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Volume 31 Issue 5
Oct.  2020
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Anatoly Kuzmich Rybin, Elena Anatol'evna Bataleva, SeKseniia Nepeina Nepeina, Pavel Alexandrovich Kaznacheev, Pavel Alexandrovich Matiukov, Pavel Nikolaevich Aleksandrov. Definition of the Seismic Field of the Underground Sources in the Ambient Seismic Noise in the Tien Shan Region Using a Three-Component Gradient System. Journal of Earth Science, 2020, 31(5): 988-992. doi: 10.1007/s12583-020-1327-5
Citation: Anatoly Kuzmich Rybin, Elena Anatol'evna Bataleva, SeKseniia Nepeina Nepeina, Pavel Alexandrovich Kaznacheev, Pavel Alexandrovich Matiukov, Pavel Nikolaevich Aleksandrov. Definition of the Seismic Field of the Underground Sources in the Ambient Seismic Noise in the Tien Shan Region Using a Three-Component Gradient System. Journal of Earth Science, 2020, 31(5): 988-992. doi: 10.1007/s12583-020-1327-5

Definition of the Seismic Field of the Underground Sources in the Ambient Seismic Noise in the Tien Shan Region Using a Three-Component Gradient System

doi: 10.1007/s12583-020-1327-5
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  • 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.
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  • Aleksandrov, P. N., 2009. The Theory of Seismic and Electromagnetic Monitoring of the Modern Geodynamic Processes. Bulletin of Kamchatka Regional Association:Earth Sciences, 2(14):49-58 (in Russian with English Abstract)
    Bataleva, E. A., Przhiyalgovskii, E. S., Batalev, V. Y., et al., 2017. New Data on the Deep Structure of the South Kochkor Zone of Concentrated Deformation. Doklady Earth Sciences, 475(2):930-934. https://doi.org/10.1134/s1028334x1708013x doi:  10.1134/s1028334x1708013x
    Buslov, M. M., De Grave, J., Bataleva, E. A. V., et al., 2007. Cenozoic Tectonic and Geodynamic Evolution of the Kyrgyz Tien Shan Mountains:A Review of Geological, Thermochronological and Geophysical Data. Journal of Asian Earth Sciences, 29(2/3):205-214. https://doi.org/10.1016/j.jseaes.2006.07.001 doi:  10.1016/j.jseaes.2006.07.001
    Karplus, M., Schmandt, B., 2018. Preface to the Focus Section on Geophone Array Seismology. Seismological Research Letters, 89(5):1597-1600. https://doi.org/10.1785/0220180212 doi:  10.1785/0220180212
    Kaznacheev, P. A., Matiukov, V. E., Aleksandrov, P. N., et al., 2019. Development of a Three-Axis Gradient System for Seismoacoustic Data Acquisition in Geodynamically Active Regions. Seismic Instruments, 55(5):535-543. https://doi.org/10.3103/s0747923919050062 doi:  10.3103/s0747923919050062
    Khavroshkin, O. B., 1999. Some Problems of Nonlinear Seismology. OIFZ RAS, 286 (in Russian)
    Korn, G. A., Korn, T. M., 1968. Mathematical Handbook:For Scientists and Engineers. McGraw-Hill Book Company, New York
    Langston, C. A., 2007. Wave Gradiometry in the Time Domain. Bulletin of the Seismological Society of America, 97(3):926-933. https://doi.org/10.1785/0120060152 doi:  10.1785/0120060152
    Lin, F. C., Li, D., Clayton, R. W., et al., 2013. High-Resolution 3D Shallow Crustal Structure in Long Beach, California:Application of Ambient Noise Tomography on a Dense Seismic Array. Geophysics, 78:45-56. https://doi.org/10.1190/geo2012-0453.1 doi:  10.1190/geo2012-0453.1
    Maeda, T., Nishida, K., Takagi, R., et al., 2016. Reconstruction of a 2D Seismic Wavefield by Seismic Gradiometry. Progress in Earth and Planetary Science, 3(1):31. https://doi.org/10.1186/s40645-016-0107-4 doi:  10.1186/s40645-016-0107-4
    Moura, R. M., Senos Matias, M. J., 2012. Geophones on Blocks:A Prototype Towable Geophone System for Shallow Land Seismic Investigations. Geophysical Prospecting, 60(1):192-200. https://doi.org/10.1111/j.1365-2478.2011.00963.x doi:  10.1111/j.1365-2478.2011.00963.x
    Picozzi, M., Parolai, S., Bindi, D., et al., 2009. Characterization of Shallow Geology by High-Frequency Seismic Noise Tomography. Geophysical Journal International, 176(1):164-174. https://doi.org/10.1111/j.1365-246x.2008.03966.x doi:  10.1111/j.1365-246x.2008.03966.x
    Schmelzbach, C., Donner, S., Igel, H., et al., 2018. Advances in 6C Seismology:Applications of Combined Translational and Rotational Motion Measurements in Global and Exploration Seismology. Geophysics, 83(3):WC53-WC69. https://doi.org/10.1190/geo2017-0492.1 doi:  10.1190/geo2017-0492.1
    Sobolev, G. A., Ponomarev, A. V., Kol'tsov, A. V., et al., 2001. Excitation of Acoustic Emission by Elastic Impulses. Izvestiya-Physics of the Solid Earth, 37(1):73-77 http://www.researchgate.net/publication/290200218_Excitation_of_acoustic_emission_by_elastic_impulses
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Definition of the Seismic Field of the Underground Sources in the Ambient Seismic Noise in the Tien Shan Region Using a Three-Component Gradient System

