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Data Set ID:
NSIDC-0292

Subglacial Topography: Airborne Geophysical Survey of the Amundsen Sea Embayment, Antarctica, Version 1

This data set includes 5 km gridded data from the Airborne Geophysical Survey of the Amundsen Sea Embayment, Antarctica (AGASEA) conducted during the 2004-2005 austral summer. Investigators derived maps of the ice sheet surface and subglacial topography, which covers the entire catchments of both the Thwaites Glacier and the Pine Islands Glacier, from airborne survey systems mounted on a Twin Otter aircraft. The surveys had sufficient density to identify critical ice dynamic transitions within the Amundsen Sea Embayment (ASE).

The ASE is the only major drainage to exhibit significant elevation change over the period of available satellite observations. Modeling of the Western Antarctic Ice Sheet (WAIS) deglaciation pinpointed the Pine Island Glacier and the Thwaites Glacier, which comprise a major portion of the ASE, as the most vulnerable features of the WAIS. Present knowledge of the ice thickness and subglacial boundary conditions in the ASE are insufficient to understand its evolution or its sensitivity to climatic change, and it is not yet determined whether these changes are evidence of ongoing deglaciation or simply a fluctuation that does not threaten the equilibrium of the ice sheet. This research will support the efforts of a community of United States and international researchers to assess the present and predict the future behavior of the ice sheet in the ASE.

These data are available via FTP.

Geographic Coverage

Spatial Coverage:
  • N: -71.7, S: -81.7, E: -84.5, W: -134.9

Spatial Resolution:
  • 5 km x 5 km
Temporal Coverage:
  • 1 November 2004 to 31 March 2005
Temporal Resolution: Not specified
Parameter(s):
  • Glaciers/Ice Sheets > Glacier Topography/Ice Sheet Topography
  • Snow/Ice > Ice Depth/Thickness
  • Bathymetry/Seafloor Topography > Seafloor Topography
  • Topography > Terrain Elevation
Platform(s) AIRCRAFT
Sensor(s): GPS, RADAR, RADAR ECHO SOUNDERS
Data Format(s):
  • Binary
  • ASCII Text
Version: V1
Data Contributor(s): John Holt, Donald Blankenship, David Morse, David Vaughan, Hugh Corr, Duncan Young

Data Citation

As a condition of using these data, you must cite the use of this data set using the following citation. For more information, see our Use and Copyright Web page.

Holt, J. W., D. D. Blankenship, and D. L. Morse. 2006. Subglacial Topography: Airborne Geophysical Survey of the Amundsen Sea Embayment, Antarctica, Version 1. [Indicate subset used]. Boulder, Colorado USA. NSIDC: National Snow and Ice Data Center. doi: http://dx.doi.org/10.7265/N59W0CDC. [Date Accessed].

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Detailed Data Description

Data Files and Format
File Name Approximate
Size
Parameters and Units Format
ASE05.lonlatthkbed.txt 1.4 MB Latitude (° S), longitude (° W), ice thickness (m), and subglacial topography (m) Tab-delimited ASCII text
ASE05icethk.bin 71 KB Ice thickness (m) Gridded two-byte binary (little endian), 
200 rows x 180 columns
ASE05bedelv.bin 71 KB Bed elevation (m) Gridded two-byte binary (little endian), 
200 rows x 180 columns
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Spatial Coverage

Southernmost Latitude: 81.7° S 
Northernmost Latitude: 71.7° S
Westernmost Longitude: 134.9° W 
Easternmost Longitude: 84.5° W

Data have a 5 km grid spacing.

Projection Description

Binary grids are in a polar stereographic projection with the latitudes of true scale 71° S. The limits (in meters) are -1800000/-900000/-900000/1000000 (left/bottom/right/top), and the cell size is 5000 using pixel registration. The reference datum is WGS-84.

Grid Description

The grid values are 2-byte little endian integers arranged by row.

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Temporal Coverage

Investigators collected the data during the austral summer of 2004 to 2005.

