Estimation of fukushima radio-cesium deposits by airborne surveys: Sensitivity to the flight-line spacing.

After Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, airborne gamma-ray detection was used for regional mapping of soil contamination. For such surveys, the flight-line spacing is an important factor controlling the quality of contamination maps. In this study, cesium-137 (Cs) ground activity is interpolated and mapped using ordinary kriging method; thereafter the error of interpolation is evaluated as a function of flight-line spacing. The analyses were conducted in six 20 km × 20 km test sites with distance of less than 80 km from the FDNPP. In each site, the ordinary kriging estimators were applied to different selections of flight-lines of decreasing density, then punctual and classification errors were calculated. It is demonstrated that these variables are highly correlated (r > 0.78): increasing the flight-line spacing for 1 km increases the errors from 3% to 9%, depending on the site location. Therefore, flight-line spacing could be designed as a function of acceptable error, determined in the monitoring objectives.