Study of the technogenesis of the Degtyarsky mine by audio-magnetotelluric express sounding

The audio-magnetotelluric express sounding was performed at four sections crossing the mine field of the currently not functioning Degtyarsky mine. Field measurements were carried out by a universal broadband receiver “OMAR-2m” with active electromagnetic field sensors developed at the Institute of Geophysics UB RAS. Based on the obtained data, deep sections of the electrophysical parameters of the medium – apparent resistivity and effective longitudinal conductivity – are drawn. The nature of the geoelectric structure of the section allows mapping of the major lithochemical contamination plume and identifying the tectonic disturbance zones that drain aggressive mine waters. The mine waters of the Degtyarsky mine are a source of dangerous technogenic pollution. Despite the neutralization of surface runoff, underground routes of acidic water migration occur along tectonic cracks, primarily in the zone of the regional Serovsko-Mauksky fault. Tectonic zones in the mine area contain contaminated fissure-vein water, which is transited at a depth of 70 to over 200 m. Discharging ascending springs of such waters can be located at a great distance from controlled hydrological objects and pollute sources of drinking and household water supply. Urban development in the western and eastern parts of Degtyarsk does not fall within the distribution zone of polluted water. The southern part of the city is located beyond the watershed of the mine water flow area, but a danger of local contamination by tectonic disturbance zones remains possible. The worst environmental situation is observed in the northern outskirts of Degtyarsk, which falls into the area of heavy pollution of underground and surface waters. Besides, acidic fumes from the flooded Kolchedanny quarry can affect the health of city residents when emitted to the atmosphere.

Introduction. Technogenesis is commonly understood as a set of various processes that change the natural environment under the influence of human activity. The largest environmental transformations are caused by the ore mining industry. The active stage of ore mining technogenesis, associated with the removal and movement of large masses of rocks, occurs during the development of deposits. Mining operations change the structure and chemical composition of the lithosphere, as well as pollute the atmosphere and natural waters. As a result, extensive litho-, atmo-, and hydrochemical pollution plumes are formed [1]. After the mine closure, a postoperational passive stage of technogenesis occurs, characterized by a lower degree of impact, but much longer duration.
In the mining industry of the Urals, a special place is taken by pyrite deposits. Ore minerals undergo hypergenic changes when the hard-to-dissolve sulfides are oxidized to well-soluble sulfates in the presence of free oxygen. A sharp decrease in the pH of water, accompanied by an increase in the redox potential, is the result of these reactions that leads to the transformation of natural geochemical processes. The consequence is a change in the chemical composition of natural waters, technogenic migration of ore elements and environmental pollution by heavy metals [12]. These processes continue for a long time after the closure of mining enterprises. The purpose of the work is to determine the scale of pollution from the currently non-functioning Degtyarsky mine in the Middle Urals. The main task is to study the features of the geoelectric structure of the mine field section using magnetotelluric technologies.
Problem statement. The Degtyarskoye copper-pyrite deposit, known since 1888, occupies the central district of the town of Degtyarsk. Initially, the Istokinsky brown iron ore (limonite) mine was developed here; in 1906, sulfide ores were discovered in the underlying strata. The deposit is located in the most narrowed part of the Tagilo-Magnitogorsky downfold and is represented by a DOI: 10.31897/PMI.2020.3.379 Vadim A. Davydov Study of the technogenesis of the Degtyarsky mine… briny, with mineralization of 3-8 g/l and pH = 2.5-3.6 [4]. Such waters have very low electrical resistivity values (< 1 Ohm·m), so the areas of their distribution should be confidently distinguished by electrical exploration methods.
Electromagnetic sounding allows tracing the zones of penetration and drainage of mine water into the host rocks by electrical conductivity anomalies. The study of the electrophysical properties of the geological section will help to identify the main plumes of aggressive underground water and determine the danger of man-made pollution of the urban area.
