U.S. DEPARTMENT OF THE INTERIOR 

U.S. GEOLOGICAL SURVEY

Central West Antarctica Aeromagnetic Data: 

A Web Site for Distribution of Data and Maps

(paper edition)

by

R.E. Sweeney1 , C.A. Finn1 , D.D. Blankenship2 , 

R.E. Bell3 , and J.C. Behrendt1

Open-File-Report 99-420

This report is preliminary and has not been reviewed for conformity with U.S. Geological 

Survey editorial standards. Any use of trade, product, or firm names is for descriptive 

purposes only and does not imply endorsement by the U.S. Government.

JU.S. Geological Survey, Denver, CO

2The University of Texas Institute for Geophysics, Austin, TX

3Lamont-Doherty Earth Observatory, Palisades, NY

1999
Abstract

This report is a listing of the pages in the following web site:

http ://greenwood.cr.us gs. gov/pub/open-file-reports/ofr-99-0420/cwantarctica.html

This web site describes the aeromagnetic data collected as a National Science Foundation 

(NSF) project entitled, "Lithospheric controls on the behavior of the West Antarctic ice 

sheet." This is a multi-institutional project that includes the Institute for Geophysics of the 

University of Texas at Austin, the Lamont-Doherty Earth Observatory, and the U. S. 

Geological Survey. The Support Office for Aerogeophysical Research (SOAR) collected the 

data. The website allows users to download (via FTP) data files as 1.5 km grids (in several 

formats), point data files used to make the grids, and plot files (in HPGL, GIF, and postscript 

formats) covering the area of Central West Antarctica. The anonymous FTP sites are:

ftp://greenwood.cr.usgs.gov/pub/open-file-reports/ofr-99-0420/data 

and

ftp://greenwood.cr.usgs.gov/pub/open-file-reports/ofr-99-0420/plots 

Table of Contents: 

Pages Content

1....................Home page - color figure of composite aeromagnetic map

2....................Non-graphical home page

3....................Aeromagnetic data index map

4....................Table 1 - Aeromagnetic survey specifications

5....................Aeromagnetic compilation

6-7.................Aeromagnetic data processing

8....................List of other aeromagnetic maps

9....................Central West Antarctica composite aeromagnetic map

10...................Central West Antarctica composite aeromagnetic map -

continued to 1500 meters above bedrock elevation 

11...................IRE & BSB composite aeromagnetic map

12...................TKD & WAZ composite aeromagnetic map

13...................WAZ composite aeromagnetic map

14...................TKD composite aeromagnetic map

15...................Reference list
Central West Antarctica Aeromagnetic Anomaly Map

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Central West Antarctica Aeromagnetic Data

This is a non-image-mapped page to allow those with older browsers to access all the individual pages 

that make up the Central West Antarctica Aeromagnetic Data web site (USGS Open-file report 99-420).

Select a destination from the list below:

Read map compilation text.

Read data processing details.

View the aeromagnetic data index map.

View the survey specifications table.

View the Central West Antarctica composite aeromagnetic map.

View the other aeromagnetic maps.

View the reference list.

Download gridded data via FTP.

Download flight-line data via FTP.

Download binary plot files via FTP.
Central West Antarctica Aeromagnetic Data Index Map

S78*

S80*-

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The Central West Antarctica aeromagnetic maps contain data from 5 separate Antarctica field seasons 

including 4 geographic areas (IRE, BSB, WAZ, TKD). The map shown above [gridname = avhrr_index] 

is an index to the names and geographic regions of the original surveys. The basemap displays AVHRR 

(advanced very high resolution radiometer) data from Ferrigno and others (1996). A data table 

summarizes the survey specifications, including dates flown and original flight elevations. The final 

data, in both point and grid format, are available to copy using FTP.
Table 1.-- Survey specifications used in the Central West Antarctica compilation.

SURVEY

CTZ2

DATES

12/91 - 1/92

AVERAGE 

BASEMAGFOR 

SURVEY

57,150nT

CTZ3 12/92-1/93 |57,070nT

RTZ4 1/95 57,260 nT

RTZ5

RTZ6

12/95 - 1/96 57,190nT

1/96 57,190nT

12/96 - 1/97 59,435 nT

AREA FLOWN

IRE/CC

IRE/NC

IRE/EC

IRE/NE

IRE/SE

IRE/SC

IRE/SW

IRE/WC

BSB/SC

BSB/NW(partial)

BSB/NC(partial)

BSB/CC(partial)

BSB/NW(completed)

BSB/NC(completed)

BSB/CC(completed)

IRE/NW

BSB/SW

BSB/WC

WAZ/NE

WAZ/NW

WAZ/SW

TKD/NE

TKD/SE

TKD/NC

TKD/SC

TKD/NW

TKD/SW

FLIGHT ELEVATIONS 

(BAROMETRIC)

2250m

2500m

2750m

2750m

2500m

2000m

1750m

1750m

2100m

2200m

2200m

2200m

2200m

2200m

2200m

1900m

1900m

2050m

1900m

1600m

1600m

3950ft

3950ft

3300ft

3300ft

3300ft

3300ft
Central West Antarctica Aeromagnetic Compilation

The aeromagnetic data presented here are one data set collected as a National Science Foundation (NSF) 

project entitled, "Lithospheric controls on the behavior of the West Antarctic ice sheet." This is a 

multi-institutional project that includes the Institute for Geophysics of the University of Texas (UTIG) at 

Austin, the Lamont-Doherty Earth Observatory (LDEO), and the U. S. Geological Survey (USGS). The 

scientific objective of this program is to understand the lithospheric framework across the West 

Antarctic rift system in order to determine the geological controls on the dynamics of the West Antarctic 

ice sheet. The Support Office for Aerogeophysical Research (SOAR) collected the data.

