1. The Austrian absolute gravity base net: 27 years of spatial and temporal acquisition of gravity data Ullrich Christian and Diethard Ruess, Federal Office of Metrology and Surveying (BEV), Vienna, Austria (christian.ullrich@bev.gv.at; diethard.ruess@bev.gv.at ) EGU General Assembly 2014 Vienna Abstract: The Federal Office of Metrology and Surveying (BEV) has been operating the absolute gravimeters JILAg-6 (since 1987) and FG5/242 (since 2010). They are used for basic measurements to determine or review fundamental gravity stations in Austria and abroad. More than 70 absolute gravity stations were placed in Austria and neighbouring countries. Some of the stations have been monitored regularly. A few stations are part of international projects e.g. ECGN (European Combined Geodetic Network) and UNIGRACE (Unification of Gravity System in Central and Eastern Europe). As a national metrology institute (NMI) the Metrology Service of the BEV maintains the national standards for the realisation of the legal units of measurement and ensures their international equivalence and recognition. Thus the BEV maintains the national standard for gravimetry in Austria, which is submitted to international comparisons. Since 1989 the two Austrian absolute gravimeters participated seven times at the ICAG (International Comparison of Absolute Gravimeters) at the BIPM in Paris and in Luxemburg and also three times at the ECAG (European Comparison of Absolute Gravimeters) in Luxemburg. The results of these ICAG and especially the performance of the Austrian absolute gravimeter are reported in this presentation. We also present some examples and interpretations of long time monitoring stations of absolute gravity in several Austrian locations. Some stations are located in large cities like Vienna or Graz and some in mountainous regions. These are at the Conrad Observatory where a SG (Superconducting Gravimeter) is permanently monitoring and in Obergurgl (Tyrolia) at an elevation of approx. 2000 m which is very strongly influenced by glacier ablation. Poster B 707

2. EGU General Assembly 2014 Vienna The Austrian absolute gravity base net: 27 years of spatial and temporal acquisition of gravity data Locations of important absolute gravity stations in Austria: ECGN Stations (blue) / Traflberg (Conrad Obsrvatory), Graz (Lustbühel)and Pfänder; Monitoring stations (red): Obergurgl and Vienna; all other absolute gravity stations in pink.

3. EGU General Assembly 2014 Vienna The Austrian absolute gravity base net: 27 years of spatial and temporal acquisition of gravity data Gravimeters at Conrad Observatory Traflberg: In front: JILAg6, in the middle FG5/242 and at the back Superconducting Gravimeter

4. EGU General Assembly 2014 Vienna The Austrian absolute gravity base net: 27 years of spatial and temporal acquisition of gravity data Gravity measurements at COBS for calibrating the SG gravimeter: 24000 drops were recorded over 9 days of maximal tide amplitudes. Measurements and data processing in cooperation with ZAMG (Zentralanstalt für Meteorologie und Geodynamik) and the University of Vienna (Prof. B.Meurers)

5. EGU General Assembly 2014 Vienna The Austrian absolute gravity base net: 27 years of spatial and temporal acquisition of gravity data Measurements and data processing in cooperation with ZAMG (Zentralanstalt für Meteorologie und Geodynamik) and the University of Vienna (Prof. B.Meurers)

6. EGU General Assembly 2014 Vienna The Austrian absolute gravity base net: 27 years of spatial and temporal acquisition of gravity data Gravity data series at COBS: the FG5/242 data series are in accordance with the other FG5 from Luxemburg and the Superconducting Gravimeter. The offset to the JILAg6 varys between 10 and 20 microgal. Offset

7. EGU General Assembly 2014 Vienna The Austrian absolute gravity base net: 27 years of spatial and temporal acquisition of gravity data Absolute gravity station Obergurgl Impressions of the absolute gravity station Obergurgl, surrounded by big glaciers