Methodology. The audio-magnetotelluric sounding (AMS), applied to study the nature of the geoelectric section, has the necessary transmission distance and has proven itself positively in the study of groundwater [7,15,18,19] and sulfide mineralization [16,17]. Field measurements were carried out in the express version of AMS with dual-channel universal receiver "OMAR-2m", designed at the Institute of Geophysics UB RAS [9].
Registration of the natural electromagnetic field was conducted in the frequency range of 100-15000 Hz on latitudinal sections in 20 m increments. The duration of continuous recording at the point was 2-3 minutes. To observe the horizontal component of the H x magnetic field, an active inductive sensor with a linearized amplitude-frequency response and variable sensitivity was used. The electrical component E y was measured using an outspread capacitive line with a pre-amplifier [6]. The ratio of the amplitude of the electrical component of the signal to the orthogonal magnetic component determines the impedance of the medium Z = E y /H x , which is proportional to the electrical resistivity of rocks. The apparent values of specific electrical resistivity (SER) were calculated using the formula adopted in magnetotelluric studies [2]: where f is the signal frequency, Hz; μ ≈ μ 0 = 4π·10 -7 is the magnetic permeability of the medium, H/m.
The stages of desk-study processing of the audio-magnetotelluric data included: • filtering of electromagnetic interference with a frequency of 50 Hz and its harmonics; • obtaining signal spectra using fast Fourier transform; • restoration of the real spectral distribution of signal amplitudes taking into account the nonlinearity of the characteristics of end-to-end measuring channels; • determination of spectral relations E y (f)/H x (f) to obtain the longitudinal impedance of the medium Z(f); • the calculation of the apparent resistivity (AR) to produce frequency curves  t (f); • transformation of frequency curves into a deep section of resistivity ρ t (h), using the original conversion algorithm [8]; • recalculation and drawing of the section of the effective longitudinal conductivity S ef .
Results. The marking of research sections is carried out in the sublatitudinal direction, intersecting the major geological structures. In total, four sections were passed in different sections of the ore field. Audio-magnetotelluric sounding is a fairly deep technique of geophysical research. The frequency range in which the measurements were made provides the information on the section at a depth of ten to several hundred meters [5]. However, the resolution of the AMS decreases with depth, so the depth of research was limited to 200-300 m to maintain a high level of detail.
The features of the geoelectric structure of the mine territory are due to the difference between the SER of rocks and ores, waterlogging, tectonics, and undermining. The study of the electrophysical properties of the section in the zone of technogenic impact of mine waters was started from the northern part of the mined field, near the flooded Kolchedannaya quarry. The section PR0 crosses the northern part of the artificial brown-colored lake with a persistent smell of sulfuric acid. In the southern part of the former quarry, mine water from the Degtyarsky mine and the sub-basement water from the Kapitalnaya-1 mine outflows constantly. From the East, the Istok River flows into the lake and, when mixed with mine waters, forms a common runoff with water mineralization levels of 3.7-12.8 g/l and pH = 2.3-3.3 [16].
Tuffaceous sandstones and green-stone shales are the major host rocks along the line of the PR0 section. Surface clay deposits are characterized by the lowest resistivity values (10-20 Ohm·m), with an increasing proportion of the fractions of sand and rock debris, the resistivity of loose rocks increases to 100-150 Ohm·m. Bedrock has higher electrical resistivity, varying from 150-200 to 1000-3000 Ohm·m (Fig.2).
Taking into account the lateral influence of the flooded quarry with a depth of more than 70 m, the obtained electrical resistivity is somewhat underrated. However, by the type of the KS section one can say that the penetration of acidic waters into the bedrock reaches a depth of more than 150 m from the daylight surface. Through tectonic cracks pollution can reach even deeper horizons. Tectonic disturbances in the geoelectric section are usually characterized by linear low-resistance anomalies. At the same time, the shape of abnormal zones is better distinguished on sections of longitudinal conductivity.
