Anisotropic backscatter in ice-penetrating radar data: potential mechanisms and implications

Airborne and ground-based radar have been used extensively in the past to measure ice thickness and to investigate the internal structure of ice sheets in terms of layering. The main reflection mechanisms for internal reflections are changes in density, conductivity, and crystal orientation fabric, which alter thepermittivity of the ice. Linking the different mechanisms to the individual reflection horizons enables thededuction of glaciological parameters like accumulation rates or age-depth estimates. If no sample material from snow pits or ice-cores are available, multi-frequency and multi-polarization measurements must be applied to distinguish between the different reflection mechanisms. The backscattered power of horizons caused by changes in conductivity varies with the center frequency whereas in the case of horizons originating from changing crystal orientation the backscattered power is dependent on the polarization plane of the carrier signal.In this study we examine a sample data set near the German summer station Kohnen (drill site for theEPICA-EDML ice core) on the Antarctic plateau. The data were acquired with an airplane sliding on ground, producing varying incident polarization with a circular profile and several cross profiles with different headings. We find that the backscattered power changes with varying antenna orientation (i.e. polarization). In the upper third of the ice column the backscatter has two maxima with a 180° symmetry. The maxima align with the direction of minimal surface strain. At approximately 900 m depth the anisotropy is shifted by 90° in heading azimuth, with the maxima now being parallel to the maximum in surface strain. This dataset is unique, as airborne systems (primarily designed for the sounding of ice thickness) are usually not used for ground-based applications. The observed anisotropy appears clearly and is intriguing as the reason for it is entirely unknown. As primary suspects we consider the role of changing crystal orientation and ellipsoidal shaped air bubbles. The effect is visible from 200 1400 m. It appears distributed along the entire interval, and not restricted to individual layers. It seems that the polarization dependence becomes visible by a changing background level of the acquired signal, which is otherwise largely dominated by layer-like, polarization independent reflections. Hence we apply a (semi-analytical) volume scattering model in order to understand the different reflection mechanisms better. From ice-core measurements it is known that the crystals in the upper hundred meters are only weakly aligned (if at all), and it is unclear how the crystal orientation changes overshort depth intervals (~10 m). The rotation of the anisotropy coincides with the clathrate transition in the ice core and thus we first focus on the effect of anisotropic air bubbles. In an in-coherent approach we treat the ice matrix as a random medium and use the vector radiative transfer theory to incorporate boundary conditions. In a second step we model the effect of crystal orientation to estimate both, the degree of alignment and the statistical variance in the permittivity tensor needed to generate the observed pattern in backscatter. Doing so, we eventually aim at pinning down the mechanisms for the anisotropy in the upper interval, lower interval and the interrelation of the two by a shift of 90°.Anisotropic air bubbles as well as aligned crystal orientation allow to deduce stress and strain rates and a potential change thereof along depth. So far it is largely unclear, how surface strain rates relate with strain rates within the ice. If one of the two suspected mechanisms can be excluded or confirmed, this study may serve as a case study for future polarimetric surveys with low-frequency radars, in order to supply ice-sheet modelling with adequate boundary conditions - including changes in the internal structure of ice sheets along depth

Determination of crystal orientation fabric from seismic wideangle data

It is known from ice core analyses that the crystal orientation fabric (COF) of ice sheets is anisotropic and changes over depth. A better understanding of these anisotropies as well as their remote detection is important to optimize flow models for ice. Here we show how seismic wideangle measurements can be used to determine the COF remotely. We demonstrate the principle formalism how observed seismic traveltimes can be related to COF properties by a forward model and then apply the formalism to field data. The eigenvalues that describe the ice fabric of the ice core EDML (Dronning Maud Land, Antarctca) are set into a relationship with the elasticity tensor. From the elasticity tensor the expected seismic velocities and reflection coefficients are calculated. Additionally we calculate the value eta from the Thomsen-parameters epsilon and delta. The value eta gives a measure of the anisotropy of vertical transverse isotropic (VTI)-media and is an important tool for the NMO-correction of anisotropic data. The approximation of reflection horizons as hyperbolas is not valid anymore in anisotropic media. The calculation of the moveout is therefore performed by a 4th order NMO-correction with the rms-velocity and the effective eta value as variables. This approach is applied to data from a wideangle survey shot at Halvfarryggen, Dronning Maud Land, Antarctica. From this data we derived rms-velocities and effective eta values. These values were than recalculated to interval velocities and interval eta values to give a hint on the measure of anisotropy of the different layers. The results give first insight into the anisotropies at Halvfarryggen

Location and distribution of micro-inclusions in the EDML and NEEM ice cores using optical microscopy and in situ Raman spectroscopy

