Deep Magnetotelluric Sounding Research Articles

RESULTS OF DEEP MAGNETOTELLURIC SURVEYS ALONG THE PUSHKINSKIYE GORY - ANDREAPOL PROFILE

In january-february of 2022 deep magnetotelluric soundings (DMTS) were carried out along the regional (300 km) profile Pushkinskie Gory - Andreapol. The field expedition included professors, graduates and students of the Geophysical department of the Geological Faculty of Lomonosov Moscow State University, as well as scientists from the Laboratory of Magnetotelluric Soundings of the Center of Geoelectromagnetic Research IPE RAS. The article presents an analysis of the results, as well as geoelectric two-dimensional models obtained during the inversion of magnetotelluric data in the ZONDMT2D program.

Three-dimensional geoelectrical model of the central part of the Zvizdal-Zaliska and Brusyliv fault zones of the Ukrainian Shield

To study the deep structure of the geoelectrically complex junction zone of three megablocks of the western part of the Ukrainian Shield (Volyn', Podil, and Ros'), a three-dimensional model of the central part of the Zvizdal-Zaliska and Brusyliv fault zones was created. It is based on modern experimental observations of the Earth's natural low-frequency electromagnetic field in a wide range of periods. Synchronous experimental data of deep magnetotelluric sounding and magnetovariational profiling, obtained by the Institutes of the National Academy of Sciences of Ukraine in 2009—2019, were analyzed. The main issues of the geoelectrical modeling methodology using the Mtd3fwd software complex were considered, such as model elements, stages, alternative models, examples of calculations and comparison of observations, errors, etc. The constructed model analysis showed that of the large number of near-surface anomalies with low resistivity (5 to 100 Ohm · m), most sink to 500 m and only a few reach a depth of 1 km and are followed up to 11 km. It was established that there are connections between conductivity and structural features of the Zvizdal-Zaliska, Brusyliv, Nemyriv fault zones, the Samgorod fault and the Kocheriv synclinorium. Most of the isolated conductors appear in a mosaic pattern along the extended fault zones and form interspersed chains of high and low resistivity. Regional anomalies were confirmed and detailed both in the deep part of the crust and in the upper mantle; part of the Zvizdal-Zaliska fault zone appeared at depths of 15—30 km as a contact zone of abnormally high and low resistivities. Anomalies are confined to elongated zones of metasomatization and graphitized rock areas, some of the surface anomalies correspond to areas of the weathering crust. Most of the anomalies coincide with ore occurrences, ore-bearing fields and mineral deposits. The conducted geological-geoelectrical analysis of the model made it possible for the first time to identify two promising areas for further study that meet the geoelectrical criteria for the mineral search: along the Zvizdal-Zaliska fault zone between the Samgorod and Unava faults; along the Samgorod fault in the area of its intersection with the Kocheriv and Taboriv faults.

Multiparameter approach in the deep geoelectrics

The main provisions of geoelectrics are described, the importance of taking into account the ambiguity of its inverse problem is emphasized. Three main methods of deep geoelectrics which use natural fields of ionospheric-magnetospheric origin are considered: geomagnetic deep sounding (GDS), magnetotelluric sounding (MTS), and magnetovariational profiling (MVP). The response functions of each method are described. Each response function carries its own specific information about some parameters of the studied object and is characterized by the degree of reliability of the information about the object extracted from it. For example, the most reliable information about electrical conductivity anomalies (if any in the study area) is contained in the MVP response functions. The horizontal tensor of the anomalous field contains information about the electrical conductivity under the observation point, and the tipper (induction vector) contains information from the surrounding areas. In general, MVP information is less susceptible to distortions than MTS information and deserves more trust. Artificial field sources in deep geoelectrics are rarely used due to their high cost. Since 1970, two powerful sources created for other purposes arised on the Kola Peninsula and were used for deep sounding. In the center of these studies found themself young talented geologist-geophysicist and organizer of major projects AbdulkhaiAzimovichZhamaletdinov. The «Khibiny» project with an MHD generator and an ultra-deep well as one of the objects of the study, the «Zeus» low-frequency emitter, the signals of which were recorded in China at a distance of 7000 km, and a number of projects conceived and organized by Zhamaletdinov and executed under his leadership: «Volgograd-Donbass» (1979, 1986), experiments «PHOENIX» (2007, 2009, 2014, 2019) and others. At the same time, methods of interpretation were developed for sounding with artificial EM sources and new response functions were obtained which make it possible to «see» the object of research in a new way. This experience must be preserved, generalized, improved and used, for example as follows. In the area of modern synchronous multipoint MTS-MVP survey, a controlled source composed of two grounded lines emits strong current (harmonics at fixed frequencies and/or pulses) which signal will be recorded by survey instruments during night-time sessions.

