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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. Index Terms--Attenuation, electromagnetic propagation, ground penetrating radar (GPR), Mars, scattering.

The question of penetration of synthetic aperture radar (SAR) signals at C-band frequency into polar glaciers is addressed by comparing ground penetrating radar (GPR) and SAR backscatter signatures. Profiles of the Kongsvegen glacier, Svalbard, were obtained with a C-band GPR. The received signal is converted to the equivalent radar cross section using the standard radar equation, thus mapping the effective scattering sources within the glacier at this frequency. The depth of the observed scattering sources is greatest in the superimposed ice where layers are clearly seen to a depth of approximately 14 m. The very high scattering properties of the upper firn layers preclude layers deeper than approximately 6 m from being imaged. Integrating the radar cross sections over the depth gives a single backscatter value that we compare with the backscatter coefficient (scattering cross section per unit area) of the processed SAR data for the same profile. The comparison indicates that for coincidentally acquired GPR and SAR data, the radar cross section measured by the GPR does represent the features that contribute to the SAR signal. Index Terms--C-band ground penetrating radar (GPR), glacier, scatter cross section, synthetic aperture radar (SAR).

The detection of antipersonnel landmines using ground-penetrating radar (GPR) is particularly hindered by the predominant soil surface and antenna reflections. In this paper, we propose a novel approach to filter out these effects from 2-D off-ground monostatic GPR data by adapting and combining the radar antenna subsurface model of Lambot et al. with phase-shift migration. First, the antenna multiple reflections originating from the antenna itself and from the interaction between the antenna and the ground are removed using linear transfer functions. Second, a simulated Green's function accounting for the surface reflection is subtracted. The Green's function is derived from the estimated soil surface dielectric permittivity using full-wave inversion of the radar signal for a measurement taken in a local landmine-free area. Third, off-ground phase-shift migration is performed on the 2-D data to filter the effect of the antenna radiation pattern. We validate the approach in laboratory conditions for four differently detectable landmines embedded in a sandy soil. Compared to traditional background subtraction, this new filtering method permits a better differentiation of the landmine and estimation of its depth and geometrical properties. This is particularly beneficial for the detection of landmines in low-contrast conditions. Index Terms--Clutter reduction, ground-penetrating radar (GPR), inverse modeling, landmine detection, phase-shift migration.

Scalar migration algorithms developed for three-dimensional seismic data are commonly used for imaging ground-penetrating radar (GPR) data. Yet, radar is a vector phenomenon, such that the GPR amplitudes and phases depend on the antenna orientations and wave propagation paths. To address this issue, vector imaging algorithms that fully account for the vector characteristics of GPR data are required. All previously developed vector imaging algorithms are based on far-field approximations of radiation patterns. We demonstrate the limited applicability of these algorithms and introduce a computationally efficient practically exact-field method that accounts for the far-, intermediate-, and near-field contributions to the radiation patterns. To compute rapidly the required 'exact' radiation patterns, inverse fast Fourier transforms are applied to their horizontal wavenumber-frequency domain formulations, balancing the tradeoff between accuracy and efficiency by an appropriate oversampling in the wavenumber-frequency domain, and taking advantage of vertical wavenumber phase shifting. We include the exact radiation patterns in a multicomponent vector imaging scheme that jointly images copolarized and cross-polarized data. This scheme is tested on synthetic and field data containing dipping planar and near-planar structures. For both suites of data, high-quality multicomponent images with reflection amplitudes that are nearly independent of the antenna and reflector orientations are obtained. Index Terms--Exact radiation patterns, Green's functions, ground-penetrating radar (GPR), imaging, migration.

