Maps Of Ice Thickness Research Articles

High-resolution ice-thickness mapping in South Greenland

AbstractAirborne radar sounding is difficult in South Greenland because of the presence of englacial water, which prevents the signal from reaching the bed. Data coverage remains suboptimal for traditional methods of ice-thickness and bed mapping that rely on geostatistical techniques, such as kriging, because important features are missing. Here we apply two alternative approaches of high-resolution (~300m) ice-thickness mapping, that are based on the conservation of mass, to two regions of South Greenland: (1) Qooqqup Sermia and Kiattuut Sermiat, and (2) Ikertivaq. These two algorithms solve optimization problems, for which the conservation of mass is either enforced as a hard constraint, or as a soft constraint. For the first region, very few measurements are available but there is no gap in ice motion data, whereas for Ikertivaq, more ice-thickness measurements are available, but there are gaps in ice motion data. We show that mass-conservation algorithms can be used as validation tools for radar sounding. We also show that it is preferable to apply mass conservation as a hard constraint, rather than a soft constraint, as it better preserves elongated features, such as glacial valleys and ridges.

Developments in frequency domain AEM: Tackling drift and noise with a ferrite-core, receiver triplet.

SUMMARY The polar oceans’ sea ice cover is a challenging geophysical target to map. Current state of practice helicopter-electromagnetic (HEM) ice thickness mapping is limited to 1D interpretation due to common procedures and systems that are mainly sensitive to layered structures. We present a new generation Multi-sensor, Airborne Sea Ice Explorer (MAiSIE) to overcome these limitations. As the actual sea ice structure is 3D and in parts heterogeneous, errors up to 50% are observed due to the common 1D approximation. With MAiSIE we present a new EM concept based on one multi frequency transmitter loop and a three component receiver coil triplet without bucking The small weight frees additional payload to include a line scanner (lidar) and high accuracy INS/dGPS. The 3D surface topography from the scanner with the EM data at from 500 Hz to 8 kHz, in x, y, and z direction, will increase the accuracy of HEM derived pressure ridge geometry significantly. Experience from two field campaigns shows the proof-of-concept with acceptable sensor drift and receiver sensitivity. The preliminary 20 ppm noise level @ 4.1 kHz is sufficient to map level ice thickness with 10 cm precision for sensor altitudes below 13 m.

High-resolution bed topography mapping of Russell Glacier, Greenland, inferred from Operation IceBridge data

AbstractDetailed 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.

X-밴드 산란계를 이용한 하천 얼음 두께 측정에 관한 연구

In this study, we setup a ground-based scatterometer using an antenna of which the center frequency is 9.5 GHz (X-band), and measured radar backscatterings from air/ice and ice/water interfaces to extract ice thickness. Both of air/ice and ice/water interfaces make strong radar backscatterings and so we can clearly identify two peaks in measured data by scatterometer. By using the distance of two peaks and refractive index of ice, we confirmed that it is possible to measure ice thickness. Field survey was performed at the downstream of Jiam River flowing into Chuncheon Lake. We measured radar backscattering from river ice along a survey path and extracted ice thickness. The ice thickness map of the downstream of Jiam River was produced by using kriging which is one of well known interpolation methods. The ice thickness was about 50 cm along the mainstream while ice was thin as 30 ~ 40 cm at a fast-flowing meander. Ice thickness was particularly thinner at some locations than that of surrounding areas even in the mainstream region of constant flow. This was because of impurities in ice or artificially formed refrozen holes after fishing. We expect that this study helps to expand utilization field of X-band SAR and airborne scatterometer system.

A mass conservation approach for mapping glacier ice thickness

The traditional method for interpolating ice thickness data from airborne radar sounding surveys onto regular grids is to employ geostatistical techniques such as kriging. While this approach provides continuous maps of ice thickness, it generates products that are not consistent with ice flow dynamics and are impractical for high resolution ice flow simulations. Here, we present a novel approach that combines sparse ice thickness data collected by airborne radar sounding profilers with high resolution swath mapping of ice velocity derived from satellite synthetic-aperture interferometry to obtain a high resolution map of ice thickness that conserves mass and minimizes the departure from observations. We apply this approach to the case of Nioghalvfjerdsfjorden (79North) Glacier, a major outlet glacier in northeast Greenland that has been relatively stable in recent decades. The results show that our mass conserving method removes the anomalies in mass flux divergence, yields interpolated data that are within about 5% of the original data, and produces thickness maps that are directly usable in high spatial-resolution, high-order ice flow models. We discuss the application of this method to the broad and detailed radar surveys of ice sheet and glacier thickness. Copyright 2011 by the American Geophysical Union.

