Elena Pettinelli

Radar Soundings of the Subsurface of Mars

The martian subsurface has been probed to kilometer depths by the Mars Advanced Radar for Subsurface and Ionospheric Sounding instrument aboard the Mars Express orbiter. Signals penetrate the polar layered deposits, probably imaging the base of the deposits. Data from the northern lowlands of Chryse Planitia have revealed a shallowly buried quasi-circular structure about 250 kilometers in diameter that is interpreted to be an impact basin. In addition, a planar reflector associated with the basin structure may indicate the presence of a low-loss deposit that is more than 1 kilometer thick.

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.

Early-Time GPR Signal Attributes to Estimate Soil Dielectric Permittivity: A Theoretical Study

High-frequency electromagnetic (EM) surveys have shown to be valuable techniques in the study of soil water content due to the strong dependence of soil dielectric permittivity with moisture content. This quantity can be determined by analyzing the average value of the early-time instantaneous amplitude of ground-penetrating radar (GPR) traces. We demonstrate the reliability of this approach to evaluate the shallow soil water content variations from standard fixed-offset GPR data by simulating the data over different likely EM soil conditions. A linear dipole model that uses a thin-wire approximation is assumed for the transmitting and receiving antennas. The homogenous half-space model is used to calculate the waveform instantaneous amplitude values averaged over different time windows. We analyzed their correlation with the soil surface dielectric parameters, and we found a clear inverse linear dependence on the permittivity values. Moreover, we evaluated how different kinds of noise affect this correlation, and we determined the influence of the electrical conductivity on the trace attributes. Finally, through a two-layered medium, we estimated the effect on the GPR signal of a shallow reflector, we analyzed how its presence can carry out inaccuracies in the soil surface dielectric permittivity estimation, and we determined the best time window to minimize these errors.

Time domain reflectrometry: Calibration techniques for accurate measurement of the dielectric properties of various materials

In this work, an extensive theoretical and experimental analysis has been carried out on the main factors that affect the accurate evaluation of dielectric properties using time domain reflectometry (TDR). Dielectric constant measurements have been performed under different experimental conditions for various types of materials having a wide range of permittivity values. Interpretation of the results on the basis of the transmission-line theory emphasizes the importance of a suitable calibration procedure that takes into account several critical aspects, some of which seem to have been disregarded or underestimated in the literature. Qualitative and quantitative information is given on the kind of corrections needed in order to significantly improve the reliability of the TDR technique for permittivity measurements.

Electromagnetic Propagation of GPR Signals in Martian Subsurface Scenarios Including Material Losses and Scattering

A study on the electromagnetic propagation in various models of the Martian subsurface is performed with a relevance to ground penetrating radar (GPR) operating onboard rover missions. Measurements of the electromagnetic properties of Mars soil simulants are obtained; on this basis, the attenuation features of the GPR signals are estimated, including both electric and magnetic losses. The effect on propagation of inhomogeneities inside the soil is also taken into account by means of a specific model with randomly distributed scatterers. The GPR performance in terms of resolution and maximum penetration depth is evaluated in the considered scenarios for different operating frequencies, thus providing a basic information for the design of systems for future subsurface sounding investigations on Mars

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.

GPR, TDR, and geochemistry measurements above an active gas vent to study near-surface gas-migration pathways

The migration of deep gas to the atmosphere along faults and associated structures is important in many fields, from studying the natural contribution of atmospheric greenhouse gases leaking from geothermal areas to ensuring the safety of man-made natural gas and carbon dioxide (C O2 ) geologic-storage sites. Near-surface geophysical and geochemical techniques were applied to a naturally occurring gas vent located along a deep terrestrial fault to better understand the structure and geophysical response of this gas-migration pathway. A number of ground-penetrating radar (GPR) profiles were first conducted across the vent. Spot samples were then measured along one of these profiles for in situ apparent permittivity (using time-domain reflectometry — TDR), complex permittivity on dried samples (using a capacitivecell), soil-gas composition, and clay and bulk mineralogy. Results show how the migrating gas induces secondary effects that modify the signature of the vent as seen in the GPR profiles. In particul...

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.