HR: 0800h
AN: G31A-0792    [Abstracts]
TI: Combining geodetic techniques to understand the seasonal effects affecting vertical positioning
AU: Nicolas, J
EM: joelle.nicolas@esgt.cnam.fr
AF: ESGT-Laboratoire de G‚od‚sie et G‚omatique (L2G), 1, Bd Pythagore, Le Mans, F-72000 France
AU: * Van Camp, M
EM: mvc@oma.be
AF: Royal Observatory of Belgium, Avenue Circulaire 3, Bruxelles, BE-1180 Belgium
AU: Nocquet, J
EM: Jean-Mathieu.Nocquet@earth.ox.ac.uk
AF: Department of Earth Sciences University of Oxford, Parks Road, Oxford, OX13PR United Kingdom
AU: Hinderer, J
EM: Jacques.Hinderer@eost.u-strasbg.fr
AF: EOST/IPGS, 5 rue Ren‚ Descartes, Strasbourg Cedex, F-67084 France
AU: Gegout, P
EM: Pascal.Gegout@eost.u-strasbg.fr
AF: EOST/IPGS, 5 rue Ren‚ Descartes, Strasbourg Cedex, F-67084 France
AU: Van Dam, T
EM: tvd@ecgs.lu
AF: European Center for Geodynamics and Seismology, 19 Rue Josy Welter, Walferdange, L-7256 Luxembourg
AB: Seasonal variations are commonly observed in site position time series derived from space geodetic techniques with larger amplitude for the vertical component. These seasonal signals are mainly due to crustal deformations induced by the mass loading effects (atmosphere, ocean, snow, and soil moisture). Time-varying absolute gravity is not only due to the crustal deformation effects but is also due to the Newtonian effect (mass attraction). To better understand the observed seasonal signals and to avoid artefacts linked to the technique or to the analysis strategy, it is essential to perform a multi-technique analysis. The OCA Grasse observatory, France, operates collocated SLR and GPS space techniques, offering long time series. Moreover, several absolute gravity measurements were performed with FG5 to monitor the long term stability of the observatory which is set up on a 1270 m high karstic plateau. In this study, we compare the time series obtained from absolute gravity measurements, from combined LAGEOS-1 and -2 SLR solutions, and from GPS weekly solution for the vertical positioning, from 1998 to 2003. The geophysical annual vertical signal presents a magnitude of 5-6 mm. The absolute gravity signal presents amplitude of several æGal. Results are compared with different mass loading models. In particular, hydrologic and atmospheric loading models are applied, taking into account the local particularities of the OCA, which is on a plateau. We study the influence of the Mediterranean closed sea, which could act as a NIBO (Non-Inverted Barometer Ocean) where all pressure effects are fully transmitted to the Earth. In the global models, and particularly for the atmospheric delay correction, the proximity of the sea (south) and of the Alps (north) is investigated. The annual vertical signal observed in SLR and GPS vertical positioning time series is mainly explained by continental scale hydrological mass loading. The seasonal gravity signal coming from regional effects is isolated from local effects such as hydrology variations in the karst. Indeed, the difference between space geodetic positioning and gravity variations gives some indications concerning the hydrological variations and particularly the ground water table located under the OCA observatory. Acknowledgements: E. Calais, J.P. Boy, M. Amalvict, B. Luck, J.J. Walch, E. Gilli, M. Llubbes, P. Exertier
DE: 1200 GEODESY AND GRAVITY
DE: 1229 Reference systems
DE: 1243 Space geodetic surveys
DE: 1247 Terrestrial reference systems
DE: 1294 Instruments and techniques
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting