Effects of cold glacier ice crystal anisotropy on seismic data


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Anja.Diez [ at ] awi.de

Abstract

Knowledge about crystal anisotropy is mainly provided by crystal orientation fabric (COF) data from ice cores. To gain a broader understanding about the distribution of crystal anisotropy in ice sheets and glaciers seismic data from Antarctica and the Swiss Alps are analysed here. Two effects are important: (i) sudden changes in COF lead to englacial reflections and (ii) the anisotropic fabric induces an angle dependency on the seismic velocities and, thus, also recorded traveltimes. A framework is presented here to connect COF data with the elasticity tensor to determine seismic velocities and reflection coefficients for cone and girdle fabrics from ice-core data. These results are compared to vertical seismic profiling (VSP) measurements form Antarctica to validate the overall approach. The best agreement between measured velocities from the VSP survey and theoretically calculated velocities from COF eigenvalues is obtained using the elasticity tensor of Gammon et al. (1983). Reflection coefficients calculated for layers of different anisotropic ice fabrics and ice-bed interfaces show the weak influence of the anisotropic fabric on the reflection coefficient. Therefore, the focus is set on the analysis of the anisotropic ice fabric using the two-way traveltimes of englacial and bed reflections. Two approaches are applied: (i) the analysis of anisotropic normal moveout velocities (NMO) velocities from normal-spread seismic data (offset/depth-ratio � 1) in combination with other data sets determining the depth of reflectors and (ii) the analysis of the anisotropy parameter h determined from long-spread seismic data (offset/depth-ratio > 1). These anisotropic NMO velocities determined for the stacking process differ from the zero-offset velocities needed for the depth conversion. For the Antarctic and Alpine site, it is found, that this difference is up to 9% for the P-wave but only up to 2% for the SH-wave. This sensitivity of the P-wave velocity to the anisotropic ice fabric is used to derive information about the COF from NMO analysis. An improved understanding of COF-induced reflections is gained by the combination of seismic, radar and ice-core data. Use is made of the fact that the common reflection mechanism of seismic and radar data in cold glacier ice below the firn ice-transition is an abrupt change in the distribution of the anisotropic ice crystals. Thus, englacial reflectors in seismic and radar data can be identified as COF induced. Additionally, a new S-wave–density relationship is derived by analysing continuously refracted SH-waves of the firn from the Alpine field site. The results show the great potential that is within the combined interpretation of seismic and radar data to identify COF-induced reflections. It is shown, that the analysis of normal spread reflection seismic data in combination with radar data and of long-spread seismic data alone gives a tool to determine the anisotropic ice fabric of glaciers and ice sheets. This is an important contribution to constrain results from the upcoming generation of ice-flow models with anisotropic rheology by remotely sensed data.



Item Type
Thesis (PhD)
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Primary Division
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Not peer-reviewed
Publication Status
Published
Eprint ID
34858
Cite as
Diez, A. (2013): Effects of cold glacier ice crystal anisotropy on seismic data PhD thesis,


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