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Parameter or Variable

Sample Data Record

These data are from the file ASE05.lonlatthkbed.txt. The columns represent the following parameters:

  1. Longitude (° W)
  2. Latitude (° S)
  3. Ice thickness (m)
  4. Subglacial topography (m)
-86.8952
-73.5416
985.953
-514.74
-86.8866
-73.5867
971.518
-389.572
-86.8779
-73.6317
975.337
-298.066
-86.8691
-73.6768
991.617
-231.923
-86.8603
-73.7219
1016.89
-181.278
-86.8515
-73.7669
1048.94
-141.303
-86.8426
-73.812
1086.4
-117.608
-86.8337
-73.8571
1128.37
-120.468
-86.8247
-73.9022
1174.22
-151.765
-86.8156
-73.9472
1223.17
-192.738
-86.8066
-73.9923
1274.04
-224.622
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Data Acquisition and Processing

Data Acquisition Methods

Investigators acquired the raw data on a 15 x 15 km grid over most of the area, augmented by seven along-flow profiles of the major trunks and tributaries. The UTIG investigators collected more than 43,500 line-kilometers of geophysical data over the catchments of both the Thwaites and Pine Island Glaciers on 77 survey flights over a seven week period. Additionally, BAS investigators acquired data from five flights focused solely on the Thwaites Glacier catchment.

Post-season Processing and Data Reduction

Investigators processed the radar data from HICARS for interpretation of the ice surface and bed interface echoes necessary for obtaining ice thickness. Coherent integration of 10 records (approximately 350 cm along track) preserved bed slopes up to six degrees without significant phase interference. Pulse compression was performed to improve range resolution. Additional incoherent integration resulted in final observations approximately every 17.5 m along track.

Investigators prioritized ice thickness measurements in post-season processing and analysis based on radar sounding data. They obtained ice thicknesses from the time difference between bed and surface echoes. They added all other sources of ice thickness data available for the area in order to create the most complete bed elevation map possible. Lastly, they subtracted the ice thicknesses from a surface digital elevation model (DEM) to generate a new bed elevation map. Travel time was converted to ice thickness assuming a wave velocity of 168.374 m/µsec with a uniform addition of 10 m to account for presence of low-density ice in the near surface.

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Sensor or Instrument Description

Thwaites Glacier

The UTIG maintains and operates the High-Capability Radar Sounder (HICARS) system. For this survey, investigators configured the HICARS system for a chirped 1 µsec pulse (52.5-67.5 MHz, peak power 8000 watts) with a repetition frequency of 6408 Hz. Coherent integrations of 32 signals were recorded approximately every 35 cm along track. The antennas were cross-track-polarized flat-plate dipoles mounted beneath each wing. Positioning was accomplished with differential carrier-phase GPS. Post-processed vertical crossover discrepancies were 25 cm root mean square; horizontal errors are smaller.

Pine Island Glacier

The aircraft was equipped with dual-frequency carrier-phase Global Positioning System (GPS) for navigation, radar altimeter for surface mapping, wing-tip magnetometers, gravity meter, and a new ice-sounding radar system, PASIN. PASIN was configured to operate with a transmit power of 4 kW around a central frequency of 150 MHz. A 0.1 µsec pulse optimized for imaging the near-surface layering was interleaved with a 4 µsec, 10 MHz chirp, which investigators used to successfully obtain bed echoes through ice more than 4200 m thick. Crossover analysis yielded root mean square differences of around 23 m in ice thickness. These differences are substantially greater than the wavelengths of 150 MHz radio waves in ice (approximately 1 m), and result largely from off-nadir reflections and interpretation/digitizing uncertainty. Post-processed GPS data allowed determination of aircraft positions to better than ±1 m.

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References and Related Publications

Contacts and Acknowledgments

John W. Holt
Institute for Geophysics 
University of Texas at Austin
4412 Spicewood Springs Rd. #600 
Austin, TX 78759-8500
USA

Donald Blankenship
Institute for Geophysics
University of Texas at Austin
4412 Spicewood Springs Rd. #600 
Austin, TX 78759-8500 USA

David Morse
Institute for Geophysics
University of Texas at Austin
4412 Spicewood Springs Rd. #600 
Austin, TX 78759-8500
USA

David G. Vaughan
British Antarctic Survey
High Cross
Madingley Road
CB3 0ET
Cambridge
United Kingdom

Hugh F. J. Corr
British Antarctic Survey
High Cross
Madingley Road
CB3 0ET
Cambridge
United Kingdom

Duncan A. Young
Jackson School of Geosciences, Institute for Geophysics
University of Texas at Austin
Bldg. 196
10100 Burnet Road (R2200)
Austin, TX 78758-4445
USA

Acknowledgements: 

The National Science Foundation (NSF) Office of Polar Programs (OPP) funded this research through award OPP-0230197. The British Antarctic Survey (BAS), the Jackson School of Geosciences at the University of Texas at Austin, and the G. Unger Vetlesen Foundation also contributed funding for this research.

Document Information

DOCUMENT CREATION DATE

October 2006

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