaging (MWI) in the medical field was the detection of breast tumors [8], [9], [10], [11], [12], [13], [14], [15], [16], [17]. Subsequently, brain stroke detection has received great attention [18],[19], [20], too. Other possible clinical applications include imaging of torso, arms, and other body parts [21], [22], [23], [24]. The standard diagnostic method are computerized tomography (CT), nuclear magnetic resonance (NMR) and X-rays. Although these consolidated techniques are able to provide extraordinary diagnostic results, some limitations still exist that stimulate the continuous research of new imaging solutions. In this context, MWI can be overcome some limitations of these techniques, such as the ionizing radiations in the CT and X-rays or the disadvantages of being expensive, in the NMR case. This motivates the study of MWI methods and systems, at least as a complementary diagnostic tools. The aim of electromagnetic diagnostic techniques is to determine physical param- eters (such as the electrical conductivity and the dielectric permittivity of materials) and/or geometrics of the objects under test, which are suppose contained within a certain space region, sometimes denoted as "investigation domain". In particular, by means of a properly designed transmitting antenna, the object under test is illuminated by an electromagnetic radiation. The interaction between the incident radiation and the target causes the so-called electromagnetic scattering phenomena. The field generated by this interaction can be measured around the object by means of one or more receiving antennas, placed in what is sometimes defined as the "ob- servation domain". Starting from the measured values of the scattering field, it is possible to reconstruct the fundamental properties of the test object by solving an inverse electromagnetic scattering problem. As it is well known, the inverse problem is non-linear and strongly ill-posed, unless specific approximations are used, which can be applied in specific situations. In several cases, two-dimensional configurations (2D) can be assumed, i.e., the inspected target has a cylindrical shape, at least as a first approximation. More- over, often the target is illuminated by antennas capable of generating a transverse magnetic (TM) electromagnetic field [25]. These assumptions reduces the problem from a vector and three-dimensional problem to a 2D and scalar one, since it turns out that the only significant the field components are those co-polarized with the incident wave and directed along to the cylinder axis. In recent years, several methods and algorithms that allow an efficient resolution of the equations of electromagnetic inverse scattering problem have been developed. The proposed approaches can be mainly grouped into two categories: qualitative and quantitative techniques. Qualitative procedures, such as the delay-and-sum technique [26], the linear sampling method [27], and the orthogonality sampling method [28], usually provides reconstructions that allows to extract only some parameters of the targets, such as position, dimensions and shape. However, they are in most cases fast and computationally efficient.On the contrary, quantitative methods allows in principle to retrieve the full distributions of the dielectric properties of the object under test, which allows to also obtain additional information on the materials composing the inspected scenario. Such approaches are often computationally very demanding [25]. Qualitative and quantitative approaches can be combined in order to develop hybrid algorithms [29], [30], [31], [32], [33], [34]. An example is represented by the combination of a delay-and-sum qualitative focusing technique [35], [36], [37] with a quantitative Newton scheme performing a regularization in the framework of the Lp Banach spaces [38], [39], [40]. Holographic microwave imaging techniques are other important qualitative meth- ods. In this case, the processing of data is performed by using through direct and inverse Fourier transforms in order to obtain a map of the inspected target. As previously mentioned, quantitative approaches aim at retrieving the distributions of the dielectric properties of the scene under test, although they can be significantly more time-consuming especially in 3D imaging. Among them, Newton- type approach are often considered [39], [40]. Recently, artificial neural networks (ANNs) have been considered as powerful tools for quantitative MWI. The first proposed ANNs were developed as shallow network architectures, in which one or at least two hidden layers were considered [41], [42]. Successively, deep neural networks have been proposed, in which more complex fully-connected architecture are adopted. In this framework, Convolutional Neural Networks (CNNs) have been developed as more complex topologies, for classification problems or for solving the inverse scattering problems [43], [44], [45], [46], [47], [48], [49]. In the inverse scattering problems, the CNNs often require a preliminary image retrieved by other techniques [43], [44], [47], [50], [51] and do not allow directly inver- sion from the scattered electric fields collected by the receiving antennas. Standard CNNs are developed for different applications. Examples are represented by Unet [52], ResNet [53] and VGG [54]. This Thesis is devoted to the application of MWI techniques to inspect the human neck. Several pathologic conditions can affect this part of the body, and a non-invasive and nonionizing imaging method can be useful for monitoring patients. The first pathological condition studied in this Thesis is the cervical myelopathy [55], which is a disease that damages the first part of the spinal cord, between the C3 and C7 cervical vertebrae located near the head [56]. The spinal cord has an important function in the body, since it represents the principal actor in the nervous system. For this reason, it is "protected" inside the spinal canal [57]. A first effect of cervical myelopathy is a reduction of the spinal canal sagittal diameter, which may be caused by different factors [58]. Some patients are asymptomatic and for this reason a continuous monitoring could be very helpful for evaluating the pathology progression. To this end, the application of qualitative and quantitative MWI approaches are proposed in this document. The second neck pathology studied in this Thesis is the neck tumor, in particular supraglottic laryngeal carcinoma [59], thyroid cancer [60] and cervical lymph node metastases [61]. These kinds of tumors are frequently occurring and shown a 50% 5-year survival probability [61],[62], [63], [64]. Fully-connected neural network are proposed for neck tumor detection. The Thesis is organized as follows. In Chapter 2, the relevant concepts of the electromagnetic theory are recalled. Chapter 3 describes the developed inversion algorithms. It also reports an extensive validation considering both synthetic and experimental data. Detailed data about the imaging approach based on machine learning are provided in Chapter 4. This chapter also reports the results obtained in a set of simulations and experiments. Finally, some conclusions are drawn in Chapter 5