These data show variations in the Earth's magnetic field caused primarily by the uneven distribution of 

the mineral magnetite in the rocks that make up the upper part of the Earth's crust. The features and 

patterns of the maps reveal details of subsurface geology including the locations of buried faults and 

magnetite-bearing rocks, which include Cenozoic magmatic rocks, and non-magnetic sedimentary and 

granitic rocks.

The aeromagnetic data were collected in 5 separate field seasons covering 4 geographic regions (IRE, 

BSB, WAZ, TKD). Each survey made simultaneous measurements of radar ice soundings, laser 

altimetry, gravity (Bell and others, 1999), and magnetics. The transects flown in all regions were 

separated by 5.3 kilometers in both the flight-line and orthogonal tie-line directions. The magnetic field 

observations are accurate to one nT. High-resolution horizontal and vertical positions were obtained 

from differential positioning techniques based on Global Positioning System (GPS) satellites. When 

GPS satellite positioning was unavailable, the laser altimetry was corrected using attitude information 

from an inertia! navigation system, and a digital pressure transducer was used to recover vertical 

positions and accelerations. The latitudes of the final composite map ranged from approximately -77.8 to 

-84.5 degrees; the longitudes from approximately -103.9 to -153.3 degrees. A data index plot gives an 

overview of the names and locations of the geographic regions. A data table summarizes the survey 

specifications, including dates flown and original flight elevations. A list of data processing steps details 

the procedures used to produce the final maps.

HPGL plot files can be downloaded for plotting at a 1:1,000,000 scale for the composite IRE/BSB, 

WAZ/TKD, and IRE/BSB/WAZ/TKD maps, and at a 1:750,000 scale for the individual WAZ and TKD 

maps. GIF and page-sized color postscript plot files can be downloaded for all maps. All maps are also 

available for screen viewing.

This project was supported by National Science Foundation Grant OPP-9319877 and by the Mineral 

Resources Team, the Global Change and Climate History Team, and the Coastal and Marine Geology 

Team, all of the USGS. The authors wish to thank our USGS colleagues Richard Saltus and Kim 

Oshetski for their assistance in preparing this report, and Robert Arko of LDEO for his assistance with 

the data processing.
Central West Antarctica Aeromagnetic Data Processing

The assembly of 5 individual aeromagnetic surveys and 4 geographic regions to create the Central West 

Antarctica compilation was done in several steps.

DATA PROCESSING STEPS

1. 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.

2. The edited field data for each transect were then sampled at 1 second intervals and merged.

3. 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.)

4. 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).

5. The data for each transect were then resampled at a 5 second interval.

6. 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.

7. 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).

8. 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 boundary differences.

9. Data quality problems were addressed.

10. Final grids were constructed from the fully-adjusted flight line data using a cell size of 1.5 

kilometers.
11. 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 filter.

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
Central West Antarctica Aeromagnetic Maps - View/Download

Select an aeromagnetic map from the list below. For a full survey description, see the page on 

aeromagnetic compilation. For a detailed data processing description, see the page on aeromagnetic data 

processing.

View composite grid of all data sets at original survey elevations, [gridname = rtz]

View composite grid of all data sets at 1,500 meters above bedrock elevations, [gridname = 

rtz_bed!500]

View composite IRE & BSB grid at original survey elevations, [gridname = irebsb] 

View composite TKD & WAZ grid at original survey elevations, [gridname = tkdwaz] 

View composite WAZ grid at original survey elevations, [gridname = waz] 

View composite TKD grid at original survey elevations, [gridname = tkd] 

Transfer to plot file directory for downloading HPGL, GIF, & postscript plot files.
Central West Antarctica Composite Aeromagnetic Map

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The Central West Antarctica composite aeromagnetic map above was constructed (see data processing 

details) from data in all the regions of Central West Antarctica shown on the data index plot. A data 

table summarizes the survey specifications, including dates flown and original flight elevations, 

[gridname = rtz]
Central West Antarctica Composite Aeromagnetic Map 

1500 Meters Above Bed Elevation

577'

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579*

380'

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The Central West Antarctica composite aeromagnetic map above was constructed (see data processing 

details) from data in all the regions of Central West Antarctica shown on the data index plot The 

continuation from the original flight elevation surface to a draped surface 1,500 meters above the 

bedrock elevation surface was performed using the chessboard method of Cordell and others (1992), 

applied with a moderate lowpass filter. A data table summarizes the survey specifications, including 

dates flown and original flight elevations, [gridname = rtz_bed!500]

