On the possible cause of long period instrumental noise (parasitic mode) of a superconducting gravimeter

The long-period instrumental noise (parasitic mode) of a superconducting gravimeter (SG) is investigated using observational data from the SG installed at Matsushiro, Japan. Although the mode emerges as a persistent peak in gravity spectra, the intensity of the peak is found to be variable with time and correlated well with the spectra of the SG’s tilt. Assuming resonant excitation of the parasitic mode by tilt, it is shown that the observed spectra could be consistently interpreted if the mode is a rotational motion of the superconducting sphere. It is unlikely that the existence of the parasitic mode is responsible for the enhanced noise level of the SG in seismic normal mode band.