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  • 1
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    Variable extent of anisotropic ice and soft sediment in the basal zone of Store Glacier (2018)
    PANGAEA
    In:  Supplement to: Hofstede, Coen Matthijs; Christoffersen, Poul; Hubbard, Bryn; Doyle, Samuel H; Young, Tun Jan; Diez, Anja; Eisen, Olaf; Hubbard, Alun L (2018): Physical Conditions of Fast Glacier Flow: 2. Variable Extent of Anisotropic Ice and Soft Basal Sediment From Seismic Reflection Data Acquired on Store Glacier, West Greenland. Journal of Geophysical Research-Earth Surface, 123(2), 349-362, https://doi.org/10.1002/2017JF004297
    Details
    Publication Date: 2023-03-16
    Description: Added are 5 seismic reflection data sets of Store Glacier, a tide water glacier in West Greenland Uummannaq Fjord. Two crossing profiles were recorded, 20140513, along the ice flow and 20140514, across the ice flow.
    Keywords: AWI_Glac; File content; File format; File name; File size; Glaciology @ AWI; Seismic reflection profile; SEISREFL; Store_Glacier; Uniform resource locator/link to file; West Greenland
    Type: Dataset
    Format: text/tab-separated-values, 25 data points
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    DATA PANGAEA
  • 2
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    Comparison of elastic moduli from seismic diving-wave and ice-core microstructure analysis in Antarctic polar firn (2019)
    In:  EPIC3Annals of Glaciology, 60(79), pp. 220-230, ISSN: 0260-3055
    Details
    Publication Date: 2019-11-11
    Description: We compared elastic moduli in polar firn derived from diving wave refraction seismic velocity analysis, firn-core density measurements and microstructure modelling based on firn-core data. The seismic data were obtained with a small electrodynamic vibrator source near Kohnen Station, East Antarctica. The analysis of diving waves resulted in velocity–depth profiles for different wave types (P-, SH- and SV-waves). Dynamic elastic moduli of firn were derived by combining P- and S-wave velocities and densities obtained from firn-core measurements. The structural finite-element method (FEM) was used to calculate the components of the elastic tensor from firn microstructure derived from X-ray tomography of firn-core samples at depths of 10, 42, 71 and 99 m, providing static elastic moduli. Shear and bulk moduli range from 0.39 to 2.42 GPa and 0.68 to 2.42 GPa, respectively. The elastic moduli from seismic observations and the structural FEM agree within 8.5% for the deepest achieved values at a depth of 71 m, and are within the uncertainty range. Our observations demonstrate that the elastic moduli of the firn can be consistently obtained from two independent methods which are based on dynamic (seismic) and static (tomography and FEM) observations, respectively, for deeper layers in the firn below ∼10 m depth.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
    Format: application/pdf
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  • 3
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    Comparison of elastic moduli from seismic diving-wave and ice-core microstructure analysis in Antarctic polar firn (2019)
    In:  EPIC3EGU General Assembly, Vienna, 2019-04-07-2019-04-12
    Details
    Publication Date: 2019-08-21
    Description: The densification of firn depends on the elastic properties of firn, processes which are still not fully explained by the usual models. Geophysical methods provide spatially distributed data, while the analysis of firn cores is restricted to finite locations, but with a different vertical resolution. In this study, we compared elastic moduli in polar firn derived from refraction seismic velocity analysis and vertical density profiles from the firn-core measurements to elastic properties derived from microstructure modelling based on firn-core data. The seismic data were obtained with a small electrodynamic vibrator source (ElViS) near Kohnen Station, East Antarctica. The analysis of divingwaves resulted in velocity–depth profiles for P-, SH- and SV-wave velocities. Elastic moduli of firn were derived by combining P- and S-wave velocities and densities obtained from firn-core measurements. P-wave velocities derived from diving-wave analysis range from 2060 m s−1at 10 m depth to 3400 m s−1at 70 m depth, S-wave velocities from 1250 m s−1 to 1700 m s−1, respectively. The structural finite-element method (FEM) was used to calculate the components of the elastic tensor from firn microstructure derivedfrom X-ray tomography of firn-core samples at depths of 10, 42, 71 and 99 m. Shear and bulk moduli range from 0.39 GPa to 2.42 GPa and 0.68 GPa to 2.42 GPa, respectively. The elastic moduli from seismic observations and the structural FEM agree within 8.5% for the values derived at a depth of 71 m, and are within the uncertainty range. Our study demonstrates that elastic moduli of firn can be consistently obtained from two independent methods, which are based on dynamic (seismic) and static (tomography and FEM) observations, respectively. The agreement of the results for both methods indicates that elastic properties in firn can be acquired as spatially distributed data with the seismic approach, supported by local density information. Thus, information about elastic properties can be derived over larger lateral distances than would be possible with the static method. This enables the analysis of the firn and conclusions of the densification models might be drawn from observations of spatial and temporal changes in elastic properties.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , notRev
