ONKALO is the underground rock characterisation built for the final disposal of spent nuclear fuel at Olkiluoto, Finland. Geology, geophysics, hydro-geology, geochemistry and rock mechanics contribute to bedrock studies conducted during repository construction to ensure the suitability of the bedrock for the final disposal. A high-resolution 3D reflection seismic imaging survey was conducted in 2009 at a depth of app. 350 m. Lines 240m long were measured, with source stations spaced at 1m and receivers at 3m, respectively. Measurements were conducted with a hydraulic impact source that was used on the tunnelwall and floor. Fractures were imaged couple of hundred meters away from the tunnel. The location of the target features was achieved by using 3-component receivers, two parallel source lines and 3D vector migration schemes. IP (Image Point) migration was found particularly useful for imaging narrow and roughly planar features of diverse orientations.
POSIVA Oy conducts bedrock investigations at the spent nuclear fuel final disposal site at Olkiluoto, in western Finland. The purpose of these efforts, which include a significant R&D component, is to ensure compliance with the requirements set forth for the long-term safety of final disposal. 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. The main objective of the seismic investigations presented here 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 250m long segment of the ONKALO tunnel.