ANALYSIS OF GEOPHYSICAL PARAMETERS VARIATIONS AND SEISMIC EVENTS AT THE POINT OF DEEP MAGNETOTELLURIC SOUNDING

The paper considers the results of processing and analysis of data from deep magnetotelluric soundings (DMT) performed in 2018. Comparison of variations in apparent resistivity, the endogenous component of the magnetotelluric field, lunar-solar tidal deformations and seismic events that were recorded during the research. The objective of the study is to detect the relationship between the appearance of variations in electromagnetic parameters of the data of magnetotelluric monitoring from the occurred earthquakes, its’ distance, energy class and position, relative to the DMT point.

ANALYSIS OF GEOPHYSICAL PARAMETERS VARIATIONS AND SEISMIC EVENTS AT THE POINT OF DEEP MAGNETOTELLURIC SOUNDING

The paper considers the results of processing and analysis of data from deep magnetotelluric soundings (DMT) performed in 2018. Comparison of variations in apparent resistivity, the endogenous component of the magnetotelluric field, lunar-solar tidal deformations and seismic events that were recorded during the research. The objective of the study is to detect the relationship between the appearance of variations in electromagnetic parameters of the data of magnetotelluric monitoring from the occurred earthquakes, their distance, energy class and position, relative to the DMT point.

Electromagnetic studies of Zvizdal-Zaliska and Brusyliv fault zones of the Ukrainian shield

Spatial-temporal distribution pattern of geomagnetic variations and electric field in the western part of the Ukrainian shield has been obtained as a result of experimental studies carried out by the methods of deep magnetotelluric sounding and magnetovariational profiling in 2018 along two profiles (15 points). It makes possible to estimate the value of electroconductivity and geoelectric structure of the section along the vertical and horizontal directions. Processing has been conducted with the help of modern software system PRC-MTMV that guarantees joint noise immunity of evaluation of impedance and induction parameters of synchronous МТ/МВ-records. Tipper appraisals have been obtained for the periods of geomagnetic variations from 50 to 3400 s, the curves of apparent resistance and phases of impedance from 20 to 10000 s. The principal result of high quality interpretation of geoelectric studies is revealing of high electro-conductivity anomalies (4 surface ones, 1 subsurface) in the Earth crust with complex system of faults in jointing zone of the Podolia and Rossinsky megablocks. Anomalies are characterized by different electroconductivity, the depth of occurrence and shape. Appraisals obtained with different approaches to formal interpretation of magnetotelluric sounding agree with regional three-dimensional geoelectric model of the western part of the Ukrainian shield and add to the concept on debatable deep structure. Surface anomalies of electroconductivity have been discovered: 1) in the central part of Zvizdal-Zalessky fault zone, 2) between the Pogrebynsky and the Kocherov faults, 3) along the Velykoerchyk and 4) Ulashev faults. Their nature is explained by heightened content of graphite and by ore formation confined to tectono-metasomatic zones. Deep anomaly of electroconductivity has been allocated with its surface projection spatially coinsiding with the area of cross-section of the Brussilov and the Nemyrov fault zones (and mineraginetic zones of the same name). Its upper edge deepens eastward from Zvizdal-Zalessky fault zone. High electroconductivity anomalies are spatially attached to tectonic fault zones and the zones of metasomatic alterations. Their territorial arrangement corresponds to metallogenic ore zones. Therefore they can be used as geoelectric criteria for the search of ore manifestations.

