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

摘要

Seismic amplitude-versus-angle (AVA) methods are a powerful means ofquantifying the physical properties of subglacial material, but seriousinterpretative errors can arise when AVA is measured over a thinly-layeredsubstrate. A substrate layer with a thickness less than1/4 of the seismic wavelength, λ, is considered "thin", and reflections from its bounding interfacessuperpose and appear in seismic data as a single reflection event. AVAinterpretation of subglacial till can be vulnerable to such thin-layereffects, since a lodged (non-deforming) till can be overlain by a thin(metre-scale) cap of dilatant (deforming) till. We assess the potential formisinterpretation by simulating seismic data for a stratified subglacialtill unit, with an upper dilatant layer between 0.1–5.0 m thick (λ / 120 to> λ / 4, with λ = 12 m). For dilatant layers less than λ / 6 thick, conventionalAVA analysis yields acoustic impedance and Poisson's ratio that indicatecontradictory water saturation. A thin-layer interpretation strategy isproposed, that accurately characterises the model properties of the tillunit. The method is applied to example seismic AVA data from RussellGlacier, West Greenland, in which characteristics of thin-layer responsesare evident. A subglacial till deposit is interpreted, having lodged till(acoustic impedance = 4.26±0.59 × 106 kg m?2 s−1)underlying a water-saturated dilatant till layer (thickness < 2 m,Poisson's ratio ~ 0.5). Since thin-layer considerations offer agreater degree of complexity in an AVA interpretation, and potentially avoidmisinterpretations, they are a valuable aspect of quantitative seismicanalysis, particularly for characterising till units.