## 2 Materials and Methods
### Survey and Data Processing Methods  
The main stages and features of the low-altitude radiometric and spectrometric survey technique used are described below. To perform low-altitude geophysical surveys, it is necessary to have a sufficiently accurate digital model of the work site's terrain. UAV flights will be operated at a constant altitude above terrain respecting the digital elevation model (DEM). The DEM can be obtained immediately before starting the UAV surveys by light detection and ranging (lidar) scanning or photogrammetry from the UAV or created in advance based on available satellite or topographic data. In this work, case 2 used a DEM with a pixel resolution of 1 m, obtained by photogrammetry, and in other cases used a DEM with a pixel resolution of 10 m, created with Intemap NEXTMap data. The digital terrain model must be accurate enough to avoid accidents and ensure accurate flow around the terrain. In the Russian Federation, there are requirements for the quality of DEMs used in UAV-magnetic surveys: the maximum vertical accuracy of DEMs should be no less than 10% of the relative height of the UAV above the terrain. For example, when surveying at the height of 50 m under the measuring system's sensor, the permissible error of the DEM is \(+/-5\) m according to the LE90 criterion. Other types of UAV-geophysical surveys do not yet have a regulatory framework, nonetheless, the same requirements for UAV gamma-ray studies were adhered to. Before starting fieldwork, the energy scale of the spectrometers was calibrated. Calibration is performed traditionally through two energy channels, and several additional lines check its quality. For a properly functioning spectrometer, the calibration graph looks like a straight line (Figure 3). Figure 3: Spectrometer energy-scale calibration. After the operators arrive at the take-off point, its altitude was determined, and flight missions were created for the UAV autopilot. It is an array of points \"longitude-latitude-relative altitude.\" Flight missions were created using the module for QGIS' SibGIS Flight Planner' ([PERSON] and [PERSON] 2017) based on the DEM. The height of the survey was selected, taking into account vegetation on the worksite. * The UAV and the spectrometer were brought into operation. * The flight mission was loaded into the UAV flight controller. The measuring system was started. Take-off and landing of the UAV were carried out in manual mode; the flight took place on autopilot. The geophysical measurements were made automatically, and the results were recorded in the recorder's memory with the exact satellite time and coordinates. The radiometer measurement frequency was fixed at 0.5 Hz; the spectrometers had a configurable frequency from 1 Hz to 0.1 Hz; usually, the recording was conducted with a frequency of 0.5 or 0.33 Hz. * After the mission was completed, the UAV battery was replaced, and the actions of the previous paragraph were repeated. * At the end of a routine survey, one control route was usually made, which was necessary to assess the data's quality. However, in this work, a control survey in the same volume as the ordinary survey was conducted, but in the opposite direction, to show no profile in the data caused by the detectors' data inaccurate spatial reference or detectors' shortcomings. * After the measurements were completed, the data were exported, processed, and mapped. The data processing technique applied had three key features. **Feature 1**. Increasing the Statistical Representativeness of the Data by Processing Multiple Energy Lines. First, in geological practice, usually only one energy line for each isotope is used to study and map the distribution of U, Th, and K-40 radionuclides. For the uranium-radium series, the 1.76 MeV line is used, for thorium--2.62 MeV, for potassium--1.46 MeV. These lines are located in relatively high energies range and are easily distinguishable from the lines of other isotopes.