We had no previous experience of performing aeromagnetic survey using unmanned aerial vehicles in our company (Neryungrygeophysics LLC), therefore the first aeromagnetic exploration works were carried out by us on the previously studied object - iron ore deposit of South Yakutia. In 2015, at this site we carried out ground-based magnetic exploration, which made it possible to reliably assess the operational capability of the flight complex, comparing the existing data of ground-based magnetic exploration with the aeromagnetic exploration data.
The first flights of the “Geoscan 401” complex with a quantum magnetometer were performed in June 2017 (Fig. 1). During one flight over a network of profiles with a step of 100 m, an area of a 1 km2 was studied. Flight time was a little more than 20 minutes.
For comparison, the study of the same area by ground-based magnetic exploration methods in 2015 required more than one month of time and a labour of two teams: topographic, for the preparation of a network of profiles, and geophysical - for ground-based magnetic exploration works.
Fig. 2 shows the anomalous magnetic fields, constructed by the data of ground-based magnetic exploration and experimental flights of the “Geoscan 401” complex with a quantum magnetometer.
The analysis of the fields showed the complete identity of the obtained anomalies. Moreover, in the northeast of the site (the farthest corner of the 3D image) in 2015, a weak anomaly was identified. Drilling works performed in the area of this anomaly, did not showed any results. If we look at the anomalous magnetic field constructed from aeromagnetic survey data, we can see that the relative intensity of this anomaly has increased, in comparison with the anomaly of ground magnetic exploration. Recalculation of the vertical gradient of the magnetic field made it possible to determine that the anomaly appeared due to a blind ore body whose upper edge is located at a depth of 200-250 m from the surface.
Experimental and methodical work demonstrated one more important advantage of the flight complex: the frequency of the magnetometer readings is 1000 Hz. The sampling frequency of the built-in GPS receiver is 10 Hz. Thus, for one positioning point, there are 100 readings of the intensity of the total magnetic field vector. The manufacturer solved the problem of summation and averaging values at the point of location "on the hardware level." Such approach makes it possible to obtain a graph of the change in the magnetic field close to ideal. Let explain: potential fields are characterized by a fairly smooth change in the field values, this applies to both magnetic and gravitational fields. Previously, the decrease in the counting step for the profile was associated with a sharp increase in the cost of field work, so the most common step was 5-10 m, in special cases, in the production of micromagnetic surveys, the step was lowered to 2 m, which was associated with a significant increase in costs for topographical works.
A graphic illustration of what has been said is provided by the graph in Fig. 3. The graph shows the anomalous magnetic fields according to ground-based magnetic prospecting and aeromagnetic survey. Crosses indicate individual counts. The counts of aeromagnetic prospecting are so dense that they hardly differ even on the enlarged fragment of the graph. On the same fragment it is clearly visible how smoothly the field gradient changes on the aeromagnetic prospecting graph in comparison with the anomalous magnetic field plot according to the data of the ground-based magnetic prospecting.
When reviewing the graph, the question may arise about the absence of high-frequency anomalies on a graph constructed from aeromagnetic survey data. First, it is worth considering the nature of these anomalies. The area of work is literally covered with boulders of magnetite, these boulders determine the presence of this high-frequency noise of near-surface anomalies. Secondly, the altitude of the flight during the production of experimental and methodical work was set at 100 m (for safety reasons). If necessary, the height can be reduced, which will allow more accurate detection of anomalies of low amplitudes.
A pleasant bonus of work with the “Geoscan-401” flight system was the possibility of creating orthophotos, topographic plans and three-dimensional terrain models - depending on the necessities - subsequentially with the performance of magnetic explorations. In Fig. 4 a fragment of a dense cloud of points of the program Agisoft Photoscan Pro is depicted, which displays one of the excavations on the site of work. While it is obviously very practical - the possibility of preparing topographic plans up to a scale of 1:500, allows the contractors to have the opportunity to improve the presentability of the results of the work, which is important nowadays.
After the successful completion of experimental and methodological work, for the first time in South Yakutia (and, probably, throughout the Far Eastern Federal District), “Geoscan-401” complex with a quantum magnetometer was used at the perfomance of industrial geological explorations at the Sutama iron ore area located in the arduous area of Sakha Republic (Yakutia).
