Ice Penetrating Research Articles

Removal of Diagonal Strip Noise in Airborne Ice Penetrating Radar Data

Airborne ice-penetrating radar (IPR) is a powerful geophysical method for detecting ice thickness, subglacial topography and internal layers in the vast and frigid Antarctic ice sheet. Data processing is an essential step in improving the signal-to-noise ratio (SNR). In order to eliminate the diagonal strip noise mainly present in airborne IPR data of the Amery ice shelf and Grove Mountain regions, traditional ground penetrating radar processing methods such as F-K (frequency-wavenumber domain) filtering and F-K migration are applied. The validity of the data processing methods is proved by the quantitative index of image quality evaluation. It provides a reference for subsequent airborne radar data processing and application.

Virtual Synthetic Aperture Radar Target Based on a Miniaturized Monostatic Digital Delay Transponder

The ice penetrating capability of low-frequency radar systems may offer a unique potential for the detection, localization, and guidance of exploratory ice-penetrating probes for cryospheric applications. As these probes exhibit a comparably small radar cross section (RCS), reliable detection, especially in high clutter surroundings, such as ice sheets, is prevented. As a solution, a miniaturized monostatic digital delay transponder is developed and tested regarding its functionality with an airborne synthetic aperture radar (SAR) sensor.

Radiometric analysis of digitized Z-scope records in archival radar sounding film

AbstractThe earliest airborne geophysical campaigns over Antarctica and Greenland in the 1960s and 1970s collected ice penetrating radar data on 35 mm optical film. Early subglacial topographic and englacial stratigraphic analyses of these data were foundational to the field of radioglaciology. Recent efforts to digitize and release these data have resulted in geometric and ice-thickness analysis that constrain subsurface change over multiple decades but stop short of radiometric interpretation. The primary challenge for radiometric analysis is the poorly-characterized compression applied to Z-scope records and the sparse sampling of A-scope records. Here, we demonstrate the information richness and radiometric interpretability of Z-scope records. Z-scope pixels have uncalibrated fast-time, slow-time, and intensity scales. We develop approaches for mapping each of these scales to physical units (microseconds, seconds, and signal to noise ratio). We then demonstrate the application of this calibration and analysis approach to a flight in the interior of East Antarctica with subglacial lakes and to a reflight of an East Antarctic ice shelf that was observed by both archival and modern radar. These results demonstrate the potential use of Z-scope signals to extend the baseline of radiometric observations of the subsurface by decades.

Surface Mass-Balance Gradients From Elevation and Ice Flux Data in the Columbia Basin, Canada

The mass-balance—elevation relation for a given glacier is required for many numerical models of ice flow. Direct measurements of this relation using remotely-sensed methods are complicated by ice dynamics, so observations are currently limited to glaciers where surface mass-balance measurements are routinely made. We test the viability of using the continuity equation to estimate annual surface mass balance between flux-gates in the absence of in situ measurements, on five glaciers in the Columbia Mountains of British Columbia, Canada. Repeat airborne laser scanning surveys of glacier surface elevation, ice penetrating radar surveys and publicly available maps of ice thickness are used to estimate changes in surface elevation and ice flux. We evaluate this approach by comparing modeled to observed mass balance. Modeled mass-balance gradients well-approximate those obtained from direct measurement of surface mass balance, with a mean difference of +6.6 ± 4%. Regressing modeled mass balance, equilibrium line altitudes are on average 15 m higher than satellite-observations of the transient snow line. Estimates of mass balance over flux bins compare less favorably than the gradients. Average mean error (+0.03 ± 0.07 m w.e.) between observed and modeled mass balance over flux bins is relatively small, yet mean absolute error (0.55 ± 0.18 m w.e.) and average modeled mass-balance uncertainty (0.57 m w.e.) are large. Mass conservation, assessed with glaciological data, is respected (when estimates are within 1σ uncertainties) for 84% of flux bins representing 86% of total glacier area. Uncertainty on ice velocity, especially for areas where surface velocity is low (<10 m a−1) contributes the greatest error in estimating ice flux. We find that using modeled ice thicknesses yields comparable modeled mass-balance gradients relative to using observations of ice thickness, but we caution against over-interpreting individual flux-bin mass balances due to large mass-balance residuals. Given the performance of modeled ice thickness and the increasing availability of ice velocity and surface topography data, we suggest that similar efforts to produce mass-balance gradients using modern high-resolution datasets are feasible on larger scales.

