Apparent calibration shift of the Scintrex CG-5 gravimeter caused by reading-dependent scale factor and instrumental drift

Abstract

Calibration measurements of the Scintrex CG-5 gravimeter were conducted using absolute gravity stations of the Japan Gravity Standardization Net, to constrain the scale factor and its temporal changes. The calibration data were obtained from a total gravity interval of 1.4 Gal through six campaigns, conducted for over 5 years between years 2012 and 2017. The scale factors varied by 1500 ppm in a range from 0.9991 to 1.0006, according to station combinations of the six campaigns. The scale factor depends primarily on the gravity reading ranges: for similar gravity reading ranges, no significant differences in the estimated scale factors were recognised, even though station combinations and observed times are different. Therefore, the gravity readings can be corrected by introducing a gravity reading-dependent scale factor. Furthermore, even though the scale factor essentially depends on gravity readings and not on time, temporal changes were observed during repeated calibration measurements at the same station combinations. A long-term instrumental drift of the CG-5 gravimeter could explain this phenomenon. In conclusion, the calibration shifts recognised during repeated measurements were apparently caused by: (1) the scale factor dependence on the gravity reading ranges and (2) the shift of the gravity reading ranges due to the instrumental drift.