Article

# Ice-sheet bed 3-D tomography

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## Abstract

Information on bed topography and basal conditions is essential to developing the next-generation ice-sheet models needed to generate a more accurate estimate of ice-sheet contribution to sea-level rise. Synthetic aperture radar (SAR) images of the ice-bed can be analyzed to obtain information on bed topography and basal conditions. We developed a wideband SAR, which was used during July 2005 to perform measurements over a series of tracks between the GISP2 and GRIP cores near Summit Camp, Greenland. The wideband SAR included an eight-element receive-antenna array with multiple-phase centers. We applied the MUltiple SIgnal Classification (MUSIC) algorithm, which estimates direction of arrival signals, to single-pass multichannel data collected as part of this experiment to obtain fine-resolution bed topography. This information is used for producing fine-resolution estimates of bed topography over a large swath of 1600m, with a 25m posting and a relative accuracy of approximately 10m. The algorithm-derived estimate of ice thickness is within 10m of the GRIP ice-core length. Data collected on two parallel tracks separated by 500m and a perpendicular track are compared and found to have difference standard deviations of 9.1 and 10.3m for the parallel and perpendicular tracks, respectively.

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... The radar was flown on a P-3 aircraft as part of NASA Operation IceBridge over the POW Icefield in April/May 2014 using a ∼5 km wide swath over Trinity and Wykeham glaciers and ∼2.5 km swath over Cadogan and Ekblaw glaciers, and downloaded from the University of Kansas CReSIS data portal (https://data.cresis.ku.edu/). This technique produces an image of the bed topography using nadir-derived ice thicknesses subtracted from surface topography (Jezek et al., 2013;Paden et al., 2010). Tomography requires a good signal-to-noise ratio in order to obtain reliable off-nadir returns (Jezek et al., 2013;Paden et al., 2010;Wu et al., 2011). ...
... This technique produces an image of the bed topography using nadir-derived ice thicknesses subtracted from surface topography (Jezek et al., 2013;Paden et al., 2010). Tomography requires a good signal-to-noise ratio in order to obtain reliable off-nadir returns (Jezek et al., 2013;Paden et al., 2010;Wu et al., 2011). Medrzycka et al. (2019) used this data set to extract bed elevation for Good Friday Bay Glacier on Axel Heiberg Island to better understand the topographic controls on glacier motion. ...
... Medrzycka et al. (2019) used this data set to extract bed elevation for Good Friday Bay Glacier on Axel Heiberg Island to better understand the topographic controls on glacier motion. Previous studies have found ice thickness uncertainty to be within 10-20 m using this method (Jezek et al., 2013;Paden et al., 2010;Wu et al., 2011). ...
Article
Full-text available
Over the last two decades, Trinity, Wykeham, Ekblaw, and Cadogan glaciers have retreated and been identified as the primary sources of iceberg flux from Prince of Wales (POW) Icefield, Ellesmere Island, accounting for ∼83% of total solid ice discharge. In this study, we used a total of 167 Radarsat‐2 Synthetic Aperture Radar scene pairs collected between 2009 and 2019 to derive winter surface velocities of these four major basins of the POW Icefield. Over this period both Cadogan and Ekblaw glaciers underwent multiyear acceleration and deceleration limited to their lower parts, consistent with characteristics of “pulse‐type” glaciers. Trinity and Wykeham glaciers are currently the fastest flowing glaciers in the CAA and are grounded below sea level for ∼40 km up‐glacier from their termini. Both glaciers underwent multiyear repeating periods of velocity acceleration between 2009 and 2019 which coincided with significant thinning at their termini. As of 2017, Trinity and Wykeham were each within ∼10 m of flotation over their lowermost 4 km. We also identified a shift in flow since 2014 on Trinity and Wykeham, after which winter flow rates began to propagate up‐glacier and were nearly identical to flow rates observed throughout the rest of the year, indicating that seasonal variability in flow has become less apparent in recent years. Our findings suggest that Trinity and Wykeham glaciers have transitioned to a flow type dominated by dynamic thinning, which is strongly influenced by subglacial topography and may be susceptible to instability of the glacier front and large‐scale collapse.
... In addition to antenna pattern synthesis, DoA estimations are used for bed-rock tomography. In [95], ice 3D tomography was demonstrated for the first time, with MIMO 3D icesounding SAR on a ground vehicle. In this publication, MUSIC is used with two expected main signals for each depth range, that may occur in the absence of layover, a distortion typical of radar that causes an object to appear closer than it is really found. ...
... In fact, BP might be considered equivalent to the highest MoCo order as possible. ω-K is used by CReSIS in their multifunction radars [65], and also in a 210 MHz MIMO 3D ice-sounding SAR on a ground vehicle [95]. BP is the processor chosen by DTU for POLARIS [97], and by the University of Kansas for a 150 MHz multichannel ice-sounding airborne SAR [12]. ...
... where P1 was taken as the reference regarding other receivers. For distinguishing left and right DoA, M is usually limited to 2 [95], but in some environments only 1 could be found [93]. In case the scattering from nadir is strong enough such as its sidelobes can leak into other far range for all the DoA angles under test, and will be a finite local maximum for M angles. ...
Conference Paper
The aim of the thesis is the software processing of data acquired by PASIN2 (Polarimetric Airborne Scientific Instrument, mark 2). It is a 150-MHz coherent pulsed synthetic aperture radar (SAR) for 3D imagery beneath the ice thickness of the Antarctic, designed and operated by the British Antarctic Survey (BAS) to map the overflown regions of the continent in a single pass. In conventional single SAR imaging (2D), along-track and range coordinates are obtained. For 3D mapping, the remaining across-track angle dimension is estimated after processing several SAR images, exploiting the multiple-input multiple-output (MIMO) capabilities, with 8 underwing elements (4 below each wing) switching between transmit- and receive-modes, and 4 receive-only below the fuselage. The array is non-uniformly distributed along the wing orientation, perpendicular to the aircraft trajectory. Using Matlab® software, the off-line processing of PASIN2 data consists firstly in amplitude, phase and delay calibration of the different channels; secondly, single SAR imaging resulting from Backprojection algorithm, assuming homogeneous ice medium, and electromagnetic propagation based on refraction and diffraction according to the surveyed area; and finally, the direction of arrival estimation, by combining the available images and applying a high-resolution non-linear technique called MUSIC. To deal with the spatial distribution of PASIN2 array, a pre-processing has been implemented to improve MUSIC outputs. The results lead to 3D map estimations of the bedrock, ice-water interface or subglacial channels, correcting the topography regarding models in which a vertical direction of arrival was wrongly assumed. These observations will be used by environmental scientist to design, optimise or validate climate models. The thesis is framed within a major project of the Natural Environment Research Council (NERC) called ‘Ice shelves in a warming world: Filchner Ice Shelf System, Antarctica’ (NERC reference NE/L013770/1), in which UCL and BAS participate, among others.
... In this mode, the Tx beamwidth is purposely spread over a wider angular range so that off-nadir targets are illuminated and a wider swath beneath the radar can be measured. Synthetic aperture radar tomography is used to process the data collected in this mode (e.g., Jezek et al., 2011;Paden et al., 2010). Although the nadir-sounding mode does not illuminate a wide swath, data collected using that mode can also be processed tomographically to generate swath images, but the achievable swath is generally narrower. ...
