Elisabetta Mattei

GPR Response From Buried Pipes: Measurement on Field Site and Tomographic Reconstructions

The identification of the physical nature of an object or target causing a ground-penetrating radar (GPR) anomaly, as well as the estimation of a targets dimensions and geometry, is rather challenging. To improve target identification, basic studies are still required, and they can be addressed primarily using a laboratory- or field-based physical model. The field model (test site) is usually expensive and difficult to build, but it provides data for controlled target properties and geometry from a natural environment that are essential for testing processing techniques. In this paper, we present the results from a field experiment where GPR data were collected on plastic and metallic pipes. The main objective is the comparison of the classical migration technique with a microwave tomography approach for reconstructing the geometrical target properties. The use of the microwave tomography approach will allow us to obtain more focused and stable images of the buried objects compared to the ones obtained using classical migration techniques.

Correlation between near-surface electromagnetic soil parameters and early-time GPR signals: An experimental study

We explore a new approach to evaluate the effect of soil electromagnetic parameters on early-time ground-penetrating radar GPR signals. The analysis is performed in a time interval which contains the direct airwaves and ground waves, propagating between transmitting and receiving antennas.Toperformthemeasurementswehaveselectedanatural test site characterized by very strong lateral gradient of thesoilelectricalproperties.Toevaluatetheeffectofthesubsoil permittivity and conductivity on the radar response we comparetheenvelopeamplitudeoftheGPRsignalsreceived in the first 12 ns within 4 ns-wide windows, with the electricalpropertiesrandDCdeterminedusingtime-domainreflectometryTDR.Theresultsshowthattheconstitutivesoil parameters strongly influence early-time signals, suggesting a novel approach for estimating the spatial variability of watercontentwithGPR.

Characterization of a CO2 gas vent using various geophysical and geochemical methods

An understanding of gas migration along faults is important in many geologic research fields, such as geothermal exploration, risk assessment, and, more recently, the geologic storage of man-made carbon dioxide (C O2 ) . If these gases reach the surface, they typically are discharged to the atmosphere from small areas known as gas vents. In a study of an individual gas vent located in the extinct Latera caldera, central Italy, near-surface geochemical and geophysical surveys were conducted to define the spatial distribution of gas-induced effects in the first few meters of the soil and, by inference, the 3D structure and geometry of the associated gas-permeable fault. Grid surveys and detailed profiles were performed across this vent using time-domain reflectometry (TDR), ground-penetrating radar (GPR), frequency-domain electromagnetics (FDEM), electrical resistivity tomography (ERT), and gas geochemistry measurements. Detailed profilesurveys indicate that the leaking C O2 has changed the physical, chemic...

Dielectric properties of Jovian satellite ice analogs for subsurface radar exploration: A review

The first European mission dedicated to the exploration of Jupiter and its icy moons (JUpiter ICy moons Explorer—JUICE) will be launched in 2022 and will reach its final destination in 2030. The main goals of this mission are to understand the internal structure of the icy crusts of three Galilean satellites (Europa, Ganymede, and Callisto) and, ultimately, to detect Europas subsurface ocean, which is believed to be the closest to the surface among those hypothesized to exist on these moons. JUICE will be equipped with the 9 MHz subsurface-penetrating radar RIME (Radar for Icy Moon Exploration), which is designed to image the ice down to a depth of 9 km. Moreover, a parallel mission to Europa, which will host onboard REASON (Radar for Europa Assessment and Sounding: Ocean to Near-surface) equipped with 9MHz and 60MHz antennas, has been recently approved by NASA. The success of these experiments strongly relies on the accurate prediction of the radar performance and on the optimal processing and interpretation of radar echoes that, in turn, depend on the dielectric properties of the materials composing the icy satellite crusts. In the present review we report a complete range of potential ice types that may occur on these icy satellites to understand how they may affect the results of the proposed missions. First, we discuss the experimental results on pure and doped water ice in the framework of the Jaccard theory, highlighting the critical aspects in terms of a lack of standard laboratory procedures and inconsistency in data interpretation. We then describe the dielectric behavior of extraterrestrial ice analogs like hydrates and icy mixtures, carbon dioxide ice and ammonia ice. Building on this review, we have selected the most suitable data to compute dielectric attenuation, velocity, vertical resolution, and reflection coefficients for such icy moon environments, with the final goal being to estimate the potential capabilities of the radar missions as a function of the frequency and temperature ranges of interest for the subsurface sounders. We present the different subsurface scenarios and associated radar signal attenuation models that have been proposed so far to simulate the structure of the crust of Europa and discuss the physical and geological nature of various dielectric targets potentially detectable with RIME. Finally, we briefly highlight several unresolved issues that should be addressed, in near future, to improve our capability to produce realistic electromagnetic models of icy moon crusts. The present review is of interest for the geophysical exploration of all solar system bodies, including the Earth, where ice can be present at the surface or at relatively shallow depths.

