scholarly journals Crevasse-induced Rayleigh-wave azimuthal anisotropy on Glacier de la Plaine Morte, Switzerland

2018 ◽  
Vol 60 (79) ◽  
pp. 96-111 ◽  
Author(s):  
Fabian Lindner ◽  
Gabi Laske ◽  
Fabian Walter ◽  
Adrian K. Doran
Keyword(s):  
Rayleigh Wave ◽  
Drainage System ◽  
Fracture Network ◽  
Wide Distribution ◽  
Azimuthal Anisotropy ◽  
Seismic Velocities ◽  
Ice Shelves ◽  
Passive Seismic ◽  
Iceberg Calving ◽  
And Inversion

AbstractCrevasses and englacial fracture networks route meltwater from a glacier's surface to the subglacial drainage system and thus influence glacial hydraulics. However, rapid fracture growth may also lead to sudden and potentially hazardous structural failure of unstable glaciers and ice dams, rifting of ice shelves, or iceberg calving. Here, we use passive seismic recordings to investigate the englacial fracture network on Glacier de la Plaine Morte, Switzerland. Glacier dynamics and the drainage of an ice-marginal lake give rise to numerous icequakes, the majority of which generate dispersed, high-frequency Rayleigh waves. A wide distribution of events allows us to study azimuthal anisotropy between 10 and 30 Hz in order to extract englacial seismic velocities in regions of preferentially oriented crevasses. Beamforming applied to a 100-m-aperture array reveals azimuthal anisotropy of Rayleigh-wave phase velocities reaching a strength of 8% at high frequencies. In addition, we find that the fast direction of wave propagation coincides with the observed surface strike of the narrow crevasses. Forward modeling and inversion of dispersion curves suggest that the azimuthal anisotropy is induced by a 40-m-thick crevassed layer at the surface of the glacier with 8% anisotropy in shear-wave velocity.

Evaluation of 3D shear-wave anisotropy based on elastic-wave velocity variations around the borehole

Geophysics ◽  
2021 ◽  
pp. 1-47
Author(s):  
Song Xu ◽  
Xiao-Ming Tang ◽  
Carlos Torres-Verdín ◽  
Zhen Li ◽  
Yuanda Su
Keyword(s):  
Shear Wave ◽  
Elastic Anisotropy ◽  
Anisotropy Parameter ◽  
Primary Source ◽  
Shear Velocity ◽  
Fracture Network ◽  
Elastic Wave Velocity ◽  
Azimuthal Anisotropy ◽  
Conventional Procedure ◽  
Radial Anisotropy

Aligned fractures/cracks in rocks are a primary source of elastic anisotropy. In an azimuthally anisotropic formation surrounding a borehole, shear waves split into fast and slow waves that propagate along the borehole and are recorded by a borehole logging tool. However, when the formation has conjugate fractures with orthogonal strike directions, the azimuthal anisotropy vanishes. Hence, azimuthal anisotropy measurements may not be adequate to detect orthogonal fracture sets. We develop a method for obtaining azimuthal and radial shear-wave anisotropy parameters simultaneously from four-component array waveforms. The method utilizes a velocity tomogram around the borehole. Azimuthal and radial anisotropy were determined by integrating shear velocity radiation profile along the radial direction at different azimuthal angles. Results indicate that this approach is reliable for estimating anisotropy properties in aligned crack systems. The advantage of this interpretation method is shown in multiple conjugate crack systems. Field data processing examples are used to verify the application of the proposed technique. Comparison of results against those obtained with a conventional procedure shows that the new method can not only provide estimates of azimuthal anisotropy, but also of the radial anisotropy parameter, which is important in fracture network evaluation.

scholarly journals Ocean-driven thinning enhances iceberg calving and retreat of Antarctic ice shelves

2015 ◽  
Vol 112 (11) ◽  
pp. 3263-3268 ◽  
Author(s):  
Yan Liu ◽  
John C. Moore ◽  
Xiao Cheng ◽  
Rupert M. Gladstone ◽  
Jeremy N. Bassis ◽  
...  
Keyword(s):  
Mass Loss ◽  
Mass Balance ◽  
Climate Forcing ◽  
Empirical Estimate ◽  
Positive Mass ◽  
Ice Shelves ◽  
Total Mass Loss ◽  
Rapid Changes ◽  
Iceberg Calving ◽  
Ice Sheet Mass Balance

