Thursday 25 August 2005

GP3
1530-1650 hours

415
Estimating gravity change and crustal motion in Dronning Maud Land, Antarctica
Koivula, Hannu¹, Mäkinen, Jaakko T¹, Ahola, Joel¹, Bilker, Mirjam¹, Poutanen, Markku¹
1 Finnish Geodetic Institute, Masala, Finland
Author email: Jaakko.Makinen@fgi.fi
In 2004 we performed absolute gravity measurements at Aboa, Sanae IV and Novolazarevskaya, using the FG5-221 of the Finnish Geodetic Institute. Measurements will be repeated in the future. We already measured at Aboa in 1994 and 2001, with the JILAg-5. Gravity is sensitive to elevation change and to density variations caused by both present and past variation in ice mass. Change in the 3-D position of the sites is being determined by GPS methods: continuous GPS (CGPS) was started at Aboa in 2003. The relation between gravity change and elevation change at bedrock depends on the uplift mechanism. For past deglaciation (viscous effect) it is about -1.5 microgals/cm. For present-day variation in ice load it is about -2.7 microgals/cm (elastic effect). The variation in ice elevations is being mapped by satellite altimeter missions, most recently by the laser altimeter of ICESAT, and soon by the Synthetic Interferometric Radar Altimeter of CryoSat. It can (with assumptions on density) be translated into variation in load and convolved with Green's functions to predict the elastic contribution at the gravity/GPS sites. The GRACE mission provides estimates of total variation in regional mass, including mantle flow due to the GIA and the present-day ice load. The elastic ratio -2.7 microgals/cm (rule-of-thumb) is based on the assumption that the variable load is a thin layer at the station elevation. In fact, the attraction of the load must be computed using the correct geometry. This is essential at the close range. At Aboa we have in 2000 started in-situ measurements of ice surface elevations and densities. The four techniques: repeated absolute gravity, CGPS, satellite altimetry, and GRACE all sense past and/or present changes in the ice mass. Using the techniques in combination provides a redundancy check and the possibility to estimate/eliminate unwanted signals and modelling errors.

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