Bed properties and hydrological conditions underneath McCall Glacier, Alaska, USA

AbstractDuring three summer field seasons (2003, 2005 and 2006) we carried out radio-echo sounding measurements with a 5MHz (central frequency) ice-penetrating radar on McCall Glacier, Arctic Alaska, USA, along the central flowline and 17 cross-profiles. Two-way travel time was, after migration, converted to ice thickness, which, in combination with a recent digital elevation model of the surface of the glaciated area, resulted in a detailed map of the bed topography. This reveals a complex basal topography in the confluence area of the different glacial cirques. The pattern of subglacial water flow following the hydraulic potential gradient was calculated for the whole glacier area and shows a confluence of subglacial water downstream from the confluence of the glacier cirques. From the ice-thickness map the total ice volume was estimated as slightly less than 0.5 km3. Bed reflection power (BRP) was determined for the glacier after correction for ice-thickness dependence. Results reveal a clear relationship between the BRP pattern and basal sliding anomalies along the central flowline.

Thickness distribution of Antarctic sea ice

Ship‐based observations are used to describe regional and seasonal changes in the thickness distribution and characteristics of sea ice and snow cover thickness around Antarctica. The data set comprises 23,373 observations collected over more than 2 decades of activity and has been compiled as part of the Scientific Committee on Antarctic Research (SCAR) Antarctic Sea Ice Processes and Climate (ASPeCt) program. The results show the seasonal progression of the ice thickness distribution for six regions around the continent together with statistics on the mean thickness, surface ridging, snow cover, and local variability for each region and season. A simple ridge model is used to calculate the total ice thickness from the observations of level ice and surface topography, to provide a best estimate of the total ice mass, including the ridged component. The long‐term mean and standard deviation of total sea ice thickness (including ridges) is reported as 0.87 ± 0.91 m, which is 40% greater than the mean level ice thickness of 0.62 m. Analysis of the structure function along north/south and east/west transects revealed lag distances over which sea ice thickness decorrelates to be of the order of 100–300 km, which we use as a basis for presenting near‐continuous maps of sea ice and snow cover thickness plotted on a 2.5° × 5.0° grid.

New ice thickness maps of Filchner–Ronne Ice Shelf, Antarctica, with specific focus on grounding lines and marine ice

AbstractFor the Filchner–Ronne Ice Shelf we have compiled measurements of meteoric ice thickness from many institutions, and several different techniques (e.g. radar and seismic sounding) to produce an improved digital map of meteoric ice thickness. This map has high-resolution compared to previous compilations and serves to highlight small-scale geographic features (e.g. ice plains, grounding-line regions). We have also produced a map of the thickness of marine ice bodies beneath the ice shelf by using borehole density data to calibrate an ice thickness to surface-elevation relation, and then comparing maps of ice surface elevation and meteoric ice thickness to infer marine ice thickness. Due to denser data coverage and the improved density-depth relation, the resulting map is a significant improvement on its predecessors and allows insight into the glaciological context of the ice shelf, in particular, into the location of the grounding lines on the southern Ronne Ice Shelf. Here the data were supplemented with barometric determination of surface elevation, which were used to locate the grounding line position. The final delineation of the grounding line position was confirmed by reference to satellite imagery, and revealed that earlier estimates were substantially in error, especially in the area of Foundation Ice Stream and Möllereisstrom.

Hurd Peninsula glaciers, Livingston Island, Antarctica, as indicators of regional warming: ice-volume changes during the period 1956–2000

AbstractJohnsons and Hurd Glaciers are the two main glacier units of Hurd Peninsula ice cap, Livingston Island, South Shetland Islands, Antarctica. They presently cover an area of about 10 km2. Johnsons is a tidewater glacier, while Hurd Glacier ends on emerged land. In this paper, we estimate the changes in ice volume during the period 1956–2000, and compare them with the regional meteorological records. The volume-change estimates are based on the comparison of digital terrain models for the glacier surface, constructed from aerial photographs taken by the British Antarctic Survey in 1956 and from our geodetic measurements in 1999/2000. The total volume estimates are based on an ice-thickness map constructed from radio-echo sounding profiles (18–25 MHz) done in 1999–2001, showing maximum ice thickness of about 200 m. We estimate the changes in ice volume during the period 1956–2000 to be –0.108±0.048km3, which represents a 10.0±4.5% decrease from the 1956 total volume of 1.076±0.055km3 and is equivalent to an average annual mass balance of –0.23±0.10mw.e. during 1956–2000. Ice-thickness changes range from –40 to +20 m, averaging –5.5±4.4 m. Most areas show ice thinning; the thickening is limited to a small area within Johnsons Glacier. All glacier fronts, except Johnsons’ calving front, show retreat. These changes are consistent with the regional meteorological records for mean summer temperature, which show a trend of +0.023±0.005˚Ca–1 during the period 1956–2000.

