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 quantifying the physical properties of subglacial material, but serious interpretative errors can arise when AVA is measured over a thinly-layered substrate. A substrate layer with a thickness less than 1/4 of the seismic wavelength, λ, is considered "thin", and reflections from its bounding interfaces superpose and appear in seismic data as a single reflection event. AVA interpretation of subglacial till can be vulnerable to such thin-layer effects, since a lodged (non-deforming) till can be overlain by a thin (metre-scale) cap of dilatant (deforming) till. We assess the potential for misinterpretation by simulating seismic data for a stratified subglacial till 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, conventional AVA analysis yields acoustic impedance and Poisson's ratio that indicate contradictory water saturation. A thin-layer interpretation strategy is proposed, that accurately characterises the model properties of the till unit. The method is applied to example seismic AVA data from Russell Glacier, West Greenland, in which characteristics of thin-layer responses are evident. A subglacial till deposit is interpreted, having lodged till (acoustic impedance = 4.26 ± 0.59 × 10 6 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 a greater degree of complexity in an AVA interpretation, and potentially avoid misinterpretations, they are a valuable aspect of quantitative seismic analysis, particularly for characterising till units.

Sensitive response of the Greenland Ice Sheet to surface melt drainage over a soft bed

“…The physical properties in the subglacial sediment model integrates recent geophysical observations, revealing where available that RG is underlain by a porous, mechanically weak sediment 34,35 , of similar character to tills produced by glaciers in Canada and Svalbard 37,45 (see Methods). The subglacial sediment model was coupled to a hydrological model, which has previously been used to calculate the routing and fluxes of water associated with the episodic drainage of subglacial lakes in Antarctica 46 , and is well suited for analysis of SGL drainage events.…”

“…In addition, by focusing explicitly on the character of the hydrological system, previous work has inherently assumed that the ice-bed interface consists of hard bedrock. However, thick subglacial sediments have been observed 34,35 and furthermore are known to exert firstorder control on flow in other glaciated regions [36][37][38][39][40][41] . To date, theoretical considerations on the implications of a soft sedimentary bed on GrIS dynamics are only starting to emerge 42 , but have never been implemented and tested in modelling studies.…”

Antarctic subglacial groundwater: a concept paper on its measurement and potential influence on ice flow

“…A growing number of glaciological applications have been using amplitude v. offset (AVO) data as powerful diagnostics of acoustic impedance -the product of seismic velocity and density -and Poisson's ratio -a measure of material stiffness calculated from compressional and shear wave velocities -as proxies for subglacial till deformation (Nolan & Echelmeyer 1999;Anandakrishnan 2003;Peters et al 2007Peters et al , 2008Booth et al 2012;Dow et al 2013;Christianson et al 2014;Kulessa et al In review). For example, lower acoustic impedances and higher Poisson's ratios diagnose weaker higher-porosity tills that dilate to Fig.…”

Approximations to seismic AVA responses: Validity and potential in glaciological applications

“…Thus, it provides insight into the material contrast (e.g., in seismic velocity, acoustic impedance, Poisson's ratio) either side of the interface (Aki and Richards, 1980). As such, AVA has been usefully applied to determine the hydrological condition of subglacial sediment beneath ice masses (Nolan and Echelmeyer, 1999), including the Greenland and Antarctic ice sheets (e.g., Anandakrishnan, 2003;Peters et al, 2007;Booth et al, 2012;Dow et al, 2013), to identify Antarctic subglacial lakes (Peters et al, 2008), and to interpret ice crystal orientation fabric (COF) from anisotropic seismic reflectivity (Horgan et al, 2011). The term amplitude variation with offset (AVO) is often used instead of AVA, but the difference is simply that AVO characterizes reflectivity in terms of the offset between seismic source and geophone, rather than the incident angle (with an accurate velocity: depth function, the offset and angle are interchangeable).…”

“…Glaciological drilling provides only a point sample of the glacier bed and/or a 1D distribution of englacial quantities (Truffer and Harrison, 2006;Lüthi et al, 2002;Gusmeroli et al, 2012;Andrews et al, 2014) and is logistically challenging (Siegert et al, 2014); hence, seismic campaigns seldom benefit from borehole observations. Approaches have therefore been developed to calibrate the reflectivity from aspects of the seismic data set alone, including estimating the initial amplitude of the seismic source wavelet via energy-loss relationships (Peters et al, 2008;Horgan et al, 2011) and constraining reflectivity by comparing the amplitude of primary and multiple arrivals (Smith, 1997;King et al, 2003;Booth et al, 2012). However, such approaches can be problematic when applied to data with a low signal-to-noise ratio (S/N) because this can occur when geophones and seismic sources are located on (or close to) a bare ice surface (Dow et al, 2013).…”