Array magnetotelluric soundings in the active seismic area of Northern Tien Shan

Chave A.D., Thomson D.J., Ander M.E.

Robust estimation of power spectra, coherences, and transfer functions is investigated in the context of geophysical data processing. The methods described are frequency-domain extensions of current techniques from the statistical literature and are applicable in cases where section-averaging methods would be used with data that are contaminated by local nonstationarity or isolated outliers. The paper begins with a review of robust estimation theory, emphasizing statistical principles and the maximum likelihood or M-estimators. These are combined with section-averaging spectral techniques to obtain robust estimates of power spectra, coherences, and transfer functions in an automatic, data-adaptive fashion. Because robust methods implicitly identify abnormal data, methods for monitoring the statistical behavior of the estimation process using quantile-quantile plots are also discussed. The results are illustrated using a variety of examples from electromagnetic geophysics.

Bielinski R.A., Park S.K., Rybin A., Batalev V., Jun S., Sears C.

[1] A 450 km long north-south magnetotelluric profile spanning the Tien Shan from Kazakhstan to western China reveals lateral variations in the resistivity of the mantle lithosphere to depths of 140 km. Minimum changes of one order of magnitude in this depth range result from variations in temperature or composition, or both. Higher resistivities beneath a central portion of the range where the Moho is half as deep as elsewhere in the Tien Shan indicate a strong lithospheric block. We propose that this block protects this region of thin crust from appreciable deformation and instead transmits stresses due to India-Asia plate convergence from the southern to the northern parts of the range.

Bullen M. ., Burbank D. ., Garver J. .

Paired apatite fission track and U‐Th/He dates provide the first Late Cenozoic cooling ages for the northern Tien Shan. These data clearly argue for pulsed deformation since the Late Miocene, with early (10–11 Ma) and late (0–3 Ma) intervals of rapid exhumation separated by an extended interval of much slower rates. By integrating these bedrock cooling rates with shortening estimates derived from a balanced section, detrital cooling ages, and geomorphological estimates of conditions before deformation, we reconstruct a four‐stage history of range growth and exhumation. Following ∼100 m.yr. of tectonic quiescence, abruptly accelerated rock uplift, exhumation, and cooling in the Kyrgyz Range commenced at ∼11 Ma with rates exceeding ∼1 km/m.yr. During the subsequent 7 m.yr., deformation and cooling rates decreased three‐ to sixfold before accelerating by comparable amounts during the past 3 m.yr. Since mid‐Miocene times, the surface elevation of the Kyrgyz Range has increased ∼2 km, consistent with the reconstructed magnitude of crustal shortening (∼11 km) and thickening (∼12 km) across the range. The highly pulsed deformation rates indicate that the locus of deformation probably shifted repeatedly within the Tien Shan from the Miocene to present. Even at their most rapid, Cenozoic shortening rates in the Kyrgyz Range were equivalent to only 10%–20% of the modern geodetic convergence rate across the entire Tien Shan. This requires several ranges within the Tien Shan to have deformed simultaneously since the Middle Miocene, a situation analogous to the distributed shortening seen today.

Goubau W.M., Gamble T.D., Clarke J.

Two new techniques for analyzing 4‐channel magnetotelluric (MT) data are described. These techniques produce estimates of the elements [Formula: see text] of the impedance tensor that are unbiased by noise in the autopowers of the electric and magnetic fields. Effectively, each technique uses one field channel as a reference signal that can be correlated with the other three channels. Method 1 obtains estimates for the [Formula: see text] in terms of crosspowers of the Fourier components of the electric and magnetic fields [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text]. Method 2 is a generalization of method 1, and obtains estimates for [Formula: see text] in terms of weighted crosspowers. Both methods fail when the geology is one‐dimensional, or two‐dimensional with one electrode oriented along the strike direction. To obtain results that are stable for any geology and that are unbiased by autopower noise, at least five channels of data are required. To also minimize bias by correlated noises, one needs six channels of data, two channels of which are for fields measured at a site that is remote from the base MT station. The analysis of MT data using a remote magnetometer as a reference is discussed.

Spichak V.V., Goidina A.G.

Spichak V.V., Goidina A.G.

Abstract—The paper presents an attempt to estimate the contribution of thermal processes in the Earth’s crust to earthquake generation. As a case study, the seismically active junction zone of the Chu depression and Kyrgyz ridge of the Northern Tien Shan is considered. The analysis is based on the previously constructed models of temperature, petrophysical characteristics, and elastic moduli. The silica content model derived from seismic tomography data is used to build a lithotype model. The thermal conductivity model is constructed and used in conjunction with the temperature model to create a depth model of the heat flow density. The shear stress and thermoelastic stress models derived from density, elastic moduli, and temperature models are compared with the distribution of earthquake hypocenters in the region. The comparison suggests that on the scale of the seismically active junction zone of the Chu depression and the Kyrgyz ridge of the Northern Tien Shan, seismicity is mainly determined by thermomechanical effect induced by hot ascending flow of acid magma rising from the upper mantle beneath the Muyunkum–Narat median massif.

