## 4 Survey Technique and Data Preprocessing  
To perform a survey using the UAV-TEM method, minimum two persons (usually 3) is required, since one person will not be able to ensure either the safety of the UAV complex landing in the space often limited due to vegetation, or the deploying and grounding of the transmitter line. The main positions of the UAV-TEM technique are set out below. They are described exactly in the version that was used when obtaining the data presented in Section 5 of this article. * To perform low-altitude geophysical survey, it was necessary to have a sufficiently accurate digital relief model of the work area, at a constant altitude over which the UAV will fly. The digital elevation model (DEM) can be obtained immediately before the start of the UAV-TEM by the method of lidar scanning from the UAV, or it can be created in advance based on the available satellite or topographic data. The example in Section 5 uses a 10 m pixel DEM generated from Intermap NextMap data. The digital elevation model must be accurate enough to avoid an accident and ensure the required data quality. So, for UAV magnetic prospecting, the regulatory framework of the Russian Agency for Subsoil Use established the maximum vertical accuracy of the DEM no more than 10% of the relative height of the UAV above the terrain, i.e., for example, when surveying at an altitude of 50 m under the sensor of the measuring system, the DEM error was considered acceptable \(\pm\)5 m (according to the LE90 criterion). For other  
Figure 4: Power supply of the complex for UAV-TEM: (**a**) transmitter line grounding of the line ends; (**b**) current switch. types of UAV-geophysical survey, which are not yet provided with approved methods,1. The division of the file with data from the whole flight mission into transient records (TR), related to individual measurement points, was carried out over specified time ranges, and was based on data from a GPS receiver. 2. Suppression of industrial noise was carried out to filter noise with a frequency of 50 Hz and its derivatives from the used alternator. The filtering procedure was based on a differentiating digital filter [29]. 3. Allocation of recessions is a software feature of hardware and software system \"MARS\" operation. One accumulation contained a signal for 0.5 s (positive pulse, pause, negative pulse, pause), which must be divided into separate TRs. 4. When ground (start-stop) worked by TEM methods using a receiving ungrounded frame, the suppression of the trend was not required, but when the receiving loop moved in space, the EMF of the Earth's magnetic field was induced on it, which must be suppressed. 5. Robust smoothing in a 2D sliding window was performed using the Hampel M-estimate, where the window length was the ADC counts in transient and the window width was neighbor transients. Several filters with different window sizes and different falloff time ranges were used for processing (Figure 6) [30]. After using robust smoothing in a 2D sliding window (Figure 6a,b), decays noise level was reduced (Figure 6c), because of which the sounding curve had less noise (Figure 6d). 6. Integration into the sounding curve--the procedure of data integration into time bins allowed one to significantly compress the occupied volumes of information in comparison with the primary data and increase the signal-to-noise ratio. Based on the pre-calculated time mesh, the M-estimate was calculated using the Hampel function in a two-dimensional window with a window width equal to the number of transients. Figure 5: UAV-TEM raw data primary processing graph. * 7. Binding of coordinates for sounding curves. Each sounding curve was snapped to its corresponding geographic coordinates. Figure 6: Example of the result of use robust smoothing in a 2D sliding window: decays before ( a) and after (b) robust smoothing in a 2D sliding window; ( c): FFT for decays before (dark gray) and after (red) robust smoothing in a 2D sliding window; ( d) stacked sounding curve.