High-accuracy gravity measurements can only be carried out at absolute gravity stations established for the purpose. Most of them are located at the FinnRef network stations used by the National Land Survey of Finland to track satellite positioning signals. The FinnRef network has been continuously expanded since 2012 and it now comprises 47 stations. As part of the expansion, some of the stations have also been adapted for gravity measurements. The station at Finström was the last of the new absolute gravity stations at which measurements had not yet been carried out. In addition to a network of GNSS stations, Finland now also has a comprehensive network of absolute gravity stations, and measurements at each station are carried out at regular intervals.
‘Gravity can be measured in two ways, in an absolute or in a relative manner. In relative measurements, we measure changes in gravity value relative to time and place. The measurements that we recently made in Åland were absolute measurements, and their purpose is to determine accurate acceleration of free fall. Absolute measurements can be carried on a campaign basis, which means that the measuring instruments do not need to measure continuously at the station. The results of such measurements can be made into time series extending over many years,’ explains Jyri Näränen, Research Manager at the National Land Survey of Finland.
With the measurements in Åland, the gravity network has now been extended to sea areas
The extension of the absolute gravity network is now complete and at all 20 stations, measurements have been carried out at least once. At most of the stations, measurements have been carried out at least twice. The measurements completing the network extension were carried out in Åland at the end of May.
With the measurements in Åland, the national gravity network is now complete. Prior to this, absolute gravity measurements had not been carried out in Finnish sea areas with such accuracy. The nearest measurement stations in mainland are in Kirkkonummi, Orivesi and Vaasa, which means that with the new measurements, a major gap in the national absolute gravity network is now closed.
The measurements are carried out using the FG5-X gravimeter of the Finnish Geospatial Research Institute (FGI) of the National Land Survey of Finland. It is the world's most accurate instrument for measuring absolute gravity. There are fewer than 50 of these instruments in the world.
As land uplift progresses, gravity in Finland is weakening
Post-glacial land uplift in Finland is continuing and this also affects gravity. Land uplift rates vary geographically and for this reason, measurements must be carried out in different parts of Finland. There are also many other factors affecting the gravity value, such as geographical location, local geology and other movements of the earth’s crust. Gravity measurements are essential for calculating geoid models.
‘Geoid describes the shape of the earth. Heights in relation to mean sea level are derived from it. As accuracy requirements for satellite positioning and navigation are increasing, more accurate geoid models are needed. This is because with geoid models, we can convert vertical coordinates produced using satellite positioning into elevations on maps, such as those used in the official Finnish N2000 height system,’ explains Mirjam Bilker-Koivula, Research Group Manager at the National Land Survey of Finland.
’A more accurate geoid model allows safer navigation, especially in Finnish coastal waters, which are both shallow and rocky and thus difficult for shipping,’ adds Timo Saari, Researcher at the National Land Survey of Finland.
Gravity is changing slowly
By studying time series of gravity measurements, we can model land uplift mechanisms more accurately and produce more accurate land uplift forecasts. This is important when determining such matters as the location of shorelines.
Researchers believe that the absolute gravity time series produced in Åland will not only show the land uplift signal but also the way in which sea level rise caused by climate change will impact gravity.
Changes in gravity are slow and the changes occurring each year can only be recorded with the world's most accurate observation instruments. For this reason, long measurement time series are needed. Long time series can be produced in measurement campaigns that are repeated at each station approximately every three years for several decades.
Research Group Manager Mirjam Bilker-Koivula, firstname.lastname@example.org
Research Manager Jyri Näränen, email@example.com
Researcher Timo Saari, +358 50 357 4315, firstname.lastname@example.org