doi: 10.1007/s12583-020-1327-5
    Corresponding author: Kseniia Sergeevna Nepeina, ORCID: 0000-0003-0725-8023, nepeina.k@mail.ru

Abstract: 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.

Anatoly Kuzmich Rybin, Elena Anatol'evna Bataleva, SeKseniia Nepeina Nepeina, Pavel Alexandrovich Kaznacheev, Pavel Alexandrovich Matiukov, Pavel Nikolaevich Aleksandrov. Definition of the Seismic Field of the Underground Sources in the Ambient Seismic Noise in the Tien Shan Region Using a Three-Component Gradient System. Journal of Earth Science, 2020, 31(5): 988-992. doi: 10.1007/s12583-020-1327-5
Citation: Anatoly Kuzmich Rybin, Elena Anatol'evna Bataleva, SeKseniia Nepeina Nepeina, Pavel Alexandrovich Kaznacheev, Pavel Alexandrovich Matiukov, Pavel Nikolaevich Aleksandrov. Definition of the Seismic Field of the Underground Sources in the Ambient Seismic Noise in the Tien Shan Region Using a Three-Component Gradient System. Journal of Earth Science, 2020, 31(5): 988-992. doi: 10.1007/s12583-020-1327-5
  • Ambient seismic noise could be effective in monitoring present-day geodynamic processes in the areas with the known geologic features. We suggest the new approach of the microseism energy estimation for underground sources with the passive seismic method of gradient system. This signal processing method is also related to the seismic gradiometer array by Langston (2007) to study the site effects on the plane or 2D seismic wavefield reconstruction as Maeda et al. (2016). Such studies have not been performed in the territory of Russia and the former USSR. In Kyrgyzstan, we have applied such a method and its practical implementation for the first time, but with seismic sounding. It is shown, that the combination of methods allows using the synchronous noise recordings, eliminates the influence of horizontal seisms on the signal, and we could numerically separate the vertical component of the wave field in the ambient seismic noise.

    In this experiment, we study the microseisms registered on the ground surface by the passive seismic method. Due to the high seismic and geodynamic activity, the inland Tien Shan Orogen is the largest geodynamic and prognostic testing site, where not only the studies of deep structure, the latest tectonics and seismicity are intensively performed, but also various approaches to the study of deformation processes are being considered. The reasons for microseism occurrence in the region could be the non-volcanic tremor, microearthquakes or seismic emission. This activity describes the earthquake preparation process. Therefore, we expect to find a source of stimulation in the Earth's deep interior. For this reason, we conducted several field experiments in the Kyrgyz Ala-Too Range in 2017 and 2018. The field recording of the seismic field was carried out by a three-component gradient measurement system using the GS-20DX geophones in the observation stations of the Northern Tien Shan. We continuously measure the wave field on the ground surface using the GS-20DX geophone array. Due to several assumptions, the control over the low-frequency signals of ambient seismic noise can be maintained. We apply the gradient system and extract part of the seismic field from the underground sources. Then we propose a quantitative measure for the energy of underground sources.

    The gradient system allows determining all components of the displacement velocity vector and up to 9 spatial derivatives. We have proposed a solution to the problem of seismic field separation by the position of sources in order to separate a field of endogenous origin among the observed fields on the daylight surface. As a result of raw data processing, we have obtained the energy characteristics of a field of endogenous origin based on the seismic wave field, observed on the daylight surface for all the measured components. We note that all the energy characteristics of the seismic field of endogenous origin for each of the three field components are changed synchronously. This indicates the availability of a powerful source that is shown on both vertical and horizontal components.

