Abstract
Abstract. A preferred orientation of the anisotropic ice crystals influences the viscosity of the ice bulk and the dynamic behaviour of glaciers and ice sheets. Knowledge about the distribution of crystal anisotropy is mainly provided by crystal orientation fabric (COF) data from ice cores. However, the developed anisotropic fabric influences not only the flow behaviour of ice but also the propagation of seismic waves. 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, recorded travel times. A framework is presented here to connect COF data from ice cores with the elasticity tensor to determine seismic velocities and reflection coefficients for cone and girdle fabrics. We connect the microscopic anisotropy of the crystals with the macroscopic anisotropy of the ice mass, observable with seismic methods. Elasticity tensors for different fabrics are calculated and used to investigate the influence of the anisotropic ice fabric on seismic velocities and reflection coefficients, englacially as well as for the ice–bed contact. Hence, it is possible to remotely determine the bulk ice anisotropy.
Highlights
Understanding the dynamic properties of glaciers and ice sheets is one important step to determine past and future behaviour of ice masses
In this paper we extend the analysis of seismic velocities beyond cone fabrics and derive the elasticity tensor, which is necessary to describe the seismic wave field in anisotropic media
Of these we will use the three most common fabrics observed in glacier ice in the following analysis of the influence of ice crystal anisotropy on seismic wave propagation: (i) the cluster distribution, (ii) the thick girdle distribution and (iii) the partial girdle distribution
Summary
Understanding the dynamic properties of glaciers and ice sheets is one important step to determine past and future behaviour of ice masses. To include anisotropy in ice-flow modelling, we need to understand the development and the distribution of the anisotropic fabric; i.e. we have to observe the variation in the COF distribution over depth, as well as the lateral extent. To determine the anisotropic seismic velocities for different cone fabrics, he calculated an average from the single crystal velocity for the encountered directions This approach was used later by Gusmeroli et al (2012) for analysing the crystal anisotropy from borehole sonic logging at Dome C, Antarctica. If we want to be able to investigate and understand the influence of the anisotropic ice fabric on the seismic wave field and develop ways to derive information from travel times and reflection signatures about different anisotropic ice fabrics from seismic data, we need to be able to derive the elasticity tensor for different COF distributions. The calculations introduced here will be applied to ice-core and seismic data from Kohnen Station, Antarctica, in Part II, Diez et al (2015)
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