빙하 크레바스 탐지용 무인지상차량 (UGV) 개발

Research activities in Antarctica face a number of risk factors, such as cold, strong winds, and isolated or dangerous environments. Among them, crevasses, which are deep cracks in glaciers, can cause fatal accidents since they are difficult to locate as they are covered with snow. In this paper, we present the development, design, and deployment of an UGV (Unmanned Ground Vehicle) that can explore ice-sheets and be used in various polar research fields. The developed UGV was customized to remotely detect crevasses and prevent related accidents. In order to adapt to the harsh terrain of Antarctica, the vehicle features ground clearance over 260 mm, four-wheel-drive ability with a 400W high-power motor, as well as passive joints and differential links to allow for four-point contact on rough terrain. The UGV is also designed to run for more than two hours on the Antarctic ice sheet at a speed of 2.5 m/sec. The control unit uses open-source hardware and software for rapid function implementation and ease of future customization. In November 2019, the UGV detected crevasses successfully by towing an IPR (Ice Penetrating Radar) along the 2.6 km section of the Browning Pass around Jang Bogo Station, Antarctica. Further improvement is required for heading angle errors due to magnetic compass malfunctions in Antarctica, and for the vibration and misalignment of the traction device for the towing research equipment.

Bias-corrected estimates of glacier thickness in the Columbia River Basin, Canada

AbstractSeveral global datasets of glacier thickness exist, but the number of observations from western Canada are sparse and spatially biased. To supplement these limited observations, we measured ice thickness with ice penetrating radar on five glaciers in the Columbia Mountains, Canada. Our radar surveys, when combined with previous surveys for two glaciers in the Rocky Mountains, total 182 km of transects that represent 34 672 point measurements of ice thickness. Our measurements are, on average, 38% thicker than previous surface inversion model estimates of glacier thickness. Using our measurements within a cross-validation scheme, we model ice thickness with the Open Global Glacier Model (OGGM) driven with recent observations of surface mass balance and glacier elevation. We calibrated OGGM ice thickness by optimizing the ice creep parameter in the model. The optimized OGGM yields an ice volume for Columbia Basin of 122.5 ± 22.4 km3for the year 2000, which is 23% greater than the range of previous estimates. At current rates of glacier mass loss for this region, glaciers would disappear from the basin in about 65–80 years. Disappearance of these glaciers will negatively affect the basin's surface hydrology, freshwater availability and aquatic ecosystems.

Multidecadal Basal Melt Rates and Structure of the Ross Ice Shelf, Antarctica, Using Airborne Ice Penetrating Radar

AbstractBasal melting of ice shelves is a major source of mass loss from the Antarctic Ice Sheet. In situ measurements of ice shelf basal melt rates are sparse, while the more extensive estimates from satellite altimetry require precise information about firn density and characteristics of near‐surface layers. We describe a novel method for estimating multidecadal basal melt rates using airborne ice penetrating radar data acquired during a 3‐year survey of the Ross Ice Shelf. These data revealed an ice column with distinct upper and lower units whose thicknesses change as ice flows from the grounding line toward the ice front. We interpret the lower unit as continental meteoric ice that has flowed across the grounding line and the upper unit as ice formed from snowfall onto the relatively flat ice shelf. We used the ice thickness difference and strain‐induced thickness change of the lower unit between the survey lines, combined with ice velocities, to derive basal melt rates averaged over one to six decades. Our results are similar to satellite laser altimetry estimates for the period 2003–2009, suggesting that the Ross Ice Shelf melt rates have been fairly stable for several decades. We identify five sites of elevated basal melt rates, in the range 0.5–2 m a−1, near the ice shelf front. These hot spots indicate pathways into the sub‐ice‐shelf ocean cavity for warm seawater, likely a combination of summer‐warmed Antarctic Surface Water and modified Circumpolar Deep Water, and are potential areas of ice shelf weakening if the ocean warms.