The works were carried out by the field group of LLC Neryungrygeophysics jointly with the specialists of the clients' company - geologists with a huge field work experience. The article is of a narrative nature, so let me quote one of the geologists: "For the first time in my life I envied geophysicists - instead of running with a magnetometer over the mountains sticking their tongues out, the guys in shirts, hiding from the sunlight under a canopy, were flying hundreds of kilometres"
As the results of the showed data processing, besides the obvious comfort in the production of works, aeromagnetic surveys of the “Geoscan-401” flight complex made it possible to obtain tangible practical results. The difficulty of studying the field, in addition to the inaccessibility of the site itself, was in a strong roughness of the terrain (elevations from 600 to 1400 meters) and in waterlogging of the lowland areas. The north of the site was completely swamped, and therefore the predecessors working on the site in the 70s - 80s of the last century simply could not access this part of the site.
The projected resources of the entire Sutama ore area at the time of work in 2017 were approximately 3 billion tons of ore [2]. Today, we are proud (I'm not exaggerating, this is very important for a real geologist and geophysicist), we can say - according to the results of aeromagnetic exploration works, the increase in the projected resources of the Sutama area amounted approximately 200-300 million tons - 10% of those, which were already identified earlier.
Fig. 5 shows the plan of the anomalous magnetic field of the work area. In the northwest of the site, two anomalies were identified, that were not detected by predecessors. It should be clarified again, which does it mean: "not detected by predecessors." During the production of previous works, geophysics performed only a ground-based survey, or an aeromagnetic (rarely) - but on a very rare network of profiles - because of the very high cost of production. Ground-based survey in some areas of the field was physically impossible - due to total swampiness of the site. It is because of the total obstruction of the site in the northwest of the field, that was not previously studied by survey operations.
The “Geoscan-401” complex with a quantum magnetometer made it possible to study a previously unexplored site in one day of the flights. The result of this day's work is a significant increase in the forecast resources of the huge iron ore deposit.
Long time ago, as a teenager, I imagined what would have happened if Russian soldiers had machine guns and tanks during the battle of Borodino, how the history would’ve turned if the northern peoples of the 18th and 19th centuries had vehicles and tools of the twentieth century. Last year, in 2017, I felt like a teenager - from my own experience I knew what it is like to go through 15-20 km. of magnetographic profiles per day. And the ease of obtaining information that the “Geoscan 401” complex provides - this is something from the area of machine guns being available to Russian troops in the Battle of Borodino - a fantasy, but in real life.
In conclusion of the article it is necessary to highlight one more important aspect - reproducibility of measurements, simply speaking - control. Let me quote the production report on the Sutama area: "The root-mean-square error calculated for an array of data without gradient intervals was 1.01 nT. In the authors practice this is the first case of such a low standard deviation of the measurements made in such a large volume."
The root-mean-square error for the whole array of control measurements was 2.63 nT, with an allowable 5 nT. The average value of the absolute errors was 0.022 nT, which indicates that there is no systematic measurement error. In Fig. 6 the results of the control measurements are presented graphically. Working measurements are displayed with a blue line on the central chart, control measurements are in the form of red markers on the same chart. The measurement error is so small that the difference in measurements can be seen only on the leader-enlarger. The upper graph shows the absolute error of the measurements, on the lower graph - the deviation of the control points of observation from the working points along the three axes of coordinates. According to the graph of the absolute measurement error (upper), it is very clearly visible that the maximum measurement errors are fixed in the intervals of high magnetic field gradient and are caused by errors in positioning of control measurements in relation to working measurements. Taking into account the recommendation of the "Instruction on magnetic prospecting ..." [1] to avoid performing control measurements in high-gradient zones, the data of these intervals have been removed from the control table and the mean-square error has been calculated anew.
As a result, as indicated above, the root-mean-square error calculated for a data array without gradient intervals was 1.01 nT.
At the end of the article some facts should be brought - for three months of operation of the “Geoscan 401” complex with a quantum magnetometer, three previously unknown ore bodies have been discovered in the iron ore deposits of Southern Yakutia. Two of these ore bodies can compete on the projected resources with an average field. Without the works of an aeromagnetic complex, such results would be unachievable - both from an economic point of view and physically.
Siasko A.A., mining engineer-geophysicist, Ph.D., Neryungrygeophysics LLC
Learn more about application of Geoscan technologies in mining by following the link.