Iceberg calving from all Antarctic ice shelves has never been directly measured, despite playing a crucial role in ice sheet mass balance. Rapid changes to iceberg calving naturally arise from the sporadic detachment of large tabular bergs but can also be triggered by climate forcing. Here we provide a direct empirical estimate of mass loss due to iceberg calving and melting from Antarctic ice shelves. We find that between 2005 and 2011, the total mass loss due to iceberg calving of 755 ± 24 gigatonnes per year (Gt/y) is only half the total loss due to basal melt of 1516 ± 106 Gt/y. However, we observe widespread retreat of ice shelves that are currently thinning. Net mass loss due to iceberg calving for these ice shelves (302 ± 27 Gt/y) is comparable in magnitude to net mass loss due to basal melt (312 ± 14 Gt/y). Moreover, we find that iceberg calving from these decaying ice shelves is dominated by frequent calving events, which are distinct from the less frequent detachment of isolated tabular icebergs associated with ice shelves in neutral or positive mass balance regimes. Our results suggest that thinning associated with ocean-driven increased basal melt can trigger increased iceberg calving, implying that iceberg calving may play an overlooked role in the demise of shrinking ice shelves, and is more sensitive to ocean forcing than expected from steady state calving estimates.

scholarly journals A statistical fracture model for Antarctic ice shelves and glaciers

2018 ◽  
Vol 12 (10) ◽  
pp. 3187-3213 ◽  
Author(s):  
Veronika Emetc ◽  
Paul Tregoning ◽  
Mathieu Morlighem ◽  
Chris Borstad ◽  
Malcolm Sambridge
Keyword(s):  
Large Scale ◽  
Damage Mechanics ◽  
Ice Sheets ◽  
Continuum Damage Mechanics ◽  
Ice Shelf ◽  
Ice Shelves ◽  
Flow Models ◽  
Iceberg Calving ◽  
Antarctic Continent ◽  
Average Success Rate

Abstract. Antarctica and Greenland hold enough ice to raise sea level by more than 65 m if both ice sheets were to melt completely. Predicting future ice sheet mass balance depends on our ability to model these ice sheets, which is limited by our current understanding of several key physical processes, such as iceberg calving. Large-scale ice flow models either ignore this process or represent it crudely. To model fractured zones, an important component of many calving models, continuum damage mechanics as well as linear fracture mechanics are commonly used. However, these methods have a large number of uncertainties when applied across the entire Antarctic continent because the models were typically tuned to match processes seen on particular ice shelves. Here we present an alternative, statistics-based method to model the most probable zones of the location of fractures and demonstrate our approach on all main ice shelf regions in Antarctica, including the Antarctic Peninsula. We can predict the location of observed fractures with an average success rate of 84 % for grounded ice and 61 % for floating ice and a mean overestimation error rate of 26 % and 20 %, respectively. We found that Antarctic ice shelves can be classified into groups based on the factors that control fracture location.

scholarly journals Efficient Location and Extraction of the Iceberg Calved Areas of the Antarctic Ice Shelves

2020 ◽  
Vol 12 (16) ◽  
pp. 2658
Author(s):  
Mengzhen Qi ◽  
Yan Liu ◽  
Yijing Lin ◽  
Fengming Hui ◽  
Teng Li ◽  
...  
Keyword(s):  
Extraction Process ◽  
Ice Shelf ◽  
Extraction Area ◽  
Ice Shelves ◽  
Textural Characteristics ◽  
Iceberg Calving ◽  
Ice Velocity ◽  
The Antarctic ◽  
Omission Errors

Continuous, rapid, and precise monitoring of calving events contributes to an in-depth understanding of calving mechanisms, which have the potential to cause significant mass loss from the Antarctic ice sheet. The difficulties in the precise monitoring of iceberg calving lie with the coexistence of ice shelf advances and calving. The manual location of iceberg calving is time-consuming and painstaking, while achieving precise extraction has mostly relied on the surface textural characteristics of the ice shelves and the quality of the images. Here, we propose a new and efficient method of separating the expansion and calving processes of ice shelves. We visualized the extension process by simulating a new coastline, based on the ice velocity, and detected the calved area using the simulated coastline and single-temporal post-calving images. We extensively tested the validity of this method by extracting four annual calving datasets (from August 2015 to August 2019) from the Sentinel-1 synthetic aperture radar mosaic of the Antarctic coastline. A total of 2032 annual Antarctic calving events were detected, with areas ranging from 0.05 km2 to 6141.0 km2, occurring on almost every Antarctic ice shelf. The extraction accuracy of the calved area depends on the positioning accuracy of the simulated coastline and the spatial resolution of the images. The positioning error of the simulated coastline is less than one pixel, and the determined minimum valid extraction area is 0.05 km2, when based on 75 m resolution images. Our method effectively avoids repetition and omission errors during the calved area extraction process. Furthermore, its efficiency is not affected by the surface textural characteristics of the calving fronts and the various changes in the frontal edge velocity, which makes it fully applicable to the rapid and accurate extraction of different calving types.