... OIB VHF radar-sounding data also advanced investigations into the potential for swath mapping of the bed topography beneath polar ice (also known as radar tomography). The years immediately before OIB saw several investigations using airborne campaigns to evaluate the feasibility of ground-based and airborne swath radar sounding of ice sheets (e.g., Jezek et al., 2011;Paden et al., 2010). Jezek et al. (2013) further advanced this possibility using a fine-resolution OIB survey from near the ice margin of southwestern Greenland Ice Sheet, which showed that even data not collected using techniques more ideal for swath mapping (e.g., beam steering) could still be used to produce swath images of the bed that were compatible with ice thickness inferred purely from nadir-sounding measurements. ...
Article
Full-text available
... In the airborne case, the array elements are typically attached under the wings and main aircraft body. Due to the small number of elements in the array, linear estimators like beam-steering [1] and MVDR (Minimum Variance Distortionless Response) [1][2] are limited in resolution, and high-resolution approaches based on autoanalysis like MUSIC (MUltiple SIgnal Classification) [2][3][4] and maximum likelihood [5] are preferred. In case the antennas follow a non-uniform distribution, the estimator performance is strongly affected, and the algorithms should be assessed to check the output under several conditions, and to develop new approaches to improve the results. ...
... (4) with the vector joining elements n-1 and n, and the unitary vector defining the DoA,  ...
Conference Paper
We propose an algorithm to evaluate the direction of arrival (DoA) estimation in the cross-track dimension of an airborne pulsed ice-sounding synthetic aperture radar (SAR), formed by a non-linear array. The conventional methods process either simulated data, meaning a lack of real scenario, or collected over regions previously mapped, where a limited number of DoAs is found. In our technique, we combine the echoes of different transmitted pulses from a real data take above the sea surface, planned to vary the roll angle of the aircraft. Due to the mirror-like behavior of the sea, when echoes at different roll angles are summed, we obtain raw data or images with several DoAs, relative to the main axes of the aircraft. This method is used to assess the estimation algorithms and choose the optimal for Antarctic bedrock 3D-imaging, identifying the DoAs to shape the true bed topography.
... We use direction of arrival (DOA) estimation to generate 3D images of ice sheet basal topography [1][2][3][4]. The topography is used to estimate the mass balance of ice sheets and, in ice sheet models, to predict future global mean sea level rise from the ice sheets. ...
... In previous ice radar tomography work, a fixed number of sources (targets) is assumed [e.g. [1][2][3][4], but this assumption does not always hold and leads to problems of underestimation or overestimation of the number of sources. Underestimation means that some sources are not detected at all and because the model cannot account for the energy from every source properly, the errors can be substantially larger. ...
Conference Paper
The performance of several methods to estimate the number of source signals impinging on a sensor array are compared using a traditional simulator and their performance for synthetic aperture radar tomography is discussed. All methods use a penalty term that increases with model order in order to prevent overestimation. We include both separate estimation of model selection and direction of arrival as well as joint estimation. We formulate a new penalty term, numerically tuned so that it gives optimal performance over our operating conditions, and compare this method as well. Simulation results show that the numerically tuned model selection criteria is optimal and that the typical methods do not do well for low snapshots. We also found that there is little sensitivity to SNR greater than 3 dB when the number of snapshots is high. We discuss some issues to applying the algorithms to data collected by the CReSIS radar depth sounder.
... Although this may be less important for numerical modelling, such detail provides valuable insights into the genesis of the subglacial landforms and the geomorphology of the bed. Recent results suggest reduced uncertainty and significantly increased spatial detail can be obtained from radar tomography (Jezek et al., 2011;Paden et al., 2010), while mass conservation approaches have also shown promise for poorly sampled outlet glaciers (Morlighem et al., 2011). Application of these techniques could significantly improve current mapping by decreasing the amount of interpolation needed over deeply incised outlet glaciers. ...
Article
Full-text available
We present a new bed elevation dataset for Greenland derived from a combination of multiple airborne ice thickness surveys undertaken between the 1970s and 2011. Around 344 000 line kilometres of airborne data were used, with the majority of this having been collected since the year 2000, when the last comprehensive compilation was undertaken. The airborne data were combined with satellite-derived elevations for non glaciated terrain to produce a consistent bed digital elevation model (DEM) over the entire island including across the glaciated/ice free boundary. The DEM was extended to the continental margin with the aid of bathymetric data, primarily from a compilation for the Arctic. Ice shelf thickness was determined where a floating tongue exists, in particular in the north. The across-track spacing between flight lines warranted interpolation at 1 km postings near the ice sheet margin and 2.5 km in the interior. Grids of ice surface elevation, error estimates for the DEM, ice thickness and data sampling density were also produced alongside a mask of land/ocean/grounded ice/floating ice. Errors in bed elevation range from a minimum of ±6 m to about ±200 m, as a function of distance from an observation and local topographic variability. A comparison with the compilation published in 2001 highlights the improvement in resolution afforded by the new data sets, particularly along the ice sheet margin, where ice velocity is highest and changes most marked. We use the new bed and surface DEMs to calculate the hydraulic potential for subglacial flow and present the large scale pattern of water routing. We estimate that the volume of ice included in our land/ice mask would raise eustatic sea level by 7.36 m, excluding any solid earth effects that would take place during ice sheet decay.
... SAR processing will migrate the along-track component, while focusing will help resolve the detail in the crosstrack component. For the HiCARS system, there is an ambiguity as to which side is the source of cross-track clutter; for radars with higher frequencies and more phase centres, beam steering and tomographic processing [72,73] can allow side discrimination and swath mapping of the bed using this clutter information. As found for Thwaites Glacier using specularity, water systems can be highly anisotropic, so cross-track clutter in one direction may not be representative of roughness or water conditions at the bed in all directions. ...
Article
Antarctica's subglacial lakes have two end member geophysical expressions: as hydraulically flat, radar reflective regions highlighted in ice surface topography and radar sounding profiles ('definite lakes'), and as localized sites of elevation change identified from repeat elevation observations ('active lakes') that are often found in fast flowing ice streams or enhanced ice flow tributaries. While 'definite lakes' can be identified readily by high bed reflectivity in radar sounding, the identification and characterization of less distinct subglacial lakes and water systems with radar sounding are complicated by variable radio-wave attenuation in the overlying ice. When relying on repeat elevation observations, the relatively short times series and biased distribution of elevation observations, along with the episodic nature of 'active lake' outflow and replenishment, limit our understanding of how water flows under the ice sheet. Using recently developed methods for quantifying the radar scattering behaviour of the basal interface of the ice, we can avoid the problem of attenuation, and observe the plumbing of the subglacial landscape. In West Antarctica's Ross Sea Embayment, we confirm that extensive distributed water systems underlie these ice streams. Distributed water sheets are upstream in the onset regions of fast flow, while canal systems underly downstream regions of fast flow. In East Antarctica, we use specularity analysis to recover substantial hydraulic connectivity extending beyond previous knowledge, connecting the lakes already delineated by traditional radar sounding or surface elevation transients.
... Increased bandwidth and signal sensitivity of radar systems have improved the detection, resolution and fidelity of radar reflections 60 . Swath radar technology, which enables (pseudo) 3D imaging of bed topography and englacial layers 61,62 , resolves basal roughness, hydrological routing and basal melt and freeze-on better than standard radar. Using the scattering characteristics of returned bed echoes, such as the specularity content 63 , trailing bed echoes 64 , the bed echo coherence index 65,66 and bed echo reflectivity variability 67 , has advanced the quantitative identification of subglacial water and understanding of subglacial drainage systems 63,[68][69][70] . ...
... Surveying the area with 3-D radar tomography (e.g. Paden et al., 2010) would also increase the spatial resolution substantially. Our maps, nevertheless, contain enough detail for a wide range of studies and can contribute to improvements in future ice sheet modelling efforts and studies of ice sheet dynamics and hydrology. ...