Assessing the detectability of Europa’s eutectic zone using radar sounding

Radar sounding is a geophysical method capable of directly imaging subsurface interfaces within the ice shell of the icy moons, including Jupiter’s moon, Europa. For this reason, both the European Space Agency’s JUpiter ICy moons Explorer and the National Aeronautics and Space Administration’s Europa Clipper missions have ice penetrating radar sounders in their payloads. In addition to the ice–ocean interface and shallow water lenses, liquid water in the eutectic zone of Europa’s ice shell could also be a target for radar sounding investigations. However, the wide range of possible configurations for eutectic-zone water bodies and the overlying ice make their absolute echo strength difficult to predict. To address this challenge, we employ a suite of simple water configurations and scattering models to bound the eutectic detectability in terms of its effective reflectivity. We find that, for each configuration, a range of physically plausible eutectic parameters exist that could produce detectable echoes, with effective reflectivity values greater than -50 dB at HF or VHF frequencies.

Estimating glacier-bed overdeepenings as possible sites of future lakes in the de-glaciating Mont Blanc massif (Western European Alps)

De-glaciating high mountain areas result in new landscapes of bedrock and debris where permafrost can degrade, persist or even newly form in cases, and of new lakes in glacier bed overdeepenings (GBOs) becoming ice-free. These landscapes with new lakes in close neighborhood to over-steepened and perennially frozen slopes are prone to chain reaction processes (e.g. rock-ice avalanches into lakes triggering impact waves, dam breach or overtopping, and debris flows) with potentially far-reaching run-out distances causing valley floors devastation. The frequency, magnitude and zonation of hazards are shifting, requiring integrative approaches combining comprehensive information about landscape evolution and related processes to support stakeholders in their adaptation strategies. In this study, we intend to setup an essential baseline for such an integrative approach in the Mont Blanc massif (MBM), which is a typical high-mountain range affected by de-glaciation processes. We first (i) predict and (ii) detect potential GBOs by combining the GlabTop model with a visual analysis based on morphological indications of glacier flow through over-deepened bed parts. We then (iii) determine the level of confidence concerning the resulting information, and (iv) estimate the approximate time range under which potential lakes could form. The location of the predicted GBOs and the shape of glacier beds are evaluated against currently forming water bodies at retreating glacier snouts, and seismic and ice penetrating radar measurements on the Argentière glacier. This comparison shows that the location of predicted GBOs is quite robust whereas their morphometric characteristics (depth, volume) are highly uncertain and tend to be underestimated. In total, 48/80 of the predicted or detected GBOs have a high level of confidence. In addition to five recently formed water bodies at glacier snouts, one of the high confidence GBOs (Talèfre glacier) which is also the most voluminous one could form imminently (during coming years), if not partially or totally drained through deeply incised gorges at the rock threshold. Twelve other lakes could form within the first half of the century under a constant or accelerated scenario of continued glacier retreat. Some of them are located below high and permanently frozen rock walls prone to destabilization and high-energy mass movements, hinting at possible hot spots in terms of hazards in the coming decades, where more detailed analysis would be required.

Experimental Investigation on the Ice Formation and Growth in Ex Vivo Bovine Liver

The paper presents a set of experiments to characterize the ice front propagation in ex vivo bovine liver samples. Based on previous successful experiments using water and agar-gel as tested materials, the methodology infers the ice evolution in the sample from temperature measures obtained at the cooling device only, with a technique known as mirror image. No analytical or numerical solutions are proposed for the phase change propagation inside the tested material, such as for instance the Stefan approach as a classical free boundary problem for a parabolic partial differential equation. Thermal events inside the tested material, such as the heat flux released during the phase transition, have been deduced by the measured temperatures and the numerical solution of the thermal field inside the cooling device, conceived to mimic some thermal features of an actual cryogenic probe. The application domain for these experiments is in the cryoablative therapy, and the aim is to help providing an increased knowledge on the thermal effects to better deduce the effectiveness of the treatment and reduce recurrences, while at the same time avoid damages in the proximal anatomical structures. Results show that it is possible to detect the triggering of ice formation at the interface between the liver samples and the cooling device and predict the ice front propagation according to a law linear on the heat subtracted per unit of area. In a few tens of seconds, the maximum ice penetration distance is about 2 mm inside the liver tissue, with a penetration rate that goes from 0.2 mm/s to 0.02 mm/s. Moreover, adopting a special sample configured as an agar-gel coating superimposed to an ex vivo liver layer, the arrival time of the ice penetrating the liver and the temperature at the interface between these materials were detected, in order to estimate the part of the heat flux useful to the ice formation with respect to that spent for cooling the surrounding medium. Based on this preliminary result, to improve the cryoablation effectiveness it could be useful to increase the heat flux per unit of surface at the beginning, instead of the ablation duration.