Nepeina K., Bataleva E., Alexandrov P.

This study focuses on microscale anisotropy in rock structure and texture, exploring its influence on the macro anisotropic electromagnetic parameters of the geological media, specifically electric conductivity (σ), relative permittivity (ε), and magnetic permeability (μ). The novelty of this research lies in the advancement of geophysical monitoring methods for calculating cross properties through the estimation of effective parameters—a kind of integral macroscopic characteristic of media mostly used for composite materials with inclusions. To achieve this, we approximate real geological media with layered bianisotropic media, employing the effective media approximation (EMA) averaging technique to simplify the retrieval of the effective electromagnetic parameters (e.g., apparent resistivity–inversely proportional to electrical conductivity). Additionally, we investigate the correlation between effective electromagnetic parameters and geodynamic processes, which is supported by the experimental data obtained during monitoring studies in the Tien Shan region. The observed decrease and increase in apparent electrical resistivity values of ρk over time in orthogonal azimuths leads to further ρk deviations of up to 80%. We demonstrate that transitioning to another coordinate system is equivalent to considering gradient anisotropic media. Building upon the developed method, we derive the effective electric conductivity tensor for gradient anisotropic media by modeling the process of fracturing in a rock mass. Research findings validate the concept that continuous electromagnetic monitoring can aid in identifying natural geodynamic disasters based on variations in integral macroscopic parameters such as electrical conductivity. The geodynamic processes are closely related to seismicity and stress regimes with provided constraints. Therefore, disasters such as earthquakes are damaging and seismically hazardous.

Nigmatullin R.R., Bataleva E.A., Nepeina K.S., Matiukov V.E.

The research is based on the typical measurement calibration procedure of magnetotelluric soundings (MTS) prior to deep Earth exploration. We suggest the testing and new possibilities of the using MTS equipment calibration. We used two Phoenix MTS station boxes with different magnetic coil sets and mixed sets of coils for verification (check-up) stations calibration and the identity sessions. As a result of the experiments carried out and careful running over sorting of standard functions (exponential, power-law, polynomial), the verification of the proposed method was performed. Eventually this study proposes a “universal” fitting curve after one-station calibration curves to be normalized and are compared to another (etalon) station calibration curves (written in the form of two polynomials ratio). This procedure can be generalized for the case x → f(x), where f(x) is known function. It enables to fit all these 4 typical normalized curves (two for amplitudes (mV) and two for phases (degrees)) by the ratio of fourth degree function containing 9 fitting parameters C0-7 and B0, simply calculated as numeric constants through linear least squares method (LLSM).

Nepeina K.S., Rybin A.K., Kaznacheev P.A.

Abstract The project of the joint seismic-and-electromagnetic monitoring in low-ambient noise areas is aimed at studying the phenomena associated with the deformation of the Earth’s crust. The arrangements set of the geophysical monitoring in low-ambient noise areas is proposed. The paper describes in detail the technological scheme being of monitoring of the modern geodynamic activity with a magnetotelluric soundings method and a small seismic array presented as the seismic gradient system (SGS). Technical explanations, safety rules, equipment calibration and maintenance, and the technological schemes of the method of obtaining and archiving data are given. Maintenance procedures using SGS with geophones in Tien Shan to determine the endogenous component of the seismic field are described. We investigate a common workflow chart for SGS and main programs for the data caption and data processing. The proposed scheme has been tested and requires the application and adoption to the passive geophysical monitoring routine.

Spichak V.V., Zakharova O.K.

Abstract —Prediction of the open porosity factor below boreholes and in the interwell space, from electromagnetic sounding and electrical logging data, has been studied. Modeling porosity data are synthesized from laboratory studies on the samples of two boreholes drilled in the Bishkek geodynamic site. The porosity prediction is carried out using specific electrical resistivity data obtained from 1D inversion of magnetotelluric sounding data collected in the vicinities of these wells. A new approach to predict porosity is suggested, based on the constructing of electromagnetic resistivity pseudo-logs at the target location. The comparison of this technique with other options indicates that its application results in substantial improvement of the predicted accuracy (in particular, relative errors of prediction in double depth of the borehole and in the interwell space could be 2 and 8%, respectively). In general, the porosity predictions, based on Archie formula, give worse results.

Rybin A.K., Bataleva E.A., Aleksandrov P.N., Nepeina K.S.

