Unmanned systems provide significant prospects for improving the efficiency of electromagnetic geophysical exploration in mineral prospecting and geological mapping, as they can significantly increase the productivity of field surveys by accelerating the movement of the measuring system along the site, as well as minimizing problems in cases where the pedestrian walkability of the site is a challenge. Lightweight and cheap UAV systems with a take-off weight in the low tens of kilograms are unable to carry a powerful current source; therefore, semi-airborne systems with a ground transmitter (an ungrounded loop or grounded at the ends of the line) and a measuring system towed on a UAV are becoming more and more widespread. This paper presents the results for a new generation of semi-airborne technology SibGIS UAV-TEMs belonging to the “line-loop” type and capable of realizing the transient/time-domain (TEM) electromagnetics method used for studying a uranium object of the paleovalley type. Objects of this type are characterized by a low resistivity of the ore zone located in relatively high-resistivity host rocks and, from the position of the geoelectric structure, can be considered a good benchmark for assessing the capabilities of different electrical exploration technologies in general. The aeromobile part of the geophysical system created is implemented on the basis of a hexacopter carrying a measuring system with an inductive sensor, an analog of a 50 × 50 m loop, an 18-bit ADC with satellite synchronization, and a transmitter. The ground part consists of a galvanically grounded supply line and a current source with a transmitter creating multipolar pulses of quasi-DC current in the line. The survey is carried out with a terrain drape based on a satellite digital terrain model. The article presents the results obtained from the electromagnetic soundings in comparison with the reference (drilled) profile, convincingly proving the high efficiency of UAV-TEM. This approach to pre-processing UAV–electrospecting data is described with the aim of improving data quality by taking into account the movement and swaying of the measuring system’s sensor. On the basis of the real data obtained, the sensitivity of the created semi-airborne system was modeled by solving a direct problem in the class of 3D models, which allowed us to evaluate the effectiveness of the method in relation to other geological cases.

The article presents the results of transient electromagnetic (TEM) prospecting surveys using an unmanned aerial system carried out at Lake Baikal, which is a unique geoelectrical setting where low-resistivity lacustrine sediments are located under a relatively isotropic water body. The task was to investigate the possibility of using a drone-based TEM survey to delineate the electrical stratigraphy of the subsurface at depths between 50 and 300 m, separated into layers and blocks. A new version of the SibGIS UAV-TEM unmanned system was used, significantly improved compared to the prototype previously described in the literature. The current switch providing bipolar current pulses connected to a grounded electrical line was the source of the electromagnetic field in the geological environment. The hexacopter carrying a measuring system consisting of 18-bit ADC and sensor—analog of 50 × 50 loop, was the receiving system. We measured survey data of 16 traverses over the Baikal going from the shore to the depths. Significant attention is being paid to a new approach to data inversion. For fast interpretation of the TEM data, we used the Sτ-method, which allows for tracing the change in the apparent longitudinal conductivity with depth. It is shown that thanks to the new sensor and current switch, the data quality has increased significantly; now, the UAV system can register sounding curves up to 1 ms. As a result, new data on the geological structure of the shelf zone of Lake Baikal were obtained. They had a good fundamental agreement with the predecessor data obtained from terrestrial measurements (from ice cover), allowing us to conclude that the UAV-TEM technology can already replace conventional ground-based electromagnetic surveys.

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 The abstract presents the results of the experimental work of the TEM with the use of drones, carried out at the Bolshoe Goloustnoye site on Lake Baikal. Baikal is a unique geoelectric polygon, where low-resistivity lacustrine sediments are located under a relatively isotropic water body. The upper part of the sedimentary sequence is represented by diatomaceous silt with high conductivity, the lower part is represented by denser clayey sediments. High-resistivity inclusions are found in both layers: gas hydrate plates and methane deposits. The fused instrumentation complex consist of: EGI-500 - generator of bipolar current pulses, Mars 4.0 which is a multi-channel electrical prospecting potential difference recorder, PDI-50 – a reciever loop, SibGIS UAS - unmanned 6-rotor aircraft with a payload up to 10 kg. The16 profiles were carried out. Six profiles were located parallel to the electrical line AB, nine are perpendicular to AB. The interpretation of the TEM data was interpreted using version, which will allow one to trace the change in the apparent longitudinal conductivity with depth. On the sections, a high-resistivity layer is clearly distinguished, corresponding to the water of Lake Baikal and sedimentary deposits, composed by several blocks of low resistivity.

