Seismic wave propagation in anisotropic ice – Part 2: Effects of crystal anisotropy in geophysical data

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 firnice 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.

Physical analysis of an Antarctic ice core—towards an integration of micro- and macrodynamics of polar ice

“…The single maximum LPO along the vertical core axis is fully developed after a sudden final collapse of the girdle between 2035 and 2045 m. This sudden change in the LPO was also detected as a clear reflector in radio-echo sounding (RES) data, which is caused by the dependence of the electromagnetic wave velocity from the crystallographic orientation [98–100]. Grain elongation direction histograms derived from vertical sections (figure 5 b left) show a broad, but distinct distribution (cone angle of 45°) with a slight tendency towards double/multiple maxima.…”

Seismic wave propagation in anisotropic ice – Part 1: Elasticity tensor and derived quantities from ice-core properties

“…This classification introduces artificial discontinuities in the velocity profile over depth, calculated from an ice core. These discontinuities only reflect the calculation method, not sudden changes in the prevailing fabric (Part II, Diez et al, 2015). This limitation, introduced by the classification of the different fabric groups, could be overcome by calculating the opening angels directly from the derived c axis vectors.…”

“…Different authors have measured (Jona and Scherrer, 1952;Green and Mackinnen, 1956;Bass et al, 1957;Brockamp and Querfurth, 1964;Bennett, 1968;Dantl, 1968;Gammon et al, 1983) and calculated (Penny, 1948) the monocrystal elasticity tensor. A comparison of the different elasticity tensors used can be found in Part II (Diez et al, 2015). There we investigate results of a vertical seismic profiling survey in comparison to quantities from measured COF eigenvalues.…”

“…This is the first part of two companion papers. The calculations introduced here will be applied to ice-core and seismic data from Kohnen Station, Antarctica, in Part II, Diez et al (2015).…”

Measuring the Elastic Modulus of Snow

“…The elastic modulus of snow is a fundamental mechanical property relating stress and strain. It is a highly relevant parameter for many snow mechanical application covering engineering aspects of tire traction (Choi et al, 2012), interpretation of seismic waves (Diez et al, 2015), snow fracture in relation to avalanche release (Van Herwijnen et al, 2016), or the validation of new constitutive models (Barraclough et al, 2017). Snow is a fragile, porous material that exists close to its melting point, making it highly rate dependent with a strong microstructural influence.…”