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.

Пржиялговский Е.С., Рыбин А.К., Морозов Ю.А., Лаврушина Е.В., Леонов М.Г., Баталева Е.А.

The article presents the results of integrated geological and geophysical research works dealt with detailed magnetotelluric (MT) sounding and the study of the morphology and spatial position of the sedimentary cover and basement structures along the key transect of the Middle Tien Shan crossing the Naryn and Atbashi depressions. The data on the distribution of deep electrical conductivity of the crust and upper mantle were compared with the seismic profiling data. The compilation of the results of structural-geological and geophysical studies provided the opportunity to draw 2D upper-crust geological structure, consistent with the structure of electrical conductivity of the crust to depths of about 10 km. The detailed geological cross-sections and the structural and geological data allow us to characterize the deformations of the Cenozoic sedimentary complex and Paleozoic basement surface associated with the alpine activation of the key segment of the Tien Shan. It is shown that the Cenozoic structural parageneses emerged during a relatively short-term phase of deformation and orogeny under conditions of horizontal compression and transpression, which did not manifest themselves during sagging that occurred previously.

Nepeina K., Bataleva E.

This work is aimed to develop a new approach to study the relationships between the spatial distribution of the electrical conductivity and earthquake hypocenters in seismically active regions such as the Tien Shan. The basis of a complex deep complex model (up to 30 km depth) of the Tien Shan lithosphere is a fault-block model. The joint analysis of the geoelectric model and seismicity provides important information for understanding the distribution of deep electrical conductivity, which is an indicator of active geodynamic processes and provides indirect estimates of the stress-strain state of the geological media. The purpose of our investigation is to extend the study of the relationship between hypocenter depth distribution and geoelectric boundaries on a 2D cross-sections in the Central Tien Shan. This paper shows complex models, built for several meridional magnetotelluric soundings (MTS) profiles located (74.3° E and 75.2° E). We use seismic bulletins for 5 years (2015–2019) and select earthquakes nearest to the MTS profile lines. Based on our previous studies, we are interested in the behavior of seismicity with energy classes K  8 separately. We calculate relative coordinates (latitudes) to the kilometers from the starting point of the profile. As a result, the graphs present colored 2D cross-section with profile distance (R) and earthquake hypocenters into the depth marked by dots. The analysis of this study indicates the presence of a certain relationship: the earthquakes hypocenters clustering attach to the boundaries of objects and outline geological bodies, or are located inside certain zones.

Bataleva E.

The results of geophysical monitoring of seismically hazardous regions are of undoubted interest for studying the deep structure of the lithosphere, regional seismicity, modern geodynamics, etc. The work used experimental material, including the results of magnetotelluric monitoring and the catalogs of the KNET (Kyrgyzstan Telemetered Network), KRNET (Kyrgyz Republic Digital Network) and ISC (International Seismological Center) networks obtained in the seismically active zones of the Bishkek Geodynamic Proving Ground (Northern Tien Shan). The analysis of electromagnetic monitoring results of the Northern Tien Shan seismic generating zone was carried out in a wide frequency range. The data on the parameters of the electromagnetic field of the Earth’s crust in the Northern Tien Shan are generalized and systematized. Based on the analysis of these data, the dependence of the response of seismic events in electromagnetic parameters on the distance of earthquake epicenters was studied. The most likely reason for the occurrence of anomalous changes in the electromagnetic field is the activation of deformation processes during the preparation of strong earthquakes. The results of the time-frequency analysis are presented and the features of manifestation are considered depending on the location of the hypoand epicenters of seismic events and their magnitude. Regularities have been established in the behavior of tipper variations for remote and regional earthquakes for the first quarter of 2016 with an energy class from 6 to 10.

Nepeina K.S.

Abstract The relationship between space weather and earthquakes occurring in seismically active areas with the seismo-electromagnetic satellite CSES-01 data are observed. The study assumes the presence of possible ionospheric precursors of strong earthquakes. The sets of measuring instruments of the satellite and data are considered. Subsequently, to prove the non-random nature of the space weather effects, the results of ground-based geomagnetic or electromagnetic observations should be used, for example, magnetotelluric soundings (MTS). Stationary and regime points of the MTS network of the Research Station RAS in Bishkek (RS RAS) are suitable for these purposes in the Northern Tien Shan. The MTS data are presented in the form of hourly frequency-time series with system rotation from 0° to 180°. To obtain information on earthquakes, it is proposed to use data from the NEIC or ISC seismic catalog, since they aggregate data from regional catalogs, including the KNET seismological network maintained by the RS RAS. It is concluded that it is necessary to select the CSES-01 trajectories and times of flight over the territory of Kyrgyzstan and download the electric field detector (EFD) data. The results of comparing satellite and ground-based observations can be used in future technologies for short-term earthquake prediction.