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  • 4
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    Physical Conditions of Fast Glacier Flow: 2. Variable Extent of Anisotropic Ice and Soft Basal Sediment From Seismic Reflection Data Acquired on Store Glacier, West Greenland (2018)
    Wiley
    In:  EPIC3Journal of Geophysical Research: Earth Surface, Wiley, 123, pp. 349-362
    Details
    Publication Date: 2018-04-03
    Description: Outlet glaciers of the Greenland Ice Sheet transport ice from the interior to the ocean and contribute directly to sea level rise because because discharge and ablation often exceed the accumulation. To develop a better understanding of these fast flowing glaciers, we investigate the basal conditions of Store Glacier, a large outlet glacier flowing into Uummannaq Fjord in West Greenland. We use two crossing seismic profiles acquired near the centreline, 30 km upstream of the calving front, to interpret the physical nature of the ice and bed. We identify one notably englacial and two notably subglacial seismic reflections on both profiles. The englacial reflection represents a change in crystal orientation fabric, interpreted to be the Holocene–Wisconsin transition. From Amplitude Versus Angle (AVA) analysis we infer that the deepest ∼80 m of ice of the parallel-flow profile below this reflection is anisotropic with an enhancement of simple shear of ∼2. The ice is underlain by ∼45 m of unconsolidated sediments, below which there is a strong reflection caused by the transition to consolidated sediments. In the across-flow profile subglacial properties vary over small scale and the polarity of the ice–bed reflection switches from positive to negative. We interpret these as patches of different basal slipperiness associated with variable amounts of water. Our results illustrate variability in basal properties, and hence ice-bed coupling, at a spatial scale of ∼100 m, highlighting the need for direct observations of the bed to improve the basal boundary conditions in ice-dynamic models.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
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  • 5
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    Deriving micro- to macro-scale seismic velocities from ice-core c axis orientations (2018)
    COPERNICUS GESELLSCHAFT MBH
    In:  EPIC3The Cryosphere, COPERNICUS GESELLSCHAFT MBH, 12(5), pp. 1715-1734
    Details
    Publication Date: 2019-05-06
    Description: One of the great challenges in glaciology is the ability to estimate the bulk ice anisotropy in ice sheets and glaciers, which is needed to improve our understanding of ice-sheet dynamics. We investigate the effect of crystal anisotropy on seismic velocities in glacier ice and revisit the framework which is based on fabric eigenvalues to derive approximate seismic velocities by exploiting the assumed symmetry. In contrast to previous studies, we calculate the seismic velocities using the exact c axis angles describing the orientations of the crystal ensemble in an ice-core sample. We apply this approach to fabric data sets from an alpine and a polar ice core. Our results provide a quantitative evaluation of the earlier approximative eigenvalue framework. For near-vertical incidence our results differ by up to 135ms−1 for P-wave and 200ms−1 for S-wave velocity compared to the earlier framework (estimated 1% difference in average P-wave velocity at the bedrock for the short alpine ice core). We quantify the influence of shear-wave splitting at the bedrock as 45ms−1 for the alpine ice core and 59ms−1 for the polar ice core. At non-vertical incidence we obtain differences of up to 185ms−1 for P-wave and 280ms−1 for S-wave velocities. Additionally, our findings highlight the variation in seismic velocity at non-vertical incidence as a function of the horizontal azimuth of the seismic plane, which can be significant for non-symmetric orientation distributions and results in a strong azimuth-dependent shear-wave splitting of max. 281ms−1 at some depths. For a given incidence angle and depth we estimated changes in phase velocity of almost 200ms−1 for P wave and more than 200ms−1 for S wave and shear-wave splitting under a rotating seismic plane. We assess for the first time the change in seismic anisotropy that can be expected on a short spatial (vertical) scale in a glacier due to strong variability in crystal-orientation fabric (±50ms−1 per 10cm). Our investigation of seismic anisotropy based on ice-core data contributes to advancing the interpretation of seismic data, with respect to extracting bulk information about crystal anisotropy, without having to drill an ice core and with special regard to future applications employing ultrasonic sounding.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
