Abstract
We demonstrate that ground penetrating radar (GPR) reflection data from a temperate glacier are accurately modeled using a Helmholtz-Kirchhoff diffraction integration technique that incorporates the radiation characteristics of point dipoles on a half-space interface. This is accomplished by comparing field data to simulated data. Our 40-MHz field data are from a 100 × 340 m (x- and y-dimensions, respectively) survey grid containing 51 parallel survey lines. The field data were collected with the dipole oriented perpendicular to the survey line (x-dipole). The synthetic data used isotropic, x-dipole, and y-dipole antennas, and reflections were calculated using a bed topography previously defined by 3D Kirchhoff migration. The comparisons between the real and synthetic waveforms show excellent agreement, including reflection arrival times, amplitude trends, interference patterns, and false layering from out-of-plane reflections. The location of reflectors determined from exploding reflector rays explains that bed reflections rapidly sink below background noise levels when reflections originate in the antenna's E-plane. This occurs in both the simulated data and field data. Our 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 more than 12-dB reduction in bottom reflection strength. Furthermore, a complicated bottom topography readily generates secondary, out-of-plane reflections that are easily confused with basal till layers.
