Englacial seismic reflectivity: imaging crystal-orientation fabric in West Antarctica

Abstract

ABSTRACT. Abrupt changes in crystal-orientation fabric (COF), and therefore viscosity, are observed near the base of the ice sheet throughout West Antarctica. We report on active-source seismic observations from WAIS Divide, mid-stream and downstream on Thwaites Glacier, and the onset region of Bindschadler Ice Stream. These data reveal a prevalence of englacial seismic reflectivity in the bottom quarter of the ice sheet. The observed seismic reflectivity is complex but largely bed-conformable, with long-spatial-wavelength features observed in the flow direction and short-wavelength features observed across flow. A correspondence of englacial structures with bed features is also observed. We determine the origin of the reflectivity to be abrupt changes in the COF of ice, based on the following: (1) observations of englacial reflectivity are consistent with current knowledge of COF within ice sheets, (2) englacial reflectivity caused by COF contrasts requires the simplest genesis, especially at ice divides, and (3) amplitude analysis shows that the observed englacial reflectivity can be explained by contrasts in seismic velocity due to COF changes. We note that the downstream increase in the quantity and complexity of observations indicates that direct observations of COF at ice divides likely underestimate the role that fabric plays in ice-sheet dynamics.

Deriving micro- to macro-scale seismic velocities from ice-core <i>c</i> axis orientations

“…Although fabric layering is regularly observed in the KCC ice core and also in the continuously sampled depth intervals in EDML, it is still unclear how representative these short-scale variations are for both a small lateral scale and larger regions in a glacier. However, evidence has been presented for abrupt COF changes as a frequent cause of seismic reflectivity (Horgan et al, 2011). Other studies do not observe such a high reflectivity due to COF but identify a high degree of gradually evolving fabric anisotropy (Picotti et al, 2015) or single strong reflections associated with transitions in fabric classes, e.g.…”

“…Current laboratory fabric measurements from an ice core drilled on an ice stream also show early indications of high fabric variability and unexpected fabric types (Jan Eichler, personal communication, 2017), offering an ideal target for extending this study to an environment with another deformation regime. Based on the presented evidence in this study the next steps should include the investigation of how a succession of short-scale fabric layers could induce englacial reflections, as has been reported and hypothesised in earlier studies (Horgan et al, 2011;Hofstede et al, 2013).…”

“…It can be hypothesised that under the increasing influence of large-scale shear deformation in the deeper regions of the glacier coherent fabric layers might develop. Accordingly, more seismic reflectivity should be expected in depth and from more dynamic settings, as proposed by Horgan et al (2011). Eisen et al (2007) show that transitions in COF in the deep ice can be followed with radio-echo sounding over longer horizontal distances of several kilometres.…”

“…This is the vertical plane underneath the horizontal seismic profile, which runs along the surface of the glacier, and may also contain the vertical ice-core axis, along which fabric information is collected. Seismic reflections occur due to sudden changes in fabric (Horgan et al, 2008(Horgan et al, , 2011Hofstede et al, 2013) and offer the chance to obtain spatially distributed information on the COF structure in various depths of the ice column, the acquisition of which would be unfeasible using direct sampling via ice coring. The motivation of this study is to improve the interpretation of seismic data by connecting the micro-and the macro-scale using the elastic properties of ice.…”

Monitoring the temperature-dependent elastic and anelastic properties in isotropic polycrystalline ice using resonant ultrasound spectroscopy

“…Seismic investigations of ice sheets (among others Bentley and Kohnen, 1976;Horgan et al, 2011Horgan et al, , 2008Picotti et al, 2015) present a potential window into the regional-scale characteristics of ice bodies. Much focus has recently been placed on understanding the physical properties of ice that influence seismic wave propagation (Maurel et al, 2015).…”