Polarimetric Characteristics of sea ice in the sea of Okhotsk observed by airborne L-band SAR

The Phased-Array L-Band SAR (PALSAR) aboard the Advanced Land Observing Satellite (ALOS) is capable of globally acquiring fully polarimetric data. In order to confirm the ability of L-band polarimetric synthetic aperture radar (SAR) to investigate sea ice before the ALOS launch, we conducted a field experiment using an airborne Polarimetric and Interferometric SAR (Pi-SAR) in the Sea of Okhotsk in 1999. This paper presents the analyzed results of data acquired in that experiment. The extracted polarimetric parameters of several ice types suggested that polarimetric coherences and phase differences between right-right (RR) and left-left (LL) are good candidates for discriminating ice types. The polarimetric anisotropy as well as the beta angle of the first eigenvector calculated in the polarimetric decomposition procedure are alternative parameters that are sensitive to ice type differences. Due to the low depolarization characteristics of open water, it could be discriminated from sea ice by scattering entropy in all incidence angle ranges. From the relation between ice thickness and the polarimetric parameters, we found that backscattering coefficients and vertical (VV) to horizontal (HH) backscattering ratio are highly correlated with ice thickness. Since the ratio is sensitive to ice surface dielectric constants, a simple simulation using the integral equation method surface model was conducted by using the physical parameters of typical sea ice. A two-dimensional ice thickness map was derived from an empirical relation between the VV-to-HH backscattering ratio and ice thickness.

Marine ice beneath the Filchner–Ronne Ice Shelf, Antarctica: a comparison of estimated thickness distributions

AbstractIn an earlier study, melt/freeze rates beneath most of the Filchner–Ronne Ice Shelf, Antarctica, were estimated using an assumption of a steady-state ice shelf applied to a velocity field derived from RADARSAT and ERS-1 interferometric synthetic aperture radar (InSAR) data and an ice-thickness map inferred from ERS-1 satellite radar altimeter data. Here, we use these basal accumulation rates and the InSAR velocity data to estimate the distribution and thickness of marine ice beneath the Filchner–Ronne Ice Shelf. These estimates are compared with a marine-ice thickness map derived from radio-echo sounding (RES) data and airborne radar altimetry. In general, we find close agreement between these estimates. In the few locations where there are significant differences, the discrepancies are largely attributable to artifacts in the radar-altimeter thickness map. With improvements such as a much smaller footprint, the next generation of altimeters should overcome many of these limitations, leading to improved marine-ice accumulation estimates. Overall, the good agreement with the RES data validates the InSAR-based estimates over much of the Ronne Ice Shelf.

Ice cover, landscape setting, and geological framework of Lake Vostok, East Antarctica

Lake Vostok, located beneath more than 4 km of ice in the middle of East Antarctica, is a unique subglacial habitat and may contain microorganisms with distinct adaptations to such an extreme environment. Melting and freezing at the base of the ice sheet, which slowly flows across the lake, controls the flux of water, biota and sediment particles through the lake. The influx of thermal energy, however, is limited to contributions from below. Thus the geological origin of Lake Vostok is a critical boundary condition for the subglacial ecosystem. We present the first comprehensive maps of ice surface, ice thickness and subglacial topography around Lake Vostok. The ice flow across the lake and the landscape setting are closely linked to the geological origin of Lake Vostok. Our data show that Lake Vostok is located along a major geological boundary. Magnetic and gravity data are distinct east and west of the lake, as is the roughness of the subglacial topography. The physiographic setting of the lake has important consequences for the ice flow and thus the melting and freezing pattern and the lake’s circulation. Lake Vostok is a tectonically controlled subglacial lake. The tectonic processes provided the space for a unique habitat and recent minor tectonic activity could have the potential to introduce small, but significant amounts of thermal energy into the lake.

