Unstable calibration factor of CG-5 relative gravimeter
21/09/2016 | 16:30 | Session 4: Global gravity field modelling
Author(s): Tõnis Oja
The accuracy evaluation of a relative gravimeter by the user is always recommended for obtaining reliable results from terrestrial gravimetric surveys. In this study the observations of digital Scintrex CG-5 relative spring gravimeters from the field campaigns on several calibration lines were analyzed to evaluate the long-term stability of the gravimeter’s calibration factor. In data pre-processing, the original scale factor of the gravimeter’s manufacturer was used to convert the raw readings into the centimeter-gram-second system (CGS) units. After the reduction of converted readings (corrected for the tides, atmosphere, the changing elevation of instrument, land uplift and polynomial drift), the parameters of user’s calibration function were estimated through the least squares (LS) adjustment, by fixing the gravity values on calibration lines. The measurements of calibration lines performed in 2004-2015 demonstrated the scale change of two CG-5 gravimeters about +500 ppm (+5∙10^-4). This estimate exceeds numerically the accuracy of calibration lines (50…150 ppm) combined with the precision of measurements (50…150 ppm). Accordingly, it was concluded that the calibration error of CG-5 gravimeters are statistically significant and corrected scale factor should be used in data pre-processing phase to avoid systematic biases in measured gravity values. Different functional models were tested to fit the long-term variability of CG-5 calibration factor. All computations, which also included calibration part, were done by using software package GRAVS2 (www.maaamet.ee/index.php?lang_id=2&page_id=544).