International Geologiical Congress - Oslo 2008

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EIE-07 High-resolution geophysical imaging of geological structures and processes in environmental studies

 

Detailed ground magnetic and resistivity measurements for a potential deep geologic repository for spent nuclear fuel in Oskarshamn

 

Leif Stenberg, Swedish Nuclear Fuel and Waste Management Co (Sweden)
 

 

Site investigations for a potential deep geologic repository for spent nuclear fuel are in progress in the municipalities östhammar and Oskarshamn. A comprehensive site investigation program was initiated to characterise the properties of the bedrock, overburden, and ecosystems. The investigations at the sites were completed in 2007 and a licence application to construct the repository at one of the sites is expected to be submitted at the end of 2009.

The candidate area Laxemar is situated by the coast in south east of Sweden, close to the Oskarshamn nuclear power plant, the interim storage facility for spent nuclear fuel (the Clab facility) and the Äspö Hard Rock Laboratory (HRL). Site investigations, which started in spring 2002, involved detailed air photography and geophysical surveys over the regional area in order to reduce the 50 km2 large candidate area to a size where the deep drilling programme could be concentrated. Accordingly, in spring 2003 the Laxemar and the Simpevarp subareas were selected and some investigations were performed in parallel on both subareas until Laxemar was prioritized. At the Laxemar subarea the investigations started in the beginning of 2004. In total, the investigation involved some 20 deep cored boreholes (with a length of c.1,000 m) and another 30; 100 - 600 m long core drilled boreholes. A typical investigation programme in these boreholes included hydraulic tests and water sampling during drilling, video imaging (BIPS), geophysical logging, borehole radar, core logging, difference flow meter logging, hydraulic injection tests and analyses of rock samples. The geophysical logging data were mainly used to support the core logging.

Ground geophysical measurements have been performed in the Laxemar area. A total of approximately 7.5 km2 were covered with measurements of the total magnetic field. The survey was performed with a point separation of 5 metres and a line spacing of 10 metres. A mosaic map of the survey results has been produced. A number of low-magnetic semi-linear features appear in the results. More than 1,800 lineaments have been possible to identify in the presented data. Three areas, 400 by 400 metres each, were covered with resistivity measurements with the ABEM Lund imaging system. The measurements were carried out along east-west oriented profiles separated by 10 metres. The electrode separation was 5 metres and a total of 81 electrodes per line were used. Measurements were performed with a gradient array protocol resulting in a depth of investigation of up to 60 metres. The soil cover thickness has been estimated from the resistivity measurements. A map of soil cover thickness and bedrock relief are presented for one area. The three survey areas were located around north-south trending lineaments. One of the interpreted deformation zones was interpreted to dip to the west while the others were interpreted to be subvertical.

 

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