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Workreport 2020-20



Updated Electrical Model of Olkiluoto


Paananen, M., Huotari, T., Suppala, I.



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This report describes the updated electrical model of the Olkiluoto study site. The preceding model of the electrical structure of the site is documented 12 years ago in, and since then large amount of additional geophysical data are gathered. Also the understanding of the site has significantly gained after 2007. Primarily the need of the model update arose in connection with the interpretations of Sampo EM monitoring of the area. These EM soundings have been used to monitor potential changes in hydogeological conditions during the excavation process. The updated electrican model serves also geological modelling of Olkiluoto.

The most important new dataset contains the supplementary resistivity logging results from deep holes OL-KR40 – OL-KR57 and pilot holes ONK-PH5 – ONK-PH29. In addition, large number of results from MAM surveys, interpretations and modellings were available from several years. Besides the new data, also the older data, used in the previous model, were utilized (Drill hole resistivity loggings, MAM results, HLEM on the ground surface). Since the electrical model is inteded to serve the Sampo EM monitoring soundings, the existing Sampo results are not used in compiling the updated model in order to avoid circular arguments in the interpretation. For the modelling work, also new inversions from the earlier resistivity soundings were done, as well as layer model inversions for AEM and ground HLEM data.

All sections with apparent resistivity less than 1000 ohmm and exceeding typically 1 m were determined and documented. In shorter pilot holes, even more narrow features (tens of cm) are determined occasionally. In many cases, there were numerous nearby narrow resistivity minima, which were combined in the interpretation to larger, more generalised conducting features. The conductive sections were also characterised according to their assessed resistivity value and probable occurrence of sulphide minerals (especially pyrrhotite). Also indications of fracturing, hydraulic conductivity, saline water, hydrothermal alteration, locations of brittle zone intersections and modelled brittle fault zones were documented. In addition to new boreholes, similar documentation was done also for all old boreholes in order to harmonise the data.

The determined conductive sections were imported to a 3D database for cross-hole correlation, utilizing e.g. MAM interpretations and the general trend of foliation. The conductors were also correlated to a number of HLEM conductors on the ground surface. The deterministic model considered altogether 41 conducting features. Their resistivity values at drill hole intersections, geological, hydrogeological and MAM indications as well as their relations to modelled brittle deformation zones were documented.

Besides the deterministic model, also numeric resistivity model was created. This was calculated separately within two domains: 1) inside and 2) outside of the determined conductors. For the calculation, the original drill hole logging data were smoothed by moving average and down-sampled to 5 m spacing. Furthermore the down-sampled dataset was supplemented by ERT, AEM and ground HLEM inversion results.


Geophysical, EM, electrical, resistivity, sounding, model, drill holes


WR 2020-20_web (pdf) (16.1 MB)


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