Rybin A.K., Bataleva E.A., Nepeina K.S., Matyukov V.E.

This article consolidates the results of studying the deep structure of the lithosphere of the Central Tien Shan, which aimed to identify the main tectonic elements in its geophysical models. We have compared the structural and geological data with the information on the deep structure obtained by geophysical methods and from the positions of earthquake hypocenters in the study area. According to geological concepts, the Tien Shan orogenic belt is characterized by longitudinal and transverse segmentation. The boundaries of the Northern, Middle, Southern Western and Eastern segments of the Tien Shan are deep-seated fault structures. In deep faults and channels of heat and mass transfer, endogenous processes are localized. High-velocity, geoelectrical and thermal models consider such faults and channels as contrasting objects that can be referred to as indicators of these processes.Our analysis of the locations of earthquake hypocenters from NNC, KNET, CAIIG, KRNET, SOME catalogues shows that seismic events are strongly confined to the fault zones and the boundaries of large blocks. A correlation between the anomalies of geophysical fields suggests the degree of inheritance of tectonic structures and the boundaries of the main tectonic segments of the Tien Shan. To compare the crustal and upper mantle heterogeneities reflected in different geophysical fields, we have analyzed seismic tomographic sections based on volumetric seismotomographic models geoelectric and velocity sections along profiles across the main tectonic elements of the study area. The sections are used to identify the zones with relatively low (i.e. reduced) seismic wave velocities and detect the deep-seated longitudinal segmentation of the folded belt. Objects showing anomalous seismic wave velocities are found in the seismotomographic sections at all the depths under consideration. The most contrasting differences in the velocities of P- and S-waves are typical of the depths of 0-5 km and 50-65 km, showing the most clearly observed Northern, Southern and Western segments of the Tien Shan. In general, the velocities of P- and S-waves at the Northern Tien Shan are higher than those at the Middle and Southern segments. We have analyzed the distribution of geoelectric heterogeneities identified from magnetotelluric sounding data in order to determine the boundaries of the main tectonic elements that are considered as the zones of increased electrical conductivity confined to the boundaries of the fault structures. The distribution of earthquake epicenters clearly reflects the segmentation of the Tien Shan into the Northern, Middle and Southern segments and shows the Western and Eastern Tien Shan relative to the Talas-Fergana fault. Ourstudies of the crust and the upper mantle of the Tien Shan have confirmed that the abovementioned tectonic segments have differences in their deep structures Based on a comprehensive analysis of the study results, we can qualitatively identify a relationship between the distribution of the velocity and geoelectric heterogeneities in the crust and upper mantle, seismicity and the stress-strain state of the crust.

Medved I., Bataleva E., Buslov M.

This paper presents new results of detailed seismic tomography (ST) on the deep structure beneath the Middle Tien Shan to a depth of 60 km. For a better understanding of the detected heterogeneities, the obtained velocity models were compared with the results of magnetotelluric sounding (MTS) along the Kekemeren and Naryn profiles, running parallel to the 74 and 76 meridians, respectively. We found that in the study region the velocity characteristics and geoelectric properties correlate with each other. The high-velocity high-resistivity anomalies correspond to the parts of the Tarim and Kazakhstan-Junggar plates submerged under the Tien Shan. We revealed that the structure of the Middle Tien Shan crust is conditioned by the presence of the Central Tien Shan microcontinent. It manifests itself as two anomalies lying one below the other: the lower low-velocity low-resistivity anomaly, and the upper high-velocity high-resistivity anomaly. The fault zones, limiting the Central Tien Shan microcontinent, appear as low-velocity low-resistivity anomalies. The obtained features indicate the fluid saturation of the fault zones. According to the revealed features of the Central Tien Shan geological structure, it is assumed that the lower-crustal low-velocity layer can play a significant role in the delamination of the mantle part of the submerged plates.

Rybin A., Bataleva E., Nepeina K., Matiukov V., Alexandrov P., Kaznacheev P.

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.

Spichak V.V.

In this Chapter we discuss the methodology for temperature forecasting from EM sounding data. To this end the data collected in two areas are used: northern Tien Shan and Hengill geothermal area. It is shown that prior knowledge of the geology and hydrological conditions in the region under study can guide the location of EM sites with respect to the locations where the temperature is to be predicted and thereby reduce the average estimation errors. The results of the electromagnetic temperature extrapolation in depth indicate that the extrapolation accuracy essentially depends on the ratio of the well depth to the extrapolation depth. Soultz-sous-Forêts (France) case study demonstrates possibility of building the temperature model of the study area up to the depth of 5 km from results of MT sounding and available temperature logs.