## 4 Survey Technique and Data Preprocessing
The final processing of the obtained data to create maps, sections and 3D models of resistances can be performed using any software for working with TEM data, including 1D and 3D inversion environments. The results presented in the next section were obtained by the 1D-inversion method in the MARS 1D software package developed by the authors [31, 32, 33]. 5. An Example of UAV-TEM Application: A Case of Mapping Structural Discordance and Tectonics  
The area is located within one of the promising uranium provinces of Eastern Siberia (Russia), in the foothills of the Eastern Sayan (Figure 7). The area is overgrown with dense forest, pedestrian permeability is dif cult due to steep slopes, rocky talus (on the satellite image they appear in white under the crowns of trees) and thickets of bushes. Difference in height at the area from 980 to 1070 m.  
There are no exact coordinates of the area boundaries, because uranium in Russia is a strategic mineral. The features of the geological structure of the area are described in more detail in the previous publications of the authors [5], in this work the geological situation at a speci c area of work will be briefy described (Figure 7b). The _e_idwork area is formed by rocks of two stratigraphic units--a granite basement and a partially eroded cover, metasedimentary deposits (sandstones). According to logging data on already drilled areas within the study area, the resistance of sandstones is signi cantly lower than resistance of granites. Electrical prospecting using the UAV-TEM method was carried out in order to clarify a priori ideas about the geological structure of the area, since the basic cartographic materials are very inaccurate--for the entire territory of Russia there are maps only at a scale of 1:200,000, real geological boundaries, zones of tectonic disturbances, etc. may differ signi cantly, especially given the low degree of bedrock exposure. According to the geological map, it is expected that in the central part of the area the cover of sandstones is eroded to the granite basement, and the thickness of the cover, marked at the western and eastern boundaries, increases in the directions to the northwest and southeast. However, since the sedimentary cover corresponds to elevated relief forms (which is noticeable in the western part), and the eastern part is in the lowland, it seems that in fact the boundary of the stratigraphic uncomformity is not located in the area, but east of it, or the thickness of the cover in this location is minimal. An error in determining the position of geological boundaries of several hundred meters is quite possible, given the scale of the geological map. However, in this regard, it became  
Figure 7: Location of the work area ( _a_) and a priori ideas about the geological structure of the work area (fragment of a geological map at a scale of 1: 200,000)(b): 1—gabbro-dolerites (RF3); 2—metasedimentary deposits (RF2-3); 3—granites (KR2); 4—areas of intensive development of ore-metasomatic formations; 5—faults; 6—contour of UAV-TEM survey. necessary to more accurately map the boundary of the structural unconformity between the granite basement and the sandstone cover, as well as to clarify the scheme of tectonic faults. Due to difficult pedestrian traffic and problems with galvanic grounding, it was decided to perform this task using the developed UAV-TEM complex. A dirt road runs along the center of the area. A transmitter line 2 km long was deployed along it. The current strength was 0.87 A, a 4 kW gasoline generator was used, 15 electrodes were grounded at each end of the transmitter line. The survey was carried out at a speed of 7.5 m/s at an altitude of 40 m under the sensor, with detailed flow around the relief. The distance between the survey lines is 40 m. Control measurements were carried out along the opposite and orthogonal routes. Orthogonal routes were also necessary to determine the distance at which the maximum departure from the transmitter line is possible for a given installation geometry and current strength.