Muriel Llubes

Crustal thickness in Antarctica from CHAMP gravimetry

Abstract CHAMP, flying at an altitude of about 400 km, is the first of a new generation of satellites dedicated to Earth gravity field observation. The high-quality data have generated new gravity field models: EIGEN-1S in 2001, and EIGEN-2S more recently. The gravitational potential is decomposed into spherical harmonic coefficients and in this study we use the free air gravity anomalies reconstituted up to degree 60, at zero altitude. The anomalies for the Antarctic continent range from −57 to 65 mGal. We have modeled the gravity effect from the ice, the ocean and the bedrock, using a 666 km cut-off filter to simulate the resolution obtained by CHAMP. Computing the differences between this terrain effect and the CHAMP map provides a map of the Bouguer anomalies. Because of the dominant influence of the crust, we first used a crustal thickness model from seismology. This gives a map of the mantle Bouguer anomalies, the range of which is still large (between −255 and 216 mGal) indicating imperfections in the crust model. By appealing to isostasy we then imposed the condition that this mantle Bouguer anomaly should vanish and therefore solve for a new resulting crustal thickness. This gravity-based crust model gives thicknesses from 8.5 to 42.6 km in the zone of interest. There is a good general agreement with seismological models, but our models shows more detail, particularly in the western part of the continent. These details are in agreement with geological studies.

Observation gravimétrique des surcharges océaniques : premières expériences en Bretagne

Abstract Ocean loading involves both the dynamic and geometric effects that result from the action of ocean water masses on the crust. Gravity variations are generated by the direct attraction of the ocean masses, by the deformation of the crust and by the redistribution potential of the masses. Such effects are still perceptible far inland. A Greens function formalism, using loading Love numbers, allows to predict the loading effects. We present here the first French experiment in coastal areas devoted to the study of this phenomenon. The campaign took place in Brest in March 1998. The absolute gravimeter of the French community recorded during four days the gravity changes. Once the standard contributions (body tides, atmospheric pressure effect, polar motion) are removed, the residual variations are mainly due to the ocean loading and they can be confronted to the models. The observed gravity variations exceed by 16 % the theoretical predictions, and we impute this fact to the raw spatial resolution of the global ocean tide models, with a strong consequence near the coasts. Improvements are needed in all the geodetic features of coastal type (tide gauge links, vertical displacement displacement, motion of reference geodetic stations), and also in the validation of the hydrodynamic models by using gravity as an integrating quantity.

Improving corrections in microgravimetric surveys through modelling of ocean loading effects

The periodic movement of the oceanic waters due to tidal forces physically affects the Earth. Under this tidalloading of the oceans, the ground surface is displaced and the redistribution of mass perturbs the gravity field. Such space-time variations can attain several mierogals of amplitude. Usually, only the gravimetrie signal of the earth tide is corrected during microgravimetric campaigns, This correction is of the order of one hundred microgals. However modern gravimeters in current use have a sensitivity of 5 microgals, which is far below the amplitude of the solid tides. We propose to improve the corrections made to the measurements by taking into account the influence of the loading effect due to oceanic tides.

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.