Waveform Attributes of Ground-Penetrating Radar Profiles of Glaciofluvial Sedimentation

We present several ground-penetrating radar (GPR) reflection and moveout profiles of a section of glaciofluvial sedimentation recorded in a gravel pit along the Connecticut River, Norwich, VT. Sediments in the pit are generally composed of silt to gravel size grains and suggest a high-energy depositional environment. Large channels are apparent in section on the pit walls. It is probable that these sediments were deposited during the last deglaciation at the mouth of the paleo-Ompompanoosuc River as it entered glacial Lake Hitchcock. The GPR profiles were recorded at 3 different pulse center frequencies with standard, commercially available short-pulse type radar. Our objective was to find the effects of the sedimentary dielectric properties on the waveform attributes of phase, pulse shape, spectral content, and horizon characteristics. We used antennas nominally rated by the manufacturer as having pulse center frequencies of 100, 400, and 900 MHz, the latter of particular interest because it is higher than previously reported for sedimentary studies. In practice, the pulse spectra were centered near 72, 300 and 600 MHz. The 72-MHz horizons are mainly resonances, while many of the 300- and 600-MHz horizons reproduce the transmitted wavelet, as expected. Given a near-surface (top 4 m) dielectric constant of 6.7, as measured by a 600-MHz moveout profile, the maximum resolution (separation of two interfaces) of the 600-MHz wavelet was about 15 cm. Localized, low frequency wavelet reflections centered at 400 MHz, for which we do not have an immediate explanation, were apparent in the 600-MHz moveout and reflection profiles. The 600-MHz reflection profile shows many solitary wavelet reflections whose phases indicate either reflections from interfaces between lower permittivity material beneath higher, or vice versa, while modeling indicates that other wavelets are responses to thin layers. More subtle yet distinct horizons at 300 MHz appear to be lost within scattering noise at 600 MHz. Theoretical study of the waveform, the pulse center frequency shifts, and the scattering noise should further aid interpretation of the geologic structure.