Scale Factor Determination of Spring Type Gravimeters in the Amplitude Range of Tides by a Moving Mass Device

This paper investigates the characteristics and the metrological limits of the calibration of spring type gravimeters by using a cylindrical test mass moved vertically around the gravimeter by a lifting device operated in the Matyashegy Observatory. The movement of the 3100 kg iron mass generates a sinusoid-like calibrating signal having a peak-to-peak amplitude of 1102 nm s(-2). The careful determination of the geometrical and physical parameters of the test mass combined with the analytical modeling of its gravitational effect and the related uncertainties provides an accuracy of 3 nm s(-2) in absolute sense. The overall accuracy, however, is influenced by several other instrumental and environmental factors which are investigated in detail. The conclusions are based on more than 400 experiments with 5 LCR G instruments. As a unique case a Scintrex CG-5 instrument was also involved in the tests what is probably the very first moving mass calibration of this type of gravimeters. Two processing methods, Max-Min and Full-Fit, based on L2 norm adjustment of the observations were developed and applied to obtain instrumental scale factor and other related parameters. The results show that the observations corrected for the disturbing effects still contain a systematic constituent with amplitude of (10-20) nm s(-2) regardless which LCR instrument was calibrated. It resembles the second time derivative of the calibrating signal that may indicate the non-uniform elastic response of the spring sensors to the rate of gravity change. Due to the problems mentioned above the overall dispersion of the resultant random and non-random residuals of the calibration observations provided by Full-Fit method are typically 10 nm s(-2). The a posteriori standard deviations of the individual scale factors provide, however, measurement accuracy of 2 nm s(-2).