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This data set contains bathymetry of Antarctic ice shelves based on measurements from the Sander Geophysics Airborne Inertially Referenced Gravimeter (AIRGrav) system. The data were collected as part of Operation IceBridge funded aircraft survey campaigns.
We kindly request that you cite the use of this data set in a publication using the following citation. For more information, see our Use and Copyright Web page.
Cochran, J.R., K. J. Tinto, and R. E. Bell. 2011. IceBridge Sander AIRGrav L3 Bathymetry, [indicate subset used]. Boulder, Colorado USA: NASA DAAC at the National Snow and Ice Data Center. http://nsidc.org/data/igbth3.html
Antarctica ice shelves
October 2009 to the present
James R. Cochran, Robin E. Bell, Kirsteen Tinto
Lamont-Doherty Earth Observatory
61 Route 9W - PO Box 1000
Palisades, New York 10964-8000 USA
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
Data collection was supported by NASA grants NNX09AR49G, NNG10HP20C, and NNX10AT69G. We thank Stefen Elieff, Kevin Charles, Marianne McLeish, Joel Dube, Eric Seme, Sean O'Rourke, Eric Renaud, John Drolet and Craig McMahon from Sander Geophysics.
The IceBridge Sander AIRGrav L3 Bathymetry data files are ASCII text XYZ grid files.
Data are available on the n4ftl01u.ecs.nasa.gov FTP site in the /SAN2/ICEBRIDGE_FTP/ directory. Within this directory, there are data subdirectories as shown in Figure 1.
Figure 1. Directory Structure
The bathymetry data files have the naming convention shown below. File name variables are described in Table 1.
|name||Glacier or ice shelf name, e.g. Larsen, Thwaites|
|bathy||Indicates bathymetry data|
|.xyz||Indicates ASCII text x, y, z data file|
Files range from 290 KB to 673 KB.
The entire data set is approximately 1 MB.
Spatial coverage for the IceBridge Sander AIRGrav L3 Bathymetry data includes Antarctic areas.
Southernmost Latitude: 90° S
Northernmost Latitude: 53° S
Westernmost Longitude: 180° W
Easternmost Longitude: 180° E
The survey over Thwaites Ice Shelf was flown as a series of parallel lines, 10 km apart, approximately perpendicular to the grounding line. Data with high horizontal accelerations due to aircraft maneuvers were excluded from the dataset. Free-air anomalies were filtered with a 70 s full wavelength filter, resulting in approximately 4.9 km half-wavelength resolution for a typical flying speed of 140 m/s (Tinto and Bell 2011).
For the survey over Larsen Ice Shelf, the spacing of north-south airborne gravity lines is generally about 20 km, while the spacing of east-west lines ranges from 15-50 km. The line spacing and the survey design limit the granularity of the gravity field and impact the spatial resolution of the final bathymetry model (Cochran and Bell 2012).
Thwaites Grid: 1 km grid spacing
Larsen Grid: 2 km grid spacing
Projection: Polar Stereographic true at 71 S, 0 up, 180 down, WGS-84 ellipsoid (EPSG 3031)
These data were collected as part of Operation IceBridge funded campaigns from October 2009 to the present.
IceBridge campaigns are conducted on an annual repeating basis. Arctic and Greenland campaigns are conducted during March, April, and May; and Antarctic campaigns are conducted during October and November.
The IceBridge Sander AIRGrav L3 Bathymetry data set contains location in Polar Stereographic easting and northing, and bathymetry measured positively downwards.
The bathymetry data files contain fields as described in Table 2.
|1||Antarctic Polar Stereographic Easting||Meters|
|2||Antarctic Polar Stereographic Northing||Meters|
Below are the first ten records of the larsen_bathy.xyz data file. The fields in each record correspond to the columns described in Table 2.
Data are available via FTP.
The data files may be opened by any ASCII text reader.
The gravity signal is extracted from an inertially based system in which a small mass is suspended within a magnetic field. Tiny variations in the acceleration of the gravimeter produce small electrical signals in the sensor as the mass moves within the magnetic field. The processed data from the AIRGrav instrument data consists of two data types: gravity and aircraft attitude.
The gravimeter is located as near the airplane center of mass as possible. Simultaneously acquired gravimeter output GPS data are recorded on hard disks on the plane. Following the flight this data is downloaded onto a PC for processing.
Inversion of the gravity data for bathymetric relief on the continental shelf underlying the Larsen Ice Shelf was carried out using the Parker-Oldenburg technique (Oldenburg, 1974). The Parker-Oldenburg inversion was employed in the form of a MATrix LABoratory (MATLAB) script that utilizes a grid of gravity anomalies as the input (Gomez-Ortiz and Agarwal 2005). In order to avoid potential artifacts in less constrained areas, the grid of depths resulting from the inversion was sampled at gravity measurement points and these values were then regridded.
