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The magnetotelluric method relies on variations of natural electromagnetic fields, which in the vicinity of human settlements are persistently distorted by anthropogenic electromagnetic noise. A large source of noise to the magnetotelluric response is caused by the harmonic oscillations of the power network utility frequency centered on 50/60 Hz along with the associated higher harmonics. Removing this type of noise is essential for high frequency magnetotelluric measurements used for shallow surveys. There are a large number of approaches for how to treat power line noise in magnetotelluric signals, however, commonly used methods do not take into account time variations/instabilities of the utility frequency. That is not serious problem in vicinity of well balanced grid networks, but can cause issues in regions with larger utility frequency variations. Under such conditions, commonly used methods loose more of the natural signal, which is undesirable especially in case of very noisy datasets. Hence, we adopted approach for removing of power line noise with respect to time variations of the utility frequency and applied it to magnetotelluric signals to preserve more of natural signal. The method is based on modelling of the grid network harmonic oscillations by the optimum utility frequency and its integer multiples. The resulting sum of sinusoidal signals is subsequently subtracted from recorded data and only particular noise frequencies are removed from the original signal with high precision, while frequency ranges around power line harmonics are cleaned.

In this paper we present a geological interpretation of magnetotelluric sounding along the southern part of the seismic 2T profile situated in the southern Central Slovakia. The complexes with higher conductivity are imaged in the shallow depths, formed by the Tertiary sediments and volcanics. In the northernmost part of the profile, the influence of non-conductive complexes composed of orthogneisses and overlying Mesozoic carbonates is significant. In the central part of the profile, the low conductive granitoid complexes are superposed over the metamorphic rocks with higher conductivity. This structure is a remnant of the Hercynian middle crust nappes. The most outstanding phenomenon of the profile is the sudden, almost step change in the conductivity parameters of the crust in the southern part. The significantly high conductivity of the crust in this area is most probably not related to its lithological composition, but by the abundant supply of fluids in the crust connected with the Neogene tectonic and volcanic processes.

Eastern margin of the Bohemian Massif is a geologically remarkable area, where three different orogenic cycles are meeting—the oldest, Cadomian, building a basement for younger cycles; a younger, Variscan; and the youngest, Alpine, covering the older units. In the past, this area was investigated by gravimetry and seismic methods. Recently, we have supplemented broadband magnetotelluric measurements within the period range of 0.001–1000 s, carried out on 29 stations distributed along a 140-km-long west-east regional profile. The profile direction was based on local geology and then confirmed by a dimensionality and directionality analysis. Data showed moderate effects of cultural noise in the signals and could be successfully processed by robust methods. We carried out a 2D inversion of the data using the REBOCC approach. The inversion results confirm the known near-surface geology and reveal deeper structures. On the west and in the central part of the profile, units of the Bohemian massif (the Moldanubian Zone, the Brunovistulicum) are interpreted. In the east, the Western Carpathians units are encountered. Short-period data agree well with the known near-surface geology of all inner smaller units and bring new knowledge in particular on their thickness. In the shallow structure, several conductive anomalies have been identified which are hypothesized to be related to graphitized layers in the Moldanubian Zone. From long-period data, a new image of the Moldanubian/Brunovistulicum contact and about the structure of the Brunovistulicum, especially the Brunovistulian Massifs, was obtained.

The paper presents the interpretation of magnetotelluric measurements along the SW–NE profile near Stará Ľubovňa (Northern Slovakia). The profile passes through the Outer Carpathian Flysch Belt, Klippen Belt and ends in the Inner Western Carpathians Paleogene NW from Ružbachy horst structure. The interpretation of the older measurements from profile Mt4 was utilized and, moreover, the 3-D geoelectrical model of studied region was constructed. The magnetotelluric data interpretations verified the northern inclination of Flysch belt structures and their smaller thickness out of Klippen Belt in direction to the Carpathian electrical conductivity zone axis. We consider this as a consequence of the flower structure—more precisely the southern branch of the suture zone related to mentioned conductivity zone. Northerly from this zone the thickness of the Outer Carpathian Flysch Belt increases and the structures have inclination to the south, i.e. to the subduction zone. The contact of Flysch Belt with Klippen Belt has a fault character and it is subvertical, slightly inclined to the North. The southern boundary between Klippen Belt and Inner Western Carpathians has also fault character and is very steep. We detect the continuation of the Ružbachy horst to the NE in the basement of Inner Western Carpathian Paleogene. The structural discordance between this horst and Klippen Belt direction is a result of younger tectonic processes. According to our results the depth distribution of the pre-Tertiary basement below the Inner Western Carpathian units is non-uniform; the basement is broken to a number of partial blocks—horsts and grabens.

In the 1-D magnetotelluric theory, a Riccati equation governs the depth variation of the impedance, or admittance, for a given distribution of the electrical conductivity. This equation can be used to compute the surface magnetotelluric functions for generally piecewise continuous conductivity profiles. In case of a simple layered medium, it provides the classical formulae for recalculating recursively the impedances between the individual layer boundaries. We present an extended version of the Riccati differential equations for generally anisotropic 1-D structures for the case of a plane wave incident field. Relation between the standard matrix propagation procedure for a layered medium and the Riccati equation approach, as a limiting case of the former, is demonstrated. In the anisotropic case, all elements of the 2 × 2 impedance tensor are present and, consequently, a system of four coupled Riccati equations has to be considered. Standard methods for the numerical solution of systems of ordinary differential equations are applied to the Riccati system, which gives an efficient alternative to the current matrix propagation procedures for the numerical evaluation of 1-D magnetotelluric impedances in anisotropic media. As an application, a synthetic study on the influence of a depth-variable regional strike on magnetotelluric decomposition results is presented, with the variable strike simulated by a variable anisotropy within the 1-D section.

