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An increasing number of practical applications of three-dimensional microwave imaging require accurate and efficient inversion techniques. In this context, a full-wave 3D inverse-scattering method, aimed at characterizing dielectric targets, is described in this paper. In particular, the inversion approach has a Newton-based structure, in which the internal linear solver is a conjugate gradient-like algorithm in lp spaces. The presented results, which include the inversion of both numerical and experimental scattered-field data obtained in the presence of homogeneous and inhomogeneous targets, validate the reconstruction capabilities of the proposed technique. [ABSTRACT FROM AUTHOR]

An innovative nonlinear inverse scattering approach in variable exponent Lebesgue spaces is proposed for microwave imaging purposes. The main objective of the approach is to overcome one of the main problems associated with reconstruction procedures developed in Lebesgue spaces with constant exponent, i.e., the need for the selection of the optimum Lebesgue-space norm parameter, which is critical for obtaining accurate reconstructions, and no exact rules exist for this choice. The approach proposed in this paper is oriented to tomographic imaging applications at microwave frequencies, for which the adopted inversion procedure is based on a Gauss–Newton method, which has been reformulated in the unconventional variable exponent Lebesgue spaces. The capabilities of the approach are demonstrated by a set of numerical simulations, in which cylindrical dielectric targets are inspected. Moreover, an experimental result is reported. [ABSTRACT FROM AUTHOR]

A new microwave tomography prototype for the inspection of wood materials is described and experimentally tested in this paper. The system prototype is composed by a set of microwave antennas, used for both illuminating the sample and collecting field measurements, a custom RF hardware, and a hybrid tomographic inversion scheme. The antennas are placed in contact with the outer structure of the sample, e.g., a tree trunk, and are optimized to radiate inside the inspected region. A switching matrix connects the set of antennas with a custom vector network analyzer, which is used for acquiring multistatic and multiview field measurements at multiple frequencies. The measured data are processed by means of a combined qualitative/quantitative inversion method. In particular, in a first step, the field scattered by the inner inhomogeneities or inclusions is estimated, producing also a qualitative image of the sample. Then, these data are utilized by a deterministic inversion algorithm in order to obtain a quantitative reconstruction of the dielectric properties of the structure under test. [ABSTRACT FROM AUTHOR]

Electromagnetic inverse scattering techniques are considered to reconstruct the permittivity and the velocity profiles of axially moving cylindrical targets. Two approaches are proposed. One of these is based on a two-step procedure. It is shown that it provides good approximations of the profiles to be reconstructed in a very efficient way, when the peak velocity is small with respect to the speed of light in vacuum. These features are obtained by neglecting the movement in the first step, which is devoted to the reconstruction of the geometric and dielectric properties of the cylindrical targets. The second approach is more classical and is defined as a global optimization problem. It is shown that the two procedures are in some way complementary in the sense that, when the two-step procedure can be applied, it is by far the best, with the general procedure giving large errors; vice versa, the two-step procedure is in trouble when the other one gives good results. In carrying out this analysis, some estimates on the effects of the movement on the scattered field components are deduced in the limit as the peak velocity goes to zero. [ABSTRACT FROM PUBLISHER]

An approach to reconstruct buried objects is proposed. It is based on the integral equations of the electromagnetic inverse scattering problem, written in terms of the Green's function for half-space geometries. The full nonlinearity of the problem is exploited in order to inspect strong scatterers. After discretization of the continuous model, the resulting equations are solved in a regularization sense by means of a two-step inexact Newton algorithm. The capabilities and limitations of the method are evaluated by means of some numerical simulations. [ABSTRACT FROM AUTHOR]

The integration of the inexact Newton method within a multiscaling strategy is assessed in the framework of the contrast-field formulation of the electromagnetic inverse scattering. By using the features of the integrated approach, the technique is expected to suitably face the nonlinearity and the illposedness/ill-conditioning issues of the imaging problem. A numerical validation dealing with experimental data related to different objects is carried out to provide evidence of the method premises pointing out features and potentialities as well as current limitations. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 53:2834-2838, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26435 [ABSTRACT FROM AUTHOR]

A new approach for the inversion of synthetic and measured scattered data is proposed in this paper. The approach is based on an iterative technique in which the nonlinear equations of the inverse-scattering problem are solved within the pth-order Born approximation. A regularization scheme based on an inexact-Newton method is applied. Several numerical simulations and experimental results are reported. Multiple separated dielectric cylinders are localized and reconstructed in a noisy environment. [ABSTRACT FROM AUTHOR]