Estimating mechanical properties of ice on Europa: Impact simulation and sensitivity analysis

The primary objective of NASA’s Europa Multiple Flyby Mission to Jupiter’s moon Europa is to determine the thickness of the ice covering the global subsurface ocean. If the ice proves too thick for the ice penetrating radar, however, an indirect method must be used to estimate the depth of the ice-water boundary. Unfortunately, the properties of Europan ice are not known precisely, with Young’s modulus having two orders of magnitude of uncertainty. It has been suggested that seismic response produced by an artificial impact could help estimating the mechanical properties of this ice. By optically measuring the induced surface motion with enough resolution, the dispersion curves can be determined. The associated elastic wave speeds are then used to estimate the moduli and density of the ice. The seismic response of the ice is modeled using a Finite Element Method solver. This presentation describes the sensitivity of the estimated mechanical properties to both temporal and spatial resolutions of the optica...

The influence of a model subglacial lake on ice dynamics and internal layering

Abstract. As ice flows over a subglacial lake, the drop in bed resistance leads to an increase in ice velocities and a draw down of isochrones and cold ice. The ice surface flattens as it adjusts to the lack of resisting forces at the base. The rapid transition in velocity induces changes in ice viscosity and releases deformation energy that can raise the temperature locally. Recent studies of Antarctic subglacial lakes indicate that many lakes experience very fast and possibly episodic drainage, during which the lake size is rapidly reduced as water flows out. Questions that arise are what effect this would have on internal layers within the ice and whether such past drainage events could be inferred from isochrone structures downstream. Here, we study the effect of a subglacial lake on ice dynamics as well as the influence that such short timescale drainage would have on the internal layers of the ice. To this end, we use a full Stokes, polythermal ice flow model. An enthalpy-gradient method is used to account for the evolution of temperature and water content within the ice. We find that a rapid transition between slow-moving ice outside the lake, and full sliding over the lake, can release considerable amounts of deformational energy, with the potential to form a temperate layer at depth in the transition zone. In addition, we provide an explanation for a characteristic surface feature commonly seen at the edges of subglacial lakes, a hummocky surface depression in the transition zone between little to full sliding. We also conclude that rapid changes in the horizontal extent of subglacial lakes and slippery patches, compared to the average ice column velocity, can create a traveling wave at depth within the isochrone structure that transfers downstream with the advection of ice, thus indicating the possibility of detecting past drainage events with ice penetrating radar.

A passive probe for subsurface oceans and liquid water in Jupiter’s icy moons

We describe an interferometric reflectometer method for passive detection of subsurface oceans and liquid water in jovian icy moons using Jupiter’s decametric radio emission (DAM). The DAM flux density exceeds 3000 times the galactic background in the neighborhood of the jovian icy moons, providing a signal that could be used for passive radio sounding. An instrument located between the icy moon and Jupiter could sample the DAM emission along with its echoes reflected in the ice layer of the target moon. Cross-correlating the direct emission with the echoes would provide a measurement of the ice shell thickness along with its dielectric properties. The interferometric reflectometer provides a simple solution to sub-jovian radio sounding of ice shells that is complementary to ice penetrating radar measurements better suited to measurements in the anti-jovian hemisphere that shadows Jupiter’s strong decametric emission. The passive nature of this technique also serves as risk reduction in case of radar transmitter failure. The interferometric reflectometer could operate with electrically short antennas, thus extending ice depth measurements to lower frequencies, and potentially providing a deeper view into the ice shells of jovian moons.

Planetary landing-zone reconnaissance using ice-penetrating radar data: Concept validation in Antarctica

The potential for a nadir-looking radar sounder to retrieve significant surface roughness/permittivity information valuable for planetary landing site selection is demonstrated using data from an airborne survey of the Thwaites Glacier Catchment, West Antarctica using the High Capability Airborne Radar Sounder (HiCARS). The statistical method introduced by Grima et al. (2012. Icarus 220, 84–99. http://dx.doi.org/10.1007/s11214-012-9916-y) for surface characterization is applied systematically along the survey flights. The coherent and incoherent components of the surface signal, along with an internally generated confidence factor, are extracted and mapped in order to show how a radar sounder can be used as both a reflectometer and a scatterometer to identify regions of low surface roughness compatible with a planetary lander. These signal components are used with a backscattering model to produce a landing risk assessment map by considering the following surface properties: Root mean square (RMS) heights, RMS slopes, roughness homogeneity/stationarity over the landing ellipse, and soil porosity. Comparing these radar-derived surface properties with simultaneously acquired nadir-looking imagery and laser-altimetry validates this method. The ability to assess all of these parameters with an ice penetrating radar expands the demonstrated capability of a principle instrument in icy planet satellite science to include statistical reconnaissance of the surface roughness to identify suitable sites for a follow-on lander mission.

Recent changes in total ice volume on Volcán Villarrica, Southern Chile

Results obtained by the first intensive airborne surveys carried out at Volcan Villarrica (39°S) in Southern Chile, are presented. These campaigns included the use of a scanner laser system, for detecting the glacier surface topography, and a helicopter-borne ice penetrating radar, for measuring ice thicknesses. These surveys allowed determining the snow and ice volume storage at this volcano, volume which is susceptible to melt during eruptive events generating dangerous fast flows (lahars). Volcan Villarrica is one of the most active volcanoes in Chile, with frequent eruptive events, many of them associated with lahars which are considered the most hazardous process at this volcano. In fact, most of the casualties and infrastructure damages incurred during historical eruptive events at the volcano are associated with lahars. With use of the radar and laser data, a total volume of 1.17 ± 0.1 km3 of water equivalent (w.eq.) at the volcano in 2012 was calculated, only 37 % of the estimated volume of 1961, a reduction mainly explained by the area shrinkage and ice thinning rates observed in the last 51 years. This total volume represents a lower boundary available for melting during eruptive events when lahars mudflows can be generated, because mainly in the winter, nearly 0.14 km3 w.eq. are potentially added to the volcano as temporal snow falls. The volume of water equivalent lost in recent decades does not mean a lower risk associated with these flows, as there has been a huge increase in populated areas in the surroundings of the volcano in recent years.

Time‐dependence of the spatial pattern of accumulation rate in East Antarctica deduced from isochronic radar layers using a 3‐D numerical ice flow model

In East Antarctica surface mass balance data can only be obtained from the sparsely distributed ice cores when considering time periods greater than a few decades. Observations of internal layers measured by airborne ice penetrating radar, in principle, permit extrapolation of mass balance information from these ice cores. We use radar survey lines gathered in the 1970s, and a three‐dimensional numerical model, to investigate the feasibility of such extrapolation, seeking to match the calculations of englacial layer geometry with observations. First, we justify the use of a three‐dimensional model by showing that simple vertical flow models cannot explain all the observations and that horizontal advection is a significant glacial process. Then we examine processes that affect calculations of layer geometry, finding that spatial accumulation‐rate patterns are extremely important while geothermal heat flux and flow mode (sliding or internal deformation) are of substantially less importance. Where the layer is from the Last Glacial Maximum (17.5 ka), we find a very good match between the spatial pattern of accumulation rates inferred from this layer and the modern spatial pattern of accumulation rates. When considering deeper layers from beyond the current interglacial, we find that a different spatial accumulation‐rate pattern must have existed, in addition to the known change in accumulation rate from ice cores. The glacial spatial accumulation‐rate pattern would have had proportionally greater accumulation at the South Pole than now, compared with the Vostok and Dome C ice cores.

