Technology of low-altitude aeromagnetic survey for prospecting for iron ores as a direction of sustainable development of modern geology

. Exploration is essential to ensure the achievement of the UN Sustainable Development Goals through the provision of ferrous and non-ferrous metal resources. At the same time, geology as an industry should be based on the principles of sustainable development. The purpose of the study is to develop and test the technology of aeromagnetic survey for prospecting for iron ores that meets the requirements of the sustainable development of modern geology. Unmanned aerial vehicles with GNSS receivers, a camera, a magnetometer, as well as Trimble Business Center, Agisoft Metashape Professional, and QM Center software were used. A search for magnetic radiation anomalies over an area of 6.20 square kilometres was carried out. Areas were established where the radiation level is 59000–65860 nanotesl (background – 57803 nanotesl), that is, there are reserves of magnetite ores. The binding of these sites to the exact geographical coordinates has been completed. The results of the low-altitude aeromagnetic survey are compared with the control data of the ground-based magnetic survey performed earlier, and the correctness of the low-altitude aeromagnetic survey data is shown. Since ground surveys provide more detailed information, it is advisable to use low-altitude aeromagnetic surveys as a method of fast, low-cost screening of large areas. This will make it possible to make decisions on the expediency of carrying out expensive ground works. The study found that labour productivity in low-altitude aeromagnetic surveys exceeds that in ground-based surveys by about 75 times. In addition, low-altitude aeromagnetic surveys have virtually no impact on ecosystems and do not lead to greenhouse gas emissions. This corresponds to the main directions of sustainable development of geology.


Introduction
Modern research in the field of ecology and environmental protection is based on the concept of sustainable development. In its most general interpretation (according to UN documents), it implies such a type of human life, including economic activity, when economic growth does not harm the environment and contributes to the resolution of social problems [1]. For sustainable development, it is necessary to achieve a balance between economic, environmental and social processes [2]. Geology and exploration of the subsoil can make a significant contribution to the achievement of the UN Sustainable Development Goals. Thus, the analysis carried out in the review by J.C. Gill, demonstrates that the 17 UN Sustainable Development Goals are directly related to 11 key aspects of geology.
In particular, such aspects of geology as "Energy" (search for raw materials needed for energy, such as iron ores for wind turbines or cadmium for photovoltaic cells), "Minerals and rocks" (using geological technologies to identify and develop natural resources ) directly affect the goals of eradicating poverty and hunger, clean energy, good jobs, economic growth, sustainable cities, responsible consumption [3]. In the development of this study, D.M. Franks, J. Keenan and D. Hailu introduced the terms mineral poverty and mineral security, which affect the possibility of achieving the Sustainable Development Goals associated with the elimination of poverty and economic growth [4]. The availability of proven mineral resources is as important to sustainable development as the availability of food. There is empirical evidence that the expansion of exploration reduces inequality, promotes inclusive growth, including because it makes it possible to maintain a constant level of natural capital stocks [5]. In modern conditions, exploration and preparation for the development of metal ore reserves is extremely important, since the successful achievement of the UN Sustainable Development Goals (as well as the implementation of the Paris Agreement) requires the introduction of energy technologies that use ferrous and nonferrous metals in large volumes [6]. This is the difference between the actively developing production of renewable "green" energy and carbon energy based on fossil fuels, causing significant environmental damage to areas of production and consumption [7].
Consequently, geology and exploration make an important contribution to sustainable development and inclusive growth by supplying the necessary resources. For example, to ensure the availability of energy and food, ferrous and non-ferrous metals are needed, which are used in various types of energy and agricultural engineering. However, activities in the field of geology must also meet the requirements of sustainable development; in particular, use a minimum of resources to obtain one unit of result. For geological exploration, it is necessary to use modern technologies in order to reduce the carbon footprint, reduce the consumption of material, financial, labour resources, as well as the impact on the environment. Traditional exploration activities most often use drilling, geochemical sampling, and various types of terrain surveys to determine signs that demonstrate the presence of commercially exploitable deposits [8]. These methods have disadvantages such as high costs, the need to carry out field work in remote areas that are difficult to access, interference with natural ecosystems, and danger to the life and health of participants in field studies (wild animals, extreme environmental conditions). In addition, there is a problem of qualification and conscientiousness of performers.
