Reanalysis of background free oscillations using recent SG data

Background free oscillations (BFOs), observed even during seismically quiet periods, have been widely recognized. Although atmospheric or/and oceanic disturbances are suggested to be the possible excitation sources, there still exist some unknown uncertainties regarding their contribution. Note that most studies used seismometer records to explore the excitation mechanism of BFOs, while only a few used superconducting gravimeter (SG) records just for detection purposes. In view of the high precision, the wide measurement range and the increase of SGs distributed worldwide, this paper aims to further explore the excitation mechanism of BFOs by combining recent gravity records at several low-noise SG stations, rather than traditional seismometer records. We first analyzed gravity records at each station to ensure the detection of BFOs from spectrograms and averaged power spectra. On the basis of the BFOs detection, annual variations of the modal energy in the averaged spectrograms stacked over all stations and over different years were calculated. By comparing them with those derived from atmospheric/oceanic observations, we proved qualitatively that the atmospheric disturbance is the major excitation source of BFOs while the oceanic disturbance plays a relatively minor role below 5 mHz for the whole period considered; however, the excitation of BFOs could be mainly attributed to the oceanic disturbance for a certain period. It is necessary to further quantitatively estimate the modal energy excited by these disturbances, but as a supplementary means of seismic exploration, the results in this paper are expected to be helpful for understanding BFOs.