Revision of the Lineament Interpretations of the Olkiluoto area in the light of acoustic-seismic data from the adjacent marine areas

Several lineament interpretations at different scales have been done in the course of the site investigations in the Olkiluoto area. The interpretations have been based on satellite images, topographic and geophysical maps. The problem with the previous interpretations has been the lack of appropriate data from the sea floor of the marine areas surrounding the Olkiluoto site. That is why the bedrock structures under the sea bottom have been poorly known until now. During year 2000, systematic acoustic-seismic soundings were carried out by the Geological Survey of Finland (GSF) on the sea area west of Olkiluoto. The area of soundings covered about 150 km2. Based on the sounding data, relief maps of the sea bottom and bedrock topography were constructed. These maps, together with detailed geological interpretations of sounding profiles, have been used for a new local lineament interpretation covering an area of 10 x 15 km around the Olkiluoto site. For more regional lineament interpretations over the area 40 x 50 km, topographic maps and relief models with oblique illumination were used. Lineaments have been classified into four size categories representing fault or fracture zones of different width and length. The acoustic-seismic data proved to be of high quality and very useful for lineament interpretations. They significantly increased knowledge of the location and size of fracture zones surrounding the Olkiluoto site. The revised local lineament interpretations (sounding offshore area 10 x 15 km, Olkiluoto site area 15 x 25 km) revealed that the Olkiluoto site is part of the regional bedrock block, 11 x 3.5 km in size and elongate in a WNW-ESE direction. The block is bordered in the north, south and east by regional fracture zones of size category II. At least two HSP-structures correlate well with these fracture zones. The regional block is cut into smaller blocks by fracture zones of size category III. The western border of the regional block is thought to be situated about 6 kilometres west of the Olkiluoto island. In that area, the relief of bedrock becomes more subdued and the thickness of Quaternary deposits increases. These changes are interpreted to mean that the crystalline basement west of that area is covered by Jotnian Satakunta sandstone (Rantataro 2000). A geological map is presented with the revised contact of the sandstone against the basement, offshore. The revised regional interpretations covering the Olkiluoto region (40 x 50 km around the Olkiluoto island) do not differ in essence from the previous results, because bedrock structures on the sea bottom outside the sounding area are still poorly known. (orig.)