Generation of 137Cs surface distribution maps using dense measurement transects of a towed gamma-ray spectrometer

In this paper, we describe a method to generate surface distribution maps of seafloor ¹³⁷Cs levels. The method interpolates the measurements of a towed gamma-ray spectrometer, which are made along intersecting line transects, by modeling the spatial variations between each observation. Since it is known that sediment type has an influence on the distribution of ¹³⁷Cs, the proposed method aims to improve the spatial auto correlation of the models by segregating the surveyed area into continuous regions in which the sediment types are judged similar. This is achieved based on the back-scatter intensity of side scan sonar images taken in the surveyed region. The method is applied to data obtained at the Abukuma river mouth located approximately 80km north from F1NPP on October 2013. The accuracy of the proposed method was evaluated by correlating the levels of ¹³⁷Cs in the generated maps with samples that were obtained in the same area. The results demonstrate that the proposed method of interpolation improves the correlation compared to conventional methods that do not account for sediment type.