Precise levelling campaigns at Olkiluoto in 2006 - 2007

The GPS observation network of Olkiluoto was constructed in 1994 for monitoring crustal deformations in the investigation area. To fulfil a better vertical control of the GPS network, precise levellings were started in the area in autumn 2003. The GPS network was first connected at Lapijoki to the precise levelling network of Finland to control the vertical movements of the whole island of Olkiluoto. Then the GPS network was levelled. It consisted of the reserve marks of eight GPS pillars and five levelling bench marks, two of which constituted the nodal bench mark pair. The second precise levelling campaign on the area was carried out in autumn 2005. Now only the GPS network added with the antenna platforms of nine GPS pillars were levelled. Compared to the other points, the elevation difference of two reserve mark pairs had changed significantly during two years, about one millimetre. The reason may be the blasting of the rock in the neighbourhood of these points and deformation of the rock after the blasting. Inspired by the observed elevation changes in 2005, micro loops were established and levelled onto the ONKALO and the VLJ Repository in autumn 2006. The micro loops consisted of seven and five bench marks the mean interval being about 300 metres. The campaign in autumn 2007 consisted of the levellings of all measured and undestroyed points of the earlier campaigns. The most interesting results were: (1) Compared to the mean theoretical land uplift the nodal bench mark 03216 near the crossing of Olkiluodontie and Satamatie had risen in four years 2.6 mm more than the nodal bench mark of Lapijoki and 1.9 mm of this occurred within the 0.8 mm long interval which separates the island and the continent. (2) During four years the northern part of the island had risen about one millimetre more than the middle part, where the before mentioned 03216 is located. (3) The elevation differences between the bench marks of the ONKALO micro loop were changed even one millimetre during one year. In the place where the land uplift gradient was most significant, the change was 0.5 mm within a bench mark interval of a little more than one hundred metres. According to the plan the line from Lapijoki to Olkiluoto will be levelled every fourth year, GPS network every second year and the mini networks every year. The latest observations prove that the levelling frequency of the bench mark interval separating the island and the continent should be increased. (orig.)