Article
Full-text available
We present ice thickness and bed topography maps with a high spatial resolution (250–500 m) of a land-terminating section of the Greenland Ice Sheet derived from ground-based and airborne radar surveys. The data have a total area of ~12 000 km2 and cover the whole ablation area of the outlet glaciers of Isunnguata Sermia, Russell, Leverett, Ørkendalen and Isorlersuup up to the long-term mass balance equilibrium line altitude at ~1600 m above sea level. The bed topography shows highly variable subglacial trough systems, and the trough of Isunnguata Sermia Glacier is overdeepened and reaches an elevation of ~500 m below sea level. The ice surface is smooth and only reflects the bedrock topography in a subtle way, resulting in a highly variable ice thickness. The southern part of our study area consists of higher bed elevations compared to the northern part. The compiled data sets of ground-based and airborne radar surveys cover one of the most studied regions of the Greenland Ice Sheet and can be valuable for detailed studies of ice sheet dynamics and hydrology. The combined data set is freely available at doi:10.1594/pangaea.830314.
... Radar instrumentation and processing is one area in which advances have been particularly apparent over the last decade, driven in part by the NASA IceBridge program. Research into beam-forming processing techniques offers the exciting potential for a multi-antennae array to survey swaths of the ice bed, similar to bathymetric multi-beam sonar surveying, vastly improving the efficiency of radar surveys (Paden et al. 2010, Wu et al. 2011. The development and application of ultra-wide-band sensors (e.g. ...
Article
Full-text available
Flood-carved landforms across the deglaciated terrain of Victoria Land, East Antarctica, provide convincing geomorphological evidence for the existence of subglacial drainage networks beneath the Antarctic ice sheet, and motivate research into the inaccessible environment beneath the contemporary ice sheet. Through this research, our understanding of Antarctic subglacial hydrology is steadily building, and this paper presents an overview of the current state of knowledge. The conceptualization of subglacial hydrological behaviour was developed at temperate and Arctic glaciers, and is thus less mature in the Antarctic. Geophysical and remote sensing observations have demonstrated that many subglacial lakes form part of a highly dynamic network of subglacial drainage beneath the Antarctic ice sheet. Recent research into subglacial water flows, other than those directly concerned with lakes, has discovered potentially significant impacts on ice stream dynamics, ice sheet mass balance, and supplies of water to the ocean potentially affecting circulation and nutrient productivity. Despite considerable advances in understanding there remain a number of grand challenges that must be overcome in order to improve our knowledge of these subglacial hydrological processes.
... Surveying the area with 3-D radar tomography (e.g. Paden et al., 2010) would also increase the spatial resolution substantially. Our maps, nevertheless, contain enough detail for a wide range of studies and can contribute to improvements in future ice sheet modelling efforts and studies of ice sheet dynamics and hydrology. ...
Article
Full-text available
We present ice thickness and bed topography maps with high spatial resolution (250 to 500 m) of a and-terminating section of the Greenland Ice Sheet derived from combined ground-based and airborne radar surveys. The data have a total area of ~12000 km2 and cover the whole ablation area of the outlet glaciers of Isunnguata Sermia, Russell, Leverett, Ørkendalen and Isorlersuup up to the long-term mass balance equilibrium line altitude at ~1600 m above sea level. The bed topography shows highly variable subglacial trough systems, and the trough of the Isunnguata Sermia Glacier is over-deepened and reaches an elevation of several hundreds of meters below sea level. The ice surface is smooth and only reflects the bedrock topography in a subtle way, resulting in a highly variable ice thickness. The southern part of our study area consists of higher bed elevations compared to the northern part. The covered area is one of the most studied regions of the Greenland Ice Sheet with studies of mass balance, dynamics, and supraglacial lakes, and our combined dataset can be valuable for detailed studies of ice sheet dynamics and hydrology. The compiled datasets of ground-based and airborne radar surveys are accessible for reviewers (password protected) at doi.pangaea.de/10.1594/pangaea.830314 and will be freely available in the final revised paper.
... Please note that, when using only the antenna in nadir, 2D flight profile echograms are collected, while when using the beam in three directions, the MCoRDS collects the continuous topological sequence of the ice sheet in the cross-track direction. For details of the MCoRDS parameters and processing, please refer to reference [24] and [40]. The radar topology sequence [41] collected by the MCoRDS is shown in Figures 3 and 4. Figure 3 shows a frame in a segment of 3D topology terrain sequence data. ...
Article
Full-text available
Analyzing the surface and bedrock locations in radar imagery enables the computation of ice sheet thickness, which is important for the study of ice sheets, their volume and how they may contribute to global climate change. However, the traditional handcrafted methods cannot quickly provide quantitative, objective and reliable extraction of information from radargrams. Most traditional handcrafted methods, designed to detect ice-surface and ice-bed layers from ice sheet radargrams, require complex human involvement and are difficult to apply to large datasets, while deep learning methods can obtain better results in a generalized way. In this study, an end-to-end multi-scale attention network (MsANet) is proposed to realize the estimation and reconstruction of layers in sequences of ice sheet radar tomographic images. First, we use an improved 3D convolutional network, C3D-M, whose first full connection layer is replaced by a convolution unit to better maintain the spatial relativity of ice layer features, as the backbone. Then, an adjustable multi-scale module uses different scale filters to learn scale information to enhance the feature extraction capabilities of the network. Finally, an attention module extended to 3D space removes a redundant bottleneck unit to better fuse and refine ice layer features. Radar sequential images collected by the Center of Remote Sensing of Ice Sheets in 2014 are used as training and testing data. Compared with state-of-the-art deep learning methods, the MsANet shows a 10% reduction (2.14 pixels) on the measurement of average mean absolute column-wise error for detecting the ice-surface and ice-bottom layers, runs faster and uses approximately 12 million fewer parameters.
... In these cases, a residual error remains due to the presence of steeply layers in both the along-track and/or the across-track directions. This could be addressed by using the along-track [9] and across-track (see [8], [17], [25]) slope-estimation techniques and/or designing a repeat survey on a grid to correct for the 3-D-slope effects on the vertical-velocity estimates [25]. However, for the purpose of this study, we focus on the areas with flat layers. ...
Article
Full-text available
Englacial layer velocity can provide insights on the vertical-velocity structure of the ice sheets. We present a repeat-pass interferometric approach that allows the estimation of the vertical englacial layer velocity using the radar sounder data. In contrast to the ground-based sensors, the airborne radar sounder data can potentially be used to estimate the layer velocity on a continental scale. When merged with the horizontal surface velocity and the numerical models, layer velocity can support the 3-D analysis of the ice flow and structure. Our aim is to provide the proof-of-concept demonstration that, similar to the side-looking synthetic aperture radar for imaging, the airborne radar sounder data can be used to estimate the subwavelength displacement of the englacial radio-stratigraphic layers. To achieve this, we use the phase and magnitude acquired repeatedly over the same region of interest. After the extraction of the crossing points, two acquisitions are finely registered. Then, we compute the interferometric phase for the englacial layers (which are shown to behave as PSs in the nadir sounding geometry), to estimate a vertical displacement and a velocity profile. We present the results over East Antarctica using data from the high-capability radar sounder (HiCARS) system. We show two scenarios that demonstrate the feasibility, limitations, and requirements of this approach.
... Indeed, AUV surveys and (or) a dense grid of seismic soundings (only obtainable from non-crevassed ice shelves) are the only way to determine bed geometry in ice-shelf cavities. New techniques such as swath radar that can image the present glacier bed in 3D (Paden et al., 2010;Jezek et al., 2011), albeit in narrow swaths, have already been employed on TG (Holschuh et al., 2020) and could be used in conjunction with offshore bathymetric data to build a betterinformed, more complete and more uniform resolution picture of basal conditions under TG and at its grounding zone. ...
