Frequency response of the superconducting gravimeter SG056

The two sensors of the SG 056 double-sphere superconducting gravimeter at BFO (Black Forest Observatory, Germany) show differences in their response to long-period seismic signals. Their frequency response deviates from the nominal GGP1-filter (8th-order Bessel low-pass). We experimentally derive parameterized models for the sensor’s full frequency response by application of square-wave and down-sweep drive signals to the feedback circuit and subsequent inversion. The latter is carried out with the program calex in the time domain which iteratively minimizes the least-squares misfit between the output signal predicted with the filter model and the actual output of the sensor. We seek for values of eigenperiod and damping of the four 2nd-order subsystems of 8th-order low-pass filters. The resulting filters deviate considerably from the nominal response of the GGP1-filter also in that they are not Bessel filters. Remaining residuals indicate that the models are not able to capture the exact response. Nevertheless, they substantially reduce amplitudes of waveform-residuals in long-period earthquake recordings by a factor of four. The filter response curves approach their DC-limit (frequency f = 0Hz) within the frequency band of the drive signals. Thus we estimate the asymptotic signal delay ΔtDC to be considered in tidal analysis to be ΔtDCG1 = 10.44s for the lower sensor G1 (heavier sphere) and ΔtDCG2 = 9.86s for the upper sensor G2 (standard sphere). The accuracy of these values appears to be not better than 0.07s. For signals recorded with voltmeters on the UIPC data-acquisition and distributed through the IGETS data center (formerly GGP) ΔtDCG1 = 9.84s and ΔtDCG2 = 9.26s.