At Thwaites glacier, inversion was performed on 2D profiles using the technique of Talwani, 1959 in the GM-SYS gravity and magnetic modelling software.
The following processing steps are performed by the data provider.
For all bathymetry data sets:
For the Thwaites data (thwaites_bathy.xyz) (Tinto and Bell 2011):
Errors are approximated as ±70 m for the Thwaites dataset, incorporating errors in gravity measurement, radar ice thickness, ATM surface elevation, 2D model pinning point and some allowance for geological structures. The model assumes the absence of sea floor sediments. If sea floor sediments are present the true bathymetry will be less deep than the model (Tinto and Bell 2011).
Errors are estimated to be about ±50 meters for the Larsen survey, based on sensitivity to possible errors in density and the mean depth of the continental shelf. The depths at the bottom of troughs extending across the continental shelf may be overestimated as much as 100 m due to the presence of low density sediments (Cochran and Bell 2012).
The gravity instrument is a Sander AIRGrav designed for airborne applications. The AIRGrav system consists of a three-axis gyro-stabilized, Schuler-tuned inertial platform on which three orthogonal accelerometers are mounted. The primary gravity sensor is the vertical accelerometer that is held within 10 arc-seconds (0.0028 degree) of the local vertical by the inertial platform, monitored through the complex interaction of gyroscopes and two horizontal accelerometers (Sander et al. 2004). An advantage of the AIRGrav system over other airborne gravimeters is that it has been shown to be capable of collecting high-quality data during draped flights (Studinger et al. 2008). The gravimeter records accelerations arising from variations in the Earth's gravity field and accelerations experienced by the airplane. These accelerations are recorded at 128 Hz. Aircraft accelerations are obtained utilizing differential GPS measurements.
Allen, Chris. 2009, updated current year. IceBridge MCoRDS L2 Ice Thickness. Boulder, Colorado USA: NASA Distributed Active Archive Center at the National Snow and Ice Data Center. Digital media. http://nsidc.org/data/irmcr2.html.
Cochran, J. R. and R. E. Bell. 2012. Inversion of IceBridge Gravity Data for Continental Shelf Bathymetry Beneath the Larsen Ice Shelf. Journal of Glaciology, 58(209):540-552(13).
Gomez-Ortiz, D. and B. Agarwal. 2005. 3DINVER.M: A MATLAB Program to Invert the Gravity Anomaly Over a 3D Horizontal Density Interface by Parker-Oldenburg's Algorithm, Computers & Geosciences, 31(4): 513-520. doi:10.1016/jcageo.2004.1011.1004.
Krabill, William B. 2010, updated current year. IceBridge ATM L2 Icessn Elevation, Slope, and Roughness. Boulder, Colorado USA: NASA Distributed Active Archive Center at the National Snow and Ice Data Center. Digital media. http://nsidc.org/data/ilatm2.html.
Oldenburg, D. W. 1974. The Inversion and Interpretation of Gravity Anomalies, Geophysics, 39: 536-536.
Sander, S., M. Argyle, S. Elieff, S. Ferguson, V. Lavoie, and L. Sander. 2004. The AIRGrav Airborne Gravity System, in Airborne Gravity 2004 - Australian Society of Exploration Geophysicists Workshop, edited by R. Lane, pp. 49-53, Geoscience Australia, http://www.ga.gov.au/image_cache/GA16642.pdf.
Studinger, M., R. E. Bell, and N. Frearson. 2008. Comparison of AIRGrav and GT-1A Airborne Gravimeters for Research Applications, Geophysics, 73: 151-161.
Talwani, Manik, J. Lamar Worzel, and Mark Landisman. 1959. Rapid Gravity Computations for Two-dimensional Bodies with Application to the Mendocino Submarine Fracture Zone, Journal of Geophysical Research, 64(1):49-59.
Tinto, K. J., and R. E. Bell. 2011. Progressive Unpinning of Thwaites Glacier from Newly Identified Offshore Ridge: Constraints from Aerogravity, Geophysical Research Letters, 38: L20503. doi:10.1029/2011GL049026.
The acronyms used in this document are listed in Table 3.
|AIRGrav||Airborne Inertially Referenced Gravimeter|
|ASCII||American Standard Code for Information Interchange|
|ATM||Airborne Topographic Mapper|
|CIRES||Cooperative Institute for Research in Environmental Science|
|CReSIS||Center for Remote Sensing of Ice Sheets|
|EPSG 3031||European Petroleum Survey Group Antarctic Polar Stereographic projection|
|FTP||File Transfer Protocol|
|GPS||Global Positioning System|
|L3||Processing Level 3|
|MCoRDS||Multichannel Coherent Radar Depth Sounder|
|NASA||National Aeronautics and Space Administration|
|NSIDC||National Snow and Ice Data Center|
|URL||Uniform Resource Locator|
|UTC||Universal Time Code|
|WGS-84||World Geodetic System 1984|
11 January 2013