Stability of the Calibration of Scintrex Relative Gravimeters as Inferred from 12 Years of Measurements on a Large Amplitude Calibration Line in Iran

In spite of the large stability of the fused quartz elastic system, which is utilized in modern relative gravimeters such as Scintrex CG-5, precise gravity measurements need regular calibration mostly because of weak changes in time of the elastic properties of the spring. Calibration of the relative gravimeters is important to avoid the systematic error of scale in relevant observations. In this study, due to the establishment of a large amplitude (of about 1200 mGal) calibration line in Iran, the stability of three CG-3 M and three CG-5 gravimeters has been continuously investigated based on the results of a 12 year (2005–2017) observations. The absolute gravity values at calibration stations were measured during the period 2005–2007 and more recently in 2017–2018 (for most of the stations) with absolute FG5 gravimeters. The results show that the Scintrex gravimeters exhibit different behaviors on the calibration line. The accuracy of determining the calibration coefficient of the gravimeters was better than 40 ppm. According to our results there is no effect of the gravity amplitude itself on the calibration factors. CG-5 #83 and CG-5 #87 have the largest changes in calibration factor (more than 1000 ppm) over the 12 year observation period while CG-3 M #20 and CG-3 M #60 have the smallest range (less than 200 ppm). The misclosure of relative gravity measurements in the first-order gravity network of Iran has been calculated before and after calibration corrections and it is shown that applying the scale factor correction reduced significantly the misclosures on the gravity network.