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Abstract—Electromagnetic investigation of stress-strain state dynamics in the geological medium is based on the analysis of two groups of physical phenomena. The first group includes phenomena associated with the dynamics of electrical properties of rocks caused by a change in their fracturing, porosity, fluid content, structure and texture, temperature and pressure, etc. The second group covers the effects of generation of endogenous (geodynamic) sources of electromagnetic field under irreversible deformations of the geological medium. In this paper, we present the practical results of the studies of the Earth's natural electromagnetic field that have been carried out at the Research Station of the Russian Academy of Sciences in Bishkek over 40 years of operation of the Bishkek Geodynamical Test Site of the Research Station and aimed at gaining new insights into the present-day geodynamics of the Tien Shan lithosphere. The results of the experimental study of the first group of phenomena are based on the analysis of temporal changes in the magnetotelluric transfer functions. In this analysis it is assumed that the present-day geodynamic processes including catastrophic ones, e.g., earthquakes, trigger the crack formation processes which, in turn, changes the pore-fracture space of the rocks and causes fluid redistribution in them, thus resulting in the changes in the electrical conductivity of the geological medium. The practical results of the studies of the second group of phenomena are based on the notion that the geodynamic processes induced in the Tien Shan crust by irreversible deformations of rocks create sources of electromagnetic radiation leading to the variations in the endogenous electromagnetic field which can be observed on the ground. [ABSTRACT FROM AUTHOR]

New results of a detailed study of the deep structure of the Central Tien Shan along the Son-Kul magnetotelluric (MT) profile crossing the Son-Kul Lake are reported. Based on the results of magnetotelluric data modeling, the regional and local geoelectric anomalies in the lithosphere are studied and their quantitative characteristics are given. Geological interpretation of the geoelectric cross-section was carried out, which supported the existing ideas about the block–hierarchical structure of the upper part of the Earth's crust. This corresponds to the tectonophysical concepts of the sequential inserted subordination of large and smaller elements of the zone–block structure consisting of stable blocks and limiting mobile zones, which are distinguished by the high dislocation of the geological substrate. The integral pattern of the distribution and morphology of zones of high electrical conductivity in this segment of the Central Tien Shan crust may reflect discretely localized palm tree–type structures associated with the evolution of transgressive suture zones of localized deformation during the Hercynian and Alpine tectogenesis. [ABSTRACT FROM AUTHOR]

This paper considers a new approach to solving the problem of quantitative estimation of the microseism energy for underground sources that is based on the synthesis of noise interferometry and the passive seismic method of the gradient system. The selection of a seismic field of the underground sources is considered in an experiment conducted in the Tien Shan region. The peculiarities of approach include the separation of vertical microseisms in the ambient seismic noise field structure according to the data of the seismic gradient system and a passive noise interferometry diagram, where microseisms from the underground sources are used as the seismic signal source. It is shown that the use of noise interferometry and passive seismic gradient system allows using the synchronous microseism recordings in a small number of points for passive medium sensing, and leads to the restoration of unknown energy parameters of the seismic field of underground sources. [ABSTRACT FROM AUTHOR]

Based on a complex study of the upper crust structure in the southern margin of Kochkor basin (Northern Tien Shan), including study of the structure of the Cenozoic sedimentary cover, the deep geoelectrical structure, the structural unconformities, and occurrences of recent deformations in the basement rocks, new geological-geophysical cross sections are constructed. The cross sections show both fault structures that penetrate the cover from the basement and flat interplate detachments with related fold-overthrust structures. The comparison of the cross sections has established the absence of common planes of fault extensions along the entire margin of the hollow, except for the zone where the margin and the hollow adjoin, which can be caused by the zones of dynamic influence of secondary faults, the zones of fracturing, and the zones of cataclasis of blockwise disintegrated granite massifs. [ABSTRACT FROM AUTHOR]

We studied the infrastructure of granite massifs of the Central Tien Shan and its correlation with the electric conductive layer of the upper crust, which made possible to reveal new peculiarities of the structure of the granite layer in the region and to clarify the nature of low resistivity layers. [ABSTRACT FROM AUTHOR]

The Ysyk-Köl Basin filled with Lower Jurassic-Quaternary sedimentary rocks is the largest intermontane negative structural unit of the northern Tien Shan. The basement of this basin is composed of Precambrian-Paleozoic rocks, largely of Ordovician and Silurian granitoids exposed in mountain ranges of the basin framework and as separate anticlinal domes situated in areas occupied by the Mesozoic-Cenozoic sedimentary cover. The postmagmatic tectonic internalstructure of the Chonkurchak (Chunkurchak), Kyzyl-Choku, Kyzyl-Bulak, and Prishib massifs emplaced in the basement, as well as their relationships to the sedimentary cover, are described in the paper. The study was carried out using the morphostructural method, detailed geological mapping, structural kinematic analysis, and petrographic examination of rocks. The internalstructure of Paleozoic granites in the basement and indications of their 3D tectonic flow are characterized. It is shown that granites underwent 3D deformation after their emplacement in the consolidated crust, and this process had a substantial influence on tectonic processes at the plate and orogenic stages of regional evolution. [ABSTRACT FROM AUTHOR]

