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| Titel |
Thin-layer effects in glaciological seismic amplitude-versus-angle (AVA) analysis: implications for characterising a subglacial till unit, Russell Glacier, West Greenland |
| VerfasserIn |
A. D. Booth, R. A. Clark, B. Kulessa, T. Murray, J. Carter, S. Doyle, A. Hubbard |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1994-0416
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| Digitales Dokument |
URL |
| Erschienen |
In: The Cryosphere ; 6, no. 4 ; Nr. 6, no. 4 (2012-08-16), S.909-922 |
| Datensatznummer |
250003701
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| Publikation (Nr.) |
 copernicus.org/tc-6-909-2012.pdf |
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| Zusammenfassung |
| 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 × 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 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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