Hard Rock High Resolution 3D Seismic Investigations at Olkiluoto, Finland

Olkiluoto was selected as the site for the final disposal of nuclear waste in Finland. Currently, ONKALO, the access tunnel and underground research facility is being built at Olkiluoto. Since 1987, comprehensive geological and geophysical investigations have been performed at Olkiluoto for the detailed characterization of the disposal site area. Various seismic surveys were done, from surface and boreholes (Enescu et al. 2004), over a period of more than 10 years. A pilot high resolution 3D reflection seismic survey was acquired, for the first time in Finland, in 2006. High quality data led to very promising results, hence 3D seismic investigations were continued in 2007, over a larger area (Cosma et al. 2007). The 3D reflection seismic method proved instrumental for determining the geometry of rock features and ultimately building a geometrical model of the investigated rock volume, consistent with previous geophysical determinations in the area. Reflectors ranging from 100 m down to at least 3 km were imaged with accuracies of less than 10 m.