    The microseism studies in the geodynamically active regions are especially interesting. Our research team is located within the Bishkek Geodynamic Polygon (BGP) in Tien Shan (Fig. 1). The Tien Shan orogenesis is relatively young (about 10 million years) and is characterized by strong seismotectonic activity (Bataleva et al., 2017; Buslov et al., 2007). This chain of mountains is divided by a system of active faults. This fact, with high seismicity, justifies the availability of the actual stress-strain behaviour of the site. There are no industrial enterprises in the territory of the BGP. For this reason, we use the internal process manifestation concept as the main one. Beyond any reasonable doubt, in Tien Shan, we can observe evidence of intraterrestrial (endogenous) activity in the wave noise field. The most interesting facility in the territory of the BGP is the Kyrgyz Ala-Too Ridge. There is a system of well-known faults in the vicinity of the Research Station RAS. Many authors have identified and proved the existence of multiple open fracture systems in the rock formations (e.g., Aleksandrov, 2009). Therefore, while determining the specifications, we can provide passive monitoring of the elastic wave propagation in the medium.

    Figure 1.  Map of study area of Bishkek Geodynamic Polygon (BGP) in the Northern Tien Shan. 1. Main experimental points; 2. cities; 3. border of Kyrgyzstan.

  • The multiple open fracture system leads to the occurrence of microearthquakes and, consequently, to the generation of electromagnetic and seismoacoustic fields. The seismoacoustic phenomenon could relate to the flow of seismic signals from the deep underground or external sources (Sobolev et al., 2001; Khavroshkin, 1999; etc.). The working hypothesis is based on the idea of fracture migration. The fracture closure and disclosure are presumed as a moving source. Further, it is assumed that the elementary sources generate the impulse-type signals, have wide frequency spectra, and are randomly distributed in space. The format of the crack should assume an ideal penny-shaped inclusion near to the ellipsoidal format. Accordingly, the dynamic propagation of microfractures leads to the elastic wave generation. The passive seismic monitoring is one of the methods for its recording. Therefore, we have decided to conduct several experiments using passive seismic methods. The main purpose of the description of destruction features is to identify and localize the deep source. Therefore, we need to separate the vertically propagating impulse signals from the lateral and external movements.

    Having considered the fact that it is difficult to assess the anisotropy parameters on a practical level, these assessments, based on the effective medium models, can be used to facilitate the interpretation of seismic anisotropy using the seismic and seismological data (earthquakes or seismic phenomena).

    The proposed method concept is to use the gradient system for selecting the signals propagating in the sub-vertical direction to the surface. Obviously, we also receive the reflected seismic fields from other sources, including the daylight surface. However, due to the long measurement interval, these fields are considered small. Then their influence on the final data processing result was not considered.

    Let us assume that the wave front of a plane wave is u(x, y, z, t), where t is time, x, y, z are the Cartesian coordinates. Talking about wave description, we could point a wave peak. It is a constant fixed point in space. We could trace the peak motion. Assuming this point is a multispatial point (x, y, z, t). If at the point in time we set this function u(x, y, z, t) as a constant u(x, y, z, t)=const(x, y, z, t), the complex function appears in the form. For this function, any partial derivative is equal to zero (Korn and Korn, 1968). If we differentiate this function in time (Korn and Korn, 1968)

    where V is the velocity vector of wave front.

    Here we assume finite-differential schema. If we set the velocity vector of the elastic wave V as follows

    where i, j, k are the unit normal vectors in the Cartesian reference system, $ \tilde u = \{ {\boldsymbol{S}_x}, {\boldsymbol{S}_y}, {\boldsymbol{S}_z}\} $ is any component of the displacement vector S. These components Sx, Sy, Sz are measured by the gradient system.

    Thus, for each component of the displacement vector S, we obtain three vectors of the phase velocity

    Vx for transverse horizontal Sx component (North-South direction), Vy for another transverse horizontal Sy component (East-West direction), Vz for vertical Sz component.

    The derivatives in (6) can be numerically determined as follows (for example, the North-South direction (N-S) horizontal component Sx of displacement vector S)

    where Δt is the data discretization; Δx is the array aperture (point-to-point distance); Δ(Sx)1 and Δ(Sx)2 are the shifts between the component values at the initial and final points on the aperture (Fig. 2).

