Davide Comite

Analysis of GPR Early-Time Signal Features for the Evaluation of Soil Permittivity Through Numerical and Experimental Surveys

Recently, various studies have been carried out in order to address the possible relationships between amplitude attributes of the “first-arrival direct wave” (the so-called “early-time signal,” ETS), propagating at the interface in bistatic ground-penetrating radar (GPR) configurations, and the relevant shallow-soil permittivity parameters (dielectric constant and conductivity). In this frame, ad hoc compared numerical analyses and experimental investigations are extensively developed and discussed here, with the aim of making clearer the distinctive features and the reliability of this technique. The accurate results achieved for the ETS behavior as a function of various GPR system parameters enable us to identify both which are the more revealing signal attributes able to give predictable correlation with the ground permittivity values and the degree of complexity of the functional relationships between ETS amplitude and system parameters. A number of indications and perspectives are, thus, outlined in order to elucidate features, potential and critical aspects of the ETS technique for effective geophysical applications.

3D Imaging of Buried Dielectric Targets with a Tomographic Microwave Approach Applied to GPR Synthetic Data

Effective diagnostics with ground penetrating radar (GPR) is strongly dependent on the amount and quality of available data as well as on the efficiency of the adopted imaging procedure. In this frame, the aim of the present work is to investigate the capability of a typical GPR system placed at a ground interface to derive three-dimensional (3D) information on the features of buried dielectric targets (location, dimension, and shape). The scatterers can have size comparable to the resolution limits and can be placed in the shallow subsurface in the antenna near field. Referring to canonical multimonostatic configurations, the forward scattering problem is analyzed first, obtaining a variety of synthetic GPR traces and radargrams by means of a customized implementation of an electromagnetic CAD tool. By employing these numerical data, a full 3D frequency-domain microwave tomographic approach, specifically designed for the inversion problem at hand, is applied to tackle the imaging process. The method is tested here by considering various scatterers, with different shapes and dielectric contrasts. The selected tomographic results illustrate the aptitude of the proposed approach to recover the fundamental features of the targets even with critical GPR settings.

An evaluation of the early-time GPR amplitude technique for electrical conductivity monitoring

In the present paper we use the recently-proposed early-time GPR (Ground Penetrating Radar) amplitude technique with the aim of detecting the variations of electric conductivity in a porous material having a uniform permittivity. A specific laboratory setup has been realised to evaluate the sensitivity of the early-time amplitudes to the variations of the subsurface salt concentration (i.e., conductivity). To assess the capacity of the early-time amplitude to follow the electrical conductivity changes, we compare the early-time results acquired using the envelope of the first part of GPR signals with the concurrent conductivity measured with TDR (Time Domain Reflectometry). The GPR survey has been carried out using a bistatic radar unit (Sensors & Software, Inc) operating at 1 GHz. Further useful information has been derived by suitably implementing a full-wave numerical modelling, able to accurately analyse the features of the waves detected by the GPR with flexible parameterization. Our results indicate that the near-surface electromagnetic properties of the material can be directly extracted from the GPR early-time amplitude technique. In particular, both experimental and numerical data show a very high correlation coefficient between the radar signal amplitude and the TDR-derived electrical conductivities.

Exciting Vorticity Through Higher Order Bessel Beams With a Radial-Line Slot-Array Antenna

In this communication, it is shown that a nondiffracting vortex beam (i.e., a higher order Bessel beam with azimuthal phase variation) can be generated in the near field by synthesizing an inward cylindrical traveling-wave distribution over a finite aperture antenna. A radial line slot array (RLSA) is then designed to prove the concept. The collimated vortex beam is excited in the proximity of the RLSA, within a region properly defined by the nondiffracting range of the generated beam. The radial dependence of the longitudinal electric field of the vortex-beam magnitude follows a first-order Bessel function, and its phase presents a linear azimuthal variation. Full-wave results validate the generation of the nondiffractive higher order Bessel beam within the radiative near field of the launcher.

