1887

Abstract

Most of the current research in ground penetrating radar (GPR) relating reflection strength to material properties,<br>such as soil moisture and the existence of contaminants, involves only single component data, i.e., the component<br>with transmitter and receiver antennas parallel. This scalar view of a vector phenomenon leaves most of the<br>information contained in the reflected wavelet untapped. Moreover, an amplitude anomaly in the parallel component<br>may be largely, or at least in part, due to a polarization effect. Anomalous polarization of the returned wavelet<br>degrades the coupling with the receiver antenna since the match between wavelet and antenna polarizations is<br>decreased. This polarization contribution is ignored by the common scalar approach which ascribes the amplitude<br>anomaly entirely to such properties as water saturation or a suspected contaminant plume. Wavelet depolarization<br>occurs, to some degree, for most cases of reflection and refraction. Indeed, the depolarizing character of a scatterer<br>can aid in its identification. Although complete depolarization, causing reflections to extinguish, was confirmed 20<br>years ago in sea ice, little attention has been given to possible depolarization effects in soil or rock.<br>We investigate the power loss in the parallel component due to anomalous polarization by using a new instantaneous<br>attribute: polarization match. The instantaneous polarization match estimates how severely depolarization<br>is affecting amplitudes while also identifying the responsible depolarizing structure. In this initial investigation<br>the instantaneous polarization match estimate is applied to data from sites of two different structural complexities.<br>Field experiments at an abandoned overpass ramp investigated depolarization for the simple cases of smooth<br>shallow dipping interfaces and a lateral change in material. Both profiles and common midpoint (CMP) soundings<br>demonstrated that the transverse magnetic (TM) wavelet depolarizes more than the transverse electric (TE) wavelet.<br>This difference is, at least in part, explained by the occurrence of the Brewster angle in the TM mode. At the<br>sites investigated, the most significant mechanisms for depolarization appear to be scattering from rough spots (a<br>S-20% power loss) and at points of wavefront interference. These surveys provide only an initial investigation, and<br>other traverse orientations, environments, and target types should be investigated, some of which are likely better<br>depolarizers than the targets in this study.<br>Although degraded polarization match is usually a secondary amplitude effect, in some cases this problem<br>could become significant. When pursuing targets of anomalous amplitude, or any other attribute, at least one<br>two-component profile should be acquired to characterize the depolarization nature of the field site.

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/content/papers/10.3997/2214-4609-pdb.205.1996_123
1996-04-28
2021-12-06
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http://instance.metastore.ingenta.com/content/papers/10.3997/2214-4609-pdb.205.1996_123
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