Harmonization and mapping of terrestrial gamma dose rate data in Belgium

With several databases available, including two sets of in situ measurements of the ambient gamma dose rate and an airborne survey of K, Th, U in soil, Belgium is a favourable case for exploring the mapping methodology for terrestrial radiation. The first step is the harmonization of the different data sets, taking in situ measurements with an ion chamber as the reference. Corrections are necessary, based on the data themselves (a) to the measurements of permanent monitoring stations, (b) to the data calculated from airborne measurements of the soil activity, due in particular to the attenuation by the forest cover, and (c) to the other data calculated from the soil activity, due to the lower activity of the upper layer. After subtracting the cosmic contribution, a harmonized database of the terrestrial gamma dose rate (TGDR) based on 379 in situ measurements was built, together with a harmonized data set of 30134 TGDR values calculated from the concentrations of K, Th, U in soil deduced from the airborne survey. The two data sets are in good agreement with each other for all statistical characteristics that were examined like basic statistics, qq-plots, analysis of variance (ANOVA) or variograms, which validates the airborne-based data set by the link with in situ ion chamber measurements. ANOVA reveals the strong relation between TGDR and the soil class, which justifies the use of a soil map as the framework for developing the TGDR map. The variograms show the absence of residual spatial correlations within soil classes. The two harmonized TGDR data sets were mapped at the nodes of a kilometric grid by the moving average method within soil groups. There is a rather good agreement between the maps, confirming the equivalence between the two data sets and the validation of the airborne based one, which can obviously give more detail. After reducing the maps to a 10 km × 10 km grid, the two data sets were used to check the accuracy of the Belgian part of the European TGDR contained in the European Atlas of Natural Radiation.