Three-dimensional deep geoelectric model of the Tarasivka structure of the Golovanivsk suture zone

In 2017 the field simultaneous areal measurements of the external low-frequency natural electromagnetic field of the Earth and the construction of a three-dimensional deep resistivity distribution in the crust of the Tarasivka structure (48032' N, 30037' E) were performed in the central part of the Yatran Block of the Golovanivsk suture zone. The analysis of experimental data (the curves of deep magnetotelluric sounding for a period of 10—10000 s and the complex induction parameters for periods of 20—6900 s) indicates a complex three-dimensional situation, which involves the presence of a nearsurface and possibly deep conductivity anomalies. According to the results of three-dimensional modeling, the Tarasivka structure is fragmentarily manifested as a low resistivity, the conductive zones with different resistivity cross it in a sublatitudinal direction, in which the lowest values from 10 Ohm·m in the south to 100 Ohm·m in the north are in its contour. In the vertical section it can be represented in as several layers: the first one is the conductor with ρ=10ч250 Ohm·m — from the surface to 10 m (south of 48030ў) to 100 m (north of 48030ў), most likely linked not only with high electrical conductivity of surface sediment deposits, but also with the zones of disintegration of rocks of the basement; the second one is a high resistivity layer with ρ=10000 Ohm·m — from 100 m to 2 km, possibly it is represented by the uniform non-differentiated thickness; the third layer is the electrical conductor with ρ=10ч250 Ohm∙m from 2—3 km to 10 km, probably linked with the special composition of the earth’s crust at these depths (graphitization, sulfidization, etc.) or fluidization of different origins, more often it is considered according to the modern data that the nature of the conductive anomalies is the result of the joint influence of the electron and ion types of electrical conductivity.

The results of deep magnetotelluric sounding for studying the Nha Trang - Tanh Linh fault

The profile of deep magnetotelluric sounding (MT) from Duc Trong - Tuy Phong has been carried out in Lam Dong and Binh Thuan provinces. The length of the Duc Trong - Tuy Phong profile is about 80 km with 15 stations and the distance between the stations measures about 5 km. Two-dimensional MT inversion was used to find a resistivity model that fits the data. The 2D resistivity model allows determining position and development formation of the Nha Trang - Tanh Linh fault. This is the deep fault, which is showed by the boundaries of remarkable change of resistivity. In the near surface of the Earth (from ground to the depth of 6 km), the angle of inclination of this fault is about 60o; in the next part, the direction of the Nha Trang - Tanh Linh faut is vertical. Geoelectrical section of the Nha Trang - Tanh Linh profile shows that the resistivity of mid-crust is higher than that of lower-crust and of upper-crust.

Deep structure of Kamchatka according to the results of MT sounding and seismic tomography

