Noise Levels of Superconducting Gravimeters: Updated Comparison and Time Stability
Résumé
Since the beginning of the Global Geodynamics Project (GGP) in 1997, the number of superconducting gravimeters (SGs) has increased to reach 30 operating sites today. Data from this network allow a comparison of the noise levels of the different contributing stations. The knowledge of the noise levels at each site is important in any combination of data to determine global Earth parameters, for example, the stacking of the data in the search for elusive signals, like the gravity variations associated with the translational mode of the inner core. We use a standardized procedure based on the computation of the residual power spectral densities (PSDs) over a quiet time period in order to evaluate the combined instrument plus site noise in the long-period seismic band (0.3 mHz-1 mHz). The experience at Strasbourg (France) has shown some improvements from the TT70-T005 full-size instrument to the C026 compact model in terms of noise reduction, while the most recent Observatory SG types, OSG044 at Bad Homburg (Germany) and OSG052 at Sutherland (South Africa), do not show any further improvement with respect to the compact models, respectively CD30 and CD037, operating at the same stations. At Black Forest Observatory (BFO) in Germany, the experience of the dual-sphere OSG with a lower sphere heavier than usual has shown that the instrumental and site conditions make this station the least noisy one at frequencies larger than 0.1 mHz. The noise analysis using the longest time-series available has shown that the noise level at these sites is mostly stable (within 1σ) over the years. The comparison with some seismological noise models shows that the best SG sites are less noisy than longperiod seismometers below 1 mHz. However, the noise level of the best SGs is still at the limit of detection of the subseismic translational mode of the inner core.