An ice-sheet-wide framework for englacial attenuation from ice-penetrating radar data

Radar inference of the bulk properties of glacier beds, most notably identifying basal melting, is, in general, derived from the basal reflection coefficient. On the scale of an ice sheet, unambiguous determination of basal reflection is primarily limited by uncertainty in the englacial attenuation of the radio wave, which is an Arrhenius function of temperature. Existing bed-returned power algorithms for deriving attenuation assume that the attenuation rate is regionally constant, which is not feasible at an ice-sheet-wide scale. Here we introduce a new semi-empirical framework for deriving englacial attenuation, and, to demonstrate its efficacy, we apply it to the Greenland Ice Sheet. A central feature is the use of a prior Arrhenius temperature model to estimate the spatial variation in englacial attenuation as a first guess input for the radar algorithm. We demonstrate regions of solution convergence for two input temperature fields and for independently analysed field campaigns. The coverage achieved is a trade-off with uncertainty and we propose that the algorithm can be "tuned" for discrimination of basal melt (attenuation loss uncertainty ∼ 5 dB). This is supported by our physically realistic ( ∼ 20 dB) range for the basal reflection coefficient. Finally, we show that the attenuation solution can be used to predict the temperature bias of thermomechanical ice sheet models and is in agreement with known model temperature biases at the Dye 3 ice core.

An ice-sheet wide framework for englacial attenuation and basal reflection from ice penetrating radar data

Radar-inference of the bulk properties of glacier beds, most notably identifying basal melting, is, in general, derived from the basal reflection coefficient. On the scale of an ice-sheet, unambiguous determination of basal reflection is primarily limited by uncertainty in the englacial attenuation of the radio wave, which is an exponential function of temperature. Most existing radar algorithms assume stationarity in the attenuation rate, which is not feasible at an ice-sheet wide scale. Here we introduce a new framework for deriving englaical attenuation and basal reflection, and, to demonstrate its efficacy, we apply it to the Greenland Ice-Sheet. A central feature is the use of a prior Arrhenius temperature model to estimate the spatial variation in englaical attenuation as a first guess input for the radar algorithm. We demonstrate regions of solution convergence for two input temperature fields, and for independently analysed field campaigns. The coverage achieved is a trade-off with uncertainty and we propose that the algorithm can be "tuned" for discrimination of basal melt (attenuation loss uncertainty ∼ 5 dB). This is supported by our physically realistic (∼ 20 dB) range for the basal reflection coefficient. Finally, we show that the attenuation solution can be used to predict the temperature bias of thermomechanical ice-sheet models.

Radar absorption, basal reflection, thickness and polarization measurements from the Ross Ice Shelf, Antarctica

Radio-glaciological parameters from the Moore’s Bay region of the Ross Ice Shelf, Antarctica, have been measured. The thickness of the ice shelf in Moore’s Bay was measured from reflection times of radio-frequency pulses propagating vertically through the shelf and reflecting from the ocean, and is found to be 576 ± 8 m. Introducing a baseline of 543 ± 7m between radio transmitter and receiver allowed the computation of the basal reflection coefficient, R, separately from englacial loss. The depth-averaged attenuation length of the ice column, 〈L〉 is shown to depend linearly on frequency. The best fit (95% confidence level) is 〈L(ν)〉= (460±20) − (180±40)ν m (20 dB km−1), for the frequencies ν = [0.100–0.850] GHz, assuming no reflection loss. The mean electric-field reflection coefficient is (1.7 dB reflection loss) across [0.100–0.850] GHz, and is used to correct the attenuation length. Finally, the reflected power rotated into the orthogonal antenna polarization is <5% below 0.400 GHz, compatible with air propagation. The results imply that Moore’s Bay serves as an appropriate medium for the ARIANNA high-energy neutrino detector.

