Noise levels of superconducting gravimeters at seismic frequencies

Until recently superconducting gravimeters (SGs) have been used principally in tidal studies (periods 6-24 hr) due to their high sensitivity and low drift rates. This paper considers the performance of these instruments as long-period seismometers, particularly in the normal mode band (periods 1-54 min). To judge their suitability in providing useful information to seismology, it is important to determine their noise characteristics compared to other established instruments such as spring gravimeters. We compare several continuously recording instruments: the SGs in Esashi (Japan), Wuhan (China), Strasbourg (France) and Cantley (Canada) and the spring gravimeter ET-19 and seismometer STS-1 at the Black Forest Observatory (BFO, Germany). We also include non-permanent instruments, the SG102 at BFO as well as the ET-ls in Metsahovi (Finland). The five quietest days out of the available records are stacked to obtain the power spectral density of the noise in the frequency band 0.05-20 mHz (50 s to 6 hr). Our reference is the New Low Noise Model designed for seismometers. Only at the BFO site were there several instruments that could be compared; even so, in order to obtain the best individual data for each instrument the records selected were not simultaneous. The noise characteristics of the different instrument-site combinations are compared, leading to conclusions about site selection, instrument modifications and the recent potential of SGs to contribute to seismic normal mode studies. We refer to our previous work on the seismic noise magnitude, a summary statistic derived from the power spectral density which has been used to rank the performance of instrument-site combinations.