Constraining variable density of ice shelves using wide-angle radar measurements
TL;DR: In this article, the authors reconstruct depth to internal reflectors, local ice thickness, and firn-air content using a novel algorithm that includes traveltime inversion and ray tracing with a prescribed shape of the depth-density relationship.
Abstract: . The thickness of ice shelves, a basic parameter for mass balance estimates, is typically inferred using hydrostatic equilibrium, for which knowledge of the depth-averaged density is essential. The densification from snow to ice depends on a number of local factors (e.g., temperature and surface mass balance) causing spatial and temporal variations in density–depth profiles. However, direct measurements of firn density are sparse, requiring substantial logistical effort. Here, we infer density from radio-wave propagation speed using ground-based wide-angle radar data sets (10 MHz) collected at five sites on Roi Baudouin Ice Shelf (RBIS), Dronning Maud Land, Antarctica. We reconstruct depth to internal reflectors, local ice thickness, and firn-air content using a novel algorithm that includes traveltime inversion and ray tracing with a prescribed shape of the depth–density relationship. For the particular case of an ice-shelf channel, where ice thickness and surface slope change substantially over a few kilometers, the radar data suggest that firn inside the channel is about 5 % denser than outside the channel. Although this density difference is at the detection limit of the radar, it is consistent with a similar density anomaly reconstructed from optical televiewing, which reveals that the firn inside the channel is 4.7 % denser than that outside the channel. Hydrostatic ice thickness calculations used for determining basal melt rates should account for the denser firn in ice-shelf channels. The radar method presented here is robust and can easily be adapted to different radar frequencies and data-acquisition geometries.
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TL;DR: A massive subsurface ice layer is discovered in an area of intense melting and intermittent ponding on Larsen C Ice Shelf, Antarctica, found to be ∼10 °C warmer and ∼170 kg m−3 denser than anticipated in the absence of ponding and hitherto used in models of ice-shelf fracture and flow.
Abstract: Surface melt ponds form intermittently on several Antarctic ice shelves. Although implicated in ice-shelf break up, the consequences of such ponding for ice formation and ice-shelf structure have not been evaluated. Here we report the discovery of a massive subsurface ice layer, at least 16 km across, several kilometres long and tens of metres deep, located in an area of intense melting and intermittent ponding on Larsen C Ice Shelf, Antarctica. We combine borehole optical televiewer logging and radar measurements with remote sensing and firn modelling to investigate the layer, found to be ∼10 °C warmer and ∼170 kg m−3 denser than anticipated in the absence of ponding and hitherto used in models of ice-shelf fracture and flow. Surface ponding and ice layers such as the one we report are likely to form on a wider range of Antarctic ice shelves in response to climatic warming in forthcoming decades.
72 citations
TL;DR: In this article, high-resolution altimetry measurements from 2010 to 2016 show that Dotson Ice Shelf (DIS), West Antarctica, thins in response to basal melting focused along a single 5 km-wide and 60 km-long channel extending from the ice shelf's grounding zone to its calving front.
Abstract: Ice shelves play a vital role in regulating loss of grounded ice and in supplying freshwater to coastal seas. However, melt variability within ice shelves is poorly constrained and may be instrumental in driving ice shelf imbalance and collapse. High-resolution altimetry measurements from 2010 to 2016 show that Dotson Ice Shelf (DIS), West Antarctica, thins in response to basal melting focused along a single 5 km-wide and 60 km-long channel extending from the ice shelf's grounding zone to its calving front. If focused thinning continues at present rates, the channel will melt through, and the ice shelf collapse, within 40-50 years, almost two centuries before collapse is projected from the average thinning rate. Our findings provide evidence of basal melt-driven sub-ice shelf channel formation and its potential for accelerating the weakening of ice shelves.
60 citations
TL;DR: In this article, a combination of inverse methods and ice-penetrating radars is used to date a recent reorganization of an ice divide in the Weddell Sea Sector, West Antarctica.
Abstract: We date a recent ice-flow reorganization of an ice divide in the Weddell Sea Sector, West Antarctica, using a novel combination of inverse methods and ice-penetrating radars. We invert for two-dimensional ice flow within an ice divide from data collected with a phase-sensitive ice-penetrating radar while accounting for the effect of firn on radar propagation and ice flow. By comparing isochronal layers simulated using radar-derived flow velocities with internal layers observed with an impulse radar, we show that the divide's internal structure is not in a steady state but underwent a disturbance, potentially implying a regional ice-flow reorganization, 2.5 (1.8–2.9) kyr B.P. Our data are consistent with slow ice flow in this location before the reorganization and the ice divide subsequently remaining stationary. These findings increase our knowledge of the glacial history of a region that lacks dated constraints on late-Holocene ice-sheet retreat and provides a key target for models that reconstruct and predict ice-sheet behavior
37 citations
Cites background from "Constraining variable density of ic..."
...…of strain rate boundary conditions (supporting information) and our assumptions of small ice-sheet surface and bed slopes, positive horizontal strain rates and steady state, horizontally uniform densities that are known to vary significantly across some ice divides [Drews et al., 2016]....