Electrical resistivity of the Earth’s crust is sensitive to a wide range of petrological and physical parameters, and it particularly clearly indicates crustal zones that have been tectonically or thermodynamically disturbed. A complex geological structure of the Alpine nappe system, remnants of older Hercynian units and Neogene block tectonics in Western Slovakia has been a target of recent magnetotelluric investigations which made a new and more precise identification of the crustal structural elements of the Western Carpathians possible. A NW-SE magnetotelluric profile, 150 km long, with 30 broad-band and 3 long-period magnetotelluric sites, was deployed, crossing the major regional tectonic elements listed from the north: Brunia (as a part of the European platform), Outer Carpathian Flysch, Klippen Belt, blocks of Penninic or Oravicum crust, Tatricum and Veporicum. Magnetotelluric models were combined with previous seismic and gravimetric results and jointly interpreted in the final integrated geological model. The magnetotelluric models of geoelectrical structures exhibit strong correlation with the geological structures of the crust in this part of the Western Carpathians. The significant resemblance in geoelectrical and crustal geological structures are highlighted in shallow resistive structures of the covering formations represented by mainly Tertiary sediments and volcanics. Also in the deeper parts of the crust highly resistive and conductive structures are shown, which reflect the original building Hercynian crust, with superposition of granitoids or granitised complexes and lower metamorphosed complexes. Another important typical feature in the construction of the Western Carpathians is the existence of young Neogene steep fault zones exhibited by conductive zones within the whole crust. The most significant fault zones separate individual blocks of the Western Carpathians and the Western Carpathians itself from the European Platform.

We present results of a study of the seismicity and the geoelectric structure of the Eastern Carpathians. After the evaluation of the seismicity, new methods of processing and analyzing seismic data are developed, which allow constructing an averaged horizontal-layered velocity model of the crust in the Carpathian region of Ukraine, tracing the seismic active faults and localizing the seismic events both in horizontal and in vertical direction with a better precision. For the study of the conductivity structure beneath the Eastern Carpathians, the collected magnetovariation and magnetotelluric data are used. The depth of electrical conductivity anomalies are estimated and the resulting quasi-3D model of the conductivity structure beneath the Carpathians is compared with the seismicity in the depth range of 10 ± 2.5 km. The comparison suggests possible geological mechanisms: the seismic events occur mainly in resistive solid rock domains which surround aseismic high-conductivity zones, consisting of at least partially melted material. Aqueous fluids, or a joint effect of several mechanisms, may also play an active role in this distribution.

The Riccati equation approach to the analysis of magnetotelluric impedances in 1-D anisotropic structures is generalized to models with non-uniform source field excitation. The problem is solved in the horizontal wave-number domain. General Riccati matrix equations for the spectral impedances of the medium are derived and their relation to the standard impedance propagation formulae in layered anisotropic models is discussed. Riccati equations give a full solution for the spectral impedances, comprising both the induction and galvanic mode. For a purely inductive excitation of the field, each wave-number harmonics of the magnetic field is strictly linearly polarized on the surface, and only one half of the spectral impedance tensor can be restored. Both induction and galvanic modes generally exist inside the anisotropic conductor and are coupled. A formal similarity between the Riccati equations for a 1-D anisotropic medium with non-uniform sources and those obtained for 2-D laterally inhomogeneous structures is demonstrated, which indicates a possible way of extending the Riccati impedance/admittance equations to multi-dimensional conductors.

Crustal structures in the northern part of Slovakia were interpreted based on magnetotelluric data with help of supplementary gravity information. Magnetotelluric measurements in northern Slovakia along the northern part of seismic profile 2T were modelled using new processing and inversion methods. Geoelectric model reveals the position and structure of the deep crustal tectonic units and identify major deep fault zones. Presented northern part of the model exhibits a significant influence of resistive complexes composed of Cadomian crystalline basement of European platform beneath Outer Western Carpathians conductive sediments of Flysch belt. The important contribution of the magnetotelluric method for the interpretation of crustal structures is the differentiation of high-resistivity and low-resistivity complexes, which can be interpreted in the southern part of the profile as inhomogeneities in the crust caused by tectonic superposition of granitoid and metamorphic complexes, as relics of crustal Hercynian tectonic structure. Young steep shear zones can be well identified by magnetotelluric method due to their conductive properties. In the gravity model we imaged denser structures of European platform and also overlaid flysch sediments, Klippen belt, Tatricum crystalline complex and Mesozoic carbonates.

In the years 2001–2003, we accomplished the experimental phase of the project CEMES by collecting long-period magnetotelluric data at positions of eleven permanent geomagnetic observatories situated within few hundreds kilometers along the south-west margin of the East European Craton. Five teams were engaged in estimating independently the magnetotelluric responses by using different data processing procedures. The conductance distributions at the depths of the upper mantle have been derived individually beneath each observatory. By averaging the individual cross-sections, we have designed the final model of the geoelectrical structure of the upper mantle beneath the CEMES region. The results indicate systematic trends in the deep electrical structure of the two European tectonic plates and give evidence that the electrical structure of the upper mantle differs between the East European Craton and the Phanerozoic plate of west Europe, with a separating transition zone that generally coincides with the Trans-European Suture Zone.