Thin-layer effects in glaciological seismic amplitude-versus-angle (AVA) analysis: implications for characterising a subglacial till unit, Russell Glacier, West Greenland

Abstract

Abstract. Seismic amplitude-versus-angle (AVA) methods are a powerful means of interpreting the physical properties of subglacial material, although interpreting an AVA response is complicated in the case of a thinly-layered substrate. A layer thinner than one-quarter of the seismic wavelength is considered seismically "thin", and reflections from its bounding interfaces are perceived as a single event. Since a lodged (non-deforming) subglacial till can capped by a thin (metre-scale) cap of dilatant (deforming) till, serious misinterpretations can result if thin layer considerations are not honoured. AVA responses for layered subglacial tills are simulated: we model dilatant layers of thickness 0.1–3.0 m (up to a quarter-wavelength of our synthetic seismic pulse) overlying a lodged half-space, assigning typical acoustic impedance and Poisson's ratios to each. If thin layer effects are neglected, the AVA response to ultra-thin (<1.0 m) dilatant layers yields incompatible physical properties (acoustic impedance and Poisson's ratio indicating, respectively, a low- and high-porosity unit). We show an interpretative strategy that identifies thin layer effects and accurately quantifies the modelled acoustic impedance of lodged till from the composite AVA response. We apply this method to example seismic AVA data from the Russell Glacier outlet of the West Greenland Ice Sheet, in which characteristics of thin layer responses are evident. We interpret a stratified subglacial deposit, with upper and lower layers of high-porosity (<1.0 m thick, Poisson's ratio >0.492 ± 0.015) and low-porosity (acoustic impedance of 4.20–4.39 × 106 kg m−2 s−1) material, respectively assumed to represent dilatant and lodged tills. Thin layer considerations are strongly advised wherever seismic AVA analyses are used to quantify subglacial material properties.