A portable superconducting gravimeter in a field enclosure: comparison to traditional observatory gravimeters

The superconducting gravimeters (SGs) are thanks to inherent high precision and low linear drift rates predetermined for observations of small gravity variations related to mass transport occurring in their close vicinity such as water storage changes. However, the full potential of SGs as instrument dedicated to continuous and integrative hydrological monitoring is mostly hindered due to the location of SGs inside observatory buildings. The drawback of such installation is the disturbances of natural flow paths causing unpredictable water storage changes directly beneath the foundations, thus in the area causing potentially highest gravity variations. This issue concerns both, geodetic and hydrological applications. To minimize the negative effect, a new enclosure was developed allowing to mount and operate a portable SG of the latest generation (iGrav) to a pillar with diameter of only 1 m in field conditions. The measuring system has been installed in Wettzell (Germany) where two gravimeters (SG029 and SG030) are in operation inside observatory buildings. We present the technical layout, discuss the emerging challenges and report on the direct comparison of iGrav recordings to SG029 and SG030. Significant differences exceeding 50% of the observed amplitude were found. Utilizing modelled hydrological effects, the major part of these discrepancies can be attributed to different umbrella effects related to pillar or building footprint and to topographic effects.