Thin‐bed AVO effects

Bakke, N.E.; Ursin, Bjørn

doi:10.1046/j.1365-2478.1998.00101.x

Cited by 52 publications

(10 citation statements)

References 14 publications

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“…The Knott-Zoeppritz equations are strictly defined for the interface between two otherwise infinite, homogeneous, isotropic, half-spaces, and the presence of thin-layering between these half-spaces violates this condition (Bakke and Ursin, 1998;Aki and Richards, 2002;Nolan and Echelmeyer, 1999). In seismic terms, a layer is "thin" if it is thinner than one-quarter of the dominant wavelength, λ, of the seismic wavelet (Widess, 1973).…”

Section: Thin-layers In Glaciological Ava Analysismentioning

confidence: 99%

“…At this threshold, termed the "tuning thickness" (Widess, 1973), reflections from the layer's bounding interfaces superpose and a single interface is perceived. For thinner layers, with thickness around λ / 8, the composite response approximates the derivative of the original signal and further thinning also reduces its amplitude (Bakke and Ursin, 1998). For such ultra-thin layers, the composite response may also be influenced by intrabed multiples and mode conversions (i.e.…”

Section: Thin-layers In Glaciological Ava Analysismentioning

confidence: 99%

“…The presence of thinlayering at that interface is a significant complicating factor in AVA interpretation (Swan, 1991;Bakke and Ursin, 1998;Nolan and Echelmeyer, 1999), as this condition can often be violated when surveying over a layered till. A till deposit can be structurally complex, with abrupt variations (both vertical and lateral) in physical properties (Evans et al, 2006) that, critically, are on a smaller spatial scale than the seismic wavelength (∼ 10 m in glaciology, depending on source characteristics and ice thickness; Smith, 2007).…”

Section: Introductionmentioning

confidence: 99%

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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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Section: Thin-layers In Glaciological Ava Analysismentioning

confidence: 99%

Section: Thin-layers In Glaciological Ava Analysismentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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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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“…There is a large number of works for a layer described by a single‐phase (solid) case. For example, Widess () and Bakke and Ursin () considered the normal incidence case for a thin layer and Juhlin and Young () studied amplitude‐versus‐offset effects of a thin layer, whereas the effect of the thickness of a sedimentary layer was investigated by Chung and Lawton (, ). Carcione () computed the scattering response of a lossy layer having orthorhombic symmetry and embedded between two isotropic half‐spaces, and Liu and Schmitt () obtained the P‐wave reflection coefficient in isotropic lossless thin layer as a function of the incidence angle.…”

Section: Introductionmentioning

confidence: 99%

Validation of the boundary conditions to model the seismic response of fractures

Corredor

Santos

Gauzellino

et al. 2016

Geophysical Prospecting

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A B S T R A C TFractures in fluid-saturated poroelastic media can be modeled as extremely thin, highly permeable, and compliant layers or by means of suitable boundary conditions that approximate the behavior of such thin layers. Since fracture apertures can be very small, the numerical simulations would require the use of extremely fine computational meshes and the use of boundary conditions would be required.In this work, we study the validity of using boundary conditions to describe the seismic response of fractures. For this purpose, we compare the corresponding scattering coefficients to those obtained from a thin-layer representation. The boundary conditions are defined in terms of fracture apertures that, in the most general case, impose discontinuity of displacements, fluid pressures, and stresses across a fracture. Furthermore, discontinuities of either fluid pressures, stresses, or both can be removed, or displacement jumps proportional to the stresses and/or pressures can be expressed via shear and normal dry compliances in order to simplify.In the examples, we vary the permeability, thickness, and porosity of the fracture and the type of fluid saturating the background medium and fractures. We observe good agreement of the scattering coefficients in the seismic range obtained with the two different approaches.

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“…Seismic AVA analysis has several key assumptions, including the requirement that a reflective interface separates two otherwise infinite half-spaces. The presence of thinlayering at that interface is a significant complicating factor in AVA interpretation (Swan, 1991;Bakke and Ursin, 1998;Nolan and Echelmeyer, 1999), as this condition can often be violated when surveying over a layered till. A till deposit can be structurally complex, with abrupt variations (both vertical and lateral) in physical properties (Evans et al, 2006) that, critically, are on a smaller spatial scale than the seismic wavelength (∼ 10 m in glaciology, depending on source characteristics and ice thickness; Smith, 2007).…”

Section: Introductionmentioning

confidence: 99%

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

Booth¹,

Clark²,

Kulessa³

et al. 2012

Preprint

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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

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Thin‐bed AVO effects

Cited by 52 publications

References 14 publications

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

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

Validation of the boundary conditions to model the seismic response of fractures

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

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