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Data Set ID:
IGBTH3

IceBridge Sander AIRGrav L3 Bathymetry, Version 1

The NASA IceBridge Sander AIRGrav L3 Bathymetry (IGBTH3) 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, and are stored in ASCII text format and are available for periodic, ongoing campaigns from 2009 to the present via FTP.

Geographic Coverage

Parameter(s):
  • Bathymetry/Seafloor Topography > Bathymetry
Spatial Coverage:
  • N: -53, S: -90, E: 180, W: -180

Spatial Resolution:
  • Varies x Varies
Temporal Coverage:
  • 1 October 2009
Temporal Resolution: Varies
Data Format(s):
  • ASCII Text
Platform(s) AIRCRAFT, DC-8, P-3B
Sensor(s): AIRGrav, GRAVIMETERS
Version: V1
Data Contributor(s): Kirsteen Tinto, Robin Bell, James Cochran
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.

Tinto, K. J. and R. E. Bell 2011. IceBridge Sander AIRGrav L3 Bathymetry, Version 1. [Indicate subset used]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: http://dx.doi.org/10.5067/B9DRKO62I5Q9. [Date Accessed].

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Detailed Data Description

Format

The IceBridge Sander AIRGrav L3 Bathymetry data files are ASCII text XYZ grid files.

See the IceBridge Sander AIRGrav L4 Bathymetry data set for CSV data files of Arctic fjords and Antarctic ice shelves bathymetry based on measurements from the Sander Geophysics Airborne Inertially Referenced Gravimeter (AIRGrav) system.

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File and Directory Structure

Data are available in the https://n5eil01u.ecs.nsidc.org/ICEBRIDGE_FTP/IGBTH3_AIRGravBathymetry_v01/ directory.

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File Naming Convention

The bathymetry data files have the naming convention shown below. File name variables are described in Table 1.

larsen_bathy.xyz
thwaites_bathy.xyz

name_bathy.xyz

Where:

Table 1. Gravity Data File Naming Convention
Variable Description
name Glacier or ice shelf name, e.g. Larsen, Thwaites
bathy Indicates bathymetry data
.xyz Indicates ASCII text x, y, z data file
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File Size

Files range from 290 KB to 673 KB.

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Volume

The entire data set is approximately 1 MB.

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Spatial Coverage

Spatial coverage for the IceBridge Sander AIRGrav L3 Bathymetry data includes Antarctic areas.

Antarctica:
Southernmost Latitude: 90° S
Northernmost Latitude: 53° S
Westernmost Longitude: 180° W
Easternmost Longitude: 180° E

Note: See the IceBridge Sander AIRGrav L4 Bathymetry data set for bathymetry of Arctic ice shelves based on measurements from the Sander Geophysics Airborne Inertially Referenced Gravimeter (AIRGrav) system.

Spatial Resolution

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).

Projection and Grid Description

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)

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Temporal Coverage

These data were collected as part of Operation IceBridge funded campaigns from October 2009 to the present.

Temporal Resolution

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.

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Parameter or Variable

The IceBridge Sander AIRGrav L3 Bathymetry data set contains location in Polar Stereographic easting and northing, and bathymetry measured positively downwards.

Parameter Description

The bathymetry data files contain fields as described in Table 2.

Table 2. Parameter Description and Units
Column Description Units
1 Antarctic Polar Stereographic Easting Meters
2 Antarctic Polar Stereographic Northing Meters
3 Bathymetry Meters

Sample Data Record

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.

sample data record

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Software and Tools

The data files may be opened by any ASCII text reader.

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Data Acquisition and Processing

Theory of Measurements

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.

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Data Acquisition Methods

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.

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Derivation Techniques and Algorithms

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.

Processing Steps

The following processing steps are performed by the data provider.

For all bathymetry data sets:

  1. The gravimeter data were filtered and decimated to 10 Hz to match the GPS data.
  2. GPS-derived accelerations were subtracted from the data.
  3. The gravity was corrected for the Eötvös effect.
  4. The expected gravity at the measurement latitude was subtracted.
  5. The resulting anomalies were decimated to 2 Hz and low-pass filtered to suppress noise.
  6. The free-air correction was applied.
  7. After evaluating the noise level with different filter lengths, a 70 s filter was used. The filter half-wavelength is approximately 5 km at the average flight speed of 275 knots (142 m/s). Features narrower than the half wavelength can be resolved, but have an attenuated amplitude.
  8. Horizontal accelerations associated with turns and other vigorous maneuvers disturb gravity measurements. A routine that examines horizontal accelerations was used to divide each flight into lines that are free of these perturbations.
For the Larsen data (larsen_bathy.xyz) (Cochran and Bell 2012):
  1. The marine free-air gravity data were gridded, upward-continued to the elevation of the Operation IceBridge flights over Larsen C and the upward-continued grid was resampled at the original measurement locations.
  2. The combined airborne and upward continued marine data set was gridded on a 2 km grid.
  3. The gridded data set was inverted for bathymetry using the Parker-Oldenburg technique.
  4. To avoid artifacts, the grid of bathymetry data was sampled at the locations of the gravity data points and then regridded.

For the Thwaites data (thwaites_bathy.xyz) (Tinto and Bell 2011):

  1. A forward model of gravity was generated according to Talwani, 1959 with ice density 0.915 g/cc, water density 1.028 g/cc and rock density 2.67 g/cc. Ice surface and base are taken from the NASA Airborne Topographic Mapper (ATM) (Krabill 2010) and the CReSIS Multichannel Coherent Radar Depth Sounder (MCoRDS) (Allen 2009).
  2. The model was pinned at a point where ice is grounded and nearby gravity variations are due only to variations in topography of the ice/rock interface.
  3. A gravity inversion was performed on the water/rock interface under floating ice. This produces a model of the sub-ice shelf bathymetry responsible for the gravity anomaly.
  4. 2D inversion results were gridded to produce a bathymetry map.

Error Sources

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).

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Sensor or Instrument Description

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.

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References and Related Publications

Contacts and Acknowledgments

James R. Cochran, Robin E. Bell, Kirsteen Tinto 
Lamont-Doherty Earth Observatory
109C Oceanography
61 Route 9W - PO Box 1000
Palisades, New York 10964-8000 USA

Acknowledgments: 

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.

Document Information

Document Creation Date

11 January 2013

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