The key features in the distribution of geoelectric and velocity heterogeneities in the Earth’s crust and the upper mantle of Kamchatka are considered according to the data of deep magnetotelluric sounding and seismotomography. Their possible origin is discussed based on the combined analysis of electric conductivity and seismic velocity anomalies. The geoelectric model contains a crustal conducting layer at a depth of 15–35 km extending along the middle part of Kamchatka. In the Central Kamchatka volcanic belt, the layer is close to the ground surface to a depth of 15–20 km, where its conductivity considerably increases. Horizontal conducting zones with a width of up to 50 km extending into the Pacific Ocean are revealed in the lithosphere of eastern Kamchatka. The large centers of current volcanism are confined to the projections of the horizontal zones. The upper mantle contains an asthenospheric conducting layer that rises from a depth of 150 km in western Kamchatka to a depth of 70–80 km beneath the zone of current volcanism. According to the seismotographic data, the low- and high-seismic-velocity anomalies of P-waves that reflect lateral stratification, which includes the crust, the rigid part of the upper mantle, the asthenospheric layer in a depth range of ~70–130 km, and a high-velocity layer confined to a seismofocal zone, are identified on the vertical and horizontal cross sections of eastern Kamchatka. The cross sections show low-velocity anomalies, which, in the majority of cases, correspond to the high-conductivity anomalies caused by the increased porosity of rocks saturated with liquid fluids. However, there are also differences that are related to the electric conductivity of rocks depending on pore channels filled with liquid fluids making throughways for electric current. The seismic velocity depends, to a great extent, on the total porosity of the rocks, which also includes isolated and dead-end channels that can be filled with liquid fluids that do not contribute to the electric-current transfer. The data on electric conductivity and seismic velocity are used to estimate the porosity of the rocks in the anomalous zones of the Earth’s crust and the upper mantle that are characterized by high electric conductivity and low seismic velocity. This estimate serves as the basis for identifying the zones of partial melting in the lithosphere and the asthenosphere feeding the active volcanoes.

The results of deep magnetotelluric sounding on profile Quan Son - Quan Hoa in the province Thanh Hoa

The results of deep magnetotelluric sounding on profile Quan Son - Quan Hoa in the province Thanh Hoa

Role of electrical measurements at deep magnetotelluric sounding of the Earth

The study of deep structure of the Earth is of great interest for both applied (e. g., mineral exploration) and scientific research. The most common method of electromagnetic (EM) exploration is magneto-telluric sounding (MTS). This method is a passive method of research, which uses a wide range of natural geomagnetic variations as a powerful source of EM induction in the Earth. Improvement of the quality of MTS demands constant increase of resolving capacity and accuracy of measuring equipment, particularly electric channels. The results of the development of a new version of electrometer with increased, comparing to the known instruments, parameters level are described in the paper and the results of experimental tests of this electrometer, as a part of the long-term magnetotelluric station LEMI-420, are given.

Indication of meta-anthracite by magnetotellurics in the Kőszeg-Rechnitz Penninic window: a test area

One of the Penninic Nappes is the Kőszeg-Rechnitz (K-R) tectonic window at the Eastern end of the Eastern Alps. It has a complicated metamorphic history from the Jurassic time. The organic material of the Penninic Ocean was transformed to electrically conductive meta-anthracite. Its amount in the chalcophyllite is estimated by geochemists to 0.2 per cent.Taking this conducting structure as a test area pilot deep magnetotelluric (MT) soundings have been carried out and we determined the structure of the conductivity anomaly due to 0.2 per cent meta anthracite in the K-R window and its surroundings the different kinds of MT distortions as lateral (side) effect of the conductor appearing in the crust and mantle the most probable depth of the conductive asthenosphere at the border of the Pannonian Basin (having extreme shallow asthenosphere). The obtained ∼140 km depth is in correlation with value of the asthenospheric map based mainly on seismic data.

New data on the structure of the Kas Block in the basement of the West Siberian Plate

A new concept concerning the structure and stages of evolution of the Kas Block of the West Siberian Plate is stated in this paper. The Kas Block is traditionally considered to be a subsided western margin of the Siberian Platform. The new concept is based on the results of the interpretation of the geophysical data recently obtained along the reference and regional profiles in this territory. The geological interpretation of the deep dynamic sections obtained by reprocessing of the CDP seismic reflection records has been performed for the first time. The structural features of the Kas Block, as well as the character of its junction with the Siberian Platform and the Paleozoic framework, are discussed. The tectonic scheme of the territory and the scheme of the pre-Late Devonian surface of the Kas Block have been compiled. The Baikalian age of the basement of the Kas Block is substantiated. The Salairian allochthonous ophiolite-basalt tectonic nappe is localized for the first time within the sedimentary cover of the Kas Block. The available information allowed us to reconstruct the development of the western margin of the Siberian continent in the Riphean and Early Paleozoic before and after the Baikalian Orogeny, respectively. The informational background of the geological and geophysical interpretation involves the results of the CDP seismic reflection profiling, including the deep dynamic seismic sections and parameters of the P-wave velocities along the reference 1-SB seismic line and the regional Vostok 10, 12, 15, and 16 seismic lines; the results of the deep seismic and magnetotelluric soundings; the gravity measurements; the magnetic exploration; and the new coherent physical geological models.