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TL;DR: In this paper, an improved technique based on satellite observations is presented to capture the small-scale variability in the basal mass balance (BMB) of ice shelves of the Roi Baudouin Ice Shelf, Dronning Maud Land, East Antarctica.
Abstract: . Ice shelves control the dynamic mass loss of ice sheets through buttressing and their integrity depends on the spatial variability of their basal mass balance (BMB), i.e. the difference between refreezing and melting. Here, we present an improved technique – based on satellite observations – to capture the small-scale variability in the BMB of ice shelves. As a case study, we apply the methodology to the Roi Baudouin Ice Shelf, Dronning Maud Land, East Antarctica, and derive its yearly averaged BMB at 10 m horizontal gridding. We use mass conservation in a Lagrangian framework based on high-resolution surface velocities, atmospheric-model surface mass balance and hydrostatic ice-thickness fields (derived from TanDEM-X surface elevation). Spatial derivatives are implemented using the total-variation differentiation, which preserves abrupt changes in flow velocities and their spatial gradients. Such changes may reflect a dynamic response to localized basal melting and should be included in the mass budget. Our BMB field exhibits much spatial detail and ranges from −14.7 to 8.6 m a−1 ice equivalent. Highest melt rates are found close to the grounding line where the pressure melting point is high, and the ice shelf slope is steep. The BMB field agrees well with on-site measurements from phase-sensitive radar, although independent radar profiling indicates unresolved spatial variations in firn density. We show that an elliptical surface depression (10 m deep and with an extent of 0.7 km × 1.3 km) lowers by 0.5 to 1.4 m a−1, which we tentatively attribute to a transient adaptation to hydrostatic equilibrium. We find evidence for elevated melting beneath ice shelf channels (with melting being concentrated on the channel's flanks). However, farther downstream from the grounding line, the majority of ice shelf channels advect passively (i.e. no melting nor refreezing) toward the ice shelf front. Although the absolute, satellite-based BMB values remain uncertain, we have high confidence in the spatial variability on sub-kilometre scales. This study highlights expected challenges for a full coupling between ice and ocean models.
33 citations
Cites background from "Constraining variable density of ic..."
...Moreover, Drews et al. (2016) used wide-angle radar measurements in conjunction with ice coring and found that firn density varies spatially over tens of kilometres scales, in particular across ice shelf channels, where surface melt water collects in the corresponding surface depressions and…...
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...Both Drews et al. (2016) and Langley et al. (2014) found evidence in the shallow radar stratigraphy that the SMB may be locally elevated in those areas, potentially reflecting the deposition of drifting snow at the bottom of surface slopes (Frezzotti et al., 2007)....
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TL;DR: In this article, the internal structure of the Larsen C Ice Shelf (LCIS) was explored using OPTV data collected from the northern and central sectors of the ice shelf during austral spring 2014 and 2015.
Abstract: We use borehole optical televiewing (OPTV) to explore the internal structure of Larsen C Ice Shelf (LCIS). We report a suite of five ~90 m long OPTV logs, recording a light-emitting diode-illuminated, geometrically correct image of the borehole wall, from the northern and central sectors of LCIS collected during austral spring 2014 and 2015. We use a thresholding-based technique to estimate the refrozen ice content of the ice column and exploit a recently calibrated density-luminosity relationship to reveal its structure. All sites are dense and strongly influenced by surface melt, with frequent refrozen ice layers and mean densities, between the depths of 1.87 and 90 m, ranging from 862 to 894 kg m−3. We define four distinct units that comprise LCIS and relate these to ice provenance, dynamic history, and past melt events. These units are in situ meteoric ice with infiltration ice (U1), meteoric ice which has undergone enhanced densification (U2), thick refrozen ice (U3), and advected continental ice (U4). We show that the OPTV-derived pattern of firn air content is consistent with previous estimates, but that a significant proportion of firn air is contained within U4, which we interpret to have been deposited inland of the grounding line. The structure of LCIS is strongly influenced by the E-W gradient in fohn-driven melting, with sites close to the Antarctic Peninsula being predominantly composed of refrozen ice. Melting is also substantial toward the ice shelf center with >40% of the overall imaged ice column being composed of refrozen ice.
21 citations
References
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01 Jan 1969
TL;DR: In this paper, the transformation of snow to ice mass balance heat budget and climatology structure and deformation of ice hydraulics and glaciers glacier sliding deformation, subglacial till structures and fabrics in glaciers and ice sheets distribution of temperature in glaciers, flow of ice shelves and ice streams non-steady flow of glaciers, ice sheets surging and tidewater glaciers ice core studies.
Abstract: The transformation of snow to ice mass balance heat budget and climatology structure and deformation of ice hydraulics and glaciers glacier sliding deformation of subglacial till structures and fabrics in glaciers and ice sheets distribution of temperature in glaciers and ice sheets steady flow of glaciers and ice sheets flow of ice shelves and ice streams non-steady flow of glaciers and ice sheets surging and tidewater glaciers ice core studies.
