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.
KEYWORDS: Antennas, General packet radio service, Finite-difference time-domain method, Interfaces, Data modeling, Dielectrics, Solids, Signal attenuation, Geometrical optics, Wavefronts
GPR dipole antenna patterns can be described by the interference of space and lateral waves. Because this is an interference phenomenon, antenna patterns are a function of frequency, distance, and electrical properties. Traditional far-field criteria based on dipoles in a whole-space are insufficient to describe dipole antennas on a half-space boundary. Whole-space criteria fail because they do not take into account the interference of space and lateral waves. The travel time difference between space and lateral waves increases as the angle of observation from vertical increases, or with increasing distance from the source. The result is increased interference and more abundant lobes with increasing distance and observation angle. Since GPR investigations are limited by attenuation and many environmental and engineering targets of interest are located within a few wavelengths of the antenna, asymptotic solutions do not accurately describe antenna patterns for most GPR applications. The exclusion of lateral waves in geometric optics solutions is another source of error for many GPR applications. Data were measured over a water filled tank to verify FDTD antenna pattern models. Asymptotic solutions predict H-plane peaks at an angular distance equal to the critical angle. Measured and modeled antenna patterns are broader and have peaks located at a larger angular distance, than predicted from asymptotic solutions. The peaks approach and decrease the rate of convergence toward the asymptotic solution with increasing distance from the source, and data modeled over water demonstrate that the peaks still do not converge to the asymptotic solution at a distance of 24 wavelengths. The low directivity of dipole antennas explains why out of the plane reflections are commonly observed in GPR data.
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