Estimation of the electric resistivity distribution (EURHOM) in the european lithosphere in the frame of the eurisgic WP2 project

In connection with the EURISGIC WP2 project the authors present those procedures which have been used to construct a map in cells on the electrical resistivity distribution in Europe at least till to the asthenosphere. The data are based on the deep magnetotelluric soundings published in the international literature. This map is the basis of the calculation of the induction risk endangering the electric network and communication systems.

Magnetovariational information to improve distortion diagnostics in deep magnetotelluric soundings

An interpretative experience from nine magnetotelluric soundings was accomplished in the central region of Argentina (32°S–34°S; 63°W–69°W), from the Andean region in the west to the platform zone in the east. To do this, magnetovariational information was used to improve the distortion diagnostics in magnetotelluric curves. Using Pilar Geomagnetic Observatory as a reference site, horizontal magnetic transfer functions were estimated, which were compared with the integrate conductivity at each location in field. As a result, a rather simple methodology is proposed to better approach the accurate positions of normal curves. Results suggest that, in this way, better formal interpretations of soundings may be reached. In addition, a more clear and comprehensible knowledge about the nature of lateral in-homogeneities is obtained; e.g., discovering 3-D effects no suspected from tectonic maps. This methodology seems to be particularly useful when-as in the present case-magnetotelluric soundings are far away each others; i.e., when effective volumes of soundings are not interpenetrated. Horizontal magnetovariational information suggests two elongate conductivity anomalies (about N25°–30°E), possibly associated with deep seated faults belonging to the South American regmatic network. These anomalies would be produced by partial melting in lower crust and possibly in the asthenospheric zone next to Andean Range. Another elongate anomaly (possibly of graphitic nature) is shown in the study region. It seems to be a marginal fault following the border between The Sierras Pampeanas dynamic zone and the South American craton. Magnetotelluric results indicate the study region can be considered as divided in a dynamic belt next to Andean Range and a cratonic zone eastward. The dynamic zone presents a well developed lower crust, with conductances ranging 300–4300 Siemens and depths of about 20–30 km. An asthenosphere close to the Andes with 1000 Siemens of conductance at 74 km depth is also observable. Heat flows of 63–70 mW/m2 are estimated next to Andes and 48 mW/m2, eastward, close to South American platform. The cratonic zone presents a first conductive layer with a conductance of 270 Siemens underneath BUL sounding site, but it does not seem to present lateral development. An intermediate conductive layer is also present in this region, but it does not have so much development. Therefore, this layer would not have asthenospheric character; so, the lithosphere would be tied to upper mantle. Heat flows ranging 40–35 mW/m2 were estimated for this cratonic region.

Anomalies in the electric field and electric conductivity of the Earth’s crust in relation to the Kultuk Earthquake on Lake Baikal

We analyze the variations in the horizontal and vertical components of the Earth’s electromagnetic field measured at nine points onshore along the southern part of Lake Baikal. It is found that variations with a period of a few tens of seconds have decayed 20 minutes before the earthquake. This is a regional effect. It is observed in the variations of the horizontal magnetic field. The intensity of the effect in the electric field is site-specific, which is due to the nonuniform distribution of electric conductivity in a medium, as revealed by the deep magnetotelluric sounding. The electric field also experiences a coseismic variation associated with the seismic wave. This effect is stronger pronounced in the vertical electric field. The bay-like anomalies that precede and accompany the Kultuk earthquake are revealed in the electric field of intraterrestrial origin and in the behavior of the electric tipper. These anomalies are concurrent with the rise of the ground water level in the borehole. The probable nature of the revealed anomalies is discussed.

