Analysis of long-term gravity records in Europe. Consequences for the retrieval of small amplitude and low frequency signals including the Earth's core resonance effects

Abstract

Temporal gravity variation measurements have been a long historical tradition in Central Europe, with some stations recording for decades. From the 80s, time varying gravity is permanently recorded at the Earth’s surface by a worldwide network of superconducting relative gravimeters within the Global Geodynamics Project of the International Association of Geodesy. In one of these stations, located in Strasbourg since the 1970s, the three main gravimeter types (relative spring gravimeter, relative superconducting gravimeter, and absolute gravimeter) have been set up. We use all these series to review the instrumental betterments. Studying the different improvements on gravimeters in the last years, mainly in terms of long term stability and instrumental drift, we show that the superconducting gravimeters can uniquely contribute to the study of the low frequency Earth's tides and small amplitudes waves. Also, the stability of the scale factor of the superconducting gravimeters is studied with the help of numerous calibration experiments carried out by collocated absolute measurements at Strasbourg Observatory. Finally, after estimating the values of the Free Core Nutation parameters, we search for the rotational normal mode called Free Inner Core Nutation (FICN), the gravity effect of which has never been observed before. For this purpose we develop a methodology to constrain the possible frequency range, through the detailed tidal analysis in the diurnal frequency band, using the 27-year superconducting gravity series recorded at J9 observatory, to separate small amplitude waves that have never been studied before, and which could be close enough to the frequency range of the FICN to be affected in terms of resonant amplitude. This work contributes to show the importance of not only the length, but also the quality of the data series to improve our knowledge of the Earth’s dynamics.