Modelling accreted ice in subglacial Lake Vostok, Antarctica

The distribution and thickness of accreted ice at the ice‐lake interface of subglacial Lake Vostok, East Antarctica, is calculated conflating various sources: (1) The modelled basal mass balance at the ice‐lake interface based on two different bathymetry models, (2) different ice flow trajectories obtained from satellite interferometry and ice penetrating radar measurements, and (3) reasonable ice flow velocity. Our results show that the accreted ice distribution is highly sensitive to the ice draft and to the used flow line directions. The volume and thickness of the accreted ice depends significantly on the ice flow velocity. According to our modelling, we estimate the accreted ice area, volume and mean thickness to be 10,800 ± 500 km2, 980 ± 200 km3, and 90 ± 45 m, respectively, for an ice flow velocity of 3.6 m/a. Only about 36 ± 2% of Lake Vostok's surface is in contact with meteoric ice, melted by about 2.65 ± 0.10 cm/a. This has impacts on the sedimentation rate and the supply of nutrients, oxygen, and/or other components only present in meteoric ice but not in accreted lake ice. We estimate the residence time of the lake water at about 32,000 ± 4000 years.

Glacial–interglacial variations in central East Antarctic ice accumulation rates

Past rates of ice accumulation in central East Antarctica are defined poorly for the Last Glacial period. Ice cores, from which current estimates are based, are limited in number (there is one deep ice core at Vostok Station, one at Dome F and two others currently being extracted at Dome C and Dronning Maud Land), and are restricted spatially (they are one-dimensional). Here, spatial variations in ice accumulation rates during the Last Glacial are calculated from isochronous internal ice sheet layering, measured by airborne ice penetrating radar data. The layers are used to identify the stratigraphy around five East Antarctic ice domes. Isochrons are dated by linking them to the Vostok ice core and, in one case, the EPICA Dome C ice core. This chronostratigraphy allows the depth–age function to be identified across the ice sheet. A simple ice flow model, using the depth–age measurements as input, is then used to determine the spatial and temporal variation in East Antarctic ice accumulation over the Last Glacial cycle. The lowest rates of ice accumulation were predicted in the full glacial period to the south of Dome A and Ridge B. There are two possible explanations for this: (1) the ice divide topographies may have caused a ‘precipitation shadow’ in the ice age; (2) with the expansion of the Ross ice shelf the area to the south of Dome A and Ridge B would be at the very centre of the ice sheet and, hence, at a maximum distance from the ocean moisture source.

Surface movement and mass balance at the Hans Tausen drill site determined by use of GPS

During the deep drilling at Hans Tausen Iskappe in Peary Land, North Greenland, the ice cap has been mapped with GPS. Kinematic GPS and ice penetrating radar measurements were done at the southeast dome extending to approximately 3 km from the drill site at the top of the dome, 82.5°N; 37.5°W (Gundestrup et al. 2001). Furthermore a strain net was established consisting of a center pole at the top of the dome and three rings, each of eight poles, at distances 0.3, 1.5, and 3.0 km from the center. The bottom and surface topography, ice surface velocities from the strain net and precipitation data were used to calculate the present mass balance of the dome. The present mass balance of the dome was found to be +0.04±0.02 m of ice/year, i.e. the ice thickness at the dome increases with a rate of about one third of the annual accumulation. The result is highly consistent and independent on assumptions regarding the ice flow. Our result shows that the central part of the ice cap is far from steady state, implying that simple models cannot be used to determine the time scale of the ice core and the thinning of annual layers.

Unusual surface morphology from digital elevation models of the Greenland Ice Sheet

In this study of the North Greenland ice sheet, we have used digital elevation models to investigate the topographic signatures of a large ice flow feature discovered in 1993 and a unique surface anomaly which we believe has not been observed previously. The small scale topography of the flow feature is revealed in striking detail in a high‐pass filtered elevation model. Furthermore, ice penetrating radar show that the sub‐stream bed is rough with undulation amplitude increasing downstream. The new feature consists of two large depressions in the ice sheet connected by a long curving trench. The low driving stresses in the region and the increased power of radio echo from the ice/bed interface over the feature clearly show presence of basal water. The shape of one of the two depressions and its location above a bedrock trough is indicative of a sub‐glacial lake but we also see evidence suggesting that water transport through a basal hydrological system may be occurring.