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  • 6
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    Seismic wave propagation in anisotropic ice - Part 2: Effects of crystal anisotropy in geophysical data (2015)
    Copernicus Publications
    In:  EPIC3The Cryosphere, Copernicus Publications, 9(1), pp. 385-398, ISSN: 1994-0416
    Details
    Publication Date: 2017-10-20
    Description: 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.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
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  • 7
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    Crystal-orientation fabric variations on the sub-metre scale in cold Alpine ice (2017)
    In:  EPIC3International Symposium on the Cryosphere in a Changing Climate, Wellington, Neuseeland, 2017-02-12-2017-02-17
    Details
    Publication Date: 2017-09-18
    Description: The macroscopic flow of a glacier is substantially influenced by the plastic anisotropy of individual ice crystals on the microscale. A preferred crystal orientation fabric (COF) develops with depth in a glacier and is subjected to the influence of the temperature, deformation and recrystallisation regime as well as the climate-dependent impurity load in the ice. Detailed knowledge about the crystal anisotropy in a glacier is thus required to better constrain the response of ice sheets in a changing climate. While the gradual change in anisotropy on a large scale of tens to hundreds of metres can mostly be explained, this is not the case for changes in the anisotropic fabric on a shorter scale of centimetres to decimetres. It is therefore essential to improve the understanding of how and why the anisotropic COF changes on a sub-metre scale in a glacier. Fabric data from an ice core of 72 m length, drilled at the high-altitude Alpine site Colle Gnifetti in Switzerland, were measured in continuously sampled sections of ca. 1 m length, covering 10 % of the entire core length. The eigenvalues of the second-order orientation tensor describing the distribution of crystal axes were analysed in high-resolution together with impurity data from meltwater analysis. It is found that the fabric anisotropy exhibits a strong variability on the short scale in all depths of the ice core with extreme eigenvalue differences within one metre of up to 0.2, often associated with small- or large-grained layers. The observation of a clear connection between the grain size variation and the impurity content leads to the conclusion that the influence of impurities on short-scale fabric variations is partially conveyed by the impurity-controlled grain size in combination with the local deformation regime.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , notRev
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  • 8
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    Approximations to seismic AVA responses: Validity and potential in glaciological applications (2016)
    In:  EPIC3Geophysics
    Details
    Publication Date: 2015-10-04
    Description: Amplitude-variation-with-angle (AVA) methods establish the seismic properties of material either side of a reflective interface, and their use is growing in glaciology. The AVA response of an interface is defined by the complex Knott-Zoeppritz (K-Z) equations, numerous approximations to which we typically assume weak interface contrasts and isotropic propagation, inconsistent with the strong contrasts at glacier beds and the vertically transverse isotropic (VTI) fabrics were associated with englacial reflectivity. We considered the validity of a suite of approximate K-Z equations for the exact P-wave reflectivity RP of ice overlying bedrock, sediment and water, and englacial interfaces between isotropic and VTI ice. We found that the approximations of Aki-Richards, Shuey, and Fatti match exact glacier bed reflectivity to within RP±0.05, smaller than the uncertainty in typical glaciological AVA analyses, but only for maximum incident angle θi limited to 30°. A stricter limit of θi≤20° offered comparable accuracy to a hydrocarbon benchmark case of shale overlying gas-charged sand. The VTI-compliant Rüger approximation accurately described englacial reflectivity, to within RP±0.01, and it can be modified to give a quadratic expression in sin2(θi) suitable for curve-matching operations. Having shown the circumstances under which AVA approximations were valid for glaciological applications, we have suggested that their interpretative advantages can be exploited in the future AVA interpretations.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
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  • 9
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    Ross ice shelf vibrations (2015)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 42 (2015): 7589–7597, doi:10.1002/2015GL065284.
