Direct evidence for continuous radar reflector originating from changes in crystal-orientation fabric

Eisen, Olaf; Hamann, Ilka; Kipfstuhl, Sepp; Steinhage, Daniel; Wilhelms, Frank

doi:10.5194/tc-1-1-2007

Cited by 85 publications

(108 citation statements)

References 28 publications

(37 reference statements)

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“…At EDML, the majority of internal reflectors below ∼900 m originate from individual conductivity peaks, with some being an interference signal of closely spaced peaks (Eisen et al, 2006). The last two detected signals in the 60 ns RES data at EDML correspond to a reflector from changes in COF at 2040 m (Eisen et al, 2007) and a conductivity peak at 2080 m. The EFZ is observed in both profiles. Figure 4a and b display an example from profile 033137 (trace 6297), where the last continuous reflector is found at 1654 m depth, 690 m above the icebed interface.…”

Section: Res Internal Structurementioning

confidence: 87%

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Layer disturbances and the radio-echo free zone in ice sheets

et al. 2009

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Abstract. Radio-echo sounding of the Antarctic and Greenlandic ice sheets often reveals a layer in the lowest hundreds of meters above bedrock more or less free of radio echoes, known as the echo-free zone (EFZ). The cause of this feature is unclear, so far lacking direct evidence for its origin. We compare echoes around the EPICA drill site in Dronning Maud Land, Antarctica, with the dielectric properties, crystal orientation fabrics and optical stratigraphy of the EPICA-DML ice core. We find that echoes disappear in the depth range where the dielectric contrast is blurred, and where the coherency of the layers in the ice core is lost due to disturbances caused by the ice flow. At the drill site, the EFZ onset at ∼2100 m marks a boundary, below which the ice core may have experienced flow induced disturbances on various scales. The onset may indicate changing rheology which needs to be accounted for in the modeling of ice sheet dynamics.

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Section: Res Internal Structurementioning

confidence: 87%

“…Changes in density and conductivity have isochronous character (Vaughan et al, 2004; Correspondence to: R. Drews (reinhard.drews@awi.de) 2004). Changing COF might have isochronous character, but is also influenced by the ice flow (Eisen et al, 2007).…”

Section: Introductionmentioning

confidence: 99%

Layer disturbances and the radio-echo free zone in ice sheets

et al. 2009

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“…Changes in crystal-orientation fabric, changes in conductivity or changes in the amount of bubbles (respectively density) are considered to cause internal reflections in glaciers and ice sheets (e.g. Harrison, 1973;Gudmandsen, 1975;Siegert, 1999;Eisen et al, 2007). Strong internal and basal reflectors can further be caused by layers of varying density within firn and snow (Bingham and Siegert, 2007), sediments (Woodward et al, 2003), debris (Arcone et al, 1995;Fukui et al, 2008) or subglacial lakes.…”

Section: Methodsmentioning

confidence: 99%

Imaging the structure of cave ice by ground-penetrating radar

Hausmann

Behm

2011

The Cryosphere

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Abstract. Several caves in high elevated alpine regions host up to several meters thick ice. The age of the ice may exceed some hundreds or thousands of years. However, structure, formation and development of the ice are not fully understood and are subject to relatively recent investigation. The application of ground-penetrating radar (GPR) enables to determine thickness, volume, basal and internal structure of the ice and provides as such important constraints for related studies. We present results from four caves located in the Northern Calcareous Alps of Austria.We show that the ice is far from being uniform. The base has variable reflection signatures, which is related to the type and size of underlying debris. The internal structure of the cave ice is characterized by banded reflections. These reflection signatures are interpreted as thin layers of sediments and might help to understand the ice formation by representing isochrones. Overall, the relatively low electromagnetic wave speed suggests that the ice is temperate, and that a liquid water content of about 2% is distributed homogenously in the ice.

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“…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. However, variations in seismic velocity on short vertical scales cannot be resolved with conventional surface-based seismic techniques with large wavelengths of the order of 10 m, depending on the source of the seismic waves and the sounding depth.…”

Section: Rapid Velocity Changes Over Short Vertical Distancesmentioning

confidence: 99%

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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Direct evidence for continuous radar reflector originating from changes in crystal-orientation fabric

Cited by 85 publications

References 28 publications

Layer disturbances and the radio-echo free zone in ice sheets

Layer disturbances and the radio-echo free zone in ice sheets

Imaging the structure of cave ice by ground-penetrating radar

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

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Direct evidence for continuous radar reflector originating from changes in crystal-orientation fabric

Cited by 85 publications

References 28 publications

Layer disturbances and the radio-echo free zone in ice sheets

Layer disturbances and the radio-echo free zone in ice sheets

Imaging the structure of cave ice by ground-penetrating radar

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

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