Observing Absolute Gravity Acceleration in the Fennoscandian Land Uplift Area A. Engfeldt 1 , L. Timmen 2 , O. Gitlein 2 , J. Mäkinen 3 , B.R. Pettersen 4 , O.C.D. Omang 5 , H. Wilmes 6 , R. Falk 6 , J. Müller 2 , H. Denker 2 , M. Bilker- Koviula 3 , J.G.Gjevestad 4 , D.I. Lysaker 4 , K. Breili 4 ,G. Strykowski 7 , B. Engen 5 , M. Lilje 1 , J. Ågren 1 , R. Forsberg 7 , H.-G. Scherneck 8 1 Lantmäteriet (National Land Survey of Sweden), Lantmäterigatan 2, SE-80182 Gävle, Sweden. Email: andreas.engfeldt@lm.se 2 Institut für Erdmessung (IfE), University of Hannover, Am Schneiderberg 50, D-30167 Hannover, Germany 3 Finnish Geodetic Institute (FGI), Geodeetinrinne 2, FIN-02430 Masala, Finland 4 Department of Mathematical Sciences and Technology, UMB, P.O. Box 5003, N-1432 Ås, Norway 5 Statens Kartverk (Norwegian Mapping Authority, NMA), Kartverksveien 21, N-3511 Hønefoss, Norway 6 Bundesamt für Kartographie und Geodäsie (BKG), Richard-Strauss-Allee 11, D-60598 Frankfurt, Germany 7 Danish National Space Center (DNSC), Juliane Maries Vej 30, 2100 Copenhagen, Denmark 8 Chalmers Univ. of Technology, Onsala Space Observatory, SE-439 92 Onsala, Sweden Abstract The Nordic countries Norway, Sweden, Denmark and Finland are a key study region for the research of glacial isostasy, and, in addition, it offers a unique opportunity for validating and testing the results of GRACE. Over a period of five years a temporal geoid variation of 3.0 mm is expected in the centre of the Fennoscandian land uplift area, corresponding to a gravity change of about 100 nm/s². This is expected to be within the detection capabilities of GRACE. With terrestrial absolute gravimetry, the gravity change due to the land uplift can be observed with an accuracy of ±10 to 20 nm/s² for a 5-year period. Thus, the terrestrial in-situ observations (ground- truth) may be used to validate and test the GRACE results. Since 2003, annual absolute gravity measurements have been performed in Fennoscandia at about 30 stations covering Sweden, Norway, Finland and Denmark. Four groups with FG5 absolute gravimeters (BKG, FGI, IfE, UMB) are engaged to survey the uplift by a mutually controlled procedure. Nearly all absolute stations are co-located with permanent GPS stations. From the 2003 to 2005 surveys, an overall accuracy of ±30 nm/s 2 is indicated for a single absolute gravimeter and a single station determination. This is in full agreement with the project goal. First results of linear gravity changes will be compared with height rates as derived from continuous GPS (BIFROST). Sites measured 2003 Sites measured 2004 Sites measured 2005 Comments to the graphs of the calculated land uplift • All FG5 measurements from 2003 and later were computed using the g- software by the observing institute. • All g-values are compared at the height 1,20 m. Observations delivered at other heights were transferred to this height by Andreas Engfeldt using the mean value of the observed gradients. • The estimated standard errors show that the land uplift has so far not been very accurately determined at most stations. More observations are required. The most accurate stations are Onsala AN, Onsala AS and Trysil AC. Since the uplift has not yet been very significantly determined, at least not in some of the stations, it is not strange that some contradicting results are obtained, for instance between Vaasa AA and Vaasa AB. • In Metsähovi 90 measurements have been made by the Finnish JILAg instrument. The trend computed using these observations is exactly the same as the trend from the FG5s, but are not included here. The inclusion