Institute of Petroleum-Gas Geology and Geophysics, SB RAS, Innovations Department, Firsov A.P. Some results of using High-Frequency Magnetometric Probe (HFMP) mounted on light UAV in Geology

Technical characteristics of magnetometric channel Sensitivity - no less than:±0.07 nT Absolute accuracy:±1 nT Dynamic range of measurement: … nT Sampling frequency: 1500 Hz Advantages: This system improves the performance of the magnetometric survey, reducing its cost and allows to scan at almost any scale in hard conditions and in the absence of any infrastructure.

Studied locations and structures In 2014 we conducted first operations using the developed complex: – in archaeological work in Vengerovsky District of Novosibirsk Region; – in geological work at several sites in the Republic of Khakassia («Samson», «Alekseevskoe», «Erbinsky» neck, «Sohochulskoe» oil field).

Archeological monument «Kurgan», NR Img. 1: A - satellite image of archaeological site "Kurgan", Novosibirsk region, Vengerovsky District. The blue line represents the flight path of unmanned aerial vehicle (UAV). B - magnetometric survey profiles at the archaeological "Kurgan". The red line denotes the magnitude of the modulus of the induction of the geomagnetic field during ground survey with proton magnetometer at a height of ≈1.5 meter; blue dots - during surveying with magnetometric complex mounted on UAV, with a series of flights at altitudes ranging from 1.5 to 3 meters. Decrease in the amplitude of the anomalous field recorded with the UAV is due to the increase in distance between the magnetometer sensor and anomaly sources. The vertical scale shows the relative values of the geomagnetic field. АВ

«Alekseevskoe» Cu-Mo deposit, Republic of Khakassia Img. 2. Maps of the anomalous magnetic field dTa (nT) for Alekseevskoe region skarn deposit, Republic of Khakassia: A - according to the ground survey (the height of the sensor m) using a proton magnetometer POS- 1; B - according to magnetometric survey complex mounted on the UAV at altitudes of 2 to 5 meters. The vertical scale shows the relative values of the geomagnetic field. AB

«Erbinsky» neck of alkaline ultrabasic rocks

«Erbinsky» neck, main line Proton magnetometer High-frequency magnetometer

Erbinsky neck, main line, comparison of data: proton magnetometer - HFM (smoothing 1:64)

Pro's And Con's Very light weight, allows the use of light class UAVs – obvious plus. Very high frequency survey – plus. The impressive density of the data obtained, which many consider excessive - plus? The possibility of surveying at different altitudes, which opens new possibilities for interpretation – plus? Not yet implemented. Insufficient accuracy of GPS location - minus. No system for data visualization - minus? Less sensitive than quantum and proton magnetometers – minus? No implemented ability of vector analysis for the magnetic field (most other types of modern magnetometers don't possess such ability even in theory) - minus. Can we make it a plus?

Compared to taking measurements on foot, surveying with the help of the UAV is much more efficient. The efficiency of using UAV increases with scale, increasing also the measurement accuracy. In general, productivity of the magnetic survey increased by an order of magnitude. When surveying Erbinsky neck with double passes with the HFMP on the same route, the performance was 11 times higher than the survey with a proton magnetometer. All this unfortunately relies on the reliability of mass- produced small UAVs.

Data redundancy vs. accuracy of the instrument If we want to know the geological structure of the object based on the knowledge of the magnetic field, a step in the measurement of the profile must be an order of magnitude smaller than the size of the object. This is especially important in mapping linear features (dikes, veins, stratal bodies). An actual magnetic field and one based on 8 points measured by an accurate magnetometer Comparison of an actual magnetic field and one observed with less precise magnetometer (100 points)

Data redundancy vs. accuracy of the instrument For the vast majority of cases, in geological & even archaeological research, the density of the observation data is more «important» than accuracy. The specific ratio of importance for these parameters is not yet clear. It is clear that for fixed-position stations monitoring the accurate change in magnetic field is more important. While in mapping magnetic field of a complex object the importance of such parameter as frequency of observations is currently strongly underestimated. Everyone wants to purchase a device with pT measurement accuracy, but the device with high-frequency nT accuracy can give much more accurate picture of the actual magnetic field structure.

Map of magnetic field for Erbinsky neck (standard treatment) Proton magnetometer High-frequency magnetometer

Map of magnetic field for «Alekseevskoe» deposit field (standard treatment) The whole field Detailed site

Map of magnetic field for «Alekseevskoe» deposit field (standard treatment) The whole fieldDetailed site

Questions of interpretation When presenting the magnetic field, based on the data aquired with high-frequency magnetometer, problems arise in interpolation between the profiles. This is due to the fact that ratio, of the distance between the points on the profile and the distance between the profiles, is greater than 1: Structure, extrapolated by standard methods gives "hummocky" structure of the field. And it is imperative to be able to reflect the linear inner structures, which usually are essential for understanding the geological structures.