Summary In order to successfully prospect for new deposits, in most cases it is necessary to use a set of geophysical methods that allow both studying the surface, and creating models of the deep structure of the near surface part of the earth’s crust. The classic universal combination is the «triad» of methods “magnetic prospecting+gamma-spectrometry+electromagnetic prospecting”, this option is used in most geophysical systems based on manned helicopters. In recent years, magnetic prospecting and gamma survey methods have already been widely used in the UAV version, so UAV-based electromagnetic soundings are of maximum interest at this stage of geophysics. In this paper, we consider a case of combining all three UAV-based methods in the case of prospecting for blind deposits of sandstone-type uranium, with special attention being paid to the use of the current version of SibGIS UAV-TEM system for time-domain electromagnetic sounding, which is significantly improved compared to the first prototypes. According to the authors, this case proves that unmanned systems, which allow performing geophysical surveys with a set of methods at low altitudes and with precise terrain drape, make it possible to almost completely abandon ground-based surveys, at present - and in the case of transient electrical prospecting too.

Summary A modified cultural algorithm (MCA) implementation for the three-dimensional (3D) magnetotelluric (MT) inverse problem is presented. MCA uses several parameters for belief space forming. Information about number of applicants, number of parents and mutation probability is used to form belief space together with min/max interval for mutation of genes. The COPROD2 data set (Canada) is used as an MT benchmark of real field data.

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.

Summary To create a complete set of UAV-based aerogeophysical methods, two lightweight UAV-electromagnetic systems were developed. The first one (VLF-type) 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 more than 300 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.

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 Geoelectric techniques are applied to identify geobodies in the shallow subsurface (<1km) that correspond to commercial ore deposits (copper-molybdene) in Kazakhstan. A combined CSEM and Induced Polarisation method is chosen to delineate anomalies in the underground. Resistivity and polarisation effects prove diagnostic. The workflow comprises steps like: EM acquisition, quality control and data preconditioning, inversion, interpretation and Principle Component Analysis. Inversion processing is done via a finite elements method solving the Cole-Cole formula simulating Maxwell’s equations. 1D inversion results serve as input for the 3D inversion. Principle Component Analysis (n-dimensional clustering and distance weighting) and computation of composite geoelectric parameters enhance the discrimination power. EM anomalies are circular (hydrothermal injection feature) and/or elongate in shape. Fracture zones and faults provide conduits/barriers and govern hydrothermal processes. Faulting in part controls the outline of the segmented IP anomalies. Three shallow well locations were proposed based on the EMS-IP data. Two of these boreholes demonstrate elevated polarisation phenomena: copper-molybdene metal ore in MN17 and pyrite enrichment in MN16. The mapped geobodies based on EM anomalies give complementary information on volume and distribution of the mineral resources. EMS-IP is a cost-effective investigation tool that deserves more attention in geoscience projects.

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 Stochastic algorithms belong to methods of global search which are able to find global minimum of discrepancy function in the presence of multiple local minima. Modified cultural algorithm is implemented for 3D inversion of EM sounding data. The algorithm is tested on data of electromagnetic investigations of detecting the gold mineralization zones. The algorithm uses information about the number of candidates for the role of a parent, the number of parents and the probability of mutation as a belief space together with information about global minimum area. Starting from a homogeneous half-space and using no priori information, results obtained by modified cultural algorithm are in good agreement with the data of the 2D inversion.

