Workreport 2008-59



Gefinex 400S (Sampo) EM-Soundings at Olkiluoto 2008


Jokinen, T. & Lehtimäki, J.



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In the beginning of June 2008 Geological Survey of Finland (GTK) carried out electromagnetic frequency soundings with Gefinex 400S equipment (Sampo) in the vicinity of ONKALO at the Olkiluoto site investigation area. The same soundings sites were first time measured and marked in 2004 and has been repeated after it yearly in the same season. The aim of the measurements is to monitor the changes of groundwater conditions by the changes of the electric conductivity of the earth at ONKALO and repository area.

The measurements form two 1400 m long broadside profiles, which have 200 m mutual distance and 200 m station separation. The profiles have been measured using 200, 500, and 800 m coil separations. Because of the strong electromagnetic noise all planned sites (48) could not be measured. In 2008 the measurements were performed at the sites that were successful in 2007 (43 soundings).

The numerous power lines and cables in the area generate local disturbances on the sounding curves, but the signal/noise also with long coil separations and the repeatability of the results is reasonably good. However, most suitable for monitoring purposes are the sites without strong surficial 3D effects. Comparison of the results of 2004 to 2008 surveys shows differences on some ARD (Apparent resistivity-depth) curves. Those are mainly results of the modified man-made structures. The effects of changes in groundwater conditions are obviously slight.


Electromagnetic frequency soundings, Sampo, Olkiluoto, ONKALO, repository, monitoring of electric conductors


Gefinex 400S (Sampo) EM-Soundings at Olkiluoto 2008 (pdf) (5 MB)


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