Spectral Method of Moments for Planar Structures With Azimuthal Symmetry

An efficient method-of-moments formulation is proposed for the analysis of planar structures in the presence of azimuthally symmetric fields. Boundary integral equations are derived for TM and TE polarizations, assuming as a reference structure a metal plate with a finite number of concentric annular slots placed above a stratified medium. The equations are discretized with a Galerkin testing scheme in the Fourier-Bessel domain, adopting suitable sub-domain basis functions that ensure a very rapid convergence of the involved spectral integrals. Numerical validation is provided by comparison with state-of-art simulation software, considering planar antennas with omnidirectional radiation patterns, in order to show the accuracy of the proposed formulation.

Forward Scatter Radar for Air Surveillance: Characterizing the Target-Receiver Transition from Far-Field to Near-Field Regions

A generalized electromagnetic model is presented in order to predict the response of forward scatter radar (FSR) systems for air-target surveillance applications in both far-field and near-field conditions. The relevant scattering problem is tackled by developing the Helmholtz–Kirchhoff formula and Babinet’s principle to express the scattered and the total fields in typical FSR configurations. To fix the distinctive features of this class of problems, our approach is applied here to metallic targets with canonical rectangular shapes illuminated by a plane wave, but the model can straightforwardly be used to account for more general scenarios. By exploiting suitable approximations, a simple analytical formulation is derived allowing us to efficiently describe the characteristics of the FSR response for a target transitioning with respect to the receiver from far-field to near-field regions. The effects of different target electrical sizes and detection distances on the received signal, as well as the impact of the trajectory of the moving object, are evaluated and discussed. All of the results are shown in terms of quantities normalized to the wavelength and can be generalized to different configurations once the carrier frequency of the FSR system is set. The range of validity of the proposed closed-form approach has been checked by means of numerical analyses, involving comparisons also with a customized implementation of a full-wave commercial CAD tool. The outcomes of this study can pave the way for significant extensions on the applicability of the FSR technique.

Multiview Imaging for Low-Signature Target Detection in Rough-Surface Clutter Environment

Forward-looking ground-penetrating radar (FL-GPR) permits standoff sensing of shallow in-road threats. A major challenge facing this radar technology is the high rate of false alarms stemming from the vulnerability of the target responses to interference scattering arising from interface roughness and subsurface clutter. In this paper, we present a multiview approach for target detection in FL-GPR. Various images corresponding to the different views are generated using a tomographic algorithm, which considers the near-field nature of the sensing problem. Furthermore, for reducing clutter and maintaining high cross-range resolution over the imaged area, each image is computed in a segmentwise fashion using coherent integration over a suitable set of measurements from multiple platform positions. We employ two fusion approaches based on likelihood ratio tests detector to combine the multiview images for enhanced target detection. The superior performance of the multiview approach over single-view imaging is demonstrated using electromagnetic modeling data.

The Role of the Antenna Radiation Pattern in the Performance of a Microwave Tomographic Approach for GPR Imaging

Microwave tomography has been exploited as a successful imaging technique for ground penetrating radar (GPR) surveys. However, an “ideal” antenna model (i.e., a filamentary current element) has usually been adopted so far to describe the involved scattering phenomena. In this study, such an assumption is removed, and the effects of a “realistic” antenna are taken into account by means of a customized numerical implementation. Specifically, we provide indications about the reconstruction capabilities expectable when the imaging is performed by using a “directional” wideband transceiver to gather GPR data. Moreover, we discuss to what extent the imaging performance is affected by a priori knowledge of the antenna model.

A Dual-Layer Planar Leaky-Wave Antenna Designed for Linear Scanning Through Broadside

A low-cost planar leaky-wave antenna (LWA) offering directive antenna beam patterns as well as linear beam scanning through broadside is proposed. The design is based on a one-sided annular slot grating placed on a dual-layer grounded dielectric slab with an integrated TM

Radially Periodic Leaky-Wave Antenna for Bessel Beam Generation Over a Wide-Frequency Range

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