Summary Detection of brine-filled undersaturated zones in the subsurface is relevant for mitigation of operational and environmental risks in salt mines. Early delineation of salt dissolvement may avoid unwanted flooding of mines and penetration of meteoric waters along various pathways. Water-filled zones are characterized by a lower electrical resistivity (Ohmm) and therefore electromagnetic sounding methods are very promising. The resistivity structure of the subsoil is hereby evaluated and identification of potentially hazardous zones with low resistivity around the salt stock is made possible. Electromagnetic sounding and induced polarization (EMS-IP) technology was successfully tested at the Verkhne-Kamsky salt mine to provide an effective EM workflow. The method consists of electric stimulation at the surface with a grounded dipole setup and registration of the earth response (electric potential difference) with a receiver electrode array. After a quality control and validation step, the measured data is inverted for resistivity, initially with a simple 1D approach. As next step the outcome, constrained by calibration data, serves as starting model in a more sophisticated 3D inversion procedure. The article has formed some methodological aspects of planning and conducting work, as well as analyzing and processing pulsed electrical prospecting data.

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 Ore body’s profile was measured with small-step system in the Turavity area (Altai Republic) to research the near-surface occurrences in-situ with electromagnetic sounding and induced polarization technology (EMS-IP) in October 2016. This profile located in combined delve (bulldozer and manual) with outcrop ore body. Earlier this profile had measured with geochemical testing. Zones of high and low polarizability were identified as a result of geophysical measurements with grounding into the ore body. These zones correlate with high copper and silver content’s zones which were obtained by geochemical testing. In results of the research we can conclude that this measurement technique is a good addition to the areal surveys modeling.

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.

Summary Kimberlite pipe exploration overlapped by loose terrigenous-sedimentary rocks and traprock in the Yakutsk diamondiferous province, were and remain one of the most pressing and difficult tasks for geologists and geophysicists. Overlying deposits are usually highly variable physical properties, which the layer thickness of tens meters, blunt the effectiveness of traditional land-based geophysical methods. Exploratory drilling on a cellular mesh is come at a price but does not always solve the task of kimberlite pipe exploration. In connection with the above, special relevance takes use of new geophysical technologies to detection zones of kimberlite manifestations. Electromagnetic sounding and induced polarization technology (EMS-IP), by means of registration of the full form pulse ( Davidenko et al 2012 ) allows you to select areas with changes in physical properties (dedicated to the manifestation of kimberlite magmatism) on the background of the enclosing rocks to a depth of 1 km even in high-resistivity medium. In the period June-July 2017 in the Sakha (Yakutia) Republic, was performed three-dimensional survey on standard zone Baytakhsky by the of EMS-IP method. Mission was to test the technology of EMS-IP to identify, delineate and localize promising areas with kimberlite-containing diatreme within the scope of measurements.

Summary Currently, more than 65% of world reserves of copper and about 60% of its world production, as well as more than 60% of molybdenum reserves and about 70% of its production occur in so-called copper-molybdenum porphyry deposits of hydatogenesis. These deposits are characterized by large accumulations of milling-grade ores, represented by systems of fine ore veinlets and small impregnations of sulfide ore minerals, among which chalcopyrite and molybdenite are widespread. The largest industrial deposits of this type are in Kazakhstan. The paper provides an assessment of the sensitivity of measured transients to changes in electrical resistivity and polarization, which makes it possible to separately determine the distribution of these parameters in the medium, as well as a comparison of the results of the inversion of pulsed electrical survey data with the geology of the area under study.

Summary In geological prospecting, searching for hydrocarbons has been currently a truly promising trend in geophysical activities. The application of pulse electric exploration coupled with seismic survey, while searching for hydrocarbon fields, essentially (up to 80% or more) reduces the expenses for exploration drilling. The induced polarization (IP) anomalies might originate due to the epigenetic pyrite formed above the deposit within the geochemical barrier hampering atmospheric oxygen penetration. However, the IP anomalies could also be caused by sulfide presence in trap intrusions widespread over the Siberian platform. The paper reports comparison of results on inversion of data on pulse electric exploration with survey of polarization characteristics of core collected within a hydrocarbon deposit in the zone of trap magmatism development.