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Revealing the effects of subsurface structure on the antenna coupling of ground penetrating radar

University of British Columbia

The character (amplitude, phase, frequency, and polarization) of a reflection in a ground penetrating radar (GPR) profile contains a wealth of information about the reflector. However, most of the current research relating reflection strength to material properties, such as soil moisture and the existence of contaminants, involves only single—parallel—component data. This scalar view of a vector phenomenon leaves most of the information contained in the reflected wavelet untapped. Moreover, an amplitude anomaly in the parallel component may be largely, or at least in part, due to a polarization anomaly. This polarization contribution is ignored by the common scalar approach which attributes the anomaly entirely to such properties as water saturation or a suspected contaminant plume. Anomalous polarization degrades the receiver antenna coupling due to a lower polarization match between antenna and wavelet. Usually more obvious, are the coupling changes due to material/structural variations within the near-field which affect antenna properties. Before character analysis can be applied reliably, the effects of variable antenna coupling must be considered. A theoretical review of the effect of ground conditions on antenna radiation patterns, and also of wavelet depolarization, provides the GPR interpreter with the insight to recognize coupling effects in the data. When a survey traverses into material having significantly higher dielectric constant, the antenna centre frequency decreases, and the radiation pattern directivity increases due to a narrower beamwidth and possibly smaller side lobes. The possible effect in the data is lower energy from out of the plane scatterers and a decrease in the maximum dip that can be imaged. An increase in conductivity will decrease the radiated power and the accompanying dispersion will smear the radiation pattern nodes, resulting in a more omnidirectional radiation pattern for pulse antennas. Wavelet depolarization occurs, to some degree, for most cases of reflection and refraction. The severity of depolarization depends on the contrast in electrical properties, incident angle, incident polarization, and the orientation of the reflecting area. Generally, wavelet depolarization increases with an increase in reflector asymmetry, such as in scattering geometry, continuity, roughness, and anisotropy. To estimate the power loss in the parallel component due to anomalous polarization, an instantaneous polarization match estimate was developed and applied to field data from two test sites of different structural complexity. In this initial investigation, the TM survey mode was confirmed to suffer a greater degree of depolarization resulting in degraded coupling compared to the TE survey mode. Generally, degraded coupling was also observed at reflector rough spots (a 5 - 20% power loss) and at points of wavefront interference. Although the polarization coupling is probably a second order effect for most cases, at least one situation was documented in 1974 where depolarization was a first order effect causing the parallel component to be extinguished. Additional target types and environments should be investigated for their depolarizing characteristics.

Thesis/Dissertation

application/pdf

2009-01-21

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http://hdl.handle.net/2429/3825

Geophysics

University of British Columbia

UBCV

DSpace