Spatial depth distribution of the subglacial bed and internal layers in the ice around NGRIP, Greenland, derived with airborne RES

In 1996 and 1997, an airborne RES survey was flown in the context of the North Greenland Ice Core Project (NGRIP). At 75.10°N and 42.30°W, a deep ice core is being drilled for palaeoclimate studies with the objective to recover an undisturbed Eemian sequence. The present survey was specially designed to determine with high lateral resolution the bed topography and the layering of internal reflectors in the vicinity around the drill site. A total of 19,000 km of profiles were flown yielding a rectangular grid with side lengths of 222 by 210 km on 16 flights. The spacing between two flight tracks is about 10 km for most of the area and about 2.5 km in the center of the grid. Detailed maps of ice thickness and subglacial topography have been produced. Moreover, a data set was acquired that enables an independent stratigraphic control on the ice core record of NGRIP. From the map of subglacial topography and from selected profiles, the bed of NGRIP can be characterized as flat, particularly when compared to the region of GRIP/GISP2. There appears to be no flow disturbances that might have altered the stratigraphic order in the lower part of the ice sheet, at least up to 30 km upstream from NGRIP. On the basis of the present study, NGRIP seems to be an ideally chosen drill site in order to recover an undisturbed Eemian sequence.

Distribution of marine ice beneath the Amery Ice Shelf

We present a map of the marine ice accreted to the base of the Amery Ice Shelf (AIS), East Antarctica. This map is obtained by converting a Digital Elevation Model (DEM) of the AIS generated from satellite radar altimeter data to an ice thickness map, assuming hydrostatic equilibrium, and subtracting from that a second ice thickness map, derived from airborne radio‐echo sounding (RES) measurements. The RES signal does not penetrate the marine ice, so the measurement is only to the meteoric‐marine ice boundary, and therefore the difference between the two maps is the marine ice thickness. The marine ice is up to 190 m thick and accounts for about 9% of the shelf volume. It is concentrated in the northwest of the shelf, a result of the clockwise ocean circulation in the cavity below.

An impulse radar measurement in NE Greenland

A snowmobile towed impulse radar measurement was used to produce detailed maps of ice-thickness in two areas, one on a local dome (the Hans Tausen ice cap), and one on an outlet glacier north thereof. A constant spatial sampling was achieved by using a device attached to the snowmobile's odometer. The bedrock reflection was automatically detected in the majority of the data, by using digital signal processing techniques. After merging with differential GPS positions and elevations, a digital database of these parameters and the bedrock reflection times is presented as contour maps.

Sea ice thickness mapping by airborne and ground EM methods

An experimental campaign was carried out in 1991-1997 in the Baltic Sea, a body of brackish water with resistivities of 3 to 4 ohm-m, to test the ability of airborne and ground electromagnetic methods to map sea ice thickness. The measurements were made as part of the European research projects Baltic Experiment for ERS-1 and Local Ice Cover Deformation and Mesoscale Ice Dynamics.Measurements on sea ice were carried out using a multifrequency, horizontal and vertical coplanar loop-loop system with a coil separation of 10 m. The purpose of these measurements was to obtain detailed information about conductivity properties of the ice, including keel ice rubble.The first interpretation was made by using a layered-earth model. To correct for small variations in coil separation which primarily influences the in-phase component, coil separation was treated as an unknown parameter in the inversion. A three-dimensional model was used to interpret more complicated ice geometry in ridge areas. The results obtained were validated by drilling.Airborne measurements were made with a vertical coplanar coil configuration where the coils were mounted on the wing tips of a Twin Otter aircraft, at a frequency of 3.1 kHz. A laser profilometer was used to map surface topography. Test measurements were made along 1 to 2 km long profiles. In good conditions the thickness accuracy achieved was ±0.2 m for undeformed ice, but worse for deformed ice with complicated geometry.EM measurements on sea ice were found to be a practical tool if the detailed structure of ice and ridges is needed whereas AEM is superior if large coverage is essential.

Radar reflections reveal a wet bed beneath stagnant Ice Stream C and a frozen bed beneath ridge BC, West Antarctica

AbstractDigital airborne radar data were collected during the 1987-88 Antarctic field season in nine gridded blocks covering the downstream portions of Ice Stream B (6km spacing) and Ice Stream C (11 km spacing), together with a portion of ridge BC between them. An automated processing procedure was used for picking onset times of the reflected radar pulses, converting travel times to distances, interpolating missing data, converting pressure transducer readings, correcting navigational drift, performing crossover analysis, and zeroing rémanent crossover errors. Interpolation between flight-lines was carried out using the minimum curvature method.Maps of ice thickness (estimated accuracy 20 m) and basal-reflection strength (estimated accuracy 1 dB) were produced. The ice-thickness map confirms the characteristics of previous reconnaissance maps and reveals no new features. The reflection-strength map shows pronounced contrasts between the ice streams and ridge BC and between the two ice streams themselves. We interpret the reflection strengths to mean that the bed of Ice Stream C, as well as that of Ice Stream B, is unfrozen, that the bed of ridge BC is frozen and that the boundary between the frozen bed of ridge BC and the unfrozen bed of Ice Stream C lies precisely below the former shear margin of the ice stream.

