Superconducting gravimeter calibration by co-located gravity observations – results from GWR C025

After a 12 years’ observation in Vienna (VI), the superconducting gravimeter GWR C025 has been transferred in autumn 2007 to the new Conrad observatory (CO) 60 km SW of Vienna. It is one of few instruments which were operated at different stations. This aspect motivated a re-analysis of all calibration experiments performed so far in VI and CO, focused on the direct impact of drift effects and noise. Noise limited the calibration accuracy achievable in VI considerably. It turns out that considering the drift even of absolute gravimeters in a common adjustment reduces the root mean square error of the averaged calibration factor essentially. Further it is shown that also spring type gravimeters have some potential to contribute to the SG calibration factor determination. The re-analysis of the calibration experiments proves that the calibration factor of GWR C025 did not significantly change during the transfer from VI to CO. The final calibration factor is calculated as weighted average over in total 9 JILAg and FG5 experiments with an accuracy of better than ±0.5 ‰. The calibration factor is temporarily stable with maximum variation less than ±0.1 ‰. Based on these results the gravity time series in VI and CO have been analyzed. The respective amplitude factors for O1, K1 and M2 agree almost perfectly at both stations after correcting for ocean loading effects. The maximum deviation from the numbers provided by the non-hydrostatic body tide models DDW and MAT01 is 0.8 ‰ at maximum.