scholarly journals Modelling the Backscattering Properties of Anisotropic Rough Snow Surfaces: Implications for the Evolution of Near Surface Features on the Ice Sheets

2021 ◽  
Author(s):  
Shashwat Shukla ◽  
Stef Lhermitte ◽  
Bert Wouters
Keyword(s):  
Ice Sheets ◽  
Near Surface ◽  
Surface Features

The imaging of the near‐surface features using earthquakes and ambient noise in the Tehran region

2017 ◽  
Vol 15 (2) ◽  
pp. 131-139 ◽  
Author(s):  
Samane Asadi ◽  
Habib Rahimi ◽  
Rouhollah Amiri Fard
Keyword(s):  
Ambient Noise ◽  
Near Surface ◽  
Surface Features ◽  
Tehran Region

Electron Beam-Induced Deposition for Atom Probe Tomography Specimen Capping Layers

2016 ◽  
Vol 23 (2) ◽  
pp. 321-328 ◽  
Author(s):  
David R. Diercks ◽  
Brian P. Gorman ◽  
Johannes J. L. Mulders
Keyword(s):  
Electron Beam ◽  
Atom Probe Tomography ◽  
Focused Ion Beam ◽  
Ion Beam ◽  
Atom Probe ◽  
Near Surface ◽  
Surface Features ◽  
Dicobalt Octacarbonyl ◽  
Electron Beam Induced Deposition

AbstractSix precursors were evaluated for use as in situ electron beam-induced deposition capping layers in the preparation of atom probe tomography specimens with a focus on near-surface features where some of the deposition is retained at the specimen apex. Specimens were prepared by deposition of each precursor onto silicon posts and shaped into sub-70-nm radii needles using a focused ion beam. The utility of the depositions was assessed using several criteria including composition and uniformity, evaporation behavior and evaporation fields, and depth of Ga+ ion penetration. Atom probe analyses through depositions of methyl cyclopentadienyl platinum trimethyl, palladium hexafluoroacetylacetonate, and dimethyl-gold-acetylacetonate [Me2Au(acac)] were all found to result in tip fracture at voltages exceeding 3 kV. Examination of the deposition using Me2Au(acac) plus flowing O2 was inconclusive due to evaporation of surface silicon from below the deposition under all analysis conditions. Dicobalt octacarbonyl [Co2(CO)8] and diiron nonacarbonyl [Fe2(CO)9] depositions were found to be effective as in situ capping materials for the silicon specimens. Their very different evaporation fields [36 V/nm for Co2(CO)8 and 21 V/nm for Fe2(CO)9] provide options for achieving reasonably close matching of the evaporation field between the capping material and many materials of interest.

scholarly journals Development of crystal orientation fabric in the Dome Fuji ice core in East Antarctica: implications for the deformation regime in ice sheets

2021 ◽  
Author(s):  
Tomotaka Saruya ◽  
Shuji Fujita ◽  
Yoshinori Iizuka ◽  
Atsushi Miyamoto ◽  
Hiroshi Ohno ◽  
...  
Keyword(s):  
Crystal Orientation ◽  
Ice Core ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Inverse Correlation ◽  
Chloride Ions ◽  
East Antarctica ◽  
Dust Particles ◽  
Near Surface ◽  
Deformation Regime

Abstract. The crystal orientation fabric (COF) of a polar ice sheet has a significant effect on the rheology of the sheet. With the aim of better understanding the deformation regime of ice sheets, the present work investigated the COF in the upper 80 % of the depth within the 3035 m long Dome Fuji Station ice core drilled at one of the dome summits in East Antarctica. Dielectric anisotropy (∆ε) data were acquired as a novel indicator of the vertical clustering of COF resulting from vertical compressional strain within the dome, at which the ice cover has an age of approximately 300 kyrs BP. The ∆ε values were found to exhibit a general increase moving in the depth direction, but with fluctuations over distances on the order of 10–102 m. In addition, significant decreases in ∆ε were found to be associated with depths corresponding to three major glacial to interglacial transitions. These changes in ∆ε are ascribed to variations in the deformational history caused by dislocation motion occurring from near-surface depths to deeper layers. Fluctuations in ∆ε over distances of less than 0.5 m exhibited a strong inverse correlation with at depths greater than approximately 1200 m, indicating that they were enhanced during the glacial/interglacial transitions. The ∆ε data also exhibited a positive correlation with the concentration of chloride ions together with an inverse correlation with the amount of dust particles in the ice core at greater depths corresponding to decreases in the degree of c-axis clustering. Finally, we found that fluctuations in ∆ε persisted to approximately 80 % of the total depth of the ice sheet. These data suggest that the factors determining the deformation of ice include the concentration of chloride ions and amount of dust particles, and that the layered contrast associated with the COF is preserved all the way from the near-surface to a depth corresponding to approximately 80 % of the thickness of the ice sheet. These findings provide important implications regarding further development of the COF under the various stress-strain configurations that the ice will experience in the deepest region, approximately 20 % of the total depth from the ice/bed interface.

scholarly journals A combined surface-and volume-scattering model for ice-sheet radar altimetry

1993 ◽  
Vol 39 (133) ◽  
pp. 675-686 ◽  
Author(s):  
Curt H. Davis ◽  
Richard K. Moore
Keyword(s):  
Ice Sheets ◽  
Ice Sheet ◽  
Altimeter Data ◽  
Wave Form ◽  
Combined Model ◽  
Volume Scattering ◽  
Near Surface ◽  
Scattering Model ◽  
Wave Forms ◽  
Antarctic Ice Sheets

