GPR Profiles of Glacial Till and its Transition to Bedrock: Interpretation of Water Content, Depth and Signal Loss from Diffractions

Abstract

We discuss GPR reflection profiles that we recorded on glacial till and a colluvial diamict at several locations in New Hampshire, and from which we interpret water contents, depths and rates of signal loss. We used pulses centered from 150–200 MHz and 300–360 MHz. The boulder-rich sediments reside over granitic and metavolcanics, the horizons of which we recognize from the relative strengths and phase of their waveforms, underlying fractures, and well-developed diffraction asymptotes. The till produced an apparent dense distribution of diffractions with limited asymptotes and dispersion, and occasional minor stratification. We use these diffractions and moveout profiles to calculate relative dielectric permittivities between 17 and 27, values which suggest up to 30% volumetric water, and likely saturation within these over-consolidated sediments. The evidence for transitions from till to bedrock ranges from a simple horizon to complex horizon segments, all characterized by diffractions and amenable to single-layer migration. A gradational loss in diffraction strength with depth suggests gradational weathering or changes in grain size as the cause. Maximum profiled depths range from 4 m to at least 10 m, with estimated scattering attenuation rates of about 3.3 dB m−1. In contrast, one and possible two colluvial diamicts, which likely contained 3-m-size boulders, show short segments of stratification, rare diffraction asymptotes, allow more than 20-m penetration and provide 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 content of 4–12%. Low, single scattering loss and deep penetration in the till are consistent with evidence of ground waves traveling up to 40 m one way. 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.