In the modern digital economy, many operations that were previously carried out by a person with limited accuracy and reliability can be performed by unmanned vehicles, a bot, and a neural network quickly, at low cost and obtaining high quality information [9]. The authors believe that part of the exploration work, in particular exploratory exploration, can be carried out using various types of aerial photography from unmanned aerial vehicles (UAVs). Of course, this cannot completely replace field work, but it will make it possible to reduce both material and labour costs and the time it takes to search for promising deposits. Existing studies note that geology is a conservative industry, where modern technologies are being introduced slowly, but there is some experience in using remote sensing of the Earth, unmanned surveys of various types to search for deposits [10]. To search for iron ores, it is advisable to use aeromagnetic surveys, since this makes it possible to determine the magnetic anomalies associated with specific deposits. The advantage of this method compared to manual magnetic field surveys is a much higher productivity and speed of surveying large areas. However, airborne magnetic surveys from a manned aircraft require large financial costs and also lead to greenhouse gas emissions. Therefore, it is advisable to carry it out using UAVs. There are a number of scientific works where methods of aeromagnetic survey from UAVs are being developed. C.A. Walter, A. Braun, G. Fotopoulos compared the results of aeromagnetic surveys from a UAV and from a helicopter at different heights when searching for gold deposits in the United States in order to verify the results. The comparison showed that the results of the survey from the UAV are quite accurate [11]. A similar approach was taken by B. Kim et al. to search for titanium and vanadium deposits in South Korea [12], J. d'Amour Uwiduhaye, J.C. Ngaruye, H. Saibi -to search for deposits of tin, tantalum and tungsten in Rwanda [13]. In a study by Y. Zheng et al. the choice of a helicopter-type UAV and a scalar magnetometer for aeromagnetic survey is justified, which ensures the highest accuracy of the results [14].
However, in the existing scientific publications, the methods of aeromagnetic survey from UAVs for the search for iron ores, which are also necessary for the development of modern energy, have not been sufficiently developed. In addition, almost all studies use airborne magnetic survey data from manned aircraft to control the results. However, according to the authors, it is more correct to verify the results by comparing them with data from a more accurate ground-based aeromagnetic survey (if they are available). Therefore, the purpose of the study is to develop and test the technology of aeromagnetic survey for the search for iron ores that meets the requirements of the sustainable development of modern geology.

Materials and methods
The study used a hardware complex, including: 1) Geoscan 201 Geodesy complex, which includes a Geoscan 201 aircraft-type UAV with a Sony RX1R II digital camera and a GNSS receiver; 2) Complex Geoscan 401. Geophysics with multi-rotor UAV Geoscan 401 and tethered quantum magnetometer Geoshark; UAV DJI Matrice 300 RTK multi-rotor type, which was used with the quantum magnetometer Geoshark after the failure of the UAV Geoscan 401.The Trimble Business Center software was used to process the results of satellite geodetic measurements. Photogrammetric data processing and construction of a digital terrain model were carried out in the Agisoft Metashape Professional program. Tracks of magnetic field measurements were built in the QM Center program. Plannedaltitude substantiation of UAV flights, aerial photography of the area were carried out in accordance with the national standard of Russia "Shooting aerial topographic. Technical requirements". The low-altitude aeromagnetic survey was carried out in accordance with the provisions of the Russian regulatory document "Methodological recommendations for performing low-altitude aeromagnetic survey". The distance between the profiles when performing a low-altitude aeromagnetic survey was no more than 50 meters, and the distance between the measurement points was no more than 5 meters. In accordance with the limits of permissible distances between observations points on the working profiles specified in the methodological recommendations, the low-altitude aeromagnetic survey being performed refers to a survey at a scale of 1:5000.
Also, before each day of work (within 7 days), magnetic-variation observations were carried out to control magnetic storms, which may affect the measurement results. Magnetic storms were not detected. The study was carried out according to the following scheme: conducting aerial photography to build a relief model of the work site; conducting a low-altitude aeromagnetic survey; data processing and interpretation of the study results; comparison of the obtained images of the magnetic field with the results of the ground magnetic survey carried out earlier. In order to preserve commercial secrets, the authors cannot disclose the area and geographical coordinates of the study, however, it should be noted that it is distinguished by difficult conditions for geological exploration. This territory has a mountainous relief (heights up to 1000 meters above sea level with a slope of up to 60 degrees), extreme continental climate of the Far North, covered with dense dark coniferous taiga. There are dangerous animals in the area, in particular, bears and moose. There is no road network. All this complicates the performance of geological exploration and requires the use of modern technologies. However, in the study area there are significant reserves of mineral resources, in particular, iron ores, which have not yet been sufficiently studied. The study was originally intended to cover an area of 15.37 square kilometres. However, due to technical reasons, only aerial photography was completed in full. The aeromagnetic survey was carried out only on an area of 6.20 square kilometres, because the Geoscan 401 UAV broke down. Next, the DJI Matrice 300 RTK UAV was used, which showed the worst performance. Although its manufacturer claims comparable flight performance, but in the mountains, as the study showed, the length of one route before recharging the battery is reduced by a factor of three. This is due to the fact that the software in conditions of altitude differences forcibly reduces the flight speed to the minimum values. In addition, at points where the flight altitude was below the starting point, the flight task was completely stopped. Therefore, it was possible to conduct an aeromagnetic survey only on an area of 6.2 square kilometres (about 40% of the planned scope of work).

