## 4 Survey Technique and Data Preprocessing
The closest to the transmitter survey line is 30 m away from it. The survey line scheme is shown in Figure 8.  
The survey was completed in fo4 ur hours, excluding the time required to deploy the transmitter. The total volume of the survey was over 20 km, taking into account the control and orthogonal lines. The curves of transient processes (sounding curves) were reduced to a grid of 15 \(\times\) 40 m, that is, at a flight speed of 7.5 m/s, only two actual records of the transient curves were averaged per one point of the final vector layer. However, it turned out that due to the high stability of the platform with an induction sensor (when the UAV had already reached cruising speed), the curves of ordinary and control observations had a fairly good convergence at times up to 100-130 \(\upmu\)s (Figure 8b), despite the fact that they were represented by only a few units of points (due to the low sampling rate of the ADC). Thus, the achieved sounding depth in this case was about 100-120 m. The measurement data obtained at the time of acceleration and deceleration of the UAV, or after its turn on the survey line, mostly turned out to be substandard, since it was difficult for the UAV to quickly stabilize a load with a floating center of gravity, comparable to its weight. Areas with substandard data were excluded from processing. They included the central part of the section, where the UAV flew in and out on missions, and the three nearest to the transmitter survey lines (No.1,2,3), where the influence of the transmitter was too strong. On survey line 4, the the quality of the data was the worst found acceptable for processing. (Figure 8a and Figure 10a). Thus, it turned out that the closest survey line must not be  
Figure 8: (**a**) Location of survey lines and transmitter line on the area 1—transmitter line, 2—contour of areas within which it was possible to obtain conditional measurements; (**b**) convergence of ordinary and control sounding curves. closer than 70-80 m to the transmitter line. In fact, only five of eight survey lines turned out to be suitable for processing (Figure 8, Figure 10 and Figure 11). An important parameter of the effectiveness of the developed UAV-TEM technology is the maximum allowable distance to which the UAV can move away from the transmitter line, since the survey area that can be performed using one take-off point depends on this parameter. The orthogonal lines were about 600 and 800 m long. Figure 9 shows the measured sounding curves for points at their ends, as well as a graph of signal changes at time delays of 0.03 and 0.04 milliseconds. It is shown that the level of the recorded signal at the extreme points was not less than 0.005 mV/A, it was possible to invert obtained curves with sufficient accuracy. The obtained results proved that at a distance of at least half the length of the transmitter line, suitable signal was registered for processing, even in case of low current in the transmitter. Therefore, the developed complex allowed us to perform a survey on a rectangular area with a short side length slightly less than the transmitter line length, and a long side length not less than the short side length. For simplicity, let us assume that this is a square, since it was not established at what distance the signal will be lost due to the insufficient length of the orthogonal survey lines. We should have made them longer, but authors did not think that with the current strength achieved, it was possible to obtain high-quality data at a distance of more than 500-600 m, the result obtained turned out to be better than expected. Thus, for example, with a transmitter length of 2 km, a survey could be performed on a square area of 1.8 by 1.8 km (3.2 km\({}^{2}\)), and with a length of 4 km--14.4 km\({}^{2}\). An acquisition made a little before reaching the ends of the transmitter line, since in areas near the ground there were strong effects from field inhomogeneity.