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Moran, M.L., 2001

Imaging and modeling of ground penetrating radar array data with application to the Gulkana Glacier, AK

Bibliographic Reference

Moran, M.L., 2001, Imaging and modeling of ground penetrating radar array data with application to the Gulkana Glacier, AK: University Park, Pennsylvania, Pennsylvania State University, Ph.D. dissertation, 138 p., illust.

Abstract

As currently practiced, geophysical methods have significant deficiencies when applied to temperate glacier investigations. We present ground penetrating radar (GPR) array processing and analysis methods that overcome many of these limitations by increasing signal-to-noise ratio, improving the depth of signal penetration, and correctly positioning reflections in three dimensions. This is accomplished by modifying state-of-the art multidimensional seismic migration techniques to consider the radiation characteristics of GPR antennas. The dissertation is a compilation of four stand-alone papers. In general, the investigation approach compares modeled data to field data collected on the Gulkana Glacier, AK. In chapter 2, a modified 3-D Kirchhoff migration method is given. This algorithm is applied to synthetic GPR data from point diffractions. This largely theoretical study demonstrates 3-D target localization, and significant noise suppression. In chapter 3, I present migration results for 50MHz GPR data collected on the Gulkana Glacier, AK. I show that the migration array has flashlight-like beam patterns (5° at the -6-dB level). This characteristic allows me to define a complex, steeply sloping bed topography. In places, the bed dip exceeds 45°. The 3-D migrations improved depth estimates of the bottom topography by 41% compared to raw data depth profiles and 16% compared to 2-D migrations. They migrations also had >12 dB better noise suppression over the standard 2-D method. Furthermore, loss of bottom reflections for depths greater than 140 m is shown to be due to the dip and curvature of the reflector surface, and not scattering losses. In chapter 4, the impact of dipole orientations on data quality is demonstrated. Using the topographic surface of the Gulkana bed, I compare synthetic reflections for x -dipole, y -dipole, and isotropic antennas having the same antenna locations as those used in the field data. The amplitude and arrival-time profiles from the modeled x -dipole data show excellent agreement with the field data (which also has an x -dipole orientation). This gives compelling evidence that the bed reflection surface is correct and that the modeling approach accurately represents GPR dipole radiation. Apparent till layer reflections, seen in both the field data and modeled data, are shown to be out-of-plane arrivals. Lastly, the modeled results are of general importance for radio-glaciology because they demonstrate that inappropriate dipole orientation, with respect to the specular reflection point, can lead to large reductions in bottom reflection amplitudes. In Chapter 5 I establish the effects of radiation patterns on migration images by comparing migrated Gulkana field data to the synthetic data discussed in Chapter 4. I also give GPR migration radiation patterns by imaging simple planar reflection surfaces. I find that the individual dual-dipole radiation patterns dominate the array's total radiation pattern and that the array geometry only has a minor influence. The results confirm that under appropriate antenna orientations (relative to the reflector position) GPR migration can image surfaces as steep as 45°. Conversely, for some antenna geometries, even shallow dips (<20°) are difficult to image. The results are explained by giving image amplitude expressions that include the effects of a finite array aperture, a dipping reflector interface, and the array radiation pattern. This study indicates that careful manipulation of GPR radiation characteristics in acquisition of field data, and subsequent migration processing, will lead to significant improvements in temperate glacier characterization as well as in other difficult GPR characterization applications.

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