    Description: Broadband seismic stations were deployed across the Ross Ice Shelf (RIS) in November 2014 to study ocean gravity wave-induced vibrations. Initial data from three stations 100 km from the RIS front and within 10 km of each other show both dispersed infragravity (IG) wave and ocean swell-generated signals resulting from waves that originate in the North Pacific. Spectral levels from 0.001 to 10 Hz have the highest accelerations in the IG band (0.0025–0.03 Hz). Polarization analyses indicate complex frequency-dependent particle motions, with energy in several frequency bands having distinctly different propagation characteristics. The dominant IG band signals exhibit predominantly horizontal propagation from the north. Particle motion analyses indicate retrograde elliptical particle motions in the IG band, consistent with these signals propagating as Rayleigh-Lamb (flexural) waves in the ice shelf/water cavity system that are excited by ocean wave interactions nearer the shelf front.
    Description: Bromirski, Diez, and Gerstoft were supported by NSF grant PLR 1246151. Stephen and Bolmer were supported by NSF grant PLR-1246416. Wiens, Aster, and Nyblade were supported under NSF grants PLR-1142518, 1141916, and 1142126, respectively. Bromirski also received support from the California Department of Parks and Recreation, Division of Boating and Waterways under contract 11-106-107. The NIB data were collected under NSF grant OPP-0229546 and were downloaded from the IRIS DMC archives.
    Description: 2016-03-16
    Keywords: Ocean wave-ice shelf interactions ; Infragravity waves ; Dispersed gravity wave arrivals ; Polarization analysis ; Rayleigh-Lamb waves ; Flexural waves
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 10
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    Seismic wave propagation in anisotropic ice - Part 1: Elasticity tensor and derived quantities from ice-core properties (2015)
    Copernicus Publications
    In:  EPIC3The Cryosphere, Copernicus Publications, 9(1), pp. 367-384, ISSN: 1994-0416
    Details
    Publication Date: 2015-02-27
    Description: A preferred orientation of the anisotropic ice crystals influences the viscosity of the ice bulk and the dynamic behaviour of glaciers and ice sheets. Knowledge about the distribution of crystal anisotropy is mainly provided by crystal orientation fabric (COF) data from ice cores. However, the developed anisotropic fabric influences not only the flow behaviour of ice but also the propagation of seismic waves. Two effects are important: (i) sudden changes in COF lead to englacial reflections, and (ii) the anisotropic fabric induces an angle dependency on the seismic velocities and, thus, recorded travel times. A framework is presented here to connect COF data from ice cores with the elasticity tensor to determine seismic velocities and reflection coefficients for cone and girdle fabrics.We connect the microscopic anisotropy of the crystals with the macroscopic anisotropy of the ice mass, observable with seismic methods. Elasticity tensors for different fabrics are calculated and used to investigate the influence of the anisotropic ice fabric on seismic velocities and reflection coefficients, englacially as well as for the ice–bed contact. Hence, it is possible to remotely determine the bulk ice anisotropy.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
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