of these observations would naturally have improved the estimated standard error for the land uplift in Metsähovi. • According to all the presently available land uplift models (based on both tide gauges and/or GPS) the station closest to the uplift centre should be the Swedish station Skellefteå; see for instance the NKG2005LU model in the below figure. • According to the observations illustrated in the graphs, the largest uplift is obtained in Trondheim. This seems clearly unrealistic. Something must have happened to the Trondheim station between the measurements in the 90s and 2003. When the uplift is computed using measurements from 2003-05, we obtain a more realistic velocity. If the proper weights are given to the observations, the uplift is -1,9+/-3,0 µGal/year. If equal weights are assumed, then we obtain -1,0 +/-3,0 µGal/year. It should be noticed, though, that the estimated standard error is 3 µGal/year in both cases, which implies that the latter results are far from significant. • In 2003, the FG5 owned by IfE delivered a systematically too low g-value than the other instruments. The situation was the opposite in 2004 and 2005. Conclusions obtained in the project • More than 30 absolute gravimetry stations in Fennoscandia recently observed • Occupied stations: 22 (2003), 24 (2004), 30 (2005). • Continuous GPS is an important complementary technique for absolute gravimetry • Four FG5 absolute gravimeters are employed to increase the reliability and accuracy of the whole network and to achieve a proper number of station occupations every year • Accuracy of a single station determination: ±2 . . . ±3 µGal • With GRACE, a new appreciation of processes in the system Earth has arisen which requires the interaction with ground based measurement techniques • The absolute gravity net is designed as a long-term monitoring system • With the implementation of a long-term reference frame, future gravimetric datum problems can be avoided (establishment of a few reference stations with SCG, GPS, GW monitoring, frequent abs. grav. measurements by different groups) Acknowledgements • We thank everyone who have participated to the project and who are not among the authors. Pictures of some of the gravity stations Some facts about the history of land uplift See A consice history of postglacial land uplift research (from its beginning to 1950) by Martin Ekman, published in Terra Nova, 3 (1993) • 1491: The Swedish town Östhammar (105 km NNE of Stockholm) complains to the Archbishop that the town can no longer be reached by boat. • 1694: Urban Hjärne sends a questionnaire to different parts of Sweden. He hypothesises that the water flows from the Baltic Sea into the Atlantic Ocean. • 1743: Anders Celsius estimates the decrease of the sea level close to Gävle to 13 mm/year, based on his etching 1731 in a seal rock (see the picture to the right). Proposes evaporation or a hole in the bottom of the sea • 1765: Ephraim Runeberg and Bengt Ferner independently suggest that the land might rise • 1774: A tide gauge is established in the Stockholm harbour • Through the mid 19th century: Various heating and cooling theories • 1837: The hypothesis of the Ice Age is proposed by Louis Agassiz • 1865: Thomas Jamieson suggests that the land uplift is related to unloading due to the melting of the ice sheet. Graphs illustrating the observations and the calculated land uplift • Five stations in Finland, four stations in Norway and five stations in Sweden have been measured continuously since the 80s or 90s. • Five organisations own the involved absolute gravimeters: BKG, FGI, IfE, NOAA