10
IRE & BSB Composite Aeromagnetic Map

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The IRE and BSB composite aeromagnetic map above was constructed (see data processing details) 

from data in the IRE and BSB regions of Central West Antarctica shown on the data index plot. A data 

table summarizes the survey specifications, including dates flown and original flight elevations, 

[gridname = irebsb]

11
TKD & WAZ Composite Aeromagnetic Map

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378*

379'

380*

381*

The TKD and WAZ composite aeromagnetic map above was constructed (see data processing details) 

from data in the TKD and WAZ regions of Central West Antarctica shown on the data index plot. A data 

table summarizes the survey specifications, including dates flown and original flight elevations, 

[gridname = ikdwaz]

12
WAZ Composite Aeromagnetic Map

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The WAZ composite aeromagnetic map above was constructed (see data processing details) from data in 

the WAZ region of Central West Antarctica shown on the data index plot. A data table summarizes the 

survey specifications, including dates flown and original flight elevations, [gridname = waz]

13
TKD Composite Aeromagnetic Map

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CO

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379*30'

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380*30'

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The TKD composite aeromagnetic map above was constructed (see data processing details) from data in 

the TKD region of Central West Antarctica shown on the data index plot. A data table summarizes the 

survey specifications, including dates flown and original flight elevations, [gridname = tkd]

14
Central West Antarctica Aeromagnetic References

 Behrendt, J.C., Blankenship, D.D., Finn, C.A., Bell, R.E., Sweeney, R.E., Hodge, S.M., and J.M. 

Brozena, 1994, Evidence of late Cenozoic volcanic activity in the West Antarctic rift system 

revealed by the CASERTZ aeromagnetic survey: Geology, v. 22, n. 6, p. 527-530.

 Behrendt, J.C., Saltus, R.W., Damaske, D., McCafferty, A., Finn, C., Blankenship, D., and R. Bell, 

1996, Patterns of late Cenozoic volcanic and tectonic activity in the West Antarctic rift system 

revealed by aeromagnetic surveys: Tectonics, v. 15, p. 660-676.

 Bell, R.E., Blankenship, D.D., Finn, C.A., Morse, D.L., and T.A. Scambos, 1998, Aerogeophysical 

evidence for geologic controls on the onset of a West Antarctic ice stream: Nature, v. 394, p. 

58-62.

 Bell, R.E., Childers, V.A., Arko, R.A., Blankenship, D.D., and J.M. Brozena, 1999, Airborne 

gravity and precise positioning for geologic applications: Journal of Geophysical Research, v. 104, 

p. 15,281-15,292.

 Blankenship, D., Bell, R., Hodge, S., Brozena, J., Behrendt, J., and C.A. Finn, 1993,

Aerogeophysical evidence for active volcanism beneath the West Antarctic ice sheet: Nature, v. 

361, n. 6412, p. 526-529.

 Cordell, L., Phillips, J.D., and R.H. Godson, 1992, U.S. Geological Survey potential-field 

geophysical software, version 2.0: U.S. Geological Survey Open-File Report 92-18A-G, 16 p.

 Ferrigno, Jane G., Mullins, Jerry L., Stapleton, Jo Anne, Chavez, Jr., Pat S., Velasco, Miguel G., 

Williams, Jr., Richard S., Delinski, Jr., George F., and D'Ann Lear, 1996, Satellite image map of 

Antarctica: U.S. Geological Survey Miscellaneous Investigations Map 1-2560, scale 1:5,000,000.

 Mittal, P.K., 1984, Algorithm for error adjustment of potential field data along a survey network, 

Geophysics, v. 49, p. 467-469.

 Saltus, R.W., and R.P. Kucks, 1992, Geomagnetic activity and its implications for the 1991-92 

CASERTZ aeromagnetic survey in Antarctica [abs.]: Eos (American Geophysical Union 

Transactions), v. 73, no. 43, suppl., p. 131.

 Sweeney, R.E., 1990, IGRFGRID--A program for creation of a total magnetic field (International 

Geomagnetic Reference Field) grid representing the earth's main magnetic field: U.S. Geological 

Survey Open-File Report 90-45, A-B, 37 p., 1 diskette.

 Sweeney, R.E., Finn, C.A., Behrendt, J.C., Blankenship, D.D., Bell, R.E., Hodge, S.M.,

Najmulski, K.A., Noonan, Maureen, Kucks, R.P., Saltus, R.W., Griffiths, Kenneth, Arko, Robert, 

and Sheila Dopplehammer, 1994, Aeromagnetic maps of the eastern Ross transect zone, West 

Antarctica, Folio A: U.S. Geological Survey Open-File Report 94-0180,4 sheets, scale 1:250,000.

 Webring, M.W., 1981, MINC--A gridding program based on minimum curvature: U.S. Geological 

Survey Open-File Report 81-1224,41 p.

15