Electromagnetic propagation features of ground penetrating radars for the exploration of martian subsurface

In this work, the effects of magnetic inclusions in a Mars-like soil are considered with reference to the electromagnetic propagation features of ground-penetrating radars (GPRs). Low-frequency and time-domain techniques, using L-C-R meters and TDR instruments, respectively, are implemented in laboratory experimental set-ups in order to evaluate complex permittivity and permeability and wave velocity for different scenarios of a dielectric background medium (silica) with magnetic inclusions (magnetite). Attenuation and maximum detection ranges have also been evaluated by taking into account a realistic GPR environment, which includes the transmitting/receiving antenna performance and the complex structure of the subsurface. The analysis and the interpretation of these results shed new light on the significant influence of magnetic inclusions on the performance of Martian orbiting and rover-driven GPRs.

Time Domain Reflectometry of Glass Beads/Magnetite Mixtures: a Time and Frequency Domain Study

The measurements of the time domain reflectometry (TDR) signal propagating along a coaxial probe, filled with dry mixtures of glass beads and magnetite, are analyzed for deriving the attenuation factor α and the bandwidth. Samples with different percentages (5%–25%) of magnetite are considered. The α values are obtained by two different methods: (i) wave amplitude at the second reflection, (ii) electromagnetic parameters and widths of the passing bands. The two methods provide consistent α values within the experimental uncertainties. The agreement supports the possibility of measuring the attenuation factor from the second TDR reflection, for anhydrous samples similar to the investigated mixtures.

Comparison of GPR and unilateral NMR for water content measurements in a laboratory scale experiment

Several factors affect antenna-soil coupling in a Ground Penetrating Radar (GPR) survey, like surface roughness, lithology, lateral heterogeneities, vegetation, antenna height from the surface and water content. Among them, lithology and water content have a direct effect on the bulk electromagnetic properties of the material under investigation. It has been recently pointed out that the wavelet of the early-time portion of a radar signal is correlated to the shallow subsurface dielectric properties of a material. This result indicates that some information on such properties can be directly extracted from the analysis of GPR early-time traces. In the present paper, we use the early-time GPR signal, in terms of average envelope amplitude computed on the first half-cycle, to map the near-surface (few centimetres) lateral distribution of dielectric parameters, induced by changing the shallow water content on a concrete slab. This controlled experiment was specifically designed to study the effect of water content variations on antenna-material coupling, minimizing the influence of both surface roughness and heterogeneity. The quantitative control of the water in the shallow portion of the slab is performed by using a portable unilateral Nuclear Magnetic Resonance (NMR) sensor, which is able to determine the water content in the material on the basis of the measured proton density. The results show a matching pattern of the physical parameters measured with the two different techniques and a very high degree of linear correlation (r = 0.97) between the radar early-time signal average amplitude and the intensity of the NMR signal, which is proportional to the proton density, i.e., to the water content. This experiment suggests that the early-time approach could be used as a fast and high- spatial resolution tool for qualitatively mapping water content lateral variations in a porous material at shallow depth, using a ground-coupled single-offset antenna configuration and that a quantitative evaluation of the moisture content would require a calibration procedure.

Non-destructive technique to investigate an archaeological structure: A GPR survey in the Domus Aurea (Rome, Italy)

The discovery and touristic fruition of the Domus Aurea (Latin for “Golden House”), and the heavy rain led to the arrival of moisture, starting the slow and inevitable process of decay and collapse. Furthermore, inside the Domus Aurea, there are a lot of parts not yet excavated, attracting a continuous and archaeological interest. In order to properly plan the restoration of this building and detect buried archaeological features, a non-destructive technique, like Ground Penetrating Radar (GPR), is extensively and profitably used for its rapid data collection and its high resolution images of contrasting subsurface structural or archaeological materials. The results show not only the presence of internal lesions and detachments of the wall and vault structure, but also the existence of buried archaeological targets; these preliminary results allow future restoration plans in order to prevent the rapid degradation of this important building.

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.

Radio wave techniques for non-destructive archaeological investigations

Geophysical techniques can be effectively applied to produce an image of buried targets in a host medium, through the detection of the ‘boundaries’ between materials having different values of a specific physical property. Over the last 30 years these methods have been increasingly used to detect the presence of archaeological ruins in the subsurface, thus reducing extensive, destructive, time consuming and expensive excavations. Recently, ground penetrating radar (GPR) has become the most important physical technique in archaeological investigations (allowing for the detection of archaeological targets with both very high vertical and horizontal resolution) and has been successfully applied also to diagnostic purposes in historical buildings and monuments. In this article an overview of the use of radio waves in archaeology is presented, first introducing the main physical concepts of GPR, then presenting some examples of its application to detect ancient buried structures, showing the potential and the limits of such a method.