    Figure 2.  Scheme of the three-component seismic gradient array system. The distance between the connected straight lines of the three-component seismic receivers is 1 m. The X axis is directed to the North.

  • The self-contained geophones have been used for decades to solve seismic problems with the active sources (for example, Karplus and Schmandt, 2018). However, technological innovations in the geophone instrument engineering have allowed them to be used for a wide range of the passive seismic surveys (for example, Lin et al., 2013). The geophones are used in two ways: as the separate sensors and as the coupled groups (for example, Schmelzbach et al., 2018; Moura and Senos Matias, 2012). The use of such tools requires the accuracy of the source location determination of approximately 1/10 of the system aperture (base, width). The experience of using geophone arrays in the different geological conditions is of considerable interest (Schmelzbach et al., 2018; Picozzi et al., 2009; Langston, 2007). For this reason, we have decided to study the seismic activity of the Bishkek Geodynamic Polygon using the geophone array (group). There are no previous similar studies in this area using the geophone arrays.

    It is assumed that the gradient system records the continuous seismic signals in the noise component on the ground surface. When designing the system, the following requirements should be considered. First of all, there are three orthogonal directions (the wave field components). For this reason, we connect three geophones at one point. These components shall be recorded independently on separate channels. Further, all data must be synchronously recorded at several points in space (at least 6 points). We have calculated the total wave field of the total ambient noise into two parts. One part represents the external and side effects. The other part consists of a nearly vertical upward flow of microseisms. This part of the wave field is the research objective. We have expected to find any disturbances in the field of ascending (vertically upward propagating) waves. It can be not only the volcanic tremor or seismic emission. Obviously, both processes specify the seismotectonic excitation. Subsequently, the results may indicate the earthquake preparation or other internal processes. In accordance with this objective, we have carried out several field experiments for the BGP area in 2017 and 2018. We have deployed the geophone array for several days at different observation points in the Kyrgyz Ala-Too Ridge, in order to verify the theory of the vertical gradient availability in the field of ambient seismic noise using the seismic record stream.

    As a result, the seismic gradient system has been developed (Kaznacheev et al., 2019). It consists of the GS-20DX geophone array (Fig. 2). The GS-20DX model manufactured by Geospace Technologies (USA) is a digital geophone. We have selected 18 out of 30 geophones with the most identical transfer specifications. For this selection, we provide the resemblance effect test in the unit conditions. These 18 geophones are connected as a triple combinations at 6 different points. All geophones are space-stabilized by three orthogonal directions (two transverse N-S, E-W and one vertical Z). They measure the drift velocity. The geophones are placed not on the plane, but in the three-dimensional space. It helps us to calculate the spatial derivatives. The recording system for this project is built using a personal computer (laptop). It maximizes interchangeability, efficiency, and operational modification. However, the use of small geophone arrays in this study does not require the earthquake location. We could use a very small system aperture (dimension or point-to-point distance) that corresponds to the expected source wavelength related to the fracturing. The system aperture is 1 m. Thus, the system dimensions cover an extremely small average volume of 1×1×1 (m). The volume can be considered isotropic in comparison with the dimensions and contrasts of the regional geological structure. The fracture scale ranges from 10-2 to 10-1 m. The generated wavelength is about from 1011 to 102 m. The data is digitized with a frequency of (0.8 Hz) and synchronized. The audio-analog-digital converter (ADC) input is used for digitizing the signal from the sensors. In order to ensure channel switching, an external industrial analog-digital signal switch AIMUX32C-2 is installed. The matching units provide the low-frequency signal filtration. Due to these factors, the signal has a significantly more uniform response in the frequency range from 10-3 to 101 Hz than the original GS-20DX. As an advantage of such a geophone connection, we get a wider amplitude-frequency characteristic (AFC) for the system (Fig. 3).

    Figure 3.  Amplitude-frequency response (AFR) for: 1. GS-20DX geophone gain, 2. high frequency seismic noise value, 3. output background signal of the one gradient array system channel. 1 and 3 were normalized to the maximum value, 2 was normalized to value at 1 mHz; a.u. arbitrary unit.

