Ice fabric in an Antarctic ice stream interpreted from seismic anisotropy

Smith, Emma C.; Baird, A. F.; Kendall, J.‐M.; Martín, Carlos; White, R. S.; Brisbourne, Alex; Smith, Andrew M.

doi:10.1002/2016gl072093

Cited by 60 publications

(138 citation statements)

References 34 publications

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“…In particular, the splitting of the shear waves can provide unique information about the anisotropy of the medium (Anandakrishnan et al, 1994;Smith et al, 2017). In the case of the evidently asymmetric fabric of the KCC ice core we observe S-wave splitting of well above 200 m s −1 in the lower half of the ice core with a maximum value of 281 m s −1 .…”

Section: Non-vertical Incidence At Kccmentioning

confidence: 83%

“…This is especially relevant for glacial environments with a complex flow pattern, for example in sloping mountain glaciers, fast-flowing polar outlet glaciers (Hofstede et al, 2018) and ice streams (Smith et al, 2017). For such sites the approximation of the fabric by opening angles centred around the vertical can deviate much more from the reality than for sites that are located in the vicinity of an ice divide.…”

Section: Azimuth-sensitive Seismic Velocitiesmentioning

confidence: 99%

“…1) that only partially cover the length of the core. However, geophysical evidence of crystalorientation fabric can also be obtained by exploiting the elastic anisotropy, which influences the propagation of seismic waves in ice (Blankenship and Bentley, 1987;Smith et al, 2017). Seismic waves propagate from a seismic source to the seismic receivers on the glacier surface within the seismic plane (Fig.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2018

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Abstract. 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 icecore 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 135 m s −1 for P-wave and 200 m s −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 shearwave splitting at the bedrock as 45 m s −1 for the alpine ice core and 59 m s −1 for the polar ice core. At non-vertical incidence we obtain differences of up to 185 m s −1 for P-wave and 280 m s −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. 281 m s −1 at some depths. For a given incidence angle and depth we estimated changes in phase velocity of almost 200 m s −1 for P wave and more than 200 m s −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 crystalorientation fabric (±50 m s −1 per 10 cm). 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.

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Section: Non-vertical Incidence At Kccmentioning

confidence: 83%

Section: Azimuth-sensitive Seismic Velocitiesmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

et al. 2018

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“…The slower S-waves provide a better resolution and are of special interest for the study of the elastic properties of ice from traditional seismic reflection profiles (Picotti et al, 2015). In particular, the splitting of the shear waves can provide unique information about the anisotropy of the medium (Anandakrishnan et al, 1994;Smith et al, 2017). In case of the evidently asymmetric fabric of the KCC ice core we observe a shear-wave splitting of well above 200 m s .…”

Section: Non-vertical Incidence At Kccmentioning

confidence: 88%

“…This is especially relevant for glacial environments with a complex flow pattern, for example in sloping mountain glaciers, fast-flowing polar outlet glaciers (Hofstede et al, 2017, in press) and ice streams (Smith et al, 2017). For such sites the approximation of the fabric by opening angles centered around the vertical can deviate much more from the reality than for sites that are located in the vicinity of an ice divide.…”

Section: Azimuth-sensitive Seismic Velocitiesmentioning

confidence: 99%

Deriving seismic velocities on the micro-scale from c-axis orientations in ice cores

Kerch

Diez

Weikusat

et al. 2018

Preprint

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Abstract. One of the greatest challenges in glaciology, with respect to sea level predictions, is the ability to gain information on bulk ice anisotropy in ice sheets and glaciers, which is urgently needed to improve our understanding of ice-sheet dynamics. Therefore, we investigate the effect of crystal anisotropy on seismic velocities in a glacier. We 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 5 ensemble in an ice-core sample. We apply this approach to fabric data sets from an Alpine (KCC) and a polar (EDML) ice core. The results allow a quantitative evaluation of the earlier approximative eigenvalue framework. Additionally, our findings highlight the variation in seismic velocity as a function of the horizontal azimuth of the seismic plane, which can be significant in case of non-symmetric orientation distributions and results in a strong azimuth-dependent shear-wave splitting. For the first time, we assess the change in seismic anisotropy that can be expected on a short spatial scale in a glacier due to a strong 10 variability in crystal-orientation fabric. 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.

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Insights into anisotropy development and weakening of ice from in situ P wave velocity monitoring during laboratory creep

et al. 2017

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Polycrystalline ice weakens significantly after a few percent strain, during high homologous temperature deformation. Weakening is correlated broadly with the development of a crystallographic preferred orientation (CPO). We deformed synthetic polycrystalline ice at −5°C under uniaxial compression, while measuring ultrasonic P wave velocities along several raypaths through the sample. Changes in measured P wave velocities (Vp) and in the velocities calculated from microstructural measurements of CPO (by cryo‐electron backscatter diffraction) both show that velocities along trajectories parallel and perpendicular to shortening decrease with increasing strain, while velocities on diagonal trajectories increase. Thus, in these experiments, velocity data provide a continuous measurement of CPO evolution in creeping ice. Samples reach peak stresses after 1% shortening. Weakening corresponds to the start of CPO development, as indicated by divergence of P wave velocity changes for different raypaths, and initiates at ≈3% shortening. Selective growth by strain‐induced grain boundary migration (GBM) of grains favorably oriented for basal slip may initiate weakening through the formation of an interconnected network of these grains by 3% shortening. After weakening initiates, CPO continues to develop by GBM and nucleation processes. The resultant CPO has an open cone (small circle) configuration, with the cone axis parallel to shortening. The development of this CPO causes significant weakening under uniaxial compression, where the shear stresses resolved on the basal planes (Schmid factors) are high.

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Ice fabric in an Antarctic ice stream interpreted from seismic anisotropy

Cited by 60 publications

References 34 publications

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

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

Deriving seismic velocities on the micro-scale from c-axis orientations in ice cores

Insights into anisotropy development and weakening of ice from in situ P wave velocity monitoring during laboratory creep

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Ice fabric in an Antarctic ice stream interpreted from seismic anisotropy

Cited by 60 publications

References 34 publications

Deriving micro- to macro-scale seismic velocities from ice-core &lt;i&gt;c&lt;/i&gt; axis orientations

Deriving micro- to macro-scale seismic velocities from ice-core &lt;i&gt;c&lt;/i&gt; axis orientations

Deriving seismic velocities on the micro-scale from c-axis orientations in ice cores

Insights into anisotropy development and weakening of ice from in situ P wave velocity monitoring during laboratory creep

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

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