Radar reflections reveal a wet bed beneath stagnant Ice Stream C and a frozen bed beneath ridge BC, West Antarctica

AbstractDigital airborne radar data were collected during the 1987-88 Antarctic field season in nine gridded blocks covering the downstream portions of Ice Stream B (6km spacing) and Ice Stream C (11 km spacing), together with a portion of ridge BC between them. An automated processing procedure was used for picking onset times of the reflected radar pulses, converting travel times to distances, interpolating missing data, converting pressure transducer readings, correcting navigational drift, performing crossover analysis, and zeroing rémanent crossover errors. Interpolation between flight-lines was carried out using the minimum curvature method.Maps of ice thickness (estimated accuracy 20 m) and basal-reflection strength (estimated accuracy 1 dB) were produced. The ice-thickness map confirms the characteristics of previous reconnaissance maps and reveals no new features. The reflection-strength map shows pronounced contrasts between the ice streams and ridge BC and between the two ice streams themselves. We interpret the reflection strengths to mean that the bed of Ice Stream C, as well as that of Ice Stream B, is unfrozen, that the bed of ridge BC is frozen and that the boundary between the frozen bed of ridge BC and the unfrozen bed of Ice Stream C lies precisely below the former shear margin of the ice stream.

An airborne electromagnetic system on a fixed wing aircraft for sea ice thickness mapping

An experimental campaign has been performed in 1991–1994 in the Baltic Sea, a brackish water basin, to test the ability of an airborne electromagnetic (AEM) system, mounted to a fixed wing aircraft (Twin Otter), to map sea ice thickness. Measurements are made with a vertical coplanar configuration of coils mounted on wing tips. The EM frequency is 3.1 kHz. A laser profilometer was integrated into the system for high frequency surface topography mapping. Test profiles were performed along 1–2 km long lines calibrated for ground truth. In good mapping conditions the thickness accuracy is ± 0.2 m for undeformed ice but worse for deformed ice with variable geometry. The raw horizontal resolution is 100 m. Analytical half-space EM models described well the overall ice thickness distribution. A 3D model was used for extraction of more detailed ice geometry. For unconsolidated deformed ice a nonzero electric conductivity must be assumed. The penetration length, which depends on the ice conductivity distribution, gives up to 70 m for fully resistive ice. The system is feasible for polar seas since the conductivity contrast between sea ice and sea water is much higher there than in the Baltic. The Twin Otter offers a wide operation area because the measurement ground speed is 90–120 knots and maximum flight duration is 6–8 hours.

Derived bedrock elevations, strain rates and stresses from measured surface elevations and velocities: Jakobshavns Isbræ, Greenland

AbstractJakobshavns Isbræ (69 °10′ N, 49 °59′ W) drains about 6.5% of the Greenland ice sheet and is the fastest ice stream known. The Jakobshavns Isbræ basin of about 10 000 km2was mapped photogrammetrically from four sets of aerial photography, two taken in July 1985 and two in July 1986. Positions and elevations of several hundred natural features on the ice surface were determined for each epoch by photogrammetric block aerial triangulation, and surface velocity vectors were computed from the positions. The two flights in 1985 yielded the best results and provided most common points (716) for velocity determinations and are therefore used in the modeling studies. The data from these irregularly spaced points were used to calculate ice elevations and velocity vectors at uniformly spaced grid points 3 km apart by interpolation. The field of surface strain rates was then calculated from these gridded data and used to compute the field of surface deviatoric stresses, using the flow law of ice, for rectilinear coordinates,X, Ypointing eastward and northward, and curvilinear coordinates.L, Τpointing longitudinally and transversely to the changing ice-flow direction, Ice-surface elevations and slopes were then used to calculate ice thicknesses and the fraction of the ice velocity due to basal sliding. Our calculated ice thicknesses are in fair agreement with an ice-thickness map based on seismic sounding and supplied to us by K. Echelmeyer. Ice thicknesses were subtracted from measured ice-surface elevations to map bed topography. Our calculation shows that basal sliding is significant only in the 10–15 km before Jakobshavns Isbræ becomes afloat in Jakobshavns Isfjord.