GPS operations at Olkiluoto in 2011

The Finnish Geodetic Institute has studied crustal deformations at Olkiluoto, Kivetty and Romuvaara in co-operation with Posiva Oy since 1995. At Olkiluoto a total of 32 GPS campaigns have been carried out at inner network since 1995 and 17 campaigns at outer network since 2003. Kivetty and Romuvaara were not measured in 2011. In the Olkiluoto inner network 80 percent of the estimated change rates are smaller than 0.10 mm/a. One third of the change rates are statistically significant. They are mainly related to the Olkiluoto permanent station (GPS1) and to the pillars GPS6 and GPS13. The change rates related to GPS6 are not realistic due to the site-specific changes affecting the time series. The maximum change rate (-0.20 mm/a ± 0.05 mm/a) is related to GPS13. The time series of GPS13 is half the length of other pillars and therefore, the change rates are more uncertain. In the Olkiluoto outer network the maximum and statistically significant change rate is between GPS1-GPS11 (0.39 mm/a ± 0.06 mm/a). Pillar GPS12 was not observed this year. The change rates of baselines GPS1-GPS14 and GPS1-GPS15 are first time statistically significant. The change rates indicate a small movement of the GPS1 pillar. The baseline GPS1-GPS11 crosses an old fracture zone locating in the direction of the Eurajoensalmi, which might be a reason for the deformation. On the other hand, the Onkalo excavations in the vicinity of the Olkiluoto permanent station (GPS1) may cause some movement. Electronic distance measurements have been performed at Olkiluoto at the baseline GPS7-GPS8 using the Mekometer since 2002. The measurements have been carried out simultaneously with GPS campaigns. Based on 19 measurements in 10 years, the trends of the two time series seems to be similar. Due to unmodelled or dismodelled geometrical offsets and the scale difference between GPS measurements and EDM there is about 0.3 mm difference between distances GPS7-GPS8 derived from GPS measurements and EDM. It is less than the uncertainty of GPS-distances and since 2006 there seems to be no difference at all. In 2010 the four new concrete pillars (GPS16, GPS17, GPS18 and GPS19) were built in Olkiluoto. The control measurements at new pillar points were carried out in July 2011. The purpose of the measurements is to determine horizontal angles and distances between pillars and control points. The distances between pillars and control markers are 5-14 m. Besides the control markers each new pillar has two prism holders and the levelling bolt established on the pillar cylinder wall. There is no need in future to remove the antenna during control measurements. FGI we will continue geodetic observations at Olkiluoto, Kivetty and Romuvaara. The Olkiluoto network is under major modernization for permanent tracking during upcoming years. The first new permanent station GPS16 became operational on 16th of December 2011. The old Olkiluoto local GPS network - including those pillars not yet tracking permanently - will be measured biannually as thus far, as well as the EDM baseline. We will also carry out measurements at Kivetty and Romuvaara every second or third year supposing the improvement of the visibility of the points of those networks. (orig.)