Results
At the first stage of the study, aerial photography was carried out to build a digital terrain model, on which the results of aeromagnetic survey would be placed. For this, the original raw digital materials of aerial photography, elements of the internal orientation of photographs, exact photographing centres, materials for controlling the accuracy of the digital terrain model were used. In the course of building a digital model, orthocorrection of aerial photographs was carried out, a sparse point digital terrain model was created, and its construction and position were controlled using control signs. Then a dense point cloud was formed, according to which a digital terrain model with a spatial resolution of 36.8 cm/px and a digital orthomosaic with a spatial resolution of 9.2 cm/px (both objects are presented in GeoTIFF format) were obtained. The digital terrain model is shown in Figure 1.  During the low-altitude aeromagnetic survey, 35 flights were performed (7 flights using Geoscan 401 and 28 flights using DJI Matrice 300 RTK). Aeromagnetic survey data were processed in a certain sequence. Initially, the results of measurements of the magnetic field (*.mgt format) and geodetic coordinates (from a GNSS receiver built into the magnetometer, *.ubx format) were imported into the QM Center program. Then, georeferenced measurements (tracks of magnetic field measurements) were generated.
After that, duplicate measurements were removed from the tracks, which occur during the take-off of the UAV, preparation for landing, and when the UAV returns to the launch site. As a result, using an orthomosaic, a visualization of low-altitude aeromagnetic survey data was obtained (Figure 2). The color legend allows you to determine that in the center of the study area there is a place with the highest intensity of the magnetic field, which indicates the presence of iron ore reserves (the background value of the magnetic field is 57803.23 nanotesla, and the values observed in the central part of the image exceed 59029.00 nanotesla). Further, in the study, the results of the low-altitude aeromagnetic survey from the UAV were verified based on their comparison with ground-based magnetic survey data. The authors used a report on the results of the work of a geological team in the same area in the 1960s. In this report there is a map with the results of a ground-based aeromagnetic survey at a scale of 1:25000. The map was digitized and matched with the model obtained in this study. On the old map, magnetic anomalies are reflected using the isolines of the vertical component of the magnetic field induction. As a result, it was found that the results of ground-based magnetic surveys and low-altitude aeromagnetic surveys are in good agreement with each other (Figure 3). Figure 3 shows the identified in the 1960s in the process of land surveying, magnetic field anomalies indicating the presence of magnetite ores. They are marked with solid lines. These lines coincide with the magnetic field anomalies found in this study from the results of the UAV survey (more intense blue color, which visualizes a higher intensity of magnetic radiation in nanoteslas). Thus, low-altitude aeromagnetic surveys give practically the same results as ground-based surveys, but they use much fewer resources and practically do not affect the ecosystem. At the same time, the degree of detail of the results for ground-based magnetic surveys is still somewhat higher. So, in the data of the 1960s the range of values of the vertical component of the magnetic field was up to 30,000 gamma (this off-system unit was used, 1 gamma is equal to 1 nanotesla). In our study, as can be seen from Figure 3, this figure is about 12,000 nanotesla. However, this degree of accuracy still makes it possible to detect magnetic anomalies that require further analysis. Fig. 3. Comparison of the results of ground magnetic survey and low-altitude aeromagnetic survey: 1 -tectonic disturbances, assumed according to the data of ground magnetic survey; 2 -axes of magnetite ore seams with a higher degree of confidence assumed from ground magnetic survey data; 3 -axes of magnetite ore seams with a lesser degree of certainty, assumed from the data of a ground magnetic survey Since the prices of the 1960s and 2020s It is practically impossible to compare directly, let's focus on labor and time costs. Ground magnetic survey work was completed in 207 days by a geological party consisting of two teams. The exact number of participants in the Coloring intensity scale (in nanoteslas) survey is not known, but the minimum size of the detachment is usually 10-15 people (it includes several specialist geologists, workers). 175.8 square kilometers of terrain were surveyed. Office data processing took about 8 more months; in general, the work took about 450 days. Low-altitude aeromagnetic survey was carried out by 2 specialists for 7 days; therefore, this method can significantly reduce costs, as well as eliminate the physical passage of routes, interference with the environment. In addition, the possibility of meeting people shooting with wild animals is almost eliminated. The productivity of the Geoscan 401 Geophysics complex under normal weather conditions is about 3 square kilometers per day. It takes about 2 hours to process one day's survey data. Consequently, 2 specialists (instead of 20 people in 2 geological teams) could survey 175.8 square kilometers in 60 days (data processing is possible at night, in bad weather conditions). Therefore, a lowaltitude aeromagnetic survey can be performed 7.5 times faster. If we take into account the difference in the number of participants in the work, labor productivity during aeromagnetic survey is at least 75 times higher. Thus, the proposed method of searching for iron ore deposits is consistent with the logic of sustainable development, as it makes it possible to quickly discover the resources necessary for energy, agriculture, reduces financial costs, and also has less impact on the environment in the areas of exploration.