Fiscal Year 1995

The mission of the Engineering Research, Development, and Technology Program at Lawrence Livermore National Laboratory (LLNL) is to develop the knowledge base, process technologies, specialized equipment, tools and facilities to support current and future LLNL programs. Engineering`s efforts are guided by a strategy that results in dual benefit: first, in support of Department of Energy missions, such as national security through nuclear deterrence; and second, in enhancing the nation`s economic competitiveness through their collaboration with US industry in pursuit of the most cost-effective engineering solutions to LLNL programs. To accomplish this mission, the Engineering Research, Development, and Technology Program has two important goals: (1) identify key technologies relevant to LLNL programs where they can establish unique competencies, and (2) conduct high-quality research and development to enhance their capabilities and establish themselves as the world leaders in these technologies. To focus Engineering`s efforts, technology thrust areas are identified and technical leaders are selected for each area. The thrust areas are comprised of integrated engineering activities, staffed by personnel from the nine electronics and mechanical engineering divisions, and from other LLNL organizations. This annual report, organized by thrust area, describes Engineering`s activities for fiscal year 1995. The report provides timely summaries of objectives methods, and key results from eight thrust areas: computational electronics and electromagnetics; computational mechanics; microtechnology; manufacturing technology; materials science and engineering; power conversion technologies; nondestructive evaluation; and information engineering

The analysis of UWB Radar System for Microwave Imaging Application.

PhDMany research groups have conducted the investigation into UWB imaging radar system for various applications over the last decade. Due to the demanding security requirements, it is desirable to devise a convenient and reliable imaging system for concealed weapon detection. Therefore, this thesis presents my research into a low cost and compact UWB imaging radar system for security purpose. This research consists of two major parts: building the UWB imaging system and testing the imaging algorithms. Firstly, the time-domain UWB imaging radar system is developed based on a modulating scheme, achieving a receiver sensitivity of -78dBm and a receiver dynamic range of 69dB. A rotary UWB antenna linear array, comprising one central transmitting antenna and four side-by-side receiving antennas, is adopted to form 2D array in order to achieve a better cross-range resolution of the target. In operation, the rotation of the antenna array is automatically controlled through the computerised modules in LabVIEW. Two imaging algorithms have been extensively tested in the developed UWB radar system for a number of scenarios. In simulation, the “Delay and Sum (DAS)” method has been shown to be effective at mapping out the metallic targets in free space, but prone to errors in more complicated environments. However, the “Time Reversal (TR)” method can produce better images in more complex scenarios, where traditionally unfavorable multi-path interference becomes a valuable asset. These observations were verified in experiment in different testing environments, such as penetration through wooden boards, clutters and a stuffed sport bag. The detectable size of a single target is 8×8×1 cm3 with 30cm distance in a stuffed bag, while DAS can achieve the estimation of 7cm cross-range resolution and 15cm down-range resolution for two targets with sizes of 8×8×1 cm3 and 10×10×1 cm3, which fits within the theoretical prediction. In contrast, TR can distinguish them with a superior 4cm cross range resolution

Journal of Telecommunications and Information Technology, 2017, nr 3

kwartalni

1-D broadside-radiating leaky-wave antenna based on a numerically synthesized impedance surface

A newly-developed deterministic numerical technique for the automated design of metasurface antennas is applied here for the first time to the design of a 1-D printed Leaky-Wave Antenna (LWA) for broadside radiation. The surface impedance synthesis process does not require any a priori knowledge on the impedance pattern, and starts from a mask constraint on the desired far-field and practical bounds on the unit cell impedance values. The designed reactance surface for broadside radiation exhibits a non conventional patterning; this highlights the merit of using an automated design process for a design well known to be challenging for analytical methods. The antenna is physically implemented with an array of metal strips with varying gap widths and simulation results show very good agreement with the predicted performance