Preparing for gravity

By: Christian Gerlach

A team from Norway and Germany has visited the sub-glacial lab, preparing for a gravity campaign next spring. This will be (as far as we know) the very first absolute gravity measurement ever to be carried out underneath a glacier.

Why gravity? Mass balance studies are classically based on geometric or meteorological data and models – quantities, which are not a direct measure of mass. In contrast, gravity observations are. On global scales, gravimetric mass balance is measured by the US/German satellite mission GRACE. On local scales, there exists no established program. Therefore the Norwegian University of Life Sciences in Ås (UMB) and the Bavarian Academy of Sciences and Humanities in Munich (BAdW) joined to carry out gravimetric observations on and underneath Svartisen glacier. The core of the observation program will be observations with UMB’s absolute gravimeter FG5. This state-of-the-art instrument is the most precise instrument on the market providing the value of gravity at the observation site with a relative precision of 10-9 (it measures 9.8 m/s2 to the 8th digit behind the comma!).

Unfortunately, FG5 is not only sensitive but also sensible – it will not accept to work at ambient tunnel temperature of 5°C, but only accepts normal room temperature around 20°C. In addition, a stable observation site free from vibrations is required. A concrete pillar of about 1 m2 will do!

On Saturday, September 14, Torsten Spohnholtz and Christian Gerlach from BAdW travelled from Munich to Bodø. Next day, Sunday 15, they met Siri Eikerol (geomatics student at UMB, who will do her Master‘s thesis on the project) and Miriam Jackson on Bodø airport for the trip to the glacier lab. The aim was to prepare a concrete pillar as observation site for FG5. On late Sunday evening one of the office containers of the sub-glacier lab was chosen to host the FG5-site and on Monday morning we started with the work. The simple plan was to cut a whole into the container floor, build a wooden case which is not in contact to the container and fill it with concrete. This sounds simple, but it took almost two full working days to do the job. Now we are waiting for next spring and FG5 to come!

Figure 1: Office container which hosts the FG5 pillar

Figure 2: Siri and Torsten with thumbs up after finalizing the pillar

Figure 3: Final action: Christian puts a marker to the concrete pillar