Discussion
The conducted study demonstrates the potential of unmanned low-altitude aeromagnetic survey for the search for iron ore deposits, and also verifies its results according to the data of ground-based aeromagnetic survey performed earlier. It is shown that magnetometric data obtained from UAV surveys make it possible to obtain a model of a magnetite-bearing stratum. With appropriate survey discreteness, individual layers can also be identified. In contrast to existing studies on the search for deposits and deposits of non-ferrous, rare, precious metals [11][12][13][14], the authors considered the problem of searching for iron (magnetite) ores, which are important for modern sustainable development (raw materials for the production of agricultural machinery, energy equipment). Our study makes a certain contribution to the literature on the use of unmanned aeromagnetic surveys as a method of automation and digitalization of geological exploration. Increasing the sustainability of development, achieving the UN Sustainable Development Goals is facilitated not only by the lower cost of work compared to ground surveys or the use of manned aircraft, but also by reducing the direct impact on the environment in the process of geological exploration. UAVs have a minimum noise level; their flights are not accompanied by greenhouse gas emissions. With aeromagnetic surveying, the need to pass routes on the ground is significantly reduced, which also contributes to the preservation of the ecosystem. Based on the results of aeromagnetic survey, many areas will be recognized as unpromising, and when using UAVs, they will hardly be subjected to technogenic impact.
Also during the study, the authors came to the conclusion that it is necessary to more flexibly select flight modes and unmanned systems, depending on specific conditions, to obtain the best result. Existing studies [11][12][13][14] recommend an aeromagnetic survey height of 50 meters, while 30 meters is the distance from the magnetometer to the terrain. However, in our study, we came to the conclusion that in conditions of complex terrain and dense forest vegetation, it is advisable to increase these values to 70 and 50 meters, respectively. Of course, low-altitude aeromagnetic survey cannot completely replace geological exploration work on the ground, since its data are limited in terms of detail. When conducting a ground-based magnetic survey, magnetic field anomalies can be determined more accurately. This determines the scope of the developed methodologymapping the area containing iron ores, to make a decision on the implementation of further field exploration work (well drilling, sampling, ground aeromagnetic survey). The level of detail of the low-altitude aeromagnetic survey is quite sufficient to exclude unpromising subsoil areas from further analysis. Moreover, in this case, there is practically no negative impact on the environment, in particular, on the forest and animals.

Conclusions
As a result of the study, it is substantiated that modern geology contributes to sustainable development in two directions: providing resources for economic development, including metals necessary for energy, agricultural engineering, as well as reducing the cost of resources for geological exploration, reducing environmental damage. In this context, the authors have developed and tested a methodology for prospecting for iron ores based on low-altitude aeromagnetic survey, which corresponds to the trends of sustainability and digitalization of the economy. In the course of the study, a high-altitude terrain model and an orthophotomap were developed, on which the results (tracks) of a low-altitude aeromagnetic survey were then placed. This made it possible to obtain a digital twin, where a higher intensity of magnetic radiation shows the place where the deposits of magnetite ores are located. With a background value of 57803 nanotesla, the radiation level in the corresponding areas is 59000 -65860 nanotesla. The exact location of areas of more intense magnetic radiation makes it possible to issue specific coordinates for field geological work. The quality and accuracy of the low-altitude aeromagnetic survey results were controlled by comparison with the results of an accurate ground-based survey conducted in the middle of the 20th century in the same area. The revealed anomalies of magnetic radiation completely correspond to each other. At the same time, labor productivity in aeromagnetic surveys is at least ten times higher. However, ground-based imaging identifies anomalies in more detail. Therefore, it is expedient to use low-altitude aeromagnetic surveys for rapid screening of large areas and for making decisions on the feasibility of expensive ground-based surveys. This method also provides the opportunity not only to drastically reduce costs, but also to minimize the negative impact on unaffected ecosystems where the presence of ores is screened.