Article
Full-text available
The geometry of the sea floor immediately beyond Antarctica's marine-terminating glaciers is a fundamental control on warm-water routing, but it also describes former topographic pinning points that have been important for ice-shelf buttressing. Unfortunately, this information is often lacking due to the inaccessibility of these areas for survey, leading to modelled or interpolated bathymetries being used as boundary conditions in numerical modelling simulations. At Thwaites Glacier (TG) this critical data gap was addressed in 2019 during the first cruise of the International Thwaites Glacier Collaboration (ITGC) project. We present more than 2000 km2 of new multibeam echo-sounder (MBES) data acquired in exceptional sea-ice conditions immediately offshore TG, and we update existing bathymetric compilations. The cross-sectional areas of sea-floor troughs are under-predicted by up to 40 % or are not resolved at all where MBES data are missing, suggesting that calculations of trough capacity, and thus oceanic heat flux, may be significantly underestimated. Spatial variations in the morphology of topographic highs, known to be former pinning points for the floating ice shelf of TG, indicate differences in bed composition that are supported by landform evidence. We discuss links to ice dynamics for an overriding ice mass including a potential positive feedback mechanism where erosion of soft erodible highs may lead to ice-shelf ungrounding even with little or no ice thinning. Analyses of bed roughnesses and basal drag contributions show that the sea-floor bathymetry in front of TG is an analogue for extant bed areas. Ice flow over the sea-floor troughs and ridges would have been affected by similarly high basal drag to that acting at the grounding zone today. We conclude that more can certainly be gleaned from these 3D bathymetric datasets regarding the likely spatial variability of bed roughness and bed composition types underneath TG. This work also addresses the requirements of recent numerical ice-sheet and ocean modelling studies that have recognised the need for accurate and high-resolution bathymetry to determine warm-water routing to the grounding zone and, ultimately, for predicting glacier retreat behaviour.
... Surveying the area with 3-D radar tomography (e.g. Paden et al., 2010) would also increase the spatial resolution substantially. Our maps, nevertheless, contain enough detail for a wide range of studies and can contribute to improvements in future ice sheet modelling efforts and studies of ice sheet dynamics and hydrology. ...
Data
Full-text available
We present ice thickness and bed topography maps with a high spatial resolution (250-500 m) of a land-terminating section of the Greenland Ice Sheet derived from ground-based and airborne radar surveys. The data have a total area of ~12 000 km^2 and cover the whole ablation area of the outlet glaciers of Isunnguata Sermia, Russell, Leverett, Ørkendalen and Isorlersuup up to the long-term mass balance equilibrium line altitude at ~1600 m above sea level. The bed topography shows highly variable subglacial trough systems, and the trough of Isunnguata Sermia Glacier is overdeepened and reaches an elevation of ~500 m below sea level. The ice surface is smooth and only reflects the bedrock topography in a subtle way, resulting in a highly variable ice thickness. The southern part of our study area consists of higher bed elevations compared to the northern part. The compiled data sets of ground-based and airborne radar surveys cover one of the most studied regions of the Greenland Ice Sheet and can be valuable for detailed studies of ice sheet dynamics and hydrology. By downloading the data set you agree to refer to the data set paper (Lindbäck et al., 2014, doi:10.5194/essd-6-331-2014) in presentations and publications arising from use of these data. The data set authors make no guarantees of product accuracy and the authors cannot be held liable for any errors, events, etc. arising from its use.
... Another source of uncertainty are off-nadir/side reflections that are difficult to distinguish from nadir reflections. Only coherent and phase-sensitive radar depth sounders with multiple cross-track antenna elements and appropriate 3D processing algorithms can compute the direction angle of the reflected signal (Paden et al., 2010). A better estimate of the effect of cross-track roughness is the scattering-derived waveform abruptness (Oswald and Gogineni, 2008;Cooper et al., 2019). ...
Preprint
The ice stream geometry and large ice surface velocities at the onset region of the Northeast Greenland Ice Stream (NEGIS) are not yet well reproduced by ice sheet models. The quantification of basal sliding and a parametrisation of basal conditions remains a major gap. In this study, we assess the basal conditions of the onset region of the NEGIS in a systematic analysis of airborne ultra-wideband radar data. We evaluate basal roughness and basal return echoes in the context of the current ice stream geometry and ice surface velocity. We observe a change from a smooth to a rougher bed where the ice stream widens, and a distinct roughness anisotropy, indicating a preferred orientation of subglacial structures. In the upstream region, the excess ice mass flux through the shear margins is evacuated by ice flow acceleration and along-flow stretching of the ice. At the downstream part, the generally rougher bed topography correlates with a decrease in flow acceleration and lateral variations in ice surface velocity. Together with basal water routing pathways, this hints to two different zones in this part of the NEGIS: the upstream region collecting water, with a reduced basal traction and downstream, where the ice stream is slowing down and is widening on a rougher bed, with a distribution of basal water towards the shear margins. Our findings support the hypothesis that the NEGIS is strongly interconnected to the subglacial water system in its onset region, but also to the subglacial substrate and morphology.
... Another source of uncertainty are off-nadir/side reflections that are difficult to distinguish from nadir reflections. Only coherent and phase-sensitive radar depth sounders with multiple cross-track antenna elements and appropriate 3D processing algorithms can compute the direction angle of the reflected signal (Paden et al., 2010). A better estimate of the effect of cross-track roughness is the scattering-derived waveform abruptness (Cooper et al., 2019;Jordan et al., 2017;Oswald & Gogineni, 2008). ...
Article
Full-text available
The ice stream geometry and large ice surface velocities at the onset region of the Northeast Greenland Ice Stream (NEGIS) are not yet well reproduced by ice sheet models. The quantification of basal sliding and a parametrization of basal conditions remains a major gap. In this study, we assess the basal conditions of the onset region of the NEGIS in a systematic analysis of airborne ultra‐wideband radar data. We evaluate basal roughness and basal return echoes in the context of the current ice stream geometry and ice surface velocity. We observe a change from a smooth to a rougher bed where the ice stream widens, and a distinct roughness anisotropy, indicating a preferred orientation of subglacial structures. In the upstream region, the excess ice mass flux through the shear margins is evacuated by ice flow acceleration and along‐flow stretching of the ice. At the downstream part, the generally rougher bed topography correlates with a decrease in flow acceleration and lateral variations in ice surface velocity. Together with basal water routing pathways, this hints to two different zones in this part of the NEGIS: the upstream region collecting water, with a reduced basal traction, and downstream, where the ice stream is slowing down and is widening on a rougher bed, with a distribution of basal water toward the shear margins. Our findings support the hypothesis that the NEGIS is strongly interconnected to the subglacial water system in its onset region, but also to the subglacial substrate and morphology.
... Such reconstructions are overwhelmingly based on records of glacial geomorphology and sediment distribution (Boulton et al., 2001;Clark, 2001, 2003;Jennings, 2006;Ottesen et al., 2008;Greenwood and Kleman, 2010;Durand et al., 2011;Rignot et al., 2011) and increasingly demand more detailed data. Recent research into the behaviour of palaeo-and modern ice sheets has shown that bedrock topography strongly controls the direction and the velocity of ice flow (Vaughan et al., 2006;Nakamura et al., 2007;Graham et al., 2009;Winsborrow et al., 2010), and information about the ice bed topography is essential for developing realistic ice sheet models (Siegert and Dowdeswell, 2004;Gogineni et al., 2007;Corti et al., 2008;Paden et al., 2010). Bedrock topography of contemporary ice sheets is difficult to access (Bamber et al., 2001;Behrendt et al., 2004;Wellner et al., 2006;Gogineni et al., 2007), and hence the study of formerly glaciated areas is of particular importance. ...
