Local hydrology and the hydrological gravity signal observed by three superconducting gravimeter sensors at Metsähovi Geodetic Fundamental Station, Finland

Variations in the local hydrology produce a significant gravity signal observable by gravimeters. The peak-to-peak gravity signal due to hydrology can reach 8microGal’s at Metsähovi, of which the local hydrology constitutes a major part. Hence changes in the local hydrology are extensively studied at the Metsähovi Geodetic Fundamental Station. The station is equipped with two separate superconducting gravimeters (SG) comprising in total of three individual SG sensors, and an absolute gravimeter (AG) FG5X-221. The SG T020 has been operational since 1994 and the OSG-073 with two individual sensors was installed in the beginning of 2014. The OSG-073 was sent back to the manufacturer (GWR) for maintenance during summer 2015 and will be installed back to Metsähovi as two separate SG’s during spring 2016. All the gravimeters are located in the same observatory on separate pillars. The SG’s are three meters apart and the AG measurements are done in a separate room approx. 5m apart. Due to the 3m baseline between the SG’s, the gravimeters are influenced differently by the local hydrological effects e.g., after heavy rain related to thunderstorms, this difference on the gravity signals of the SG’s can reach microGal-level. To monitor these local hydrological changes we have installed around the gravity laboratory several hydrological sensors with automatic registration, e.g., soil moisture arrays; multiple boreholes and since 2014 a passive gamma-ray sensor for snow water equivalence measurements. In addition, in 2015 a new weather station was installed to Metsähovi with an additional rain gauge, two pyranometers and a wind sensor. To study the varying snow cover also manual measurements of the snow water equivalent are done around the laboratory and on the roof of the laboratory. Complicated composition of the surrounding stratum and bedrock together with the heterogeneous snow conditions nearby and on the roof of the gravity laboratory makes the modelling of the hydrological gravity signal very complex task. In this presentation we present recent results from improved hydrological observations and implications to the gravity signal observed by the gravimeters.