## 2 Materials and Methods
### Survey and Data Processing Methods
It is easy to isolate them even for equipment with low energy resolution, such as old analogue spectrometers or modern BGO detectors. However, with an increase in gamma rays' energy, the penetrating power of radiation also increases. Therefore, the registration of these lines by small detectors is difficult. Simultaneously, the uranium-radium and thorium series also include a relatively large set of energy lines of various radionuclides, some of which have even greater quantum yield than traditionally used isotopes (Table 2). Some of them are located in lower energies range, i.e., they are effectively detected by scintillation detectors with a thickness of several centimetres. The additional counting of gamma rays along these lines, as shown in the next section, significantly increases the data's statistical representativeness. However, low-energy lines decay faster with height, since air is also an absorbing layer. For example, for 0.609 MeV energy lines, the free path of a gamma-ray quantum (half attenuation) is 66 m, and for 1.76 MeV energy, it is already about 100 m. Also, at a low detectors' resolution, their energy windows may overlap since the lines are quite close. In this regard, to process energy lines as 0.609 MeV, 1.12 MeV (uranium series), 0.91 (thorium series), detectors with an energy resolution of at least 8% are required, and the flight altitude must be minimized. Thus, to partially compensate for the detector's small sensitive volume, rather than three classical energy windows to process the gamma-ray survey data, six were used--one for potassium-40, two for thorium, and three for uranium. **Feature 2**. Spectrometer Calibration Method. The second important position of this UAV aerogammaspectrometry technique is the spectrometers calibration method. To create cartographic materials that reflect the relative radionuclide concentrations distribution in the study area, to identify anomalies of low and high values at a constant relative flight altitude, it is enough to sum the number of gamma quanta along all energy lines related to each radionuclide. However, to obtain maps of radionuclide concentrations for comparison with results of other works or a more accurate geological interpretation, it is necessary to find a matrix of coefficients linking the spectrometer's specific count rate in each channel with the concentrations of radionuclides and taking into account the unequal height attenuation of each energy line. Spectrometers used in ground-based surveys calibration are performed on standard samples, and the in-height field attenuation can be calculated analytically. Nevertheless, it is necessary to take into account the different geometry of the measurements additionally. A more straightforward method can be used: near the geological base of the Irkutsk National Research Technical University, a site of distribution of formation granites with a size of 150 \(\times\) 150 m with relatively uniform chemical composition and good exposure was selected, and the site was studied to determine the real concentrations of the main radionuclides. It is known that an in air located spectrometer registers uranium, thorium and potassium gamma-rays from the underlying surface corresponding to the base of the cone, which has a vertex in the detector and an included angle of 120\({}^{\circ}\) (the radius of the base of the cone is referred to as the \"effective radius\"). Thus, the selected area allows the count's relationship in the spectrometer channels with the radionuclides concentrations to be correctly established at altitudes up to 40-45 m since the effective radius reaches 70-80 m.  
Large-Scale Survey and Anti-Aliasing Interpolation. When surveying with small detectors, the variance of the recorded values, especially at a low level of gamma background, is relatively high, even though more than one energy line is involved in the data processing.