[Display omitted] • NOCFASS-method makes additional excess of frequencies for the fitting procedure. • The fitting parameters for trendless fluctuations of ambient seismic noise are given. • The reduced dispersion law is expressed as a linear function Ω k = a⋅k + b. • The relative error evaluating the fitting procedure is less than 2 %. Non-Orthogonal Combined Fourier Analysis of the Smoothed Signals (NOCFASS) helps to extract the reduced/invariant spectrum from the total Fourier-spectrum that contains a small number of frequencies with the linear dispersion law Ω k = a⋅k + b. The paper presents a new technique to treat original data of natural processes like seismic ambient noise, measured by the seismic gradient system with geophones on the surface. The authors show an example of the signal processing (July 14th-16th 2018) in the Ukok-2 point located in Tien Shan. A wide set of signals with a trend that is obtained from the set of initial trendless sequences (TLS) by means of integration procedure. This selected spectrum with eliminated high-frequency components is considered as an invariant spectrum, which contains only the basic low-frequency modes. The results are useful for the further detection of different random factors that disturb the TLS behavior. [ABSTRACT FROM AUTHOR]

Abstract: High-density array MT soundings of the crust in the seismically active northern Tien Shan were performed using Phoenix MTU-5 stations in the Bishkek Geodynamic Polygon, at the junction of the Chu basin and the Kyrgyz Range. The MT transfer functions were determined to an accuracy of 1–2% (amplitude) and about 0.5–0.8 deg (phase) in most of 145 soundings. Preliminary analysis of the collected data aimed at estimating the geoelectrical dimensionality. The Bahr decomposition analysis indicated the presence of local 3D structures in the crust of the area superposed on the regional 2D structure. [Copyright &y& Elsevier]

Based on example of the Sujak basin in the South-Western Tien Shan, it is shown that magnetotelluric sounding may successfully reveal tectonic disturbances, which are hidden under the Jurassic sediment, and zones of connate waters during construction of surface and underground objects. [ABSTRACT FROM AUTHOR]

The original version of this article unfortunately contained a mistake. The presentation of an author's name was incorrect. The corrected one is given below. [ABSTRACT FROM AUTHOR]

This paper discusses a range of issues related to the interpretation of time variations of geophysical fields as a response of the geophysical environment to lunar-solar tidal influences and seismic events under conditions of submeridional compression. A model linking the mechanism of crack formation and generation of geophysical fields has been proposed. The concept of the energy characteristics of a seismic field of deep underground (endogenous) origin along the seismic wave field observed on the Earth's surface for three mutually perpendicular components has been introduced. The results of field studies of simultaneous recording of a seismic wave field by a three-component gradient system and a magnetotelluric field at monitoring sites of the Bishkek Geodynamic Proving Ground (BGPG in Northern Tien Shan) are presented. The test area is situated in an active intracontinental mountain Central Asian Orogenic Belt with the frequent seismicity. For the experiment purposes, we select the Ukok regime monitoring point in the Kochkor basin of Kyrgyz Tien Shan. Because the location is characterized by low-level of seismic noise and the technical-and-industrial influence. The applicability of such a joint approach in geophysics is discussed. According to the results of the experiments performed, we establish a possible causal relationship between lunar-solar tides and activation of the deep underground (endogenous) source. Based on the correlation analysis of gravitational tidal effects, and the results of magnetotelluric monitoring, the azimuthal dependence of the electrical conductivity of the medium was revealed. We detect characteristic changes in the studied electromagnetic parameters, which we identify as a result of the redistribution of fluids between the fracture systems, were. The joint analysis of data from regime magnetotelluric observations and seismicity distribution confirms the high sensitivity of the method to changes in the stress-strain state of the geological environment. The confinement of seismic events to zones with high gradients of variations of apparent electrical resistivity is shown. The work may be of interest to specialists in the field of Earth Sciences, dealing with the interpretation of experimental data from geophysical test sites. • Geophysical characteristics change over time and are associated with deformation processes. • The electrical resistivity variations in time are observed in orthogonal azimuths in experimental data for the Tien Shan. • The time-frequency series of electrical resistivity variations and its' circle diagrams are novelty of the work. • The experiment of the seismic gradient system proves the applicability of the approach for endogenous component extracting. [ABSTRACT FROM AUTHOR]

Abstract: Electrical conductivity of rocks being closely related to their temperature could serve as a proxy parameter to be used for indirect temperature estimation from the surface electromagnetic (EM) data. Studies are carried out aimed at estimating the feasibility of indirect temperature estimation in the geologically complicated areas of the Earth crust from the electromagnetic data collected at the surface. Basing on the neuronet analysis of magnetotelluric (MT) and temperature data measured at the Bishkek geodynamical testing ground in the northern Tien Shan, optimal methodologies for calibration and application of indirect electromagnetic geothermometer are developed. It is shown that the temperature estimation my means of the EM geothermometer calibrated by 6–8 temperature logs results in 12% average relative error (instead of 30% achieved using only temperature logs). The availability of prior geological information about the region under study and preliminary analysis of the local heterogeneities'' indicators determined from the available MT data make it possible rejection of inappropriate site locations that may, in turn, decrease average error to only 11%. The results of the electromagnetic temperature extrapolation in depth indicate that the extrapolation accuracy essentially depends on the ratio between the well length and the extrapolation depth. In particular, in extrapolation to a depth twice as large as the well depth the relative error is 5–6%, and in case of its threefold excess the error is around 20%. This result makes it possible to increase significantly the deepness of indirect temperature estimation in the Earth interior (in particular, for geothermal exploration) based on the available temperature logs. [Copyright &y& Elsevier]

Highlights: [•] Dust monitoring at seasonal and diurnal scale at the northern slopes of Tien Shan. [•] Particle size composition was measured for 31 grain size classes at 1-min resolution. [•] Particles >2.5μm were collected using a high-volume-impactor at 3-day intervals. [•] Analytical data including the isotope composition of the attached NO3 are presented. [•] We infer relationships between dust composition and hemispherical circulation. [Copyright &y& Elsevier]