R.E. Bell1, V.A. Childers1,3, R.A. Arko1, M. Studinger1
1Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York
2Institute for Geophysics, University of Texas, Austin, Texas
3Naval Research Laboratory, Washington, D.C.
An airborne geophysical program over West Antarctica was designed to study the linkage between the West Antarctic Rift System and the dynamic evolution of the overlying West Antarctic Ice Sheet. The integrated data acquisition system mounted on a DeHavilland Twin Otter consists of an airborne gravity system, a towed aeromagnetic system, an ice penetrating radar, and a laser altimeter (see picture above). The aerogeophysical data set covers a 300,000 km2 region in West Antarctica (Figure 1). The survey area is covered by a grid of orthogonal flight lines spaced 5.3 km apart in both directions consisting of 150,000 line kilometers. Flight elevation varied from 1600 m to 2500 m (check here for details).
The work described above is a project at Lamont-Doherty Earth Observatory in collaboration with the Institute for Geophysics of the University of Texas at Austin. Airborne geophysical surveys within this program were carried out by the Support Office for Aerogeophysical Research (SOAR), a National Science Foundation facility of the Office of Polar Programs located at the University of Texas. Funding for this project was provided by the US National Science Foundation.
A detailed technical description of the airborne gravimetry can be found in Bell et al., Airborne gravity and precise positioning for geologic applications, Journal of Geophysical Research, Vol. 104, No B7, 15281-15292, 1999.
The gravimeter was mounted at the center of gravity of the Twin Otter aircraft. The platform enclosure was bolted to the floor, and the sensor was shielded from vibration by rubber shock mounts within the platform assembly. The gravity instrumentation has included both a Bell Aerospace BGM-3 gravity meter and a LaCoste & Romberg "S" gravity meter modified by ZLS Corporation. The BGM-3 gravity meter was made available through an agreement between NSF and NAVO. For both gravimeters, the sensor was mounted on a two-axis, gyro-stabilized platform that aligns the sensitive axis of the accelerometer with the time-averaged local vertical.
Data reduction steps included the subtraction of the vertical acceleration of the aircraft (Aaircraft) from the gravity measurement (Ameasured). The Eötvös correction for airborne measurements was calculated to compensate for measuring gravity from a moving platform on a rotating Earth and was added to the measurement. The anomalous gravity was determined by then subtracting the predicted gravity for that latitude at the ellipsoid (Gtheo) and adding the free-air correction (FAC) to correct the predicted gravity to the altitude of the aircraft. These corrections combine to yield the free-air anomaly (FAA):
FAA = Ameasured - Aaircraft + Eötvös + FAC - Gtheo
Aggressive low-pass filtering is required to minimize the high amplitude noise that remains in the free-air anomaly even after corrections are applied. Noise attenuation was optimized by a cosine taper applied as a filter in the frequency domain that begins its roll off at 0 Hz (dc) and reaches infinite attenuation at 0.006 Hz.
Crossover errors have been calculated at profile intersections and an appropriate dc shift and drift rate have been applied to the profiles in order to minimize the overall standard deviation in a least square sense. The accuracy of the airborne gravity data was estimated from the evaluation of crossover errors (± 2.98 mGal) and the evaluation of repeat measurements (± 1.39 mGal). The spatial resolution is 5.5 km.
Gravity measurements have been tied to the International Gravity Standardization Network (IGSN-71) at McMurdo Station (BLDG57, position 77.8477°S, 166.6820°E).
|Projection type||Lambert conformal conic|
|A axis radius||6378.137|
|B axis radius||6356.752|
|First standard parallel||-81.5|
|Second standard parallel||-83.5|
|Map projection unit||kilometer|
File format is 3 column ASCII file with x [km] y [km] z [mGal] for the projected data and longitude [°] latitude [°] z [mGal] for the files with geographical coordinates. You can choose between gzipped compressed versions (highly recommended) and uncompressed ASCII files:
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The picture below shows the free-air gravity data using a histogram equalized color table and an illumination to reveal smaller details.
US Geological Survey Open File Report 99-0420 with the magnetic data set. Homepage of the West Antarctic Ice Sheet Initiative. A multidisciplinary study of rapid climate change and future sea level. Ongoing work regarding extensional tectonics and ice sheet dynamics (Michael Studinger's homepage).