GROUND-PENETRATING RADAR PROFILES OF TILL, BEDROCK, AND THEIR TRANSITION IN NEW HAMPSHIRE

We discuss GPR reflection profiles of glacial till, colluvial diamict, bedrock, and the till-bedrock transition at several locations in New Hampshire. We mainly used pulses centered from 150−200 MHz. The boulder-rich diamicts reside over granites and metamorphosed volcanics, the horizons of which are recognized by the relative strength and phase of their waveforms, underlying fractures, and well-developed diffraction asymptotes. The till produced an apparent dense distribution of diffractions with limited asymptotes, limited dispersion, and minor stratification. We exploit these diffractions, and use moveout profiles to calculate relative dielectric permittivities from 17 to 27, which suggest likely saturation within these over-consolidated sediments. The transitions from till to bedrock range from a simple horizon to complex segmented horizons, all characterized by diffractions and amenable to single-layer migration. Maximum profiled till depths ranged from 4 to at least 10 m with estimated attenuation rates of about 3.3 dB m^–1 caused by scattering. In contrast, one and possibly two colluvial diamicts, which contained boulders likely up to 3 m in dimension, show short segments of stratification, rare diffraction asymptotes, more than 20 m penetration and scattering losses of about 0.5 dB m^–1. We measured extremely low conductivity and calculated permittivities ranging from 9–12, which suggest high densities and volumetric water contents of 4–12 %. Scattering appears to be a significant cause of signal loss in the till but not in the colluvial diamict, for which single scattering and deep penetration are consistent with surface ground waves traveling up to 40 m. The phase polarity of waveforms within till and colluvial events show they may originate from either high or low dielectric contrasts, likely related to water or large boulders, respectively.