Advancements in the development of gamma-ray spectrometers (GRS) have led to small and lightweight spectrometers that can be used under unmanned aerial vehicles (UAVs). Airborne GRS measurements are used to determine radionuclide concentrations in the ground, among which the natural occurring radionuclides 40K, 238U, and 232Th. For successful applications of these GRS sensors, it is important that absolute values of concentrations can be measured. To extract these absolute radionuclide concentrations, airborne gamma-ray data has to be corrected for measurement height. However, the current analysis models are only valid for the height range of 50-250 m. The purpose of this study is to develop a procedure that correctly predicts the true radionuclide concentration in the ground when measuring in the UAV operating range of 0-40 m. An analytical model is developed to predict the radiation footprint as a function of height. This model is used as a tool to properly determine a source-detector geometry to be used in Monte-Carlo simulations of detector response at various elevations between 0 and 40 m. The analytical model predicts that the smallest achievable footprint at 10 m height lies between 22 and 91 m and between 40 and 140 m at 20 m height. By using Monte-Carlo simulations it is shown that the analytical model correctly predicts the reduction in full energy peak gamma-rays, but does not predict the Compton continuum of a spectrum as a function of height. Therefore, Monte-Carlo simulations should be used to predict the shape and intensity of gamma-ray spectra as a function of height. A finite set of Monte-Carlo simulations at intervals of 5 m were used for the analysis of GRS measurements at heights up to 35 m. The resulting radionuclide concentrations at every height agree with the radionuclide concentration measured on the ground.

Heavy duty unmanned aerial vehicles (UAVs) have made it possible to fly with large gamma-ray spectrometers that weigh several kilograms. Moreover, they can be purchased at an affordable price. These large UAV-borne gamma-ray detection systems are used to map the naturally occurring radionuclides ⁴⁰K, ²³⁸U, ²³²Th. Such platforms have the advantage that they can be deployed over terrain that is difficult to access, while still maintaining a high spatial resolution. In contrast to UAV-borne radioactive pollution studies, the naturally occurring radionuclides have a much lower activity and therefore require longer integration time, slower flying speed or a larger detector, in order to effectively determine the spatial radionuclide distribution. Therefore, the question arises: what is the minimum practical detector size required to successfully map ⁴⁰K, ²³⁸U and ²³²Th concentrations from UAV platforms. In this study an agricultural field has been mapped with three different scintillator-based gamma-ray spectrometers: a 2000 ml, 1000 ml, and 350 ml detector. They were mounted together on the same UAV. At a flying height of 20 m and a speed of 5.6 m s^-1 the field was mapped. The various aerial measurements were compared to each other and to the ground-based measurements. The field had a low spatial variation in the ⁴⁰K concentration (relative standard deviation (RSD) = 9%) and a larger variation for ²³⁸U and ²³²Th concentrations (RSD = 24% and 31% respectively). Radionuclide concentrations have been extracted from the survey data by Full Spectrum Analysis (FSA). Uncertainties and variances of the radionuclides have been determined by using two methods. Firstly, they are calculated directly from the FSA output and secondly they are extracted from a variogram. The latter incorporates spatial variation and was shown to provide a lower uncertainty. When using small detectors, the former approach could lead to the conclusion that the uncertainty is larger than the variance, while the variogram approach does capture the spatial variation. All three detectors were able to characterize the spatial distribution of the ²³²Th concentration. It is shown that the ²³²Th concentration is a good predictor of the sand and clay fraction of the topsoil in the field. By comparing the UAV-borne measurements to the ground-based measurements it is found that UAV-borne measurements at 20 m height are less sensitive to extreme values than ground-based measurements and they have the tendency to shift to the mean concentration of the area. The results of this study can be used to optimize the detector volume, survey height, and survey speed to maintain an acceptable accuracy for gamma-ray studies with small UAV-borne detectors.

This study was realized in the frame of an IAEA Coordinated Research Project for the evaluation of sediment dynamics, applying in-situ radiometric methods accompanied with a theoretical model. The in-situ methods were validated using lab-based high-resolution gamma-ray spectrometry. Sediment dynamics assessments were performed based on the measured and mapped activity concentrations of specific ²³⁸U progenies (²¹⁴Bi or ²¹⁴Pb), ²³²Th progenies (²⁰⁸Tl and ²²⁸Ac), and ⁴⁰K along the shoreline of the beach. The maps of the activity concentrations of natural radionuclides were produced rapidly using software tools (R language v4.5). The sediment dynamics of the studied area were also investigated through numerical simulations, applying an open source model considering land–sea interactions and meteorological conditions and the corresponding sediment processes. The assessments, which were conducted utilizing the detailed data from the natural radioactivity maps, were validated by the simulation results, since both were found to be in agreement. Generally, it was confirmed that the distribution of radionuclides reflects the selective transport processes of sediments, which are related to the corresponding processes that occur in the study area. Legrena Beach in Attica, Greece, served as a pilot area for the comparative analysis of methods and demonstration of their relevance and applicability for studying coastal processes.

IntroductionSoil erosion, sediment loss, and catchment degradation are typical of the combined effects of sea level rise and climate change at the coastline and further inland. We describe an integrated, multidisciplinary, nature-based solutions (NBSs) approach to resolving some of the most pressing issues facing coastal communities affected by hydrological impacts due to increasing water levels and saline intrusion. Our blue and green infrastructure solutions are developed for coastal Africa and are equally applicable and scalable to coastlines and hydrological systems around the world where sediment starvation, changing rainfall patterns, and saline intrusion of the coastal aquifer are causing major changes to natural ecosystems, human habitats, and agriculture.MethodsWe use a systems approach that combines modeling and data collection to examine the complete system, from upstream riverine inputs and reservoirs to coastal wetlands, the littoral system, and the coastal aquifer. We emphasize the requirement to understand the interactions between existing natural and artificial assets as a precursor to recommending any management changes to the natural or anthropogenic infrastructure systems in place. A combination of NBSs and novel management techniques are proposed to restore natural sediment and freshwater supplies to coastal communities. We introduce two new green infrastructure types, which have been extensively tested and have the potential to significantly reduce erosion while simultaneously providing desalinated water (and electricity) to nearby coastal communities. We hypothesize that it is possible to reduce the extent of land degradation caused by saline intrusion through better management of water abstraction in the coastal aquifer.    ResultsWe expect our approach to deliver significant environmental and socio-economic co-benefits to coastal and inland communities and ecosystems, many of which are already suffering from the effects of land degradation and a lack of investment. When our NBSs are combined, their synergy greatly increases benefits on a regional scale.