Morphology of Polyethylene/Montmorillonite Nanocomposites Prepared by In Situ Polymerization

Polyethylene/montmorillonite (PE/MMT) nanocomposites were prepared by in situ polymerization. The morphology of MMT/MgCl2/TiCl4 catalyst and PE/MMT nanocomposites was investigated by scanning electron microscopy (SEM). It can be seen that MMT/MgCl2/TiCl4 catalyst remained the original MMT sheet structures and many holes were found in MMT and the morphology of PE/MMT nanocomposites is part of the sheet in the form of existence, as most of the petal structure. X-ray diffraction (XRD) and transmission electron microscopy (TEM) were carried out to characterize all the samples. XRD results reveal that the original basal reflection peak of PEI1 and PEI2 disappears completely and that of PEI3 become very weak. MMT/MgCl2/TiCl4 catalyst was finely dispersed in the PE matrix. Instead of being individually dispersed, most layers were found in thin stacks comprising several swollen layers.

Characterization of a Mg-rich and low-charge saponite from the Neogene lacustrine basin of Eskisşehir, Turkey

The saponite examined occurs as two 0.1 m thick layers in a Pliocene sequence consisting of dolomite and dolomitic marl. To characterize this material, mineralogical and structural analyses (XRD, SEM and FTIR), thermal analyses (DTA, TG) and chemical analyses (ICP-ES) were performed. From XRD patterns of randomly-oriented powder samples, the first basal reflection appears as an asymmetric and broad peak with d001 values varying between 16.55 and 17.32 Å. In oriented and air-dried samples, this reflection occurs between 14.45 and 16.42 Å and is fairly symmetrical with FWHM of 2.7º2θ. Oriented and ethylene glycol-solvated samples produce a rational series of basal reflections, where 001 occurs at ~17.8 Å as an intense, narrow (1.1º2θ) and fairly symmetrical reflection. Upon solvation with glycerol, the 001 reflection shifts to ~18.7 Å.The chemical composition of this saponite is similar to stevensite. However, the structural formula of Na0.114Ca0.013K0.003(Mg2.957Al0.004Fe0.028Ti0.004)(Si3.826Al0.174)O10(OH)2 indicates that vacancies in the octahedral sheet do not exist. The negative layer charge arises nearly entirely from the substitutions in the tetrahedral sheet, with the net layer charge of –0.148, smaller than for common smectites.Due to the XRD characteristics and particularly the layer-charge distribution, it was concluded that this mineral is a Mg-rich saponite with low layer charge. The saponite was formed by direct precipitation in an alkaline lake environment from Mg- and Si-rich solutions at high pH.

Evaluation of expandability for randomly interstratified illite/smectite using interstratificational peak broadening

This study attempted to quantify the interstratificational broadening of the randomly interstratified illite/smectite (random I∕S) basal reflection and to evaluate the percentage of the interstratified illite layers (%I) from the result. The interstratificational broadening was quantified using the distributional discrepancy (D) defined as D=[∑t∣ft(obs)−ft(ref)∣]∕2, where ft(obs) is the frequency of a crystallite containing thickness, t (the number of layers), measured from a basal reflection broadened by interstratifications, and ft(ref) is the frequency for a basal reflection with no interstratificational broadening. The basal reflections at 5.2° 2θ under glycolation and 8.84° 2θ under thermal dehydration provided the ft(obs) and ft(ref) of random I∕S. The linear relation, D=2.17%I+2.49(0⩽%I⩽30), was obtained from simulations for SWy-2 (Wyoming, USA).

Equations for quantifying Fe and K within the illite structure using X-ray powder diffraction

This study was performed to suggest convenient equations for quantifying Fe and K within the illite structure using relative intensities for the illite basal reflections. The 002/005 and 003/005 ratios were available for Fe and K quantifications, and equations could be derived as the following: CFe=(ln I002/005−1.807 ln I003/005+1.29)/(1.241 ln I003/005−3.843), CK=−(ln I002/005+0.121 ln I003/005−2.592)/(1.308 ln I003/005+1.984). The equations may obtain Fe and K contents within ca. ±0.05 atoms per half-unit cell, if incident radiation loss is minimized and basal reflection modification caused by expandable layers is eliminated. © 2004 International Centre for Diffraction Data.