Workreport 2013-58



Analysis of Temperature Data at the Olkiluoto


Sedighi, M., Bennett, D., Masum, S. A., Thomas, H. R. & Johansson, E.



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As part of the rock mechanics monitoring programme 2012 at Olkiluoto, temperature data have been recorded. Temperature data have been measured, collected and monitored at the Olkiluoto site and in ONKALO in various locations, by different methods and in conjunction with other investigations carried out at the site. This report provides a detailed description of the investigation and analysis carried out on temperature datasets. This report aims to provide a better understanding of the in-situ temperature of the rock and soil at the site.

Three categories of datasets have been analysed and studied from the Posiva thermal monitoring programme. These consist of: i) data collected from the various drillholes during geophysical logging and Posiva Flow Log (PFL) measurements, ii) measurements in the ONKALO ramp, the investigation niche located at elevation -140m and a technical room located at 437m below the surface, and iii) surface temperature measurements from four weather stations and four measurement ditches.

Time-series data obtained from the groundwater temperature measurements during the ‘Posiva Flow Log’ (PFL) tests in drillholes OL-KR1 to KR55 at different depths and years have been analysed. Temperature at a depth of 400m was found to be in the range of 10 to 11 oC The geothermal gradient obtained from the PFL data without pumping was found to be approximately 1.4 oC/100m with relatively uniform temporal and spatial patterns at the repository depth, i.e. at 400m.The geothermal gradient obtained from the results of the PFL measurements and geophysical loggings indicate similar temperature values at the repository depths, i.e. 400m.

The characteristics of the time series data related to the ONKALO measurements, have been obtained through a series of Non-uniform Discrete Fourier Transform analysis Datasets related to the various chainages and investigation niche at ONKALO have been studied. The largest variation in the temperature amplitude of data collected from the tunnel chainages corresponds to the two locations nearest to the surface. No major frequency behaviour has been deduced from the de-trended time-series.

Datasets containing the soil temperature at the locations of the weather stations and measurement ditches, recorded from 0.1m to 0.9m below the ground surface were analysed. Temperature variations recorded at weather stations FIP4, FIP10 and FIP11 follow a similar trend whilst the recorded temperature at station FIP14 is on average higher than that of the other three locations. The time series data recorded at the four ‘measurement ditches’, i.e. OL-MP1 to OL-MP4, were studied. Surface temperature data show a seasonal cycle with no major shift. The temperature peaks recorded in these stations were found to be higher than the maximum temperature measured at all depths at FIP4, FIP10 and FIP11 locations. This trend is thought to be related to the proximity of these stations to the sea.


Olkiluoto, rock mechanics, monitoring, temperature measurements, ONKALO, data analysis.


Analysis of Temperature Data at the Olkiluoto (pdf) (10.4 MB)


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