Subglacial Water Flow Paths Under Thwaites Glacier, West Antarctica

This data set contains subglacial water flow paths beneath Thwaites Glacier, West Antarctica, interpreted from ice thickness and bed elevation measurements collected between 7 December 2004 and 31 January 2005 by the Airborne Geophysical Survey of the Amundsen Embayment (AGASEA) expedition.

Contacts and Acknowledgments
Detailed Data Description
Data Access and Tools
Data Acquisition and Processing
References and Related Publications
Document Information

Citing These Data

The following example shows how to cite the use of this data set in a publication.

Carter, S. P., D. A. Young, and D. D. Blankenship. 2012. Subglacial Water Flow Paths Under Thwaites Glacier, West Antarctica, [list the dates of the data used]. Boulder, Colorado USA: National Snow and Ice Data Center. http://dx.doi.org/10.7265/N5RJ4GC8.

Overview

Platform	Aircraft
Sensor	Hi-Capability Radar Sounder (HiCARS), Version 1
Spatial Coverage	Thwaites Glacier, West Antarctica Northernmost Latitude: 75°S Southernmost Latitude: 80°S Westernmost Longitude: 120° W Easternmost Longitude: 90° W
Spatial Resolution	15 km x 15 km ice thickness grid, sampled at 15 m along track
Temporal Coverage	7 December 2004 to 31 January 2005
Parameters	Northing (m) Easting (m) Longitude (dec. deg.) Latitude (dec. deg.) Hydraulic Head (m) Surface Elevation (m) Bed Elevation (m) Flow Path
Data Format	ASCII text (.txt) Portable Document Format (.pdf)
Data Access	FTP

1. Contacts and Acknowledgments

Investigators Name and Title

Dr. Sasha P. Carter
Scripps Institution of Oceanography
University of California, San Diego
9500 Gilman Drive
La Jolla, CA 92093
USA

Dr. Duncan A. Young
University of Texas at Austin
Institute for Geophysics
Mail Code R2200
10100 Burnet Road
Austin, TX 78758
USA

Dr. Donald D. Blankenship
University of Texas at Austin
Institue for Geophysics
Mail Code R2200
10100 Burnet Road
Austin, TX 78758
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

This research was supported by the National Science Foundation (NSF) Office of Polar Programs (OPP) grant number 0636724.

2. Detailed Data Description

Format

Data are provided as an ASCII text file with a corresponding .pdf map.

File and Directory Structure

The FTP directory ftp://sidads.colorado.edu/pub/DATASETS/AGDC/nsidc0518_carter_V01 contains two files:

drains.xyz, an ASCII text file that contains the flow path data;
drains.xyz.pdf, a Portable Document Format file that contains a map of the flow paths.

File Size

drains.xyz and drains.xyz.pdf are 48 KB and 125 KB, respectively.

Spatial Coverage

Thwaites Glacier, West Antarctica:

Northernmost Latitude: 75°S
Southernmost Latitude: 80°S
Westernmost Longitude: 120° W
Easternmost Longitude: 90° W

Spatial Resolution

Ice thickness data are spaced on a 15 km by 15 km ice thickness grid over the entire catchment of the glacier and sampled at approximately 15 m along track.

Temporal Coverage

The AGASEA radar data from which this data set was derived were obtained between 7 December 2004 and 31 January 2005.

Parameters

Table 2 below provides a description of each column in drains.xyz.

**Table 2.** Column descriptions for drains.xyz
Column	Description
1	Northing (polar stereographic -71 degrees)
2	Easting (polar stereographic -71 degrees)
3	Longitude (decimal degrees)
4	Latitude (decimal degrees)
5	Hydraulic Head (meters, WGS-84)
6	Surface Elevation (radar derived; meters, WGS-84)
7	Bed Elevation (radar derived; meters, WGS-84; 169 m/usec, no firn correction)
8	Flow Path Name (text)

Sample Data

ASCII data are arranged in blocks, each representing a separate flow path. Blocks are separated by a line that begins with the ">" character followed by the flow path name, as illustrated by the following example from drains.xyz:

Figure 1 is a map showing the flow paths. The file drains.xyz.pdf contains a high-resolution version.

Figure 1: Flow paths map from Carter, S. P. et al., 2012

3. Data Access and Tools

Data Access

Data are available via FTP.

Software and Tools

Data can be read using standard spreadsheet software or with Global Mapping Tools, an open source collection of approximately 65 tools for manipulating geographic and Cartesian data sets.

4. Data Acquisition and Processing

Data Acquisition Methods

Radar data were acquired by the Airborne Geophysical Survey of the Amundsen Embayment (AGASEA) expedition between 7 December 2004 and 31 January 2005, using the High Capability Radar Sounder Version 1 (HiCARS 1) VHF ice-penetrating radar system developed by the University of Texas Institute of Geophysics. HiCARS, which uses an 8 kW transmitter with a 1 μs FM chirp and a pulse repetition frequency of 6400 Hz, was operating with a 60MHz center frequency and 15 MHz of bandwidth. Across track beam width is controlled by the antenna beam pattern, and has a main cross-track half-power beam width of 12 degrees and side lobes at ±22 degrees.

Derivation Techniques and Algorithms

Processing Steps

Radar data were down converted to a 10 MHz center frequency and recorded on two gain offset channels sampled at 50 MHz. Total dynamic range between the 2 channels is 90 dB. The data were coherently stacked 32 times to yield 16-bit, 3200-sample coherent records at 200 Hz, and during post-processing were range compressed with a synthetic chirp and filtered to remove 10 MHz coherent system noise.

Subglacial water flow paths were inferred by first using gridded ice thicknesses and bed elevations to generate a course map of hydrostatic potential, and then converting line-based ice thicknesses and bed elevations, derived from focused radar data, into lines of hydrostatic potential. Flow lines were constructed by selecting minima at obvious nickpoints and connecting them along fall lines of the grid.

5. References and Related Publications

Holt, J. W., D. D. Blankenship, D. L. Morse, D. A. Young, M. E. Peters, S. D. Kempf, T. G. Richter, D. G. Vaughan, and H. Corr. 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.

Peters, M. E., D. D. Blankenship, S. P. Carter, D. A. Young, S. D. Kempf, and J. W. Holt. 2007. Along-Track Focusing of Airborne Radar Sounding Data from West Antarctica for Improving Basal Reflection Analysis and Layer Detection, IEEE Transactions on Geoscience and Remote Sensing, 45(9), 2725-2736, doi: 10.1109/TGRS.2007.897416.

Carter, S. P., H. A. Fricker, D. D. Blankenship, J. V. Johnson, W. H. Lipscomb, S. F. Price, and D. A. Young. 2011. Modeling Five Years of Subglacial Lake Activity in the MacAyeal Ice Stream (Antarctica) Catchment Through Assimilation of ICESat Laser Altimetry, Journal of Glaciology, 57(206), 1098-1112, doi: 10.3189/002214311798843421.

Related Data Collections

Related Web Sites

6. Document Information

Acronyms and Abbreviations

Acronyms used in this document are listed below in Table 3:

**Table 3.** Acronyms and Abbreviations
Acronym	Description
AGASEA	Airborne Geophysical Survey of the Amundsen Embayment
AGDC	Antarctic Glaciological Data Center
FTP	File Transfer Protocol
HiCARS	Hi-Capability Radar Sounder
NSIDC	National Snow and Ice Data Center
PDF	Portable Document Format

Document Creation Date

May, 2012

Document URL

http://nsidc.org/data/docs/agdc/nsidc0518-carter/

Service Interruption

National Snow and Ice Data Center