## 1 Introduction
An ungrounded transmitter loop is used as a source of the electromagnetic field--a copper wire closed to the generator, laid in the form of a square (for ground surveys) or round (for aerial surveys, Figure 1) loops ranging in size from a few meters to hundreds of meters. A pulsed magnetic field is formed around the loop. According to [PERSON]'s law, a change in the magnetic field over time leads to the appearance of eddy electric currents in conductive rocks. Eddy currents in the ground propagate according to the law of diffusion. At the moment of switching off the current in the generator loop, an almost mirror-image current ring appears directly under the loop. Then, this current ring gradually begins to sink to a depth and simultaneously expand, covering an ever larger volume of rocks. As a result, eddy currents are completely attenuated due to heat losses. The position of the current ring depends on both time and conductivity. The rate of immersion and expansion of the eddy current is determined only by the electrical conductivity of rocks; the higher the electrical conductivity (lower resistance), the slower the current sinks and dissipates. This creates sounding effect. The eddy current creates a secondary magnetic field around itself, which also changes in time and space. This magnetic field creates an induction effect in the receiving magnetic antenna (or loop) on the earth's surface. The intensity of the signal in the receiving antenna and the rate of its attenuation over time depends on the distribution of electrical conductivity in the ground. Measurements are carried out using a measuring loop or frame. The curves of the transient of the secondary electromagnetic field (EMF) in the rocks below the measurement point are recorded and studied in the time domain [14]. The depth of the TEM methods depends on the current in the transmitter loop, the moment (size) of the receiving loop, the maximum recording time of the receiving signal, and ranges from 10 meters to several kilometers. The problem for TEM methods is the complexity of reconstructing precisely the first meters (or tens of meters) of the geological section, since the current in the transmitter  
Figure 1: GeoTech VTEM (**a**) and SkyTEM geophysical surveys (**b**)—examples of modern systems for airborne electromagnetic sounding. morphological situation is not very complex, otherwise their use is not economically justified. At the same time, the time for deploying an unmanned complex is usually longer than a ground one. What is the minimum distance from the take-off point is it necessary to be able to move away with UAV-electromagnetic soundings in order for such a complex to be of practical importance? With the first prototype, the issue was solved empirically. The authors began to develop and apply UAV-geophysical systems, first for scientific purposes, and then in geological exploration for a long time, when UAV systems had significantly reduced flight time in compare to modern models. The operating radius of the first versions of complexes for magnetic prospecting and gamma-spectrometry did not exceed the first hundred meters and gradually increased over the years. Systems with a range of hundreds of meters could be used only for scientific purposes, but when the operating radius reached about 1.5-1.7 km, it turned out that it was already possible to successfully solve commercial problems with reducing the cost of work by half compared to the ground-based survey. With such a radius, it is possible to obtain with each take-off point about of several square kilometers of dense survey per day. This is an excellent result in terms of performance compared to land surveys. Therefore, this minimum range of the UAV system was taken as one of the basic characteristics of the complex being created for electrical exploration. Another problem of aerial electrical exploration is the need to accurately maintain the horizontal position of the measuring loop, since its oscillations affect the measurement results of the vertical component of the EM field.