scholarly journals Viscous and elastic buoyancy stresses as drivers of ice-shelf calving

2020 ◽  
Vol 66 (258) ◽  
pp. 643-657 ◽  
Author(s):  
Cyrille Mosbeux ◽  
Till J. W. Wagner ◽  
Maya K. Becker ◽  
Helen A. Fricker
Keyword(s):  
Ice Sheet ◽  
Elastic Model ◽  
Ice Shelf ◽  
Ice Shelves ◽  
Bending Stresses ◽  
Iceberg Calving ◽  
Basal Melting ◽  
The Antarctic ◽  
The Impact ◽  
Maximum Stresses

AbstractThe Antarctic Ice Sheet loses mass via its ice shelves predominantly through two processes: basal melting and iceberg calving. Iceberg calving is episodic and infrequent, and not well parameterized in ice-sheet models. Here, we investigate the impact of hydrostatic forces on calving. We develop two-dimensional elastic and viscous numerical frameworks to model the ‘footloose’ calving mechanism. This mechanism is triggered by submerged ice protrusions at the ice front, which induce unbalanced buoyancy forces that can lead to fracturing. We compare the results to identify the different roles that viscous and elastic deformations play in setting the rate and magnitude of calving events. Our results show that, although the bending stresses in both frameworks share some characteristics, their differences have important implications for modeling the calving process. In particular, the elastic model predicts that maximum stresses arise farther from the ice front than in the viscous model, leading to larger calving events. We also find that the elastic model would likely lead to more frequent events than the viscous one. Our work provides a theoretical framework for the development of a better understanding of the physical processes that govern glacier and ice-shelf calving cycles.

scholarly journals Relationship between tidewater glacier calving velocity and water depth at the calving front

1991 ◽  
Vol 15 ◽  
pp. 115-118 ◽  
Author(s):  
Mauri S. Pelto ◽  
Charles R. Warren
Keyword(s):  
Water Depth ◽  
Reasonable Accuracy ◽  
Ice Streams ◽  
Ice Shelves ◽  
Tidewater Glaciers ◽  
Tidewater Glacier ◽  
Iceberg Calving ◽  
Calving Rate ◽  
The Relationship ◽  
Water Depths

An analysis of the relationship between iceberg calving rates and water depth has been completed for 22 tidewater glaciers. A linear relationship provides reasonable accuracy, with a correlation coefficient of 0.85, for all tidewater glaciers examined, whether they be polar or temperate. The polar glaciers have a slightly lower calving rate for a given water depth. This relationship indicates a lower calving rate for water depths over 50 m than determined by Brown and others (1982). It is based only on glaciers or ice streams and cannot be applied to ice shelves.

scholarly journals The Recent Advance of the Ross Ice Shelf Antarctica

1986 ◽  
Vol 32 (112) ◽  
pp. 464-474 ◽  
Author(s):  
S. S. Jacobs ◽  
D. R. Macayeal ◽  
J. L. Ardai
Keyword(s):  
Large Scale ◽  
Melting Rate ◽  
Spreading Rate ◽  
Ice Shelf ◽  
Ice Shelves ◽  
Ross Ice Shelf ◽  
Front Position ◽  
Iceberg Calving ◽  
Basal Melting ◽  
Available Information

AbstractThe seaward edge of the Ross Ice Shelf advanced northward at a minimum average velocity of 0.8 km a–1 between 1962 and 1985. That advance approximated velocities that have been obtained from glaciological data, indicating little recent wastage by iceberg calving. West of long. 178° E., the ice shelf has attained its most northerly position in the past 145 years, and has not experienced a major calving episode for at least 75 years. Since 1841 the ice-front position has advanced and retreated within a zone from about lat. 77° 10’S. (near long. 171° E.) to lat. 78° 40’ S. (near long. 164° W.). The central ice front is now farthest south but has the highest advance rate. Calving may occur at more frequent intervals in that sector, which also overlies the warmest ocean currents that flow into the sub-ice-shelf cavity. Available information on ice-shelf advance, thickness, spreading rate, and surface accumulation indicates a basal melting rate around 3 m a–1 near the ice front. These data and independent estimates imply that basal melting is nearly as large a factor as iceberg calving in maintaining the ice-shelf mass balance. In recent years, the Ross, Ronne, and Filchner Ice Shelves have contributed few icebergs to the Southern Ocean, while projections from a contemporaneous iceberg census are that circumpolar calving alone may exceed accumulation on the ice sheet. Large-scale ice-shelf calving may have preceded historical sightings of increased numbers of icebergs at sea.