Impurities control a variety of physical properties of polar ice. Their impact can be observed at all scales – from the microstructure (e.g., grain size and orientation) to the ice sheet flow behavior (e.g., borehole tilting and closure). Most impurities in ice form micrometer-sized inclusions. It has been suggested that these μ inclusions control the grain size of polycrystalline ice by pinning of grain boundaries (Zener pinning), which should be reflected in their distribution with respect to the grain boundary network. We used an optical microscope to generate high-resolution large-scale maps (3 μm pix^-1, 8 x 2 cm^2) of the distribution of micro-inclusions in four polar ice samples: two from Antarctica (EDML, MIS 5.5) and two from Greenland (NEEM, Holocene). The in situ positions of more than 5000 μ inclusions have been determined. A Raman microscope was used to confirm the extrinsic nature of a sample proportion of the mapped inclusions. A superposition of the 2-D grain boundary network and μ-inclusion distributions shows no significant correlations between grain boundaries and μ inclusions. In particular, no signs of grain boundaries harvesting μ inclusions could be found and no evidence of μ inclusions inhibiting grain boundary migration by slow-mode pinning could be detected. Consequences for our understanding of the impurity effect on ice microstructure and rheology are discussed

A Review of the Microstructural Location of Impurities in Polar Ice and Their Impacts on Deformation

Insoluble and soluble impurities, enclosed in polar ice sheets, have a major impact on the deformation behaviour of the ice. Macro- and Micro-scale deformation observed in ice sheets and ice cores has been retraced to chemical loads in the ice, even though the absolute concentration is negligible. And therefore the exact location of the impurities matters: Allocating impurities to specific locations inside the ice microstructure inherently determines the physical explanation of the observed interaction between chemical load and the deformational behaviour. Both, soluble and non-soluble impurities were located in grain boundaries, triple junctions or in the grain interior, using different methods, samples and theoretical approaches. While each of the observations is adding to the growing understanding of the effect of impurities in polar ice, the growing number of ambiguous results calls for a dedicated and holistic approach in assessing the findings. Thus, we here aim to give a state of the art overview of the development in microstructural impurity research over the last 20 years. We evaluate the used methods, discuss proposed deformation mechanisms and identify two main reasons for the observed ambiguity: 1) limitations and biases of measurement techniques and 2) the physical state of the analysed impurity. To overcome these obstacles we suggest possible approaches, such as the continuous analysis of impurities in deep ice cores with complementary methods, the implementation of these analyses into established in-situ ice core processing routines, a more holistic analysis of the microstructural location of impurities, and an enhanced knowledge-transfer via an open access data base

The new frontier of microstructural impurity research in polar ice

Deciphering the localisation of solid and dissolved impurities on the micron-scale in glacial ice remains a challenge, but is critical to understand the integrity of ice core records and internal deformation. Here we report on the state-of-the-art in microstructural impurity research by high- lighting recent progress in bringing together cryo-Raman spectroscopy and laser ablation induct- ively coupled plasma mass spectrometry (LA-ICP-MS). We show the potential of both methods and discuss possibilities to improve inter-method approaches aiming for a more holistic under- standing of the evolution of impurity localisation throughout the ice column, including post-depositional processes. In this framework, we elaborate on future research priorities such as LA-ICP-MS imaging on firn samples and integrating a large cryo-cell with imaging capabilities

What radar reveals about crystal orientation: A study from Greenland Ice Sheet

Deformation processes dominated by dislocation activity within ice sheets take place on small scale: ice crystals are effectively re-oriented to minimize resistance when deformation takes place. The analysis of Crystal Orientation Fabric (COF) of c-axes in ice cores is a well-established technique to investigate these processes within ice sheets. Due to the extensive infrastructure required for drilling and protracted analysis of ice cores, the amount of information of COF of the Antarctic and Greenland Ice Sheet is limited, both, in aerial coverage as well as in depth resolution. Indirect measurements, such as geophysical techniques, provide complementary information. Depending on whether the COF is isotropic or anisotropic, a radar signal is propagating differently in terms of angle of incidence and polarization, and, partially reflected when COF properties change along the direction of travel. We study the ability of phase-sensitive radar measurements to infer an overall pattern of COF by comparing our results to COF derived from the EastGRIP ice core, drilled into the North East Greenland Ice Stream (NEGIS). If radar measurements allow to reveal information about the COF as analyzed ice cores do, this would provide important additional information on the (an)isotropy at locations where no ice core is available. Furthermore, it has the ability to offer a quasi-continuous spatial coverage and to greatly improve our understanding of the evolution of anisotropy along ice-flow trajectories, from ice divides to the calving front of outlet glaciers