The anatomy of AVO crossplots

Simm, Rob; White, Robert S.; Uden, Richard

doi:10.1190/1.1438557

Cited by 42 publications

(15 citation statements)

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“…accommodate basal slip, while the opposite applies to stiff non-deforming tills. Glaciological AVO analysis is analogous to similar applications in the hydrocarbon sector that routinely uses AVO attributes as indicators of changes in reservoir lithology, and especially of porosity, fluid type and saturation (Sheriff & Geldart 1995;Simm et al 2000;Booth et al 2016).…”

Section: Hydrological and Mechanical Conditions At The Ice-sheet Bedmentioning

confidence: 99%

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

Siegert

Kulessa

Bougamont

et al. 2017

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Is groundwater abundant in Antarctica and does it modulate ice flow? Answering this question matters because ice streams flow by gliding over a wet substrate of till. Water fed to icestream beds thus influences ice-sheet dynamics and, potentially, sea-level rise. It is recognized that both till and the sedimentary basins from which it originates are porous and could host a reservoir of mobile groundwater that interacts with the subglacial interfacial system. According to recent numerical modelling, up to half of all water available for basal lubrication, and time lags between hydrological forcing and ice-sheet response as long as millennia, may have been overlooked in models of ice flow. Here, we review evidence in support of Antarctic groundwater and propose how it can be measured to ascertain the extent to which it modulates ice flow. We present new seismoelectric soundings of subglacial till, and magnetotelluric and transient electromagnetic forward models of subglacial groundwater reservoirs. We demonstrate that multifaceted and integrated geophysical datasets can detect, delineate and quantify the groundwater contents of subglacial sedimentary basins and, potentially, monitor groundwater exchange rates between subglacial till layers. The paper thus describes a new area of glaciological investigation and how it should progress in future.Gold Open Access: This article is published under the terms of the CC-BY 3.0 license. Water beneath the ice sheetAntarctic ice-sheet flow is fundamentally affected by water at the bed, as it reduces basal friction to encourage sliding and weakens till to enable bed deformation. Subglacial hydrology -the flow of water beneath the ice -is therefore a key element of the ice-sheet system. Studies to date on subglacial hydrology, and its impact on ice flow, have concentrated on water at or very near to the bed of the ice sheet.Basal water modulation of ice flow can be achieved in a number of ways. Over an impermeable bed, water can flow through channels cut either downwards into the substrate or upwards into the ice. Enhanced basal water pressures may occur where the channels and their linkages are distributed, increasing overriding ice flow through a reduction in the substrate's effective pressure. Conversely, where a well-organized channel system is formed, water pressures are lower and the hydrological effect on ice flow is reduced. If the ice stream rests on permeable subglacial till, its strength can affect ice flow as controlled by porewater pressures. High pressures lead to a reduction in material strength by pushing till grains apart, reducing bed friction and thus

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Section: Hydrological and Mechanical Conditions At The Ice-sheet Bedmentioning

confidence: 99%

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

Siegert

Kulessa

Bougamont

et al. 2017

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“…The characteristics of numerous AVA responses can be summarised on an "AVA cross-plot" (Simm et al, 2000) ( Fig. 1c for previous model curves).…”

Section: Reflection Coefficients and Amplitude-versus-angle Responsesmentioning

confidence: 99%

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

et al. 2012

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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.

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“…Superscript S denotes terms specific to the Shuey approximation. The Shuey coefficients are the usual foundation of AVA crossplots (Simm et al, 2000) and classification schemes (Castagna and Swan, 1997) applied in industries. Although small contrasts between density and P-wave velocity are still required, no assumption is made for the Poisson's ratio contrast (Shuey, 1985).…”

Section: Shuey Approximationmentioning

confidence: 99%

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

Booth

Emir²,

Diez

2016

GEOPHYSICS

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Amplitude-variation-with-angle (AVA) methods establish the seismic properties of material either side of a reflective interface, and their use is growing in glaciology. The AVA response of an interface is defined by the complex Knott-Zoeppritz (K-Z) equations, numerous approximations to which we typically assume weak interface contrasts and isotropic propagation, inconsistent with the strong contrasts at glacier beds and the vertically transverse isotropic (VTI) fabrics were associated with englacial reflectivity. We considered the validity of a suite of approximate K-Z equations for the exact P-wave reflectivity R P of ice overlying bedrock, sediment and water, and englacial interfaces between isotropic and VTI ice. We found that the approximations of Aki-Richards, Shuey, and Fatti match exact glacier bed reflectivity to within R P AE 0.05, smaller than the uncertainty in typical glaciological AVA analyses, but only for maximum incident angle θ i limited to 30°. A stricter limit of θ i ≤ 20°offered comparable accuracy to a hydrocarbon benchmark case of shale overlying gas-charged sand. The VTI-compliant Rüger approximation accurately described englacial reflectivity, to within R P AE 0.01, and it can be modified to give a quadratic expression in sin 2 ðθ i Þ suitable for curve-matching operations. Having shown the circumstances under which AVA approximations were valid for glaciological applications, we have suggested that their interpretative advantages can be exploited in the future AVA interpretations.

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The anatomy of AVO crossplots

Cited by 42 publications

References 0 publications

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

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

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

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

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