Raffaele Persico

A linear inverse scattering algorithm for realistic GPR applications

The paper deals with a microwave-tomography-based solution algorithm tailored for use with GPR data-processing applications. The algorithm tackles an inverse scattering problem in the frequency domain through the use of a linear model of the electromagnetic scattering, based on the Born approximation. In particular, we evaluate the reconstruction capabilities of the linear inversion algorithm in terms of the retrievable spatial variations of the unknown contrast function, whilst considering the problem of choosing an optimal frequency measurement step, theoretically, using diffraction tomography arguments. A numerical analysis of the technique is performed by means of the singular-value decomposition tool, which allows us to extend the theoretical results to more realistic cases involving lossy soils. Finally, we present a series of reconstructions, obtained using synthetic and experimental data, which show the performance of the method under realistic conditions.

Some examples of GPR prospecting for monitoring of the monumental heritage

In this paper three case histories of ground penetrating radar (GPR) for the monitoring of historical buildings are presented. They aim to present the specific valence of the GPR in the field of the diagnostic of historical buildings, which is a promising field of research, due to the increasing awareness of the relevance (even economic) of the cultural heritage. The presented GPR prospecting cases have been performed on three different constructive elements typical of historical buildings (a wall, a masonry pillar and a marble column) in order to be the answer to different problems such as the characterization of the masonry, the detection of cracks and the imaging of metallic reinforcement bars.

A Microwave Tomographic Imaging Approach for Multibistatic Configuration: The Choice of the Frequency Step

This paper deals with the application of a frequency-domain microwave-tomography-imaging algorithm to a multibistatic configuration in ground-penetrating-radar applications. First, the authors formulate the inverse problem as a linear one by exploiting the Born approximation. Then, the reconstruction capabilities of the solution algorithm are investigated, and an optimal frequency step to be adopted for the reconstruction is discussed and determined. Finally, inversion results are presented with synthetic data obtained from time-domain simulations and then Fourier transformed in frequency domain

Inverse scattering under the distorted Born approximation for cylindrical geometries

The problem of reconstructing dielectric permittivity from scattered field data is dealt with for scalar two-dimensional geometry at a fixed frequency by use of a linearized approximation about a chosen reference permittivity profile. To investigate the capabilities and limits of linear inversion algorithms, we analyze the class of retrievable profiles with reference to some canonical geometries for which either analytical or numerical details can be worked through easily. The tool for such an analysis consists of the singular-value decomposition of the relevant scattering operators. For a constant reference permittivity function, the different behavior of linear inversion algorithms with respect to either radial or angular variations of the permittivity profiles is pointed out. In the last-named case the general situation of a multiview radiation is accounted for, and, unlike for the Born approximation, profiles that cannot be reconstructed by linear inversion comprise slowly varying functions. Moreover, the effect of an angularly varying reference profile is examined for a thin circular shell, permitting the possibility of reconstruction of rapidly varying angular profiles by linear inversion. Numerical results of linear inversions that confirm the predictions are shown.

Effects of Background Removal in Linear Inverse Scattering

This paper deals with the effect of the background removal procedure on ground-penetrating radar data in the framework of a Born-based inverse-scattering solution approach for a scalar 2-D geometry. The theoretical investigation is performed by resorting to diffraction tomography reasoning, and then it is numerically confirmed thanks to the singular value decomposition of the operator that connects the unknown contrast function to the scattered field data achieved under background removal. Numerical and experimental tests are presented.

A Microwave Tomography Approach for a Differential Configuration in GPR Prospecting

The capabilities of a microwave inverse scattering algorithm for ground penetrating radar (GPR) data processing are analyzed. The algorithm is based on the Born approximation (BA) and exploits a differential measurement configuration. This configuration exploits measurements achieved as the difference of the electrical fields gathered in two symmetrical positions with respect to the source. We will discuss how this choice allows to mitigate some bad effects of uncertainties on the knowledge of the background scenario but, on the other hand, affects in a negative way the properties of the relevant scattering operator

GPR and sonic tomography for structural restoration: the case of the cathedral of Tricarico

In this paper, we present the results of a diagnostics survey, based on the exploitation of ground penetrating radar (GPR) and sonic prospecting, to characterize the deterioration status of the pillars of the cathedral of Tricarico, in the Basilicata region (Southern Italy). The prospecting falls within the more general framework of investigating the structural conditions of this monument, which is affected by heavy instability problems. This study case points out the great effectiveness of the two employed diagnostic methods, when used in an integrated way, for detecting cracks and inhomogeneities in the inner structure of masonry building elements. With regard to GPR prospecting, a comparison is made between the results obtained by a standard processing and those obtained by means of an inverse scattering algorithm. For one of the investigated pillars, the results obtained from non-invasive tests are compared with those of direct inspection. This is performed by coring the pillar and examining both the core and the hole (the latter by means of an endoscope). The seismic investigation allowed us to prove the mediocre or bad state of conservation of the pillars.

A Reconfigurative Approach for SF-GPR Prospecting

We study the impedance parameters and the energy transmitted and received by a couple of antennas working in a nonhomogeneous background. The focus is on stepped frequency ground penetrating radar (SF-GPR) prospecting. In particular, we propose a reconfiguration of the GPR system versus the frequency that accounts for the background scenario, and we show that the reconfiguration can improve the frequency behavior of the antennas significantly. Tests performed on two bow-tie antennas will be shown.

Automated Detection of Reflection Hyperbolas in Complex GPR Images With No A Priori Knowledge on the Medium

In this paper, we propose an automated detection algorithm for well- and ill-shaped ground-penetrating radar reflection hyperbolas for complex media, calibrated with human recognition principles. The algorithm detects the apex of the hyperbolas by fitting an analytical function of a hyperbola to the profile edge dots detected with a Canny filter. The existence of a hyperbola is determined using a set of carefully chosen criteria calibrated in order to fit the ambiguity zone for the human brain. The inherent misshapedness of field hyperbolas is further considered by defining a buffer zone around the theoretical hyperbola. First, the method was tested in the laboratory over tree roots and PVC pipes and on field images over tree root systems. Both time- and frequency-domain radars were used on-ground. After around 1-3 min of computation time for 10 000 edge dots in a MATLAB environment (single 1.96-GHz processor), the results showed rates of false alarm and nondetection of maximum 20% and 28%, respectively. In comparison with the semiautomated hyperbola detection provided by a commercial software, these rates were lower. Second, we conducted a sensitivity analysis to estimate the validity of the fitting of a hyperbola equation neglecting the object radius. The fitting was close, but the derivation of the relative permittivity from the analytical equation neglecting the radius led to high errors. In conclusion, owing to the low computational time and its good performances, the proposed algorithm is suitable for complex environments.

Microwave tomography for processing of GPR data at Ballachulish

In this paper we present the results of a microwave tomography algorithm applied to the GPR data gathered at the archaeological site of North Ballachulish (Scotland). The GPR survey had the aim of mapping the contours of the peat basin and investigating the possibility of archaeological remains. A linear inverse scattering algorithm has been applied on the data, and the results are compared with those achieved by means of the Reflex code. The results of this work show the possibility of applying the linear inverse scattering approach to a massive amount of data for large-scale investigations, and also of showing the possibilities offered in some cases by pre-processing the data (interpolation procedure) so as to improve the result of the overall diagnostics procedure.