8. EGU General Assembly 2014 Vienna The Austrian absolute gravity base net: 27 years of spatial and temporal acquisition of gravity data Gravity data series at Obergurgl located in Tyrol at an height of 1935 m ASL. The 27 year gravity monitoring shows an increasing absolute gravity, which is mainly caused by glacier ablation in the Alps (global warming). See: Gravity effect of glacial ablation in the Eastern Alps – observation and modelling; P. Arneitz, B. Meurers, D. Ruess, C. Ullrich, J. Abermann, M. Kuhn; The Cryosphere;03/2013; 7:491–498. The difference between spring and autumn measurements on an average is 5 microgal, which shows the influence ot the snow load signal on the gravity. The offset between JILAg6 and FG5 is around 15 microgal but both gravimeters show the same trend concerning increasing gravity and the seasonal signal described above. offset JILAg6 FG5

9. EGU General Assembly 2014 Vienna The Austrian absolute gravity base net: 27 years of spatial and temporal acquisition of gravity data Offset of the JILAg6 absolute gravimeter from Comparison Reference Value (CRV) at ICAG at BIPM. The standard deviation of the offsets is s = +/- 6  Gal. see: 20 years of International Comparison of Absolute Gravimeters (ICAG) at the Bureau International des Poids et Mesures (BIPM) in Paris with participation of the BEV; D.Ruess, Ch. Ullrich; VGI, Östereichische Zeitschrift für Vermessung & Geoinformation, 2/2011, pp. 154-161.

10. EGU General Assembly 2014 Vienna The Austrian absolute gravity base net: 27 years of spatial and temporal acquisition of gravity data JILAg6 FG5/242 Offset of absolute Gravimeters of BEV at ECAG: Jilag6 (2003 and 2007); Fg5/242 (2011) Absolute Gravimeter Fg5/242 at ECAG 2011 in Luxemburg

11. EGU General Assembly 2014 Vienna The Austrian absolute gravity base net: 27 years of spatial and temporal acquisition of gravity data Gravity data series at Graz (Observatory Lustbühel): the variation of the gravity is presumably caused by varying hydrology. There is no clear constant offset visible between the two gravimeters.

12. EGU General Assembly 2014 Vienna The Austrian absolute gravity base net: 27 years of spatial and temporal acquisition of gravity data Conclusion: From the metrological point of view the figure with the ICAG results is conclusive, which shows the deviation of the Austrian absolute gravimeter JILAg6 from the Comparison Reference Value (CRV). The standard deviation of the offset is s = +/- 6  Gal. This value can also be interpreted as a measurement uncertainty. If we look back at the combined uncertainty estimation of the JILAg-6 from the technical protocol 2009, we will find a value of 7.9  Gal. This value matches the measurement uncertainty of the JILAg-6. From experimental measurements at Conrad Observatory similar results were derived for the uncertainty of the JILAg-6 gravimeter. The combined uncertainty u c is the square root of the sum of the squared instrumental uncertainty, the site-depended uncertainty and the experimental standard deviation. The expanded uncertainties are then used for the evaluation of the weight of each measurement result in the CRV calculation. Furthermore the results at ECAG and all other figures here show the conclusive improvement of measurement uncertainty due to the use of the FG5 as the new standard for measuring earth gravity in Austria. The combined measurement uncertainty of the FG5/242 is approximately 2.5 to 2.8 microgal, which is almost 3 times better than the JILAg6 u c. A lot of small gravity fluctuations which are detected in JILAg6 time series must be interpeted in the context of the measurement uncertainty. Therefore it is difficult to calculate an accurate offset between both instruments (JILAg6 and FG5). The first examinations presented here show that the offset differs between stations and time. More measurements have to be done at other stations to be able to make a conclusive statement. Anyway the results of the FG5/242 can be used as a perfect standard for gravity measurements at the microgal level in Austria.

13. EGU General Assembly 2014 Vienna The Austrian absolute gravity base net: 27 years of spatial and temporal acquisition of gravity data Thank you ! Poster B 707