Central West Antarctica Aeromagnetic Data
The assembly of 5 individual aeromagnetic surveys and 4 geographic
regions to create the Central West Antarctica compilation was done
in several steps.
Antarctica mag ||
DATA PROCESSING STEPS
- Original unsampled field data were edited and filtered, as
required, to remove any anomalous noise. This process was applied
at varying times to all of the data transects provided, including
those from the aircraft magnetometer, base magnetometer, and
longitude, latitude, and elevation from the navigation systems.
- The edited field data for each transect were then sampled at 1
second intervals and merged.
- The diurnal component of the magnetic field was removed by
first subtracting the base magnetometer values from those of the
aircraft magnetometer, and then adding an average base
magnetometer value, preferably quiet-time, to adjust the resultant
residual field amplitudes to reasonable levels. For the CTZ2
survey, we used the average quiet-time geomagnetic value from the
recordings of two ground base station magnetometers located at the
base camp near the center of the survey (Saltus and Kucks, 1992).
For all other surveys, we used the average geomagnetic value from
the base magnetometer recordings for each complete survey. (See the
data table for survey
specifications, including the average base magnetometer values for
each survey, and the data index plot
for names and locations of all the geographic regions.)
- The internal (main-field) component of the magnetic field,
represented by the 1995 revision of the International Geomagnetic
Reference Field (IGRF), which includes the Definitive Geomagnetic
Reference Field (DGRF) for 1990, was subtracted from the aircraft
measurements (Sweeney, 1990).
- The data for each transect were then resampled at a 5 second
- All data were synthesized into a consistent data base
by equating the differing magnetic values of the flight-lines and
tie-lines at their intersection locations. The remaining
data in each line were then adjusted for consistency with the new
intersection values (Mittal, 1984). This process not only reduces
the effects of a non-lithospheric component in the magnetic
data due to the transects being flown at differing times,
but it also makes adjustments due to differences in elevation.
- Preliminary grids were constructed from the sampled aeromagnetic
transects with a cell size of 1.5 kilometers (between 1/3 and 1/5 of
the flight-line spacing of each survey), using a minimum curvature
gridding algorithm (Webring, 1981).
- Data in adjacent geographic regions were combined, not by
application of some smoothing algorithm between adjacent grids, but
by adjustment of all transect data that overlapped into the regions
occupied by the adjacent grids, using a modification of the
algorithm of Mittal (1984). This adjustment was performed by
finding the locations where the overlapping data intersected the
orthogonal flight lines in the adjacent area, and forcing them to
match the mag values at those adjacent intersection locations.
Because of the care taken in selecting the average base
magnetometer (datum) levels, the correct application of the
IGRF/DGRF suite of coefficients, and the adjustment of all transect
data in the regions of adjacent survey overlap, no additional datum
level adjustment was applied to any of the regional data to minimize
- Data quality problems were addressed.
- Final grids were constructed from the fully-adjusted flight
line data using a cell size of 1.5 kilometers.
- The composite IRE/BSB/WAZ/TKD grid was continued from its
original flight elevation surface to a draped surface 1,500 meters
above the bedrock elevation surface using the chessboard
method of Cordell and others (1992), applied with a moderate lowpass
GRID PROJECTION SPECIFICATIONS
- Projection = Polar Stereographic
- Central meridian = 110 degrees W
- Base latitude = 90 degrees S
- True scale latitude = 80 degrees S
- Semi-major ellipsoid axis = 6378206.4 m
- Eccentricity squared = 0.0067686579973
U.S. Department of the Interior
U.S. Geological Survey
This page is <URL: http://greenwood.cr.usgs.gov/pub/open-file-reports/ofr-99-0420/cwantarctica.html>
Maintained by: Gene Ellis
Last Modified Thursday, 19-Nov-1998 11:19:34 MST
For more information about this report contact: Ronald