High Resolution 3D Tunnel Seismic Reflection at Olkiluoto, Finland

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 reflection seismic imaging pilot test was conducted in 2007, followed by a more ample survey in 2009. Lines 100m and 240m long were measured, with source stations spaced at 1m and receivers at 1m and 3m, respectively. Measurements in 2007 were conducted with a hand-held electro-mechanical source held against the tunnel wall. In 2009 a more energetic hydraulic source and was used on the tunnel wall and floor. Fractures were imaged several tens, respectively hundreds of meters from the tunnel. In 2007, the one-dimensional tunnel layout left a certain ambiguity regarding azimuth relative to the tunnel. The location of the target features was improved in 2009, 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.