Geostatistical interpolation methods (block and point ordinary kriging) were used to produce soil contamination maps based on airborne gamma-ray measurements of cesium-137 deposit acquired in November 2013 with a flight-line distance of <1 km. The methods were applied to six different square tiles of ~20 km length located within 100 km from the Fukushima nuclear power plant to quantify the influence of an increase in the flight-line distance on the quality of the interpolated maps; indeed, smaller the flight-line distance, higher the quality and cost of produced maps. Therefore, kriging estimators were applied over different selections of flight-lines of decreasing density.

The quality of the produced maps was quantified by the means of a correctness rate, calculated by comparing the predicted contamination surfaces, above some threshold, to that best estimated from all the flight-lines. The results showed that the calculated correctness was highly correlated with the flight-line distance (r2=0.83): increasing the flight-line distance for every 1 km decreased the correctness of the contamination map by ~1%. In fact, the correctness also depends on variogram modeling which is a subjective step of the kriging methods.

The influence of variography was also analyzed: the contamination maps were reproduced, this time using a fixed variogram model (that of all the flight-lines) to find out how much flight-line distance influences the correctness, while the variogram model is unchanged. It was found that correctness is influenced by the variogram model by less than 6%, meanwhile at least 94% being attributed to the flight-line distance. Hence the relation of correctness versus flight-line distance is reliable.

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Poster presented at the 3rd Europe Radiological Protection Research Week ERPW 2018 in Rovinj-Rovigno, Croatia

Pedram MASOUDI (1), Mathieu LE COZ (1), Marc-André GONZE (2) & Charlotte CAZALA (1)
(1) IRSN/PSE-ENV/SEDRE, Fontenay-aux-Roses, France
(2) IRSN/PSE-ENV/SEREN, St Paul-les-Durance, France

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