Jacques Hinderer

First absolute gravity measurements at the French station Dumont d’Urville (Antarctica)

We present the first series of absolute gravity measurements at the French station Dumont d’Urville in Antarctica where GPS, DORIS and tide gauge continuous series are available. These gravity observations have been performed with the absolute gravimeter FG5#206 during one week in February-March 2000. We report on the field conditions of the experiment for which a thermally regulated shelter has been built and discuss the quality of the results. A special attention is paid to the influence of tidal ocean loading and different corrections according to existing models are tested. We also use this opportunity to establish a gravimetric link with the tide gauge of Dumont d’Urville in order to provide a geodetic Refserence. The tide gauge belongs to the ROSAME network and we used a Scintrex CG3-M relative gravimeter to perform this measurement. In the long term, it is essential to be able to distinguish between any vertical motion of tectonic origin and the true sea level changes. Furthermore, we have established a precise gravimetric link between Hobart (Tasmania) and Dumont d’Urville, which will be useful for future marine geophysical campaigns in this region.

Relative Gravimetry and the Study of the Earth’s Structure and Dynamics

This paper reviews how the study of surface gravity changes is able to provide useful information on the Earth’s structure and global dynamics. The spectral range which is observable with superconducting gravimeters is broad from the seismic frequency band to periods larger than one year. We first investigate the seismic and sub-seismic bands with special attention paid to the gravity detection of core modes. We show how accurate tidal measurements allow to infer constraints on various phenomena like mantle (an-)elasticity or core-mantle coupling from the Free Core Nutation resonance in the diurnal band. Another resonance is also theoretically predicted for the rotation of the solid inner core (Free Inner Core Nutation) but yet unobserved. Surface loading mechanisms induce gravity changes over a wide spectral range and gravity measurements are able to sense very small ocean height changes or atmospheric pressure changes. Finally long-period gravity variations due to polar motion or hydrogeology can also be observed by superconducting gravimeters because of their low instrumental drift.

Methodological Investigation of the Processing of Absolute Gravity Data

This paper is devoted to the influence of several parameters (modelling of solid and ocean tides, air pressure, vertical gradient, site conditions) affecting the determination of the value of the absolute gravity. The confidence in the results relies on the dependence of the gravity (mean value and time-varying residues) on small changes in these parameters and the way we can constrain our choice of them. With the help of data sets obtained from absolute gravimeter (AG) FG5#206 at different stations, we will thus study the influence of i) the modelling of solid tides by the use of several models based on recent tidal developments, ii) the modelling of the oceanic loading tides including models derived from satellite altimetry, iii) the modelling of the pressure correction by the use of a local barometric admittance value versus a global loading computation, iv) the vertical gradient (do we have to measure it each time or is it better to use always the same value even if it is a theoretical one?). A methodological investigation of the consequences of these corrections will help answering the fundamental question of the duration which is needed in order to achieve convergence in the mean g value. We will also show the impact on the gravity residues of these parameters.

Time stability of gravity at different sites in France

This paper deals with the stability of gravity measurements in time of some specific sites in France (J9-Strasbourg, Cerga (Grasse), Brest, Welschbruch (Vosges Mountains)) as observed by regular gravity measurements since February 1997 with the absolute gravimeter FG5#206. The rate of repetition of absolute gravity measurements is variable ranging from one experiment every six weeks at Strasbourg to one per year at Brest. The variations of the gravity value are compared with the variations of the vertical position deduced from different collocation positioning techniques (Global Positioning System, Satellite Laser Ranging, Lunar Laser Ranging). This comparison will be analysed in order to bring new elements to a discussion on the discrimination between the gravity contribution resulting from vertical motion in the Earth’s vertical free air gradient field and the one originating from mass redistribution.