Comparison of Keithley, Quanterra, and Symres 24 bit data acquisition systems for use with the GWR C021 Superconducting Gravimeter for low frequency seismology and geodesy

Reference instrumentation such as superconducting gravimeters (SG) and barometers have become important data sources in modern seismic networks, augmenting data from traditional seismometers. However, careful attention to the measurement physics, operational accuracy, and traceable calibration of these additional instruments is important for obtaining valid data. When calibrated to 0.1% accuracy in amplitude and 0.01 sec in phase, SG instruments play a useful role in the improved estimation of large earthquake source parameters, as well as investigating Earth's gravest free oscillations. Achieving these results is not easy. This paper compares the performance of 24 bit data loggers from three different companies when connected to the SG GWR C021 gravimeter (Membach station, Belgium). The data loggers are the Keithley K2000, Quanterra Q330, and Symmetric Research PAR4CH. We specifically investigate the ability of these loggers to ensure reliable DC measurement and stable calibration factor at the 0.1% level. The noise contribution of the data acquisition systems is also investigated in the 0.01-10 mHz frequency band, which includes the tidal and seismic free oscillation frequencies. It appears possible to record meaningful results with all three.