[en] POSIVA Oy conducts bedrock investigations at the spent nuclear fuel final disposal site at Olkiluoto, in western Finland. The excavation of the access tunnel to the repository hosts the ONKALO underground rock characterization facility. The investigations carried out at ONKALO focus on the bedrock and groundwater conditions prevailing on the final disposal site and how construction work affects them. Tunnel seismic investigations were carried out in July 2009, as an extension of similar work performed in December 2007. The main objective of the tunnel seismic investigations have been to demonstrate the possibility to detect, locate and image cost effectively steeply and gently dipping fractures, at the side and/or below the tunnel and to characterize the volume of rock surrounding a 250 m long segment of the ONKALO tunnel. The survey was conducted at a depth of 350 m, over a 240 m long line of 3-components receivers, spaced at 3m intervals. Seismic signals were produced along two lines, on the tunnel wall and floor, with source points spaced at 1m. A timedistributed swept-impact, the Vibsist-250 hydraulic source, was used. The source was hosted on a mini excavator. Receiver holes approximately 0.4 m deep were drilled prior to the survey, horizontally into the tunnel wall. One of the procedures used for data stacking and migration is based on a proprietary method combining the DMO (Dip Move Out) correction and an expression of the Radon Transform. Horizontal and vertical migrated profiles were computed both for the P wave and S wave reflected wave fields. A true 3D migration technique (Image Point migration) was used to create 3D migrated sections oriented to incremental azimuths around the tunnel, the result being a cylindrical imaging volume. A general conclusion is that seismic surveys along the tunnel can economically be used for rock mass characterization. High quality results can be obtained by operations in tunnel working conditions, provided that due attention is given for the design of the work flow. A particular task of the seismic survey carried out in the ONKALO access tunnel was to test the suitability of 2D/3D reflection seismics for detecting and locating geological features of diverse character and orientations. The results obtained by the novel 3D IP migration algorithm proved this to be a successful test. Due to the two source-line survey geometry and the use of three-component geophones, it was possible to detect fracture zones or single fractures of certain orientations up to a distance of 200 m. As with previous work performed in 2007, creating a good and detailed model (or prediction) of the geological and hydrological features of the repository area requires that integrated modeling is carried out using all geological, hydrological and geophysical data. Preliminary, tentative interpretations were done by comparing the processed seismic data with known geological, geophysical and hydrological features observed in the tunnel. From these it appears to be possible to locate known features by seismics, from site-scale (e.g. brittle fractured zones) to tunnel-scale (e.g. single long fractures). (orig.)