Due to the increase in the volume of cracks filled with water, tectonic zones are marked by increased electrical conductivity (from 0.5 S or more). In the section of effective longitudinal con-  (Fig.2, b), several abnormally conducting structures are distinguished. Of these, three subvertical linear anomalies located in the area of PK10, PK18, and PK27 pickets can be associated with tectonic zones. Horizontal electrical conductivity anomaly at the depth of 5-15 m corresponds to temporary perched ground water. Technogenic pollution spreads westward in the near-surface layer of soil water, coming back into the Istok River. A neutralization station is situated downstream, at the confluence of the Istok and Degtyarka. The total river and mine runoff is neutralized here by lime milk, which reduces the pH to 6.5-8.5. After neutralization, the waste water flows into the Yelchovka River, next to the settling pond.
Water from the Yelchovsky pond is discharged into the Volchikhinskoye reservoir, which is the main source of water supply in Yekaterinburg. The environmental situation with the surface flow is under control -the water condition in the rivers Degtyarka, Istok, and Yelchovka, as well as in the settling pond, is being monitored. However, uncontrolled migration routes of polluted water through tectonic zones identified by electrophysical sections are possible (Fig.2). Cracks of tectonic genesis contain fractured-vein water, which is transited in the lower part of the hydrogeological section at a depth of 100-250 m. Areas of discharge of such waters may be located outside of controlled hydrological objects, in the direction of the Volchikhinsky reservoir.
The next research section PR1 is located in the vicinity of the Kapitalnaya-1 mine, 1 km southward of PR0. It runs along the northern slope of the Labaz mountain, near the northern part of the quarry abandoned after mining of the "iron hat". In the initial part of the section, surface failures were recorded due to the collapse of underground workings. In this section, the RES of bedrock has small values (50-400 Ohm·m), the increase in resistances to 1000-6000 Ohm·m is observed in the second half of the section (Fig.3).
Electrical resistivity can vary greatly even in rocks of the same composition and origin. This variability is due to hydrothermal changes near ore zones, scattered sulfide inclusions, fracturing and humidity of rocks, as well as mineralization of pore moisture. Acid pollution, according to the resistivity measurements, embraces a layer of about 100 m from the daily surface. In the section of effective longitudinal conductivity (Fig. 3, b), a characteristic tectonic disturbance (PK4), known from the data of the mine documentation as a thrust dislocation that crosscuts the ore body, is well distinguished. The near-surface horizontal conducting zone of up to 0.7 S (PK0-PK5) is apparently associated with the sub-basement waters of the Kapitalnaya-1 mine dump. Zones with weakly increased electrical conductivity (0.2-0.3 S) are caused by local bedrock fracturing.
The PR2 research section is located 1.5 km southward of PR1 and runs along the southern slope of the Labaz (Karaulnaya) mountain. Its beginning is 300 m eastward of the Kapitalnaya-2 mine, at approximately the same distance southward of the flooded part of the quarry where the "iron hat" of the Degtyarskoye deposit was mined. In the surrounding area, intensive work was carried out at all horizons but no failures of underground workings were detected along the observation  line. The projection on the surface of the eastern boundary of the mining area approximately corresponds to the PK18 section. The electrical characteristics of the undermined area (PK0-PK18) and the area not affected by mining operations (PK18-PK34) are very different (Fig.4). At the first pickets, the resistivity of the bedrock is 100-500 Ohm·m, increasing to 1000-1500 Ohm·m in the middle of the section at deep horizons. When moving to the area of undisturbed monolithic rocks, resistivity increases sharply up to 5000-8000 Ohm·m. On the site that was subjected to underground work, the main area of acidic water circulation corresponds to the underground horizon of 70 m (abs. level of 290 m). Judging by the section of longitudinal conductivity, the upper horizons are additionally fed from the lower horizons (130 m and 190 m), similar to the situation observed at the nearest Krylatovsky mine [11]. Flows between horizons are carried out along the vertical shafts of liquidated mines and in the tectonic zone (PK6). The ridge of the Labaz mountain and its gently sloping continuation (PK22) serves as the watershed of the Degtyarka River and the Ikbulat Lake; so, groundwater contamination of an urban area eastward of the mountain does not occur (Fig.1).
