Stanley J. Radzevicius

Near-field dipole radiation dynamics through FDTD modeling

We use finite-difference time-domain (FDTD) numerical simulations to study horizontal dipole radiation mechanisms and patterns near half-space interfaces. Time snapshots illustrating propagation of wavefronts at an instance in time are included with antenna patterns to provide a visualization tool for understanding antenna radiation properties. Near-field radiation patterns are compared with far-field asymptotic solutions and the effects of electrical properties, antenna height, and observation distance are investigated through numerical simulations. Numerical simulations show excellent agreement with measured data collected over a water-filled tank. Near-field H-plane radiation patterns are broader and contain radiation maxima beyond the critical angle predicted by far-field solutions. A large amplitude E-plane radiation lobe is located directly below the antenna in all simulations, while the two large amplitude sidelobes are less distinct and occur at larger incidence angles than predicted by far-field solutions. Radiation patterns resemble far-field solutions by a distance of 10 wavelengths, except near the critical angle where H-plane radiation maxima and E-plane sidelobes occur at larger incidence angles than predicted by far-field solutions.

Electromagnetic Induction and GPR Measurements for Creosote Contaminant Investigation

Multifrequency EM induction and GPR parallel dipole (co-pole) and orthogonal dipole (cross-pole) surveys were conducted to assist in the characterization of a former industrial site prior to it being remediated by the Ohio EPA and the U.S. EPA. The site has been a major concern to both agencies for the past decade due to high concentrations of creosote present in clay-rich surficial soils, resulting from many years of wood treating at the site. Information provided on the approximate extent of contamination at the site and the locations of several contaminant-filled structures determined through the use of quadrature phase EM data and cross-pole GPR data served as the basis for an efficient, comprehensive and cost-effective site remediation plan. Geophysical data interpretations were confirmed through exploratory trenching and soil sampling subsequent to the completion of this study. This study demonstrates the potential for mapping the extent and variation with depth of resistive compounds under circumst...

Computer programs for application of equations describing elastic and electromagnetic wave scattering from planar interfaces

MATLAB programs are presented which solve equations describing the scattering of plane elastic and electromagnetic waves from a planar interface separating homogenous, isotropic, and semi-infinite geologic media. The PSHSV program calculates and plots amplitude (reflection and refraction/transmission) coefficients, square root energy ratios, energy coefficients, and phase changes for elastic waves of P-, SH- or SV-type incident on an interface between elastic media. The EHEV program calculates and plots amplitude coefficients, square root energy ratios, energy coefficients, and phase changes for electromagnetic waves of EH- or EV-type incident on an interface between dielectric media. The applicability of the programs is demonstrated through the presentation of solutions (plotted as a function of incidence angle) obtained for geologic environments commonly encountered in seismic and ground penetrating radar applications.

Practical 3-D Migration and Visualization for Accurate Imaging of Complex Geometries with GPR

Obtaining high-quality three-dimensional images that accurately represent target geometries is crucial for reliable interpretation of ground penetrating radar data. Dense data acquisition, three-dimensional migration processing, and visualization are needed to recognize complex target geometries. A novel implementation of the diffraction summation migration algorithm efficiently images three-dimensional geometries by utilizing the efficient properties of fast Fourier transforms to compute diffraction summations. The algorithm decomposes the diffraction summation calculations into spatial and temporal summations. The spatial summations are computed by convolving the data with two-dimensional convolution kernels computed from the three-dimensional point spread functions. Computation times are reduced by performing convolutions in the wavenumber domain using two-dimensional fast Fourier transforms. The migration aperture is easily adjusted in the algorithm to increase computational speeds and reduce processing artifacts associated with large amplitude spikes. Microsoft Visual Studio .NET and C# are used to create efficient and low-cost applications optimally designed for the processing and display of ground penetrating radar data. Migration algorithms are combined with volume rendering and visualization algorithms that utilize DirectX to visually recognize complex geometries in three-dimensions. A gravel test bed containing buried spheres, letters cut from metal sheets, pipes, and drums is imaged with dense and orthogonal 400 and 1,500-MHz ground penetrating radar surveys. A linear array composed of fourteen variably spaced spheres is used to empirically determine the resolution capabilities of commercial ground penetrating radar systems in favorable soils. In addition, letters cut from aluminum sheets are used to test imaging capabilities similar to those used to test human vision. Observed quarter-wavelength spatial resolutions are consistent with theoretical predictions. While horizontal slices work well for displaying horizontal objects, volume rendering is more effective for visualizing dipping objects. Increasing antenna frequency improves spatial resolution and produces less pronounced polarization differences between data acquired with orthogonal surveys.

Least-squares curve fitting for velocity and time zero

Indirect least-squares algorithms developed for bistatic antenna configurations are derived to simultaneously invert for propagation velocity, time zero, depth, and lateral offset for buried objects approximating point diffractors. An alternative direct algorithm is derived for antenna geometries approximating the monostatic antenna configuration. Least-squares fitting for velocity, depth, and lateral offset using multiple diffraction curves is combined with a time zero grid search algorithm to provide a robust solution when inverting for time zero.

Non-geologic events in single- and cross-hole radar data

Summary Accurate analysis of borehole radar data depends upon the proper identification of events and the precise measurement of amplitudes and travel times. This paper discusses and provides data examples of non-geologic events that can influence borehole radar measurements and complicate data interpretation. Data demonstrate that refracted air events can arrive prior to direct arrivals in cross-hole surveys, and that conductive cable-related effects can introduce artifacts and multiple events into records. Additionally, nontraditional variable offset soundings (VOS) are shown to be useful for studying propagation characteristics, recognizing possible cable-related effects, and providing insight in regards to coupling mechanisms between antennas and cables.

Development of a Portable Internal 3D Rail Flaw Imaging System

Internal rail flaws continue to present a significant issue for the railroad industry. Current practices to mitigate the risk of catastrophic failure of the rails involve inspection for internal flaws and cracks using ultrasonic rail flaw detector technology, usually employed on rail-based vehicles. Following the identification of a suspected defect by an inspection vehicle, a detailed inspection of the suspect area is usually conducted to confirm the presence of the defect prior to the remedial action. Current portable or ultrasonic devices require significant expertise to allow test personnel to make a good judgment regarding the presence and severity of the flaw. A new generation of handheld detector is being developed to provide accurate three-dimensional images of the suspected flaw, thereby eliminating uncertainties in the inspection and providing operators with accurate sizing and orientation information. This paper presents a prototype ultrasonic handheld rail flaw inspection device designed and developed through support provided by the Federal Railroad Administration’s Office of Research and Development by ENSCO, Inc., which can produce accurate three-dimensional images of suspected flaws for verification and characterization proposes.© 2012 ASME

Processing ground penetrating radar data from curvilinear interfaces

Ground penetrating radar investigations of buildings and other man-made structures often require data collection along curvilinear interfaces having various radii of curvature and geometry. Data collected along non-planar interfaces and containing multiple targets are often difficult to interpret without the application of migration. Migration is not commonly applied to surveys along non-planar interfaces because most published algorithms and available software assume data collection along planar interfaces. We provide a versatile imaging strategy that migrates data by using a kernel derived from the forward model. Modeled and measured data from columns are used to illustrate the utility of our imaging approach.