4,450 citations
"Constraining variable density of ic..." refers background in this paper
...The firn–ice transition, marked by the depth at which air bubbles are isolated, occurs at a density of approximately 830 kgm−3 at depths typically ranging from 30 to 120 m in polar regions (Cuffey and Paterson, 2010, Chapter 2)....
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TL;DR: In this article, a method of seismic traveltime inversion for simultaneous determination of 2-D velocity and interface structure is presented that is applicable to any type of body-wave seismic data, as opposed to trial-and-error forward modelling, is that it provides estimates of model parameter resolution, uncertainty and non-uniqueness, and an assurance that the data have been fit according to a specified norm.
Abstract: SUMMARY A method of seismic traveltime inversion for simultaneous determination of 2-D velocity and interface structure is presented that is applicable to any type of body-wave seismic data. The advantage of inversion, as opposed to trial-and-error forward modelling, is that it provides estimates of model parameter resolution, uncertainty and non-uniqueness, and an assurance that the data have been fit according to a specified norm. In addition, the time required to interpret data is significantly reduced. The inversion scheme is iterative and is based on a model parametrization and a method of ray tracing suited to the forward step of an inverse approach. The number and position of velocity and boundary nodes can be adapted to the shot-receiver geometry and subsurface ray coverage, and to the complexity of the near-surface. The model parametrization also allows ancillary amplitude information to be used to constrain model features not adequately resolved by the traveltime data alone. The method of ray tracing uses an efficient numerical solution of the ray tracing equations, an automatic determination of take-off angles, and a simulation of smooth layer boundaries that yields more stable inversion results. The partial derivatives of traveltime with respect to velocity and the depth of boundary nodes are calculated analytically during ray tracing and a damped least-squares technique is used to determine the updated parameter values, both velocities and boundary depths simultaneously. The stopping criteria and optimum number of velocity and boundary nodes are based on the trade-off between RMS traveltime residual and parameter resolution, as well as the ability to trace rays to all observations. Methods for estimating spatial resolution and absolute parameter uncertainty are presented. An example using synthetic data demonstrates the algorithm's accuracy, rapid convergence and sensitivity to realistic noise levels. An inversion of refraction and wide-angle reflection traveltimes from the 1986 IRISPASSCAL Nevada, USA (Basin and Range province) seismic experiment illustrates the methodology and practical considerations necessary for handling real data. A comparison of our final 2-D velocity model with results from studies using other 1-D and 2-D forward and inverse methods serves as a check on the validity of the inversion scheme and provides estimates of parameter uncertainties that account for the bias introduced by the modelling approach and the interpreter.
1,465 citations
TL;DR: In this paper, a simple but accurate formula is developed for the quick calculation of interval velocities from the known x2-T2 technique, which is obtained through a correlation of velocity with rock type and depth.
Abstract: The purpose of this paper is to discuss field and interpretive techniques which permit, in favorable cases, the quite accurate determination of seismic interval velocities prior to drilling. A simple but accurate formula is developed for the quick calculation of interval velocities from “average velocities” determined by the known x2-T2 technique. To secure accuracy a careful study of multiple reflections is necessary and this is discussed. Although the principal objective in determining velocities is to allow an accurate structural interpretation to be made from seismic reflection data, an important secondary objective is to get some lithological information. This is obtained through a correlation of velocities with rock type and depth.
1,232 citations
"Constraining variable density of ic..." refers background or methods in this paper
...…ray paths such that both the reflector depth and the radiowave propagation speed may be calculated using methods such as the Dix inversion (Dix, 1955), semblance analysis (e.g., Booth et al., 2010, 2011), interferometry (Arthern et al., 2013), or traveltime inversion based on ray tracing…...
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...Initial guesses for reflector depths are based on standard linear regression in the traveltime2–offset2 diagrams (Dix, 1955); r0 = 0.033 m−1 and A= 460 kgm−3 stem from the 2010 OPTV density profile (Hubbard et al., 2013)....
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...In multi-offset surveys, the traveltime of internal reflectors increases hyperbolically with increasing offset (e.g., Dix, 1955), while the surface wave (traveling in the firn column directly from transmitter to receiver) has a linear moveout. www.the-cryosphere.net/10/811/2016/ The Cryosphere, 10,…...
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...…benefit of this approach for interpreting the radar results A common problem when using the Dix inversion or semblance analysis is that the applied normal moveout (NMO) approximation presupposes small reflection angles (to linearize trigonometric functions) and small velocity contrasts (Dix, 1955)....
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TL;DR: In this paper, the Bruggeman and Bottcher equation for the dielectric constant of a heterogeneous system is reconsidered and a theoretical model is designed from which both formulae can be derived.
1,106 citations
"Constraining variable density of ic..." refers methods in this paper
...In all cases, density can be inferred from the radar-wave speed using density–permittivity relations (e.g., Looyenga, 1965; Wharton et al., 1980; Kovacs et al., 1995)....
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