A two-year analysis of the iOSG-24 superconducting gravimeter at the low noise underground laboratory (LSBB URL) of Rustrel, France: Environmental noise estimate
Résumé
Since July 2015 a Superconducting Gravimeter (SG) of the latest generation, the iOSG-24, has been recording continuously the time-varying gravity field at the low noise underground laboratory (LSBB URL) of Rustrel, France. This instrument designed for observatory purpose has a levitated Niobium sphere weighting 17.7 g instead of 4.3 g. The advantage of increasing the mass of the sphere is to reduce the thermal noise due to Brownian motion inside the sensor. A comparison of the noise levels shows that the combination of this iOSG-24 with the environmental condition at the LSBB makes this site one of the quietest worldwide SG sites. Parallel measurements with an absolute FG5 gravimeter give a calibration factor of −451 ± 3 nm/s2/V and a negligible linear drift is observed 3 months after the installation. Tidal analyses are consistent with theoretical predictions and have shown a strong S1 thermal wave. Influence of the hydrological water content, mostly related to the Fontaine de Vaucluse catchment, is clearly visible on the SG residuals after tides removal. However the hydrological influence modelled using the MERRA2 products does not fully explain the observed noise level at periods between 1 and 10 days. Longer gravimetric time-series are necessary to further study the seasonal hydrological effects.