Article
This paper presents the first high quality maps of the bedrock surface topography and the Quaternary sediment distribution for the Late Weichselian glaciation area on the East European Craton (EEC) and the reasoning behind the estimated sediment distribution. Geographic information system (GIS)-based three-dimensional model of the bedrock surface topography, the current digital terrain model (DTM), and the ice flow pattern of the last glaciation provided the main source data for the analyses and interpretations. In the current continental area of the western EEC, which was covered by the last Scandinavian Ice Sheet (SIS), the bedrock lies overwhelmingly (85%) above the contemporary sea level, with an average altitude of ~ 54 m above sea level. The topography of the bedrock surface is very well reflected in the current topography, and an 80-m difference in the ice bed surface altitudes results in the topographic focusing of the ice flow, and thus all late Weichselian ice streams were accommodated to the depressions in the subglacial surface. Such depressions are characterised by a thin (< 10-m) layer of sediment with one till bed. Just 50 m upslope from the bottom of the depressions, the thickness of the sediment increases two- to threefold and comprises five or more till layers, indicating relief enhancement through subsequent glaciations. The average calculated thickness of the Quaternary deposits in the study area was ~ 49 m, and the total volume of sediments was ~ 20,280 km3.
... Increased bandwidth and signal sensitivity of radar systems have improved the detection, resolution and fidelity of radar reflections 60 . Swath radar technology, which enables (pseudo) 3D imaging of bed topography and englacial layers 61,62 , resolves basal roughness, hydrological routing and basal melt and freeze-on better than standard radar. Using the scattering characteristics of returned bed echoes, such as the specularity content 63 , trailing bed echoes 64 , the bed echo coherence index 65,66 and bed echo reflectivity variability 67 , has advanced the quantitative identification of subglacial water and understanding of subglacial drainage systems 63,[68][69][70] . ...
Article
Subglacial lakes store ancient climate records, provide habitats for life, and modulate ice flow, basal hydrology, biogeochemical fluxes and geomorphic activity. In this Review, we construct the first global inventory of subglacial lakes (773 total): 675 from Antarctica (59 newly identified in this study), 64 from Greenland, 2 beneath Devon Ice Cap, 6 beneath Iceland’s ice caps, and 26 from valley glaciers. We use this inventory to evaluate subglacial lake environments, dynamics, and their wider impact on ice flow and sediment transport. Lake behaviour is conditioned by their unique subglacial setting and the hydrologic, dynamic and mass balance regime of the overlying ice mass. We predict that in regions where climate warming causes ice-surface steepening there will be fewer and smaller lakes, but increased activity with higher discharge drainages of shorter duration. Coupling to surface melt and rainfall inputs will modulate fill-drain cycles and seasonally enhance oxic processes. Higher discharges cause large, transient ice-flow accelerations, but might result in overall net slowdown due to development of efficient subglacial drainage. Future subglacial lake research requires new drilling technologies, and the integration of geophysics, satellite monitoring and numerical modelling, which will provide new insight into their wider role in a changing Earth system.
Chapter
This chapter provides an introduction to radar sounding and imaging of the polar ice sheets and the application of Interferometric Synthetic Aperture Radar (InSAR) for ice dynamics and topography measurements. It begins with a brief review of the electrical properties of ice that are important to the design of radars for sounding ice and interpreting results. The chapter discusses the trade-offs between radar resolution and the signal-to-noise ratio, and the approaches used to improve the signal-to-noise ratio via hardware and software techniques. It also explains the use of SAR processing and pulse compression in a radar for improving along-track resolution and obtaining the high sensitivity required to sound ice around ice-sheet margins. The chapter presents a brief overview of antenna arrays and their use on both short- and long-range aircraft. Finally, it describes the principle of InSAR operation, the steps involved in generating InSAR images, and measurement errors.
Article
We developed a multichannel wideband synthetic aperture radar (SAR) that operates over a frequency range of 190–450 MHz for measurements over the ice sheets in Antarctica and Greenland. The antenna-array, which consists of eight elements housed in a certified external structure for a BASLER aircraft, was used for measurements during the 2013–2014 Antarctic field season. We performed measurements with this system in conjunction with two ultra-wideband radars operating over a frequency range of 2–8 GHz and 12–18 GHz on Siple Coast ice streams in West Antarctica during December 2013 and January 2014. We sounded ice thicker than 2 km with a signal-to-noise ratio (SNR) of more than 20 dB in an area with two-way ice loss of about 27 dB/km. The same system also simultaneously mapped near-surface internal layers with submeter resolution from the ice-surface to a depth of about 1100 for 1200 m thick ice. In this paper, we provide a detailed overview of the radar instrumentation and signal processing algorithms and present a few sample results. The radar will be operated over a frequency range of 150–550 MHz with a 24-element antenna-array for wide-ranging measurements over the Greenland and Antarctic ice sheets, starting around August 2015.
Article
Airborne radar sounding is an established tool for observing the bed conditions and subglacial hydrology of ice sheets and glaciers. The specularity content of radar bed echoes has also been used to detect the hydrologic transition of a subglacial water system from a network of distributed canals to a network of concentrated channels beneath the Thwaites Glacier. However, the physical dimensions of the distributed water bodies in these networks have not been constrained by observations. In this letter, we use a variety of simple radar scattering, attenuation, and cross-sectional models to provide a first estimate of the subglacial water body geometries capable of producing the observed anisotropic specularity of the Thwaites Glacier catchment. This approach leads to estimates of ice/water interface root mean square roughnesses less than about 15 cm, thicknesses of more than about 5 cm, lengths of more than about 15 m, and widths between about 0.5 and 5 m.
Article
Radio-echo sounding (RES) is a radar technique widely employed in Antarctica and Greenland to define bedrock topography but, over the last decade, it has also played an important role in subglacial lake exploration and hydrogeological studies at the bedrock/ice interface. In recent studies, bedrock characterization has been improved through analysis of radar power echoes to evaluate the electromagnetic (EM) properties of the interface and allow the distinction between wet and dry interfaces. The RES received signal power depends on ice absorption and bedrock reflectivity, which is closely linked to the specific physical condition of the bedrock. In this paper, an evaluation of EM ice absorption was conducted starting from RES measurements collected over subglacial lakes in Antarctica. The idea was to calculate ice absorption starting from the radar equation in the case of subglacial lakes, where the EM reflectivity value is considered a known constant. These values were compared with those obtained from analysis of ice-core dielectric profiles from EPICA ice-core drilling data. Our analysis reveals that the ice absorption rate calculated from RES measurements has an average value of 7.2 dB km–1, and it appears constant, independent of the subglacial lake depth in different zones of the Dome C area.
Article
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. Detailed maps of bed elevation and ice thickness are essential for understanding and projecting the evolution of the ice sheets. Such maps are traditionally obtained using airborne radar-sounding profiler data interpolated onto regular grids using geostatistical tools such as kriging. Here we compare three mapping techniques applied to a dense radar survey of Russell Glacier, West Greenland, by NASA Operation IceBridge: (1) radar tomography (RT) processing of the radar data to map the bed elevation, (2) interpolation of radar-derived thickness by ordinary kriging (KR) and (3) reconstruction of ice thickness based on the principles of mass conservation (MC) combining radar-sounding profiler and ice motion data. RT eliminates ambiguities caused by off-nadir reflections, but is spatially limited. KR yields a standard error in bed elevation of 35 m, but large errors (>300 m a–1) in flux divergence when combined with ice motion data. MC yields a comparable performance in bed elevation mapping, and errors smaller than 1 m a–1 in flux divergence. When the number of radar-sounding tracks is reduced, the performance of KR decreases more rapidly than for MC. Our study site shows that MC is capable of maintaining precision levels of 60 m at 400 m posting with flight tracks separated by 5 km.