Two common Fourier imaging algorithms used in ground penetrating radar (GPR), synthetic aperture radar (SAR), and frequency-wavenumber (F-K) migration, are reviewed and compared from a theoretical perspective. The two algorithms, while arising from seemingly different physical models: a point-scatterer model for SAR and the exploding source model for F-K migration, result in similar imaging equations. Both algorithms are derived from an integral equation formulation of the inverse scalar wave problem, which allows a clear understanding of the approximations being made in each algorithm and allows a direct comparison. This derivation brings out the similarities of the two techniques which are hidden by the traditional formulations based on physical scattering models. The comparison shows that the approximations required to derive each technique from the integral equation formulation of the inverse problem are nearly identical, and hence the two imaging algorithms and physical models are making similar assumptions about the solution to the inverse problem, thus clarifying why the imaging equations are so similar. Sample images of landmine-like targets buried in sand are obtained from experimental GPR data using both algorithms. Index Terms--Frequency-wavenumber (F-K) migration, ground penetrating radar (GPR), inverse wave problem, synthetic aperture radar (SAR).

A microgenetic algorithm has been applied to design a new ultrawideband thin-wire bow-tie antenna for ground-penetrating radar applications. The broadband performance of the antenna is achieved by resistive loading and by optimizing the number of wires and the angular distances between those wires. The radiation characteristics of the optimized antenna are discussed, and its performance is compared to that of a restively loaded Wu-King dipole. Index Terms--Genetic algorithms (GAs), ground-penetrating radar (GPR), GPR antenna design, thin-wire antennas, ultrawideband antennas.

In this paper, we discuss the influence of a resonance on estimating direction of arrival (DOA) with a circular dipole array in a borehole (CAB). The resonance is caused by the phase-sequence of currents on the dipole antennas. Making use of method of moments (MoM) analysis, we predict resonant frequencies of the CAB and describe a mechanism for the resonance theoretically, making use of the fact that a plane wave can be broken down into cylindrical harmonics. In order to examine the agreement between the MoM and experimental data, we propose a signal processing method to extract phase-sequence components from the actual received array signal. This method is useful for confirming excitation of the phase-sequence resonance in experimental data, as predicted by the MoM. Using the MoM, we examine the influence of the resonances on DOA estimation, and we conclude that the resonances give some spurious solutions. In order to confirm these results, we conduct experiments at a field test site. We arrange the CAB, which consisted of seven receiving dipole antennas. In this experiment, a plane wave is incident on the CAB from a transmitter in another borehole. We apply the proposed signal method, and the measured results demonstrate the existence of the phase-sequence resonance in a physically real CAB. The spurious solutions found to occur in the DOA estimation at the resonant frequencies are also found experimentally. Index Terms--Circular array, cylindrical layers, dipole antenna, directional borehole radar, method of moments (MoM), mutual coupling, phase-sequence resonance.

An analytical solution is presented for the electromagnetic scattering from a dielectric circular cylinder embedded in a dielectric half-space with a slightly rough interface. The solution utilizes the spectral (plane-wave) representation of the fields and accounts for all the multiple interactions between the rough interface and the buried cylinder. First-order coefficients from the small perturbation method are used for computation of the scattered fields from the rough surface. The derivation includes both TM and TE polarizations and can be easily extended for other cylindrical buried objects (e.g., cylindrical shell, metallic cylinder). Several scattering scenarios are examined utilizing the new solution for a dielectric cylinder beneath a flat, sinusoidal, and arbitrary rough surface profile. Results indicate that the scattering pattern of a buried object below a slightly rough surface differs from the flat surface case only when the surface roughness spectrum contains a limited range of spatial frequencies. Furthermore, the illuminated area of the incident wave is seen to be a critical factor in the visibility of a buried object below a rough surface. Index Terms--Buried objects, ground penetrating radar, rough surfaces, scattering.

