Abstract
Abstract. We investigate the propagation of seismic waves in anisotropic ice. Two effects are important: (i) sudden changes in crystal orientation fabric (COF) lead to englacial reflections; (ii) the anisotropic fabric induces an angle dependency on the seismic velocities and, thus, recorded travel times. Velocities calculated from the polycrystal elasticity tensor derived for the anisotropic fabric from measured COF eigenvalues of the EDML ice core, Antarctica, show good agreement with the velocity trend determined from vertical seismic profiling. The agreement of the absolute velocity values, however, depends on the choice of the monocrystal elasticity tensor used for the calculation of the polycrystal properties. We make use of abrupt changes in COF as a common reflection mechanism for seismic and radar data below the firn–ice transition to determine COF-induced reflections in either data set by joint comparison with ice-core data. Our results highlight the possibility to complement regional radar surveys with local, surface-based seismic experiments to separate isochrones in radar data from other mechanisms. This is important for the reconnaissance of future ice-core drill sites, where accurate isochrone (i.e. non-COF) layer integrity allows for synchronization with other cores, as well as studies of ice dynamics considering non-homogeneous ice viscosity from preferred crystal orientations.
Highlights
To understand the behaviour of glaciers and ice sheets, we need measurements to determine the conditions of glaciers at the surface, at the base and within the ice mass
We present results of a vertical seismic profiling (VSP) measurement carried out within the EDML borehole in Sect. 4 and compare the velocity profile derived from the travel times of the direct waves to the velocities we derive from the crystal orientation fabric (COF) eigenvalues of the EDML ice core
Based on these observations we conclude that the changes in COF are laterally much more variable than changes in conductivity. This intuitively makes sense, as changes in COF are developed in response to the local stress field within the ice, partly constrained by impurities, whereas changes in conductivity in the vertical resolution of our methods are formed by homogeneous deposition at the 10 to 100 km scale at the surface, with only slight post-depositional modification. This finding is important for revisiting the physical properties of the echo-free zone (EFZ), which appears below ∼ 2200 m depth, where no clear events are observable in the radar data
Summary
To understand the behaviour of glaciers and ice sheets, we need measurements to determine the conditions of glaciers at the surface, at the base and within the ice mass. The firn–ice transition the common mechanism influencing the propagation of seismic and radar waves is a preferred orientation of the anisotropic, hexagonal ice crystals This fabric anisotropy is normally described in the form of the COF eigenvalues obtained from ice-core measurements. In order to calculate seismic velocities and reflection coefficients for different anisotropic ice fabrics, we presented a framework to derive the anisotropic polycrystal elasticity tensor from COF eigenvalues in Part 1 of this work (Diez and Eisen, 2015). We apply this methodology here to calculate seismic velocities from COF eigenvalues measured along the EDML ice core, retrieved at Kohnen Station, Dronning Maud Land, Antarctica (EDML: EPICA Dronning Maud Land; EPICA: European Project for Ice Coring in Antarctica). This allows us to identify purely conductivity-induced reflections in the radar data, which are layers of equal age and can, be used safely to laterally extrapolate the age of the ice along the reflections
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