An airborne gravity survey south of the Prince Charles Mountains, East Antarctica
Report of the project:
| The Lambert Glacier - Amery Ice Shelf in East Antarctica is a north-northeast trending graben, which is host to the largest glacier in the world. While the present configuration of this structure suggests a failed rift, some workers have suggested the Lambert region also preserves evidence of an earlier Cambrian suture between at least two Pre-Cambrian blocks that collided during the assembly of Gondwana. Very little previous gravity data exists in the Prince Charles Mountains and no previous gravity data has been collected south of this region. A joint Australian / German expedition under the banner of PCMEGA (Prince Charles Mountains Expedition of Germany and Australia) was established to develop our understanding of this structure. |
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Besides geological and geochemical investigations an aerogeophysical survey was undertakten. The methods used simultaneously were magnetics, radar ice thickness measurements and gravity. The results of this airborne gravity survey are presented in here.
The gravity system consisted of a ZLS LaCoste & Romberg gravity meter mounted on a stabilised platform. Initial processing of the gravity data to the stage of free-air anomaly values was carried out by the equipment supplier.
| Survey specifications | |||
|---|---|---|---|
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| flight line separation | 5 km | ||
flight heights (above sea level) | 2160 2760 3360 m | ||
| total | ~21000 km | ||
| size of the area | ~450x250 km | ||
| airplane | Twin Otter | ||
| gravity instrument | LaCoste& Romberg S | ||
| contractor | FUGRO Airborne Surveys | ||
The main problem with aerogravity is the separation of the gravitational attraction of the Earth's body to be studied and the accelerations generated by the movements of the airplane. The latter can easily be greater by a factor 103 than the actual gravity signal. A series of corrections, as instrument drift, latitude, altitude, Eötvös, accelerations due to the movements of airplane calculated from GPS data etc., are necessary in order to extract the unfiltered Free-Air anomaly (above to the left) form the raw data.
Intelligent filtering removes all the small wave length noise and finally supplies an interpretable map of the Free-Air anomaly (to the right aside). The aplitudes oscilate between ±90 mGal and the shortest wave lengths still remaining are about 20 km.
This Free-Air gravity image is characterised by long wavelength anomalies which represent in part still the ice cover but also deep seated crustal structures. The extension of the Lambert rift has been imaged by in the free-air gravity and is interpreted to extend off to the southwest of the survey grid. A second sub-ice canyon has been interpreted to extend towards the southeast.
Complete Bouguer anomaly
Source: BGR
A very important correction of gravity data in the ice covered environment of Antarctica is the ice correction, as the density of ice is much less than the one of rocks. For this correction a good knowledge of the ice thicknesses is essential. As mentioned, the airborne operation of PCMEGA included a radar ice thickness survey which yielded very good and detailed results. These could be used for calculationg the corrections due to the undulating ice/rock interface. An additional correction takes into consideration the surface topography and finally we end up with the complete Bouguer anomaly (to the right). Both corrections mentioned were calculated in the wave number domain after 2D Fourier transformation of the subice and the surface topographies respectively.
Residual isostatic Bouguer anomaly
Source: BGR
This complete Bouguer anomaly map is characterised by a regional north-south gradient, i.e. gravity becomes smaller towards the south (green and blue colours). Most likely this is due to the increasing thickness of the crust towards the interior of the continent.
The residual Bouguer image (left) shows the gravity data which has been isostatically corrected by subtracting the gravitational effect of the changing thickness of the crust. One can identify areas of positive and negative amplitudes mostly between ±40 mGal. A large scale regional trend from SW to NE is apparent. Interestingly, this northeast trend correlates to some degree with the northeast trend in the magnetic data which had been surveyed simultaneously during PCMEGA. Areas of positive gravity seem to roughly correlate with areas of higher effective elevations: the south-eastern corner (zone W) coinciding with the known borderland of the Gamburtsev Subglacial Mountains, and the central NE-SW striking zone (zone X) similarly distinguished by high altitudes. The cause for residual anomalies must be density inhomogeneities at depth. Positive residual gravity must be caused by excess of masses and conversely, negative anomalies by mass deficits. The mountain areas with positive residual gravity just mentioned may well be composed of denser rocks, whereas the areas in between may be covered by lighter sediments. Interestingly, the prominent sub-ice canyons (dotted lines) are overprinted by the northeast trending gravity high at a low angle.
Literature:
M. McLean & G. Reitmayr: An Airborne Gravity Survey South of the Prince Charles Mountains, East Antarctica. Terra Antartica, 12 (2), 2005 [Abstract]