The imaging of dielectric targets hidden behind a wall is addressed in this paper. An analytical solver for a fast and accurate computation of the forward scattered field by the targets is proposed, which takes into account all the interactions of the electromagnetic field with the interfaces of the wall. Furthermore, an inversion procedure able to address the full underlying non-linear inverse scattering problem is introduced. This technique exploits a regularizing scheme in Lebesgue spaces in order to reconstruct an image of the hidden targets. Preliminary numerical results are provided in order to initially assess the capabilities of the developed solvers. [ABSTRACT FROM AUTHOR]

The subject of this paper is the possible use of a network of microwave sensors to achieve a map of the electromagnetic properties of unknown targets. The basic idea is to use a set of microwave sensors to illuminate a region of interest and to measure the resulting axial component of the electric field. Measurements are then processed by means of a technique based on inverse-scattering, which provides an estimate map of the dielectric values of the area under examination, allowing to discriminate among possible targets. In order to initially evaluate the feasibility of the proposed approach, numerical results in a simulated environment are preliminarily considered and discussed. Furthermore, an initial test on experimental data in a simplified configuration is also presented. [ABSTRACT FROM AUTHOR]

Despite the significant advances in electromagnetic imaging techniques, the analysis of Ground Penetrating Radar (GPR) data is still an open challenge, and most commercial systems adopt qualitative approaches because of their simplicity and robustness. However, such processing methods do not usually provide a complete characterization of the dielectric properties of the targets, but often only some information about target shape and location can be obtained. The typical aspect-limited GPR configurations, in which measurements are not collected all around the target, are frequently an obstacle for the application of more advanced quantitative methods [1]. In this work, a novel quantitative technique is applied to invert Ground Penetrating Radar data. It consists of an adaptive and automatic procedure developed in the nonconventional mathematical framework of Lebesgue spaces with variable exponents, which is exploited inside an inexact-Newton loop. With respect to the previous approaches of the same class, it frees the user from the selection of a fixed Lebesgue-space exponent, which is case-specific and difficult to be performed without a-priori information [2]. The proposed reconstruction approach is preliminarily validated by means of GPR data obtained from numerical simulations.[1] A. Fedeli, M. Pastorino, and A. Randazzo, "Advanced inversion methods for Ground Penetrating Radar," Journal of Telecommunications and Information Technology, vol. 3/2017, pp. 37-42, Sep. 2017.[2] C. Estatico, A. Fedeli, M. Pastorino, and A. Randazzo, "Quantitative microwave imaging method in Lebesgue spaces with nonconstant exponents," IEEE Transactions on Antennas and Propagation, vol. 66, no. 12, pp. 7282–7294, Dec. 2018. [ABSTRACT FROM AUTHOR]

In principle, the dielectric reconstruction of structures with a multi-layer configuration can have several practical implications. Layered scenarios are indeed present in a quite large number of circumstances, from geophysical inspections [1] to road pavements evaluation [2]. In all these contexts, the ground-penetrating radar (GPR) is a valuable instrument to acquire the back-scattered electromagnetic signals arising from buried targets and discontinuities. However, starting from GPR data, the retrieval of geometric and dielectric properties of such structures is not a straightforward task and is frequently left to operator's interpretation skills or accomplished in qualitative ways. Conversely, the possibility of an accurate characterization of the dielectric properties of objects buried in multi-layer environments may represent a further step toward their automated recognition and classification. The main obstacle in this direction is the need to resolve an ill-posed nonlinear inverse scattering problem, which requires advanced processing techniques with proper regularization capabilities. In free-space and half-space configurations, promising results have been obtained by adopting Newton-type inversion methods in Banach spaces, in particular with multi-frequency formulations [3]. This kind of technique is extended in order to reconstruct targets in three-layer scenarios. Such an extension includes changes in the kernels of the adopted integral operators, which reflect in an increased complexity of the relationship between scattered-field data and target properties. Initial numerical simulations are proposed to validate the inversion procedure and evaluate its performance in a fully controlled environment.[1] I. Catapano, A. Affinito, A. D. Moro, G. Alli, and F. Soldovieri, "Forward-looking ground-penetrating radar via a linear inverse scattering approach," IEEE Trans. Geosci. Remote Sens., vol. 53, no. 10, pp. 5624–5633, Oct. 2015.[2] F. Tosti, L. Bianchini Ciampoli, F. D'Amico, A. M. Alani, and A. Benedetto, "An experimental-based model for the assessment of the mechanical properties of road pavements using ground-penetrating radar," Constr. Build. Mater., vol. 165, pp. 966–974, Mar. 2018.[3] C. Estatico, A. Fedeli, M. Pastorino, and A. Randazzo, "A multifrequency inexact-Newton method in Lp Banach spaces for buried objects detection," IEEE Trans. Antennas Propag., vol. 63, no. 9, pp. 4198–4204, Sep. 2015. [ABSTRACT FROM AUTHOR]