Sensor Differences of Dual Sphere Superconducting Gravimeters

We analyse the difference signal of dual sphere superconducting gravimeters (SG) to identify systematic instrumental disturbances, which would otherwise go unnoticed. Compared to classical spring gravimeters, SGs excel by their superior long-term stability. However, SG measurements also suffer from systematic errors. One possibility for characterising and quantifying these errors is the analysis of sensor differences, either of dual sphere instruments or between two colocated instruments. For perfect instruments, the sensor difference should vanish. We study the sensor differences of all dual sphere SGs in the database of the International Geodynamics and Earth Tide Service (IGETS). As expected, they show a relative drift between the sensors and steps related to operator interventions. However, we can also identify periods of unexpected drift rate changes that last for several months. Afterwards, the drift rate returns to its old value. The observed differences are too big to be caused by local gravity gradients. Therefore, we think, they indicate more complex systematic disturbances of SGs. These disturbances are at the level of a few tens of nm/ and could not be clearly identified in the gravity residuals of only one sensor. ... mehrThese findings are corroborated by the analysis of differences between colocated single sphere SGs at the J9 observatory in Strasbourg. Knowledge of the characteristics and size of these disturbances is important if gravity changes of a few tens of nm/ are studied on long time scales, like signals from hydrology or polar motion.