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

Summary

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.

Citing These Data

Holt, J. W., D. D. Blankenship, and D. L. Morse. 2006. Subglacial topography: airborne geophysical survey of the Amundsen Sea Embayment, Antarctica. Boulder, Colorado USA: National Snow and Ice Data Center. http://dx.doi.org/10.7265/N59W0CDC.

The following example shows how to cite the use of this data set in a publication. For more information, see our Use and Copyright Web page.

Overview Table

Category Description
Data format Tab-delimited ASCII text format and gridded 2-byte binary (little endian)
Spatial coverage and resolution

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.

Temporal coverage and resolution Austral summer of 2004-2005
File size File sizes range from 70 KB to 1.32 MB.
Parameters Latitude (° S), longitude (° W), ice thickness (m), and sub glacial topography (m)
Procedures for obtaining data Data are available via FTP.

Table of Contents

1. Contacts and Acknowledgments
2. Detailed Data Description
3. Data Access and Tools
4. Data Acquisition and Processing
5. References and Related Publications
6. Document Information

1. Contacts and Acknowledgments

Investigators

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

Technical Contact

NSIDC User Services
National Snow and Ice Data Center
CIRES, 449 UCB
University of Colorado
Boulder, CO 80309-0449  USA
phone: +1 303.492.6199
fax: +1 303.492.2468
form: Contact NSIDC User Services
e-mail: nsidc@nsidc.org

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.

2. 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
ASE05bedelv.bin 71 KB Ice thickness (m) Gridded two-byte binary (little endian),
200 rows x 180 columns
ASE05icethk.bin 71 KB Bed elevation (m) Gridded two-byte binary (little endian),
200 rows x 180 columns

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

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.

Temporal Coverage

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

Parameters

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

Quality Assessment

See Data Acquisition and Processing.

3. Data Access and Tools

Data Access

Data are available via FTP.

See Also

4. Data Acquisition and Processing

Instrument Descriptions

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.

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.

5. References and Related Publications

Holt, J. W., D. D. Blankenship, D. L. Morse, D. A. Young, M. E. Peters, S. D. Kempf, et. al. 2006. New boundary conditions for the West Antarctic Ice Sheet: subglacial topography of the Thwaites and Smith glacier catchments. Geophysical Research Letters 33, L09502. doi:10.1029/2005GL025561.

Vaughan, D. G., H. F. J. Corr, F. Ferraccioli, N. Frearson, A. O'Hare, D. Mach, J. Holt, et. al. 2006. New boundary conditions for the West Antarctic Ice Sheet: subglacial topography beneath Pine Island Glacier. Geophysical Research Letters 33, L09501. doi:10.1029/2005GL025588.

6. Document Information

Acronyms and Abbreviations

The following acronyms and abbreviations are used in this document:

ASCII American Standard Code for Information Interchange
AGASEA Airborne geophysical survey of the Amundsen Sea Embayment, Antarctica
BAS British Antarctic Survey
DEM Digital elevation model
FTP File Transfer Protocol
GPS Global Positioning System
HICARS High-Capability Radar Sounders
NSF National Science Foundation
NSIDC National Snow and Ice Data Center
OPP Office of Polar Programs
URL Uniform Resource Locator
UTIG University of Texas, Institute for Geophysics
WAIS West Antarctic Ice Sheet
WGS-84 World Geodetic System 1984

Document Creation Date

October 2006

Document URL

http://nsidc.org/data/docs/agdc/nsidc0292_holt/index.html