Summary Nowadays magnetic and gamma surveys using UAV technologies are widely used in the practice of geological exploration. However, until now electromagnetic sounding in the UAV version is practically not used, but is very promising. In this paper, we consider the results of testing the UAV-TDEM technology. This technology is a time-domain transient electromagnetic sounding on a lightweight UAV. A measuring system with an inductive sensor (receiving loop) is towed by a UAV, and a galvanically grounded power transmitter is on the ground and connected to a pulse generator. The testing place was not chosen by chance: Baikal Lake is a unique geological formation with a rather simple one-dimensional geoelectric model at first glance: a fresh water layer with a resistivity of about 200 Ohm m, under which there are low-resistivity bottom sediments. 3D modeling, performed taking into account bathymetry, has shown that under a water layer with a thickness of 200–300 m a high-resistance objects are found in conductive sediments to a depth of at least 200 m. Some of them are extensions of coastal geological structures. As a result, the depth of the UAV-TDEM technology in these geoelectric conditions is estimated of about 500 m.

Summary In 2012-2015, on the Norwegian shelf, a marine electromagnetic sounding surveys were carried out using the differential-normalized electro prospecting method (DNME) with length of several thousand kilometers. The effectiveness of the method is confirmed in reference fields such as Troll. More than 20 wells were drilled, a significant part of forecasts was confirmed. However, new deposits were later discovered at the survey sites, which anomalies had not previously distinguished according to the inversion data. The fact of lack of anomalous effects made us look at the results of the shooting of those years in a new way. For one of the survey lines crossing the reference field, the following results are presented: a multivariate statistical analysis applied to the reprocessed data; one-dimensional inversion of transients simultaneously with the apparent resistivity curve with a more detailed step; inversion within the 2.5D model. The research carried out shows that at the first stage, it is advisable to highlight the area of most significant interest based on the result of multivariate statistical analysis. The results of the multivariate statistical analysis do not contradict the results of 2D modeling.

Nowadays in solving geological problems, the technologies of UAV-geophysics, primarily magnetic and gamma surveys, are being increasingly used. However, for the formation of the classical triad of airborne geophysics methods in the UAV version, there was not enough technology for UAV-electromagnetic sounding, which would allow studying the geological environment at depths of tens and hundreds of meters with high detail. This article describes apparently the first technology of UAV-electromagnetic sounding in the time domain (TDEM, TEM), implemented as an unmanned system based on a light multi-rotor UAV. A measuring system with an inductive sensor—an analogue of a 20 × 20 or 50 × 50 m receiving loop is towed by a UAV, and a galvanically grounded power transmitter is on the ground and connected to a pulse generator. The survey is carried out along a network of parallel lines at low altitude with a terrain draping at a speed of 7–8 m/s, the maximum distance of the UAV’s departure from the transmitter line can reach several kilometers, thus the created technology is optimal for performing detailed areal electromagnetic soundings in areas of several square kilometers. The results of the use of the unmanned system (UAS) in real conditions of the mountainous regions of Eastern Siberia are presented. Based on the obtained data, the sensitivity of the system was simulated and it was shown that the developed technology allows one to collect informative data and create geophysical sections and maps of electrical resistivity in various geological situations. According to the authors, the emergence of UAV-TEM systems in the near future will significantly affect the practice of geophysical work, as it was earlier with UAV-magnetic prospecting and gamma-ray survey.

Summary In 2012–2015, on the Norwegian shelf, a marine electromagnetic sounding surveys was carried out using the differential-normalized electro prospecting method (DNME) with length of several thousand kilometers. The effectiveness of the method confirmed in reference fields such as Troll. More than 20 wells were drilled, a significant part of forecasts was confirmed. However, new deposits were later discovered at the survey sites, which had not previously been distinguished by anomalies according to the inversion data. This made us look at the results of the shooting of those years in a new way. For one of the survey lines crossing the reference field, the following results are presented: multivariate statistical analysis applied to the reprocessed data; one-dimensional inversion of transients simultaneously with the apparent resistivity curve with more detailed step; inversion within the 2.5D model. The research carried out shows that at the first stage it is advisable to highlight the area of greatest interest based on the result of multivariate statistical analysis. At the second stage,1D and 2.5D inversions are performed within the constricted model with chargeability. 1D inversion allows to detect deposits with high resistivity at a depth of 1200 m, while 2.5D inversion up to 2500–3000 m

Summary To create a complete set of UAV-based aerogeophysical methods, two lightweight UAV-electromagnetic systems were developed. The first one is designed to first-stage geophysical mapping over large areas; it can be performed simultaneously with magnetic and gamma surveys on one UAV. In this case, the receiver with mutually orthogonal ferrite antennas provides measurement of the vertical and horizontal components of the electromagnetic field in the audio frequency range (10 - 48 kHz), at the far field region of the electromagnetic field of navigation radio stations. The second variant is the UAV-TEM, where the time-domain transmitter located on the ground and connected to a galvanically grounded line several kilometers long, and the receiver with coil is towed by the multirotor UAV. From one line placement, it is possible to perform surveys in areas up to several square kilometers. This variant is optimal for detailed electromagnetic soundings in local sites identified at the previous stage of geophysical mapping, it provides the possibility of data inversion to a depth of 600 meters. Together with UAV-magnetic prospecting and gamma-spectrometry, the created systems provides a solution to a wide range of geological tasks in any complexity of landscape conditions.

Summary The transition zone is a zone of shallow water immediately adjacent to the coastline of offshore areas, where, due to the shallow depth of the sea (less than 10–15 m.) and marine geophysics are ruther complicated; at the same time, the useage of ground-based start-stop EM sounding is impossible because of the deep water (more than 2 m). In this paper, we will consider some technological aspects of start-stop measurements in the transit zone on the example of test work in the shallow waters of the Black Sea.

Summary To reduce risks in the construction and operation of technical facilities in the shelf zones, a multichannel electromagnetic sounding system was developed for engineering surveys. The developed system has passed two-stage testing on the Black Sea. The first stage showed the sensitivity of the system to local anomalies with low values of resistivity, and the second stage showed the possibility of registering structural heterogeneities of the seabed.