scholarly journals Subcritical crack propagation as a mechanism of crevasse formation and iceberg calving

2004 ◽  
Vol 50 (168) ◽  
pp. 109-115 ◽  
Author(s):  
Jérôme Weiss
Keyword(s):  
Crack Propagation ◽  
Subcritical Crack Growth ◽  
Body Wave ◽  
Critical Crack ◽  
Ice Shelves ◽  
Wave Speeds ◽  
Linear Elastic ◽  
Iceberg Calving ◽  
Elastic Fracture ◽  
Natural Way

AbstractRecent investigations of crevassing on alpine glaciers and ice shelves have been based on linear elastic fracture mechanics (LEFM). However, LEFM is unable to explain some aspects of crevasse formation such as the initiation of crevasse propagation from crystal-scale (mm) microcracks, the slow propagation of large fractures in ice shelves, and the acceleration of crevasse opening before breaking of the ice terminus. Here another mechanism to account for these observations is proposed: subcritical crevassing. Subcritical crack growth, documented in many materials though not yet explored in ice, is characterized by a crack velocity that scales as a power of the tensile stress intensity factor, but is much less than that associated with critical crack propagation. This mechanism allows crevasse propagation from mm-scale microcracks at velocities much lower than body wave speeds, and explains crevasse-opening accelerations in a natural way. Subcritical crevassing is theoretically explored for several simplified situations but is limited by a lack of available data on crevasse evolution.

Combination of Active and Passive Seismic Analyses for Embankment Characterization

2015 ◽  
Vol 771 ◽  
pp. 179-182 ◽  
Author(s):  
Yekti Widyaningrum ◽  
Sungkono ◽  
Alwi Husein ◽  
Bagus Jaya Santosa ◽  
Ayi S. Bahri
Keyword(s):  
Rayleigh Wave ◽  
High Frequency ◽  
Low Frequency ◽  
Wave Dispersion ◽  
Standard Penetration Test ◽  
Near Surface ◽  
Combining Data ◽  
Passive Seismic ◽  
Estimate Dispersion ◽  
Rayleigh Wave Dispersion

Rayleigh wave dispersion is intensively used to determine near surface of shear wave velocity (Vs). The method has been known as non-invasive techniques which is costly effective and efficient to characterize subsurface. Acquisition of the Rayleigh wave can be approached in two ways, i.e. passive and active. Passive seismic is accurate to estimate dispersion curve in low frequency, although it is not accurate for high frequency. While active seismic is vice versa of passive seismic. The high frequency of Rayleigh wave dispersion reflects to near surface and vice versa. Therefore, we used the combination of both passive and active seismic method to overcome the limitations of each method. The Vs which is resulted by inversion of the combining data gives accurate model if compared to log and standard penetration test (N-SPT) data. Further, the approach has been used to characterize LUSI (Lumpur Sidoarjo) embankments. The result shows that embankment material (0-12 m) has higher Vs than that lower embankment material.

scholarly journals The statistical physics of iceberg calving and the emergence of universal calving laws

2011 ◽  
Vol 57 (201) ◽  
pp. 3-16 ◽  
Author(s):  
J.N. Bassis
Keyword(s):  
Probability Distribution ◽  
Statistical Physics ◽  
Difficult Problem ◽  
Large Spatial Scale ◽  
Ice Shelves ◽  
Environmental Regimes ◽  
Discrete Fracture ◽  
Iceberg Calving ◽  
Semi Empirical ◽  
Scale Limit

AbstractDetermining a calving law valid for all glaciological and environmental regimes has proven to be a difficult problem in glaciology. For this reason, most models of the calving process are semi-empirical, with little connection to the underlying fracture processes. In this study, I introduce methods rooted in statistical physics to show how calving laws, valid for any glaciological domain, can emerge naturally as a large-spatial-scale/long-temporal-scale limit of an underlying continuous or discrete fracture process. An important element of the method developed here is that iceberg calving is treated as a stochastic process and that the probability an iceberg will detach in a given interval of time can be described by a probability distribution function. Using limiting assumptions about the underlying probability distribution, the theory is shown to be able to simulate a range of calving styles, including the sporadic detachment of large, tabular icebergs from ice tongues and ice shelves and the more steady detachment of smaller-sized bergs from tidewater/outlet glaciers. The method developed has the potential to provide a physical basis to include iceberg calving into numerical ice-sheet models that can be used to produce more realistic estimates of the glaciological contribution to sea-level rise.

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