The PR3 section is the longest (1100 m) and passes through all the main geological structures of the Degtyarskoye ore field. The section is located on the southern section of the Degtyarskoye field, passing near the Komsomolskaya mine. The beginning of the section consists of igneous rocks of the Tagilsky downfold, belonging to the Zyuzelskaya suite. After that, the section crosses from West to East the zone of the Serovsko-Mauksky fault, behind which the ore-bearing volcanicsedimentary rocks of the Degtyarskaya suite of the East Ural uplift lie. All the mentioned structures are well displayed on the sections of geoelectric parameters (Fig.5). The volcanites of the Zyuzelskaya suite (PK0-PK30) are distinguished by the highest RES on the section, their resistivity varies depending on the fracture from 1000 to 10000 Ohm·m. The zone of influence of the Serovsko-Mauksky fault (PK30-PK58), which is characterized by the lowest values of RES (up to 50 Ohm·m), is most well defined, and the eastern fault slope at an angle of about 70° is quite clearly defined. The outcrop of the ore body to the surface is confined to a swampy lowland and was located in the range of the PK64-PK65 points; at present, the swamp is almost completely covered with lime production waste. The ore body is not identified by the geoelectric section but a vertical low-resistance anomaly associated with the near-ore tectonic zone is observed at the PK75 picket, which has already been shown on the PR1 and PR2 sections. The specific electrical resistivity of rocks of the Degtyarskaya suite are sufficiently sustained and make up 3000-8000 Ohm·m.
In the area of PK88-90, a deep anomaly of low resistivity occurs probably due to strong rock fracturing. This zone has an eastward drop at an angle of about 80° and is characterized by the electrical conductivity of up to 1 S, which indicates its water saturation. The regional Serov-Mauk fault is the most conductive object of the section, which effective longitudinal conductivity is from 1 to 6.5 S. Given such high values, we can conclude that there is a clear inflow of polluted mine water into the fault zone, with the possibility of their further migration over long distances.
Analyzing the results of audio-magnetotelluric sounding, taking into account the available data on the structure of the area, a schematic hydrogeological section of the current state of the Degtyarsky ore field was drawn (Fig.6).
The obtained electrophysical parameters allow to determine the boundary between active and passive water exchange. The zone of active water exchange is characterized by a constant oxygen supply leading to the oxidation of sulfides and sulfuric acid leaching of rocks. Due to this, the acidity of ground water remains at a high level, and mineralization increases due to the penetration of atmospheric waters and the dissolution of accumulated sulfates, which leads to a general decrease in the electrical resistivity of the medium. At a greater depth, in the zone of reduced water exchange, leaching processes are poorly developed and the electrophysical properties of rocks are close to Outside the field, the hydrogeological situation is gradually returning to its natural state.
Conclusions. The Degtyarsky mine drainage is a source of dangerous technogenic pollution of the environment and water resources. Despite the neutralization of surface runoff, underground routes of acidic water migration occur along tectonic cracks, primarily in the zone of the regional Serovsky-Mauksky fault. Tectonic zones in the mine area contain contaminated fissure-vein water, which is transited at a depth of 70 to over 200 m. Discharging ascending springs of such waters can be located at a great distance from controlled hydrological objects and pollute sources of drinking and household water supply.
Urban development in the western and eastern parts of Degtyarsk fits closely, but does not fall within the distribution zone of polluted water. The southern part of the city is located beyond the watershed of the mine water flow area, however, there is a danger of local contamination by tectonic disturbance zones. The worst environmental situation is observed in the northern outskirts of Degtyarsk falling into the area of heavy pollution of underground and surface waters. In addition, acidic fumes from the flooded Kolchedannaya quarry can affect the health of city residents when emitted to the atmosphere.