AbstractOver the last 15 years, satellite-altimeter data have been used to produce surface-elevation maps of the Greenland and Antarctic ice sheets with a 2 m accuracy. Analysis of Seasat and Geosat cross-over points showed that satellite altimeters can measure changes in the mass balance of the ice sheets. The retracking algorithm used to extract surface elevations from Seasat and Geosat return wave forms is based upon a modified form of the Brown surface-scattering model. Recent work has shown that altimeter wave forms over higher-altitude regions of the ice sheets are affected by sub-surface volume-scattering. Here, we develop a theoretical model for altimeter return wave forms over the ice sheets that is based on a combination of surface-and volume-scattering. By approximating the altimeter’s antenna pattern and transmitted pulse shape with Gaussian functions, we derive a closed-form analytical solution for the return-power volume-scattered from beneath the ice-sheet surface. We then combine the volume-scattering model with the Brown model and apply it to average wave forms from the Greenland and Antarctic ice sheets. The results show that the combined model accurately describes variations in altimeter wave-form shapes that are produced by differing contributions of surface-and volume-scattering to the received power. The combined model is then used to simulate return wave forms from a dual-frequency altimeter. The simulation shows that a two-frequency system can provide quantitative estimates of the absorption and scattering coefficients for near-surface snow.

The evolution of FWI and its perceived benefits

2019 ◽  
Vol 59 (1) ◽  
pp. 432
Author(s):  
Tony Martin ◽  
Andrew Long
Keyword(s):  
High Resolution ◽  
Seismic Imaging ◽  
Waveform Inversion ◽  
Low Frequencies ◽  
Near Surface ◽  
Surface Features ◽  
Case Examples ◽  
Velocity Models ◽  
Attribute Analysis ◽  
Seismic Volumes

Despite the mathematics behind full waveform inversion (FWI) being published in the early 1980s, it was 30 years before the method could be efficiently implemented on the scale of conventional 3D marine seismic volumes. FWI has evolved from using only transmitted waves and being constrained because towed streamer data lacked the very long offsets and ultra-low frequencies necessary to derive stable velocity updates beyond shallow depths. FWI now uses the full seismic wavefield (both transmitted and scattered wavefields), recovers deep velocity updates for standard offsets and frequencies and increasingly uses a wider range of frequencies that contribute to seismic imaging. We use several case examples to consider the benefits and caveats for robust FWI application: for resolving near-surface features and reducing seismic imaging uncertainty in areas with complex overburden heterogeneities; for resolving near-surface features and improving volumetric estimates; for using an enlarged bandwidth to resolve small model features; for updating the velocity in high contrast regimes; and for the creation of survey-wide, high-resolution models to reduce imaging uncertainty, complement attribute analysis, estimate elastic properties and prospect derisking. Collectively, we demonstrate how to produce high-resolution velocity models when conventional methods cannot and how to generate earth models in an accelerated fashion to reduce project turnaround. We describe pragmatic limits to what maximum FWI frequencies are reasonable and suggest ways that may soon by-pass signal processing and obtain direct earth attributes.

scholarly journals The significance of vertical moisture diffusion on drifting snow sublimation near snow surface

2017 ◽  
Vol 11 (6) ◽  
pp. 3011-3021 ◽  
Author(s):  
Ning Huang ◽  
Guanglei Shi
Keyword(s):  
Ice Sheets ◽  
Moisture Diffusion ◽  
Blowing Snow ◽  
Polar Ice ◽  
Sublimation Rate ◽  
Near Surface ◽  
Drifting Snow ◽  
Polar Ice Sheets ◽  
Snow Model ◽  
Semi Arid

Abstract. Sublimation of blowing snow is an important parameter not only for the study of polar ice sheets and glaciers, but also for maintaining the ecology of arid and semi-arid lands. However, sublimation of near-surface blowing snow has often been ignored in previous studies. To study sublimation of near-surface blowing snow, we established a sublimation of blowing snow model containing both a vertical moisture diffusion equation and a heat balance equation. The results showed that although sublimation of near-surface blowing snow was strongly reduced by a negative feedback effect, due to vertical moisture diffusion, the relative humidity near the surface does not reach 100  %. Therefore, the sublimation of near-surface blowing snow does not stop. In addition, the sublimation rate near the surface is 3–4 orders of magnitude higher than that at 10 m above the surface and the mass of snow sublimation near the surface accounts for more than half of the total snow sublimation when the friction wind velocity is less than about 0.55 m s−1. Therefore, the sublimation of near-surface blowing snow should not be neglected.

Growth of surface flaws induced by the environment and machining stresses in unalloyed uranium

2005 ◽  
Vol 40 (2) ◽  
pp. 139-150 ◽  
Author(s):  
A Wallwork ◽  
G Burnell ◽  
S Morris ◽  
A Rowe ◽  
I Clarke ◽  
...  
Keyword(s):  
Residual Stress ◽  
Stress Field ◽  
Computer Modelling ◽  
Crack Nucleation ◽  
Initial Assessment ◽  
Residual Stress Field ◽  
Surface Residual Stress ◽  
Near Surface ◽  
Surface Features ◽  
Subsequent Propagation

Computer modelling techniques are used to predict the distribution of residual stresses in a machined uranium surface. The predictions are used to address the ageing of uranium exposed to inert gas based environments in terms of microcrack initiation and subsequent propagation. Metallographic observations of microcracking are used as the basis for the initial assessment of ageing behaviour. It is proposed that the near-surface residual stress field produced by machining influences the occurrence of microcracking. It is also suggested that corrosion-induced surface features act as initiation sites for microcracks, which begin to propagate by an environmentally assisted mechanism when the surface features reach a critical depth within the residual stress field of between 5 and 10 μm. However, the majority of the microcracks appear to arrest at about 150 μm. This behaviour is discussed in terms of the predicted threshold stress intensity for crack nucleation, uranium metallurgy, and the possible effects of crack coalescence on growth.

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