Sebastian Lauro

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.

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.

Thermal and electromagnetic models for radar sounding of the galilean satellite icy crusts

The icy satellites of Jupiter are known to host a water ocean beneath an ice shell. The ice penetrating Radar for Icy Moon Exploration (RIME) housed on board the JUpiter ICy moons Explorer (JUICE) mission is expected to probe the crust of Europa, Ganymede and Callisto up to a depth of 9 km. The main objectives of RIME are the detection of the relic Brittle-Ductile Transition in the ice shell of Ganymede and Callisto, and the identification of melt materials on Europa. The penetration of the radar signal is strictly connected to the electromagnetic properties of the ice, that in turn depends on the presence of contaminants and temperature profile inside the crust. Laboratory measurements carried out in the temperature range of (100-273)K provided the dielectric properties of pure, salty and dusty ices, whereas temperature profiles are obtained taking into account the heat conduction and thermal convection models for the Galilean satellites. The combination of electromagnetic and thermal properties of the icy crusts allowed us to generate simulated radar data at the operation frequency of RIME (9 MHz). Such simulations are important to determine the radar performance, estimating the signal penetration and the capability to resolve buried layers.

GPR measurements and FDTD simulations for landmine detection

Among the technologies used to improve landmine detection, Ground Penetrating Radar (GPR) techniques are being developed and tested jointly by “Sapienza” and “Roma Tre” Universities. Using three-dimensional Finite Difference Time Domain (FDTD) simulations, the electromagnetic field scattered by five different buried objects has been calculated and the solutions have been compared to the measurements obtained by a GPR system on a (1.3 × 3.5 × 0.5) m3 sandbox, located in the Humanitarian Demining Laboratory at Cisterna di Latina, to assess the reliability of the simulations. A combination of pre-calculated FDTD solutions and GPR scans, may make the detection process more accurate.

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.

Coaxial-Cage Transmission Line for Electromagnetic Parameters Estimation

We present the measurements performed with a custom coaxial-cage line designed to determine the complex dielectric permittivity of large samples, made of granular and/or liquid materials. The open structure of the cage facilitates the filling of the line, and allows the uniform compactness of the granular materials having large grain size. The electromagnetic parameters of the samples are retrieved using the line scattering coefficients ( S-parameters), measured with a vector network analyzer in the frequency range 1 MHz to 3 GHz. The measurements are carried out on water, ethanol, and glass beads using the Boughriet method, which, for non magnetic materials, optimizes the Nicolson-Ross-Weir algorithm. The results confirm that such a device accurately estimates the material complex permittivity in a wide frequency band. However, at very low frequencies and at frequencies multiple of the line half-wavelength resonances, the accuracy significantly reduces. In particular, the accuracy gets worse for materials with high permittivity and low losses. Nevertheless, for natural geo-materials, usually characterized by appreciable losses, the coaxial-cage line can effectively be used to accurately estimate the material electromagnetic properties in a wide frequency band.

Radar Signal Penetration and Horizons Detection on Europa Through Numerical Simulations

We propose a strategy to evaluate the performance of a radar sounder for the subsurface exploration of the Europa icy crust and, in particular, the possibility to detect liquid water at the base of the ice shell. The approach integrates the information coming from experimental measurements of the dielectric properties of icy materials, thermal models related to different crustal scenarios, and numerical simulations of radar signal propagation. The radar response has been evaluated in terms of cumulative attenuation, signal-to-noise ratio (SNR), and reflectivity. Our simulations indicate that a subsurface radar operating at 9 MHz can identify shallow-buried targets and to detect the ice/water interface in various thermal scenarios. Under our assumptions the ice/water interface can be detected almost down to a depth of 15 km under a conductive ice shell, whereas for a convective ice shell, the maximum depth is about 12 km (in the cold downwelling plume). We also discuss the possibility to detect shallow targets associated with interfaces between pure water ice and MgSO4 · 11H2O ice mixtures at various salt contents, using the data of laboratory dielectric measurements.