and UMB. The Celsius seal rock -> • Annual abs. gravity surveys over 6 years (2003: 22 stat., 2004: 24 stat., 2005: 30 stat.); • Close cooperation between five FG5-expert teams (2003: BKG, FGI, IfE, since 2004: FGI, IfE, UMB, from 2007: also LM); • Continuous GPS at almost all stations (BIFROST); • Geodetic levelling between absolute gravity points and eccenters (control of local variations, direct connection to GPS); • Geometric connections to tide gauges by GPS; • Continuous GPS at almost all stations (BIFROST); • Integration of already existing geodetic data sets (e.g. Wilmes et al. 2004, Kuo et al. 2004); • Comparison with GRACE. Products: 1. Temporal changes of gravity and the gravity disturbances at the measurement locations, 2. Area model for gravity disturbances and for geoid changes. MSL Tide Gauge Rod FG5 GPS Height Bench Mark Gravity Bench Mark GPS FG5 Gravity Bench Mark Rod Height Bench Mark GPS, VLBI, Laser Reference (Permanent Stations) Tide Gauge Control Geodynamics Research Local Network Local Network Integrated Regional Network MSL Tide Gauge Rod FG5 GPS Height Bench Mark Gravity Bench Mark GPS FG5 Gravity Bench Mark Rod Height Bench Mark GPS, VLBI, Laser Reference (Permanent Stations) Tide Gauge Control Geodynamics Research Local Network Local Network Integrated Regional Network Skellefteå AA Trend: -1,8 +/- 0,2 μGal/year -15 -10 -5 0 5 10 15 jan- 91 jan- 93 jan- 95 jan- 97 jan- 99 jan- 01 jan- 03 jan- 05 jan- 07 [μGal] Onsala AS Trend: -0,7 +/- 0,1 μGal/year -15 -10 -5 0 5 10 15 jan- 93 jan- 95 jan- 97 jan- 99 jan- 01 jan- 03 jan- 05 jan- 07 [μGal] Onsala AN Trend: -0,8 +/- 0,1 μGal/year -15 -10 -5 0 5 10 15 jan- 93 jan- 95 jan- 97 jan- 99 jan- 01 jan- 03 jan- 05 jan- 07 [μGal] Kiruna AA Trend: -0,8 +/- 0,4 μGal/year -15 -10 -5 0 5 10 15 jan- 93 jan- 95 jan- 97 jan- 99 jan- 01 jan- 03 jan- 05 jan- 07 [μGal] Mårtsbo AA Trend: -1,2 +/- 0,2 μGal/year -15 -10 -5 0 5 10 15 jan- 93 jan- 95 jan- 97 jan- 99 jan- 01 jan- 03 jan- 05 jan- 07 [μGal] Stavanger AA Trend: -0,1 +/- 0,3 μGal/year -15 -10 -5 0 5 10 15 jan- 91 jan- 93 jan- 95 jan- 97 jan- 99 jan- 01 jan- 03 jan- 05 jan- 07 [μGal] Tromsö AA Trend: -0,2 +/- 0,3 μGal/year -15 -10 -5 0 5 10 15 jan- 91 jan- 93 jan- 95 jan- 97 jan- 99 jan- 01 jan- 03 jan- 05 jan- 07 [μGal] Trysil AC Trend: -1,1 +/- 0,1 μGal/year -15 -10 -5 0 5 10 15 jan- 93 jan- 95 jan- 97 jan- 99 jan- 01 jan- 03 jan- 05 jan- 07 [μGal] Joensuu Trend: -1,7 +/- 0,8 μGal/year -15 -10 -5 0 5 10 15 jan- 93 jan- 95 jan- 97 jan- 99 jan- 01 jan- 03 jan- 05 jan- 07 [μGal] Sodankylä Trend: -1,6 +/- 0,3 μGal/year -25 -20 -15 -10 -5 0 5 10 15 20 25 jan- 87 jan- 89 jan- 91 jan- 93 jan- 95 jan- 97 jan- 99 jan- 01 jan- 03 jan- 05 jan- 07 [μGal] Vaasa AA Trend: -2,2 +/- 0,2 μGal/year -25 -20 -15 -10 -5 0 5 10 15 20 25 jan- 87 jan- 89 jan- 91 jan- 93 jan- 95 jan- 97 jan- 99 jan- 01 jan- 03 jan- 05 jan- 07 [μGal] Vaasa AB Trend: -1,7 +/- 0,3 μGal/year -15 -10 -5 0 5 10 15 jan- 93 jan- 95 jan- 97 jan- 99 jan- 01 jan- 03 jan- 05 jan- 07 [μGal] MÅRTSBO ONSALA SKELLEFTEÅ KIRUNA VAASA AB METSÄHOVI SODANKYLÄ Trondheim AA Trend: -2,3 +/- 0,3 μGal/year -15 -10 -5 0 5 10 15 jan- 93 jan- 95 jan- 97 jan- 99 jan- 01 jan- 03 jan- 05 jan- 07 [μGal] Metsähovi, only FG5 Trend: -0,5 +/- 0,2 μGal/year -15 -10 -5 0 5 10 15 jan- 93 jan- 95 jan- 97 jan- 99 jan- 01 jan- 03 jan- 05 jan- 07 [μGal] Apparent land uplift from the NKG2005LU model The Current Project “Absolute Gravimetry to Observe the Fennoscandian Land Uplift” - project realisation - Combination of Vestöl´s mathematic and Lambeck´s geophysical model View publication stats View publication stats