Since this year, the contours of the National Gravimetry Observation Service have changed significantly, and now include in addition to two international services of the International Association of Geodesy (the International Gravimetric Bureau and the International Geodynamics and Earth Tide Service), time series of gravity variations measured by superconducting gravimeters at five sites in France (Larzac, Rustrel, Strasbourg, Trappes) and in Africa (Djougou in Benin) and repeated measurements at about twenty sites by the different French absolute gravimeters (see Figure 1, for the location of the different sites).
The temporal variations of gravity on the historical site of the Gravimetric Observatory of Strasbourg (fort J9) have been measured continuously since October 1987, first by the instrument GWR T005 until June 1996, then GWR CO26 operational until November 2018. A newer GWR iOSG #23 model was installed in February 2016, thanks to the support of Résif. The Djougou site (Benin) was developed within the framework of the ANR GHYRAF project, and has been instrumented continuously since July 2010.
The iOSG #24 GWR gravimeter was installed at the Low Noise Subterranean Laboratory (LSBB) in Rustrel in October 2015 within the framework of the Equipex MIGA, whose main objective is the construction of an atomic interferometer for the observation of the Earth’s gravitational field and to provide a new tool to detect gravitational waves. The monitoring of these three instrumented sites is under the direct responsibility of the Strasbourg Gravimetric Observatory.
The iGrav #2 instrument was installed in May 2011 on the Larzac plateau and is managed by Géosciences Montpellier and OSU OREME (Observatoire de REcherche Méditerranéen de l’Environnement); the main objective of this instrument is the determination of water storage variations in a karstic system in combination with other geophysical measurements.
The Laboratoire National de Métrologie et d’Essais (LNE) site in Trappes has been instrumented with iGrav #5 since February 2013, as part of the Kibble balance project (formerly watt balance). The latter, via the conversion between mechanical and electrical power, allows the determination of the Planck constant and has been used since 2018 in the new definition of the kilogram.
These different instruments are part of the international service IGETS (International Geodynamics and Earth Tide Service),whose objective is the measurement, archiving and distribution of long time series of superconducting gravimeters, but also inclinometers, extensometers, etc., and which are used to measure, archive and distribute long time series of superconducting gravimeters. The EOST and the Gravimetric Observatory in Strasbourg play a fundamental role, hosting the Central Office and ensuring the production, as an Analysis Centre, of level 2 and 3 data. EOST also centralises the various raw data (level 1) in France and manages the sending to the data centre.
The various level 1 (raw data of temporal variations in gravity and atmospheric pressure), level 2 (data pre-processed to allow tidal analyses) and level 3 (residues after geophysical corrections) products are available at the data centre hosted by the GFZ in Potsdam, Germany. In collaboration mainly with Géosciences Montpellier, we have also proposed and obtained the labeling of a new service task of repeated measurements of absolute gravimetry with the two current national instruments FG5 #206 (Strasbourg) and FG5 #228 (Montpellier). The Muquans cold atom absolute gravimeter, purchased as part of Résif, will be added to these two ballistic instruments after reception and validation. The analysis of absolute gravimetry series, combined with precise positioning (GNSS) and tide gauge measurements, provides unique information and constraints in different fields of Earth sciences.
At present, about twenty sites have been identified (see Figure 1), including of course sites instrumented by superconducting relative gravimeters, tide gauge sites (Brest, La Rochelle, l’ile d’Aix and Marseille), some measured regularly since 1997, sites in the Alps, as well as in the French Southern and Antarctic Territories in the framework of the IPEV project carried out by the EOST.
This article appeared in the Newsletter Résif n°16 – June 2019 (pdf in French)