Article
This paper presents a radar sensor package specifically developed for wide-coverage sounding and imaging of polar ice sheets from a variety of aircraft. Our instruments address the need for a reliable remote sensing solution well-suited for extensive surveys at low and high altitudes and capable of making measurements with fine spatial and temporal resolution. The sensor package that we are presenting consists of four primary instruments and ancillary systems with all the associated antennas integrated into the aircraft to maintain aerodynamic performance. The instruments operate simultaneously over different frequency bands within the 160 MHz-18 GHz range. The sensor package has allowed us to sound the most challenging areas of the polar ice sheets, ice sheet margins, and outlet glaciers; to map near-surface internal layers with fine resolution; and to detect the snow-air and snow-ice interfaces of snow cover over sea ice to generate estimates of snow thickness. In this paper, we provide a succinct description of each radar and associated antenna structures and present sample results to document their performance. We also give a brief overview of our field measurement programs and demonstrate the unique capability of the sensor package to perform multifrequency coincidental measurements from a single airborne platform. Finally, we illustrate the relevance of using multispectral radar data as a tool to characterize the entire ice column and to reveal important subglacial features.
Article
Conference Paper
Airborne remote sensing systems utilized for imaging ice sheets in Antarctica and Greenland have distinct advantages over ground and satellite systems, but strict structural limitations impose antenna performance-degrading objects near the antennas. These metallic objects required for the aircraft degrade the antenna bandwidth which is critical for accurate imaging of the internal layers and basal conditions of the ice. To improve the bandwidth of the system, a bow-tie antenna modified with parasitic elements has been developed for use with an airborne ice-penetrating and imaging radar. A controlled measurement study took place to examine and minimize the effects of antenna integration with these metallic objects providing valuable information on improving antenna integration when external aircraft mounting is required.
Article
A 15-element wideband dipole antenna array was developed for operation with the Multichannel Coherent Radar Depth Sounder/Imager on board the National Aeronautics and Space Administration P-3B aircraft. The array, aligned in the cross-track direction, was designed for applying digital beam forming and direction of arrival estimation algorithms to improve clutter suppression and for 3-D imaging of ice sheets. The antenna array is embedded inside an aerodynamic fairing structure designed for airborne operation. While the fairing meets all the structural and aircraft requirements, initial measurements performed on the original prototype array revealed the adverse impact of the fairing structure on antenna performance. The materials used for the construction of the fairing produced electrical loading effects on the radiating structure, which adversely impacted the bandwidth and return loss characteristics of individual antenna elements. This paper describes a set of modifications to the original antenna design based on computer simulations and laboratory measurements, aimed at optimizing antenna return loss and bandwidth while reducing mutual coupling. The final antenna and fairing structure achieved a fractional bandwidth of 40% at a center frequency of 195 MHz with a demonstrated peak power handling capability of 150 W. We were able to reduce the mutual coupling between antenna elements by a factor of two through modification of the dipole ends.
Article
Ice thickness estimates using advanced nadir sounding and tomographic radar processing techniques are compared and combined in a study of Isunnguata Sermia glacier, Greenland. Using an ensemble of Operation IceBridge flight lines spaced at 500 m intervals and running approximately along the flow direction, we find there is a statistically excellent comparison between subglacial terrains derived from two-dimensional tomography and gridded nadir sounding. Analysis shows that tomographic data better capture short wavelength (1–2 km) patterns in basal terrain, but interpolated nadir sounding data yield more spatially extensive and continuous coverage across the glacier, especially in deep subglacial troughs. Using derived surface and basal topography maps, we find that driving stress and measured and modeled surface velocity comparisons indicate that basal sliding is an important component of the glacier motion, but is also only weakly coupled to the detailed bed topography save for the deepest troughs. As might be expected for this land-terminating, relatively slow-moving glacier, the subglacial and proglacial topography is similar, suggesting the erosional processes acting on the modern glacier bed once helped sculpt the now exposed land.
Conference Paper
The basal topography of most of the glaciers that drain the ice caps of the Canadian Arctic Archipelago is largely unknown. To measure the basal topography, NASA Operation IceBridge flew a radar depth sounder in a wide swath mode with three transmit beams to image the glacier beds during three flights over the archipelago in 2014. We describe the measurement setup of the radar system, the algorithms used to process the data to produce a 3D image of the glacier bed, show digital elevation model (DEM) results of the beds, and provide a basic assessment of the tracking algorithm used to extract the DEM.
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Ice core drillings have been performed in various zones in Antarctica and Greenland to obtain climatological information, study ice properties, or analyze air and dust encapsulated in the ice during the quaternary period. During these procedures, a set of measurements to characterize the ice and to evaluate its physical and chemical properties are usually performed in situ. In particular, using known temperature and dielectric profiles (DEP measurements), it is possible to evaluate the ice electromagnetic power absorption profile, valid at the drilling site. In the last decades, bedrock characterization through radio echo sounding surveys has been improved by the analysis of the power of radar echoes. This way, analysis of the electromagnetic properties of bedrock interfaces makes it possible to assess the physical characteristics and to distinguish between wet and dry conditions. Power variation of the received echoes also depends on ice absorption and on bedrock reflectivity due to specific physical conditions of the ice. In this paper, the propagation of electromagnetic waves through the ice sheet is examined, and in particular, a new method for establishing the electromagnetic absorption profile for ice from core drilling measurements is proposed and discussed. Variation in the ice absorption is deduced, starting from the analysis of ice core data from the European Project for Ice Coring in Antarctica (EPICA) at the Concordia station (Antarctica) and from the Greenland Ice Core Project (GRIP) site (Greenland). This direct method of measurement is proposed with the aim of defining common characteristics of the ice absorption rate that are valid both in Antarctica and in Greenland.
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Ice sheets reshape Earth’s surface. Maps of the landscape formed by past ice sheets are our best tool for reconstructing historic ice sheet behavior. But models of glacier erosion and deposition that explain mapped features are relatively untested, and without observations of landforms developing in situ, postglacial landscapes can provide only qualitative insight into past ice sheet conditions. Here we present the first swath radar data collected in Antarctica, demonstrating the ability of swath radar technology to map the subglacial environment of Thwaites Glacier (West Antarctica) at comparable resolutions to digital elevation models of deglaciated terrain. Incompatibility between measured bedform orientation and predicted subglacial water pathways indicates that ice, not water, is the primary actor in initiating bedform development at Thwaites Glacier. These data show no clear relationship between morphology and glacier speed, a weak relationship between morphology and basal shear stress, and highlight a likely role for preexisting geology in glacial bedform shape.
Article
Full-text available
We present ice thickness and bed topography maps with high spatial resolution (250 to 500 m) of a and-terminating section of the Greenland Ice Sheet derived from combined ground-based and airborne radar surveys. The data have a total area of ~12000 km2 and cover the whole ablation area of the outlet glaciers of Isunnguata Sermia, Russell, Leverett, Ørkendalen and Isorlersuup up to the long-term mass balance equilibrium line altitude at ~1600 m above sea level. The bed topography shows highly variable subglacial trough systems, and the trough of the Isunnguata Sermia Glacier is over-deepened and reaches an elevation of several hundreds of meters below sea level. The ice surface is smooth and only reflects the bedrock topography in a subtle way, resulting in a highly variable ice thickness. The southern part of our study area consists of higher bed elevations compared to the northern part. The covered area is one of the most studied regions of the Greenland Ice Sheet with studies of mass balance, dynamics, and supraglacial lakes, and our combined dataset can be valuable for detailed studies of ice sheet dynamics and hydrology. The compiled datasets of ground-based and airborne radar surveys are accessible for reviewers (password protected) at doi.pangaea.de/10.1594/pangaea.830314 and will be freely available in the final revised paper.
Article
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Airborne radio-echo sounding (RES) surveys are widely used to measure ice-sheet bed topography. Measuring bed topography as accurately and widely as possible is of critical importance to modelling ice dynamics and hence to constraining better future ice response to climate change. Measurement accuracy of RES surveys is influenced both by the geometry of bed topography and the survey design. Here we develop a novel approach for simulating RES surveys over glaciated terrain, to quantify the sensitivity of derived bed elevation to topographic geometry. Furthermore, we investigate how measurement errors influence the quantification of glacial valley geometry. We find a negative bias across RES measurements, where off-nadir return measurement error is typically −1.8 ± 11.6 m. Topographic highlands are under-measured an order of magnitude more than lowlands. Consequently, valley depth and cross-sectional area are largely under-estimated. While overall estimates of ice thickness are likely too high, we find large glacier valley cross-sectional area to be under-estimated by −2.8 ± 18.1%. Therefore, estimates of ice flux through large outlet glaciers are likely too low when this effect is not taken into account. Additionally, bed mismeasurements potentially impact our appreciation of outlet-glacier stability.
Article
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Recent advances in the measurement of bedforms beneath active ice streams have been made using ground-based grid profiling using impulse radar systems operating with centre frequencies in the 3–5 MHz range. Surveys of Rutford Ice Stream and Pine Island Glacier have shown that features such as mega-scale glacial lineations with topographic relief of as little as 3 m can be traced for many kilometres downstream under more than 2 km of fast-moving ice. In the discussion of these data, it is often asked ‘How is it possible to map such fine-scale topography with such a low-frequency radar’. In answering that question, the key point is the distinction between the precision of a radar range measurement to a single, isolated reflective interface and the ability to resolve the presence of two closely-spaced interfaces of similar reflectivity (commonly referred to as the vertical resolution). This paper will discuss and illustrate this distinction and use the case study of data acquired over Pine Island Glacier to examine the limits of precision of the radar range measurement.
Article
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Radio echo sounding of polar ice sheets provides important information on the ice bed topography and internal layers. These data have been used by scientists to create 3D maps of polar ice sheets for climate modeling as well as to reconstruct the climate history that dates back to hundreds of thousands of years. In this paper, we present the design, and development of three surface-based multi-channel radars in the VHF and UHF bands. We provide results from radar data multi-frequency and polarization radar data collected over the Greenland ice sheet. All the three radars shared the same digital waveform generator and digitizer, and were installed in and operated on a tracked vehicle. The radars are operated with 3 different antenna arrays designed for operation over 170-230 MHz, 180-340 MHz and 600-900 MHz. The results we sounded more than 2.7 km thick ice with radars operating at frequencies as high as 850 MHz with more than 40 dB signal-to-noise ratio.
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Multichannel radar depth sounding systems are able to produce two-dimensional (2D) and three-dimensional (3D) imagery of the internal structure of polar ice sheets. Information such as ice thickness and surface elevation is extracted from these data and applied to research in ice flow modeling and ice mass balance calculations. Due to a large amount of data collected, we seek to automate the ice-bottom layer tracking and allow for efficient manual corrections when errors occur in the automated method. We present improvements made to previous implementations of the Viterbi and sequential tree-reweighted message passing (TRW-S) algorithms for ice-bottom extraction in 2D and 3D radar imagery. These improvements are in the form of novel cost functions that allow for the integration of further domain-specific knowledge into the cost calculations and provide additional evidence of the characteristics of the ice sheets surveyed. Along with an explanation of our modifications, we demonstrate the results obtained by our modified implementations of the two algorithms and by previously proposed solutions to this problem, when compared to manually corrected ground truth data. Furthermore, we perform a self-assessment of tracking results by analyzing differences in the estimated ice-bottom for surveyed locations where flight paths have crossed and, thus, two separate measurements have been made at the same location. Using our modified cost functions and preprocessing routines, we obtain significantly decreased mean error measurements from both algorithms, such as a 47% reduction in average tracking error in the case of 3D imagery between the original and our proposed implementation of TRW-S.
Article
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Previous studies reported that Good Friday Glacier had been actively surging in the 1950–60s, 1990s and again in 2000–15. Based on observations of terminus position change from air photos and satellite imagery, we fill the gaps between previous studies and conclude that the glacier has been advancing continuously since 1959. Ice surface velocities extracted from optical and synthetic aperture radar satellite images show higher flow rates than on most other marine-terminating glaciers in the region. This behaviour contrasts with the regional trend of glacier retreat over this period. Possible explanations involve a delayed response to positive mass-balance conditions of the Little Ice Age, or a dynamic instability. There is, however, insufficient evidence to attribute this behaviour to classical glacier surging as suggested in previous studies. Based on present-day ice velocity and glacier geometry patterns in the terminus region, we reconstruct the evolution of ice motion throughout the advance, and suggest that what has previously been interpreted as a surge, may instead have been a localised response to small-scale perturbations in bedrock topography.
Article
The phase-sensitive radio-echo sounder (pRES) is a powerful new instrument that can measure the depth of internal layers and the glacier bed to millimetre accuracy. We use a stationary 16-antenna pRES array on Store Glacier in West Greenland to measure the three-dimensional orientation of dipping internal reflectors, extending the capabilities of pRES beyond conventional depth sounding. This novel technique portrays the effectiveness of pRES in deriving the orientation of dipping internal layers that may complement profiles obtained through other geophysical surveying methods. Deriving ice vertical strain rates from changes in layer depth as measured by a sequence of pRES observations assumes that the internal reflections come from vertically beneath the antenna. By revealing the orientation of internal reflectors and the potential deviation from nadir of their associated reflections, the use of an antenna array can correct this assumption. While the array configuration was able to resolve the geometry of englacial layers, the same configuration could not be used to accurately image the glacier bed. Here, we use simulations of the performance of different array geometries to identify configurations that can be tailored to study different types of basal geometry for future deployments.
Article
Understanding the processes occurring at the ice sheets requires reliable three-dimensional (3-D) models of the ice sheet geometry. To address this challenge, we propose a technique for the 3-D reconstruction of the ice sheet geometry that uses radar sounder (RS) and altimeter (ALT) data to automatically identify the scale (or grid size) for interpolation. Existing studies derive the interpolation scale empirically, by qualitatively analyzing the RS data sampling and often neglecting the surface topography effects. Our method initially performs the interpolation of RS data at several potential scales. At each scale, it uses the ordinary kriging interpolation method that enables the quantitative analysis of both the RS data sampling and the surface topography. The optimal scale for the estimation of the surface map is identified according to an objective criterion that minimizes the difference to a subset of reference ALT data. Thereafter, the identified optimum scale on the surface is used to estimate the bedrock and ice thickness maps. Thus, the technique is a best-effort approach to the reconstruction of the ice sheet geometry, given the reference surface data and in the absence of reference bedrock data. Results obtained by applying the method to RS and ALT data acquired over the Byrd Glacier in Antarctica, in four regions characterized by different RS sampling and surface topography, confirm its effectiveness. Moreover, they point out that the method could be used for guiding future RS surveys, since the identified optimal scales are typically larger than those needed for addressing specific science objectives.
Article
In this paper, we analyze the direction-of-arrival (DOA) of the ice-sheet data collected over Jakobshavn Glacier with the airborne Multichannel Radar Depth Sounder (MRDS) during the 2006 field season. We extracted weak ice-bed echoes buried in signals scattered by the rough surface of the fast-flowing Jakobshavn Glacier by analyzing the DOA of signals received with a five-element receive-antenna array. This allowed us to obtain ice thickness information, which is a key parameter when generating bed topography of glaciers. We also estimated ice-bed roughness and bed slope from the combined analysis of the DOA and radar waveforms. The bed slope is about 8° and the roughness in terms of rms slope is about 16°.
Article
Geophysical surveys have played a fundamental role in glaciology, climatology, and geology studies conducted at and around the site of the EPICA Dome C Ice Core (Antarctica). Analysis of radio-echo sounding (RES) data collected during the 2009 and 2011 Italian Antarctic Expeditions, in the immediate vicinity (2 km) of the Core site, reveals the presence of small-scale bedrock structures. Data acquired during the 2011 campaign, which applied the latest updates of our instrument, show double echoes from the ice bottom separated by very small time intervals. These reflections are distributed close to the topographic dome and do not always correspond to particular features of the bedrock structure, but instead may correspond to small-scale subglacial water storage. The unprecedented spatial resolution of the basal environment for Dome C from these recent surveys allows us to identify this and other features that have been missed by previous larger scale RES surveys of this area.
Article
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We are developing a multifrequency multistatic synthetic aperture radar (SAR) for determining polar ice sheet basal conditions. To obtain data for designing and optimizing radar performance, we performed field measurements with a network-analyzer-based system during the 2003 field season at the North Greenland Ice Core Project camp (75.1 N and 42.3 W). From the measurements, we determine the ice sheet complex transfer function over the frequency range from 110-500 MHz by deconvolving out the system transfer function. Over this frequency range, we observe an increase in total loss of 8±2.5 dB using a linear regression to the log-scale data. With the ice sheet transfer function and an ice extinction model, we estimate the return loss from the basal surface to be approximately 37 dB. These measurements have broad applicability to interpreting radar-sounding data, which are widely used in glaciological studies of the polar ice sheets. These data have also been used in the link budget for the design considerations of the multifrequency multistatic SAR system.
Article
In this paper we present results from a three-dimensional image reconstruction algorithm for impulse radar operating in monostatic pulse-echo mode. The application of interest to us is the nondestructive evaluation of civil structures such as bridge decks. We use a multi-frequency diffraction tomography imaging technique in which coherent backward propagations of the received reflected wavefield form a spatial image of the scattering interfaces within the region of interest. This imaging technique provides high-resolution range and azimuthal visualization of the subsurface region. We incorporate the ability to image in planarly layered conductive media and apply the algorithm to experimental data from an offset radar system in which the radar antenna is not directly coupled to the surface of the region. We present a rendering in three-dimensions of the resulting image data which provides high-detail visualization.
Article
Experimental results are presented from a traverse over the ice sheet of north western Greenland in 1964, during which a continuously recorded profile of ice thickness was obtained for the first time. Interpretation of data from this traverse is consistent with results of subsequent work to December 1967. The parameters of the apparatus are presented briefly, while the details of electronic circuits are being published separately. Theoretical problems of radio wave propagation in an ice sheet and, in particular, the factors affecting accuracy are discussed. The uncertainty in depth, over a small area, is $\pm$ 5 m $\pm$ 1.5% and this is verified by comparison with the seismic results for a range of depths up to 1.5 km. It is found that the only real uncertainty arises in irregular terrain. The effectiveness of the radio echo technique is dependent on the absorption of radio waves in ice. Temperature, and to a lesser extent the impurity content of ice, appear to be the main variables affecting field performance. Earlier laboratory results on the variation of absorption with temperature for ice cores from northwest Greenland, together with theoretically predicted temperature distributions throughout the ice mass, have provided estimates of the total loss by absorption. These estimates are reasonably consistent with the observed echo strengths over most of the traverse. Consequently, it is predicted that echoes can be obtained over considerable areas of the ice sheets of Greenland and Antarctica, as has been verified by subsequent observations. The reflexion coefficient at the ice/rock interface is of the order of -15 dB. It could rise to 0 dB for an ice/water interface and one area was found in Greenland where it appeared to fall to -30 dB. Results from this traverse have shown that local surface slopes on the ice sheet are largely controlled by variations of longitudinal stress along the line of flow. Regional slopes over several kilometres vary with the velocity of movement of the ice, but appear to be less dependent on basal ice temperatures than laboratory results would suggest. The velocity of ice movement increases in proportion to the square or cube of the basal shear stress, but the stress itself shows no obvious dependence on basal ice temperature. Partially reflecting layers discovered within the ice mass are discussed mainly in terms of small density variations between adjacent layers of ice. One particularly prominent layer is calculated to be about 1000 years old and its variation of depth with position provides evidence in favour of the steady state model of the ice sheet.
Article
Shallow cores to about 17 m depth were recovered at nine sites distributed on a 150 km × 150 km survey grid centered Summit in central Greenland. Measurements of the stable oxygen-isotope ratio and the gross β activity as a function of depth enable annual summer horizons to be identified and dated. From these data, average accumulation rates were computed with an uncertainty of about 5–8%. The contours of constant accumulation rate in the region suggest a primary moisture flux from the southwest of the grid. Similarly, contours of the average oxygen-isotope ratio are consistent with the progressive isotopic depletion of a precipitating air mass moving from the southwest, and automatic weather-station data indicate that the moisture flux tends to be largest from the southwest quadrant. The annual variation in the total accumulation over the grid shows no persistent trend over the interval 1964–87 common to all cores.
Article
We provide an overview of a project to develop and demonstrate radars for imaging the ice–bed interface, mapping shallow and deep internal layers, and measuring ice thickness. We developed an ultra-wideband very high-frequency (VHF) radar (120 to 300 MHz) for simultaneously measuring ice thickness, mapping internal layers at depth, and imaging the ice–bed interface, and an ultra high-frequency (UHF) ultra-wideband radar (500 to 2000 MHz) for fine-resolution mapping of near-surface internal layers. We used these radars to collect data at Summit Camp, Greenland, during July 2005, and the West Antarctic Ice Sheet (WAIS) deep drill core site during January 2006. These results provide us with an unprecedented view of basal characteristics and internal layers. We also developed an outreach program, a communications system, and rovers to facilitate geophysical measurements in the future.
Article
Feasibility and potential of tomography by Ground Penetrating Radar are investigated through experiments on laboratory models. The aim is the development of radar tomography procedures for inspection of structures like walls or pillars in historical buildings. Two different approaches are explored to satisfy high-resolution requirements. The first approach improves the results of classical traveltime (TT) and amplitude tomography (AT) on thin straight or curved rays through a progressive reduction of the null space of the problem. TT is a quantitative tool based on the thin ray assumption that allows a good tradeoff between robustness and resolution. AT is as robust as TT, but its results have only qualitative contents, since the energy transferred to the medium is basically unknown and the scattering effects are not taken into account. In the second approach, GPR is considered as a diffracting source, so that migration (MIG) and diffraction tomography (DT) are applied to overcome the geometrical optic approximations. While DT is in principle the best tool to invert the scattered field and to achieve the maximum resolution, MIG can be a more robust solution that requires less preprocessing of the data. All these advantages and drawbacks of the different approaches are discussed with some examples on synthetic and real data.