Deep electromagnetic (MT and AMT) sounding of the suture zones of the Ukrainian shield

The article discusses the results of deep magnetotelluric sounding of suture zones in the Precambrian structures of the Ukrainian shield in the context of collisional settings. All the considered suture zones (Orekhovo-Pavlograd, West Ingulets-Krivoi Rog, Golovanevsk-Yadlov-Traktemir, Nemirov-Kocherov or Brusilov) are associated with the regional anomalies of increased electric conductivity, which mark deep fault zones and characterize collisional settings. The nature of such deep anomalies might be due to the transport of the ore components together with the fluids from the crust and the mantle during the tectonomagmatic activation, which offers the possibility to predict the prospective areas of endogenic mineralization. A correlation has been established between the endogenic ore deposits located close to or within the suture zones and the low-resistivity anomalies.

Preliminary magnetotelluric study of Ambato and Valle Fértil lineaments in Bermejo Basin and Sierra de Valle Fértil, San Juan, Argentina

In the western region of Argentina, 30° S Lat., near the Andean Range, two Precambrian large faults or lineaments are present: Ambato (AL) (NE-SW) and Valle Fertil (VFF)(∼NS). They are deep-seated faults in the crust, with extend develop in time, crossing each other in the region where Bermejo Basin and Valle Fertil Range (Sierra de Valle Fertil, SVF) are present. The study of these lineaments is important to better know the tectonic evolution of this region. Three deep magnetotelluric soundings were carried out in this zone using induction coils and a flux-gate magnetic variometer. Principal results show a thickness up to 7000 m and saline character for the Bermejo Basin, with resistivities range frm 3 Ωm to 13 Ωm, and an integrated conductivity of 1600 Siemens. High conductivity is also suggested for the AL from the top of basement to higher depths. Westward of VFF, beneath the Bermejo Basin, a conductive lower crust is suggested with a longitudinal conductance at least of 1600 Siemens and seated at 43 km depth; but this layer does not seem to be present eastward of the fault. A possible phase transition zone at 300 km depth is also suggested. From these conductive layers, a heat flow estimate of 36–40 mW/m2 is made for the region, using empirical formulas. Graphite is suspected to be the cause of lower crust conductivity. If correct, it would be suggesting a strong ancient tectonic activity, with probable dipping and lifting of significant amplitude in the region.

Interpretation of the deep magnetotelluric soundings along the Austrian part of the CELEBRATION-007 profile

In the continuation of the CELEBRATION-007 deep seismic profile from Hungary to Austria a series of deep magnetotelluric soundings has been carried out, using the instruments (from GFZ MT instrument pool). In spite of the high noise level, the relatively good imaging of the structure of the sedimentary Graz Basin and possibly fluid filled conducting fractures in the resistive rock matrix of the Eastern Alps have been indicated. They might be potential source of geodynamics (earthquakes?).

Water in the mantle: Results from electrical conductivity beneath the French Alps

A deep magnetotelluric sounding in the French Alps provided a vertical electrical conductivity profile between ∼200–1000 km. Two prominent features are observed. First, the conductivity in the depth range 400–800 km is smaller than the conductivity of a pyrolite mantle obtained from laboratory results for a normal geotherm. Second, the data do not require the conductivity to change throughout the transition zone (410–660 km). In this part of the mantle, a temperature of 350–450 C less than normal explains the magnetotelluric conductivity profile. At 200–400 km, our model favors a cold mantle with 1000–1500 ppm of water dissolved in olivine. If correct, this model suggests that the subducted slab is dehydrated before reaching the transition zone.