Absolute Gravity Measurements in the Southern Indian Ocean

Abstract

In March – April 2005, Absolute Gravity (AG) has been measured by the Strasbourg gravimetry team using gravimeter FG5#206 at 4 French scientific stations in the Indian Ocean. Crozet, Kerguelen and Amsterdam are isolated Islands located approximately 3000 km south of La Reunion Island. Four times a year, the multi-purpose ship Marion-Dufresne II connects them to La Réunion. Measurements were taken at Crozet from 8 to 10 March, Kerguelen from 14 to 18 March, and Amsterdam from 20 to 24 March. Moreover AG was measured at the Volcanic Observatory of the Piton de la Fournaise, La Réunion Island, from 31 March to 4 April.

The 2005 AG measurements were the first ever made at Amsterdam Island. In contrast, AG measurements were previously done at Crozet in 2003, Kerguelen in 2001 and 2003, and finally at La Réunion in 2001 and 2003.

Here, we report on the first AG measurement at Amsterdam, present the 2005 AG values at each station and compare them to previous ones. Then we pay special attention to the comparison of the gravity changes with available vertical velocities derived from precise positioning techniques (GPS, DORIS) at Kerguelen and La Réunion. The uncertainty on AG variations is too high for inferring any change and stability seems quite likely. DORIS solutions show a small uplifte (a few mm/yr) at both locations.