Martine Amalvict

Isostatic stability of the East Antarctic station Dumont d’Urville from long-term geodetic observations and geophysical models

Geodetic measurements of the vertical crustal displacement collocated with absolute gravity changes provide a discriminatory measurement of present-day glacial changes, versus more deeply seated rock motions caused by glacial isostatic adjustment (GIA). At the East Antarctic station of Dumont d’Urville, we compare the displacements derived from continuous DORIS (1993.0– 2006.0) and Global Positioning System (GPS) (1999.0–2005.7) data, and observed changes in absolute gravity (2000–2006), with the predicted vertical displacement and change in gravity from GIA modelling. The geodetic results have mutual self-consistency, suggest station stability and provide upper bounds on both GIA and secular ice mass changes. The GIA models tend to predict amplitudes of rock motion larger than those observed, and we conclude that this part of Antarctica is probably experiencing a slight gain in ice mass, in contrast to West Antarctica.

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.

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.

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.

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.

Lithospheric Deformation and Asthenospheric Pressure

We present here a model of lithospheric deformation caused by a radial pressure acting at the base of the lithosphere. Such a pressure may arise, for instance, as a result of convective motions occurring in the Earth’s mande. As a first approximation, we consider a rather simple Earth model consisting of a fluid interior, the mantle or asthenosphere, overlain by an elastic lithosphere. The model as a whole is assumed to be incompressible and have a constant mass density throughout. These simplifying assumptions allow us to find analytical solutions to the equations of gravito-elasticity which we use here in a peculiar way. We believe that these solutions provide a sufficiently meaningful description of the global yield of the actual terrestrial lithosphere, and thus may be used to model the evolving shape of the lithosphere.

Static displacement fields due to a seismic source in a nonhomogeneous (dimorphic) medium. Geophysical applications

A new method for finding static displacement fields due to seismic sources (forces or dislocations) in a nonhomogeneous medium (called “dimorphic”) is presented. Previously known results can be easily calculated, as well as new results for sources located at the interface between two media with different elastic properties. The ratio of displacement magnitudes in homogeneous and dimorphic media, which can be as great as 15%, is calculated. Consequences for the seismic moment and some other geophysical applications are discussed.