It is often advantageous to acquire pulse-echo gound-penetrating radar (GPR) measurements from antennas that are offset from the air-ground interface by a nonnegligible distance, either because the ground's surface is rough, or because measurements must be collected remotely. This paper describes the effects of incorrectly estimating the location of the air-ground interface on GPR images created with the plane-to-plane backpropagation (PPB) method. To compensate for a nonplanar air-ground interface, the wideband time-domain synthetic aperture method is reviewed and reformulated, using Shell's law of refraction, to account for measurements taken some distance above an undulating interface. A modification of the PPB algorithm that accounts for a nonplanar interface is also formulated, and examples of imaging with both methods are given to validate the algorithms' performance for surface variations that are relatively smooth with respect to the radar waveform wavelengths. Additionally, a method for inferring the surface topography from the radar returns is described. Finally, the modified PPB is demonstrated by imaging buried targets sensed in field-acquired data. Index Terms--Ground-penetrating radar, microwave tomography, synthetic aperture radar.

This paper presents a full-wave analysis of a ground penetrating radar (GPR) using the finite-difference time-domain (FDTD) method. The antenna treated here consists of a resistor-loaded bow-tie antenna, which is covered with a rectangular conducting cavity of which inner walls are coated partially or fully with ferrite absorber. Some techniques are introduced into the FDTD analysis to obtain the accurate results and to save the computer resources. The validity of the FDTD analysis is confirmed experimentally. Furthermore, the effects of the ferrite absorber on the GPR characteristics are theoretically investigated in detail. The FDTD results indicate that the remarkable improvement of the antenna characteristics for the GPR system cannot be attained by the ferrite absorber. Index Terms - Buried object detection, cavity-backed antennas, dipole antennas.

Battelle and The Ohio State University ElectroScience Laboratory have built and demonstrated a ground-penetrating radar (GPR) system for locating buried unexploded ordnance (UXO). The system is ground based and towed by an autonomously controlled vehicle as part of the Subsurface Ordnance Characterization System (SOCS). This paper presents the results of the synthetic aperture radar (SAR) processing of the radar data acquired at Tyndall Air Force Base, FL, and Jefferson Proving Ground (JPG), IN. A companion paper by Chen and Peters presents the results of the complex natural-resonance processing used to identify the size of the ordnance items. Index Terms - Buried object detection, synthetic aperture radar.

The method of moments is used to analyze short-pulse plane-wave scattering from perfectly conducting bodies of revolution buried in a lossy, dispersive half space. The analysis is performed in the frequency domain, with the time-domain fields synthesized via Fourier transform. To make this analysis efficient, the method of complex images is used to compute the frequency-dependent components of the half-space dyadic Green's function. Results are presented for short-pulse scattering from buried spheres and cylinders, using measured frequency-dependent soil parameters (permittivity and conductivity).

Due to the similar dielectric constants of buried nonmetallic targets and dry soils, it is often difficult to detect and identify nonmetallic targets with ground penetrating radar. The addition of properly chosen chemical agents to modify soil properties can potentially provide improved detection. Previous studies using waveguide experiments have shown that the addition of water improves dielectric contrasts but also increases loss so that target detectability is not necessarily improved. The addition of liquid nitrogen to wet soils can reduce background medium loss and restore target visibility, and waveguide studies of target detection through controlled depth of nitrogen penetration have shown that scattering can be significantly enhanced if an optimal amount of nitrogen is added. In this paper, a simple physical optics (PO) model for scattering from a three-dimensional target buried below a half space is presented, and it is shown that the radar cross section of the target depends on the dielectric contrast with and attenuation in the background medium. The model is validated through comparison with a Method of Moments code and found to yield accurate predictions for near normal incidence geometries. Analytical studies of target detection with two concepts of soil modification are then described: obtaining an 'optimal' homogeneous soil water content and the addition of a large quantity of water along with an optimal amount of liquid nitrogen. Finally, initial tests of these soil modification techniques with a dielectric rod antenna ground penetrating radar are performed and demonstrate that the addition of liquid nitrogen to excessively wet soils can reduce loss and enhance target visibility. Index Terms--Ground penetrating radar, landmine detection, radar cross section, subsurface sensing.