COMPARISON OF NOISE LEVELS OF THE NEWEST GENERATION OF RELATIVE GRAVIMETERS

Since the beginning of the Global Geodynamics Project in 1997, the number of superconducting gravimeters (SGs) has increased to reach 25 operating sites today. Data from the network allows a comparison of the noise levels of the different contributing stations. Knowledge of the noise levels of each station is important in a number of studies that combine the data to determine global Earth parameters. We cite for example the stacking of the data to determine the period of the free core nutation and the Chandler wobble, and the use 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 processing procedure to evaluate the combined instrument plus site noise in the long-period seismic band (0.3 mHz - 1 mHz) and in the sub-seismic frequency band (0.03 mHz – 0.3 mHz), based on computing residual power spectral densities (PSDs) over a quiet time period. The experience at Strasbourg (France) has shown improvements from the T005 full-size instrument to the C026 compact model in terms of noise and drift, while the most recent type Observatory SG OSG044 at Bad-Homburg (Germany) does not show any improvement with respect to the compact CD30. Besides the development of new models of SGs by GWR, a new generation of portable Earth tide meters was developed by Micro-g LaCoste. A 6-month inter-comparison performed at Strasbourg shows that the newest generation of portable Earth tide meter (gPhone-54) is 20 dB noisier than the SG C026 and that its instrumental drift cannot be modeled by a simple polynomial function, preventing the use of the gPhone for long-term gravity studies.