  • It as known that the wave propagation direction is determined by the normal vector $ \boldsymbol{n} = \frac{\boldsymbol{V}}{{\left| \boldsymbol{V} \right|}}$, where Vx, Vy, Vz are the velocity vectors, calculated in Eq. (6). Then, for each orthogonal component, we obtain the following normal vectors:

    These vectors nx, ny, nz do not depend on the offset value, have the unit length, and describe only the wave propagation direction. The seismic data is measured in the Cartesian coordinates, but for the issue, it is easier to use the spherical coordinate system. Due to this conversion, we can select the impulse signals from the lower half-space to the observation station. Therefore, after such data processing, we get the resulting image of a set of points in the spherical coordinate system. An example of such an analysis is shown in Fig. 4. Further, we emphasize only those impulses that approach the sector with the angular dimensions of about π/5 steradians. It means almost vertical direction. Therefore, when selecting these vectors, we collect information about the activities of underground sources.

    Figure 4.  Source data (top 6 graphs, (channels 7–12)) and gradient array system results (bottom 6 graphs, (channels 1–6)) as measured by component: (а) Sx; (b) Sy; (с) Sz. Chon-Kurchak experiment site. Bottom part of the figure presents seismic field of deep underground sources. The dots indicate vectors nx, ny, nz the direction of the seismic wave front, determined for each registration component.

    Each selected impulse signal contributes to the full low energy field. Their contribution in total leads to the total ambient seismic noise of the internal part of the Earth. These components ($\widetilde {{\boldsymbol{S}_x}}, \widetilde {{\boldsymbol{S}_y}}\widetilde {, {\boldsymbol{S}_z}}$) separated from the full field Sx, Sy, Sz are called the seismic field of underground sources (of endogenous origin). For its quantitative measurement, we propose to measure the amplitude value. We introduce the energy characteristics of the seismic field of underground sources, calculated for each measured component $\widetilde {{\boldsymbol{E}_x}}\left(\boldsymbol{t} \right), \widetilde {{\boldsymbol{E}_y}}\left(\boldsymbol{t} \right)\widetilde {, {\boldsymbol{E}_z}}\left(\boldsymbol{t} \right)$. It is calculated as the average sum

    where ($\widetilde {{\boldsymbol{S}_x}}, \widetilde {{\boldsymbol{S}_y}}\widetilde {, {\boldsymbol{S}_z}}$) are the components of the seismic field of underground sources; τ is the time interval (equal to 1 h).

  • Finally, we observe three energy characteristics. It shall be noted that the energy characteristics for the initial full wave field are almost equal to zero. On the contrary, the energy characteristics of the separated seismic field of underground sources are different from zero. The energy characteristic curves (Fig. 5) demonstrate synchronous changes in time with three orthogonal components. This fact justifies the evidence of the time dependence of deep sources. Certainly, the gradient system is an excellent tool for studying the internal processes of the Earth.

    Figure 5.  The energy characteristic curves for seismic field of deep underground sources, calculated at each one hour: 1. Sx; 2. Sy; 3. Sz. Chon-Kurchak experiment site. Observation time interval is 60 h.

  • We have proposed the working hypothesis of the occurrence of impulse-type seismic signals that can be generated by the irreversible geodynamic processes in the stress-strain medium, for example, by the fracturing processes. We have developed the theoretical framework for studying the deformation processes in the Earth's lithosphere. In order to study the deformation processes in the Earth's crust of the seismically active zone in the Northern Tien Shan (Kyrgyz Ala-Too Ridge), we have performed monitoring observations of the low-frequency microseismic oscillations recorded by the three-component gradient measurement system using an array of GS-20DX geophones. The experience of using the GS-20DX geophone arrays in the complex geological conditions of the Tien Shan intracontinental orogen for researches has been gained.

    The data obtained are the low-frequency signals of ambient seismic noise. They may indicate the earthquake preparation or other internal processes occurring in the crust of the Northern Tien Shan.

    The seismic data relating to the seismic field separation by the source position has been processed in order to separate the components of the field of endogenous origin. The processing algorithm is based on the seismic field filtration in the direction of the elastic field propagation and selection of sub-vertically propagating waves. The main advantage of the proposed method is the avoidance of using the frequency domain in calculations, since we use only the time domain. It helps us to save information about the behaviour of field characteristics. As a result of field record processing, we have obtained the energy characteristics of the field of endogenous origin based on the seismic wave field observed on the Earth's surface for all measured components.

    Having analyzed it, we notice the non-zero contribution of internal sources. They are synchronously changed in time and show the underground activity. This activity specifies the deep underground process. On the basis of the developed technology, in the future it is planned to conduct a comprehensive analysis of the geological and geophysical information. These results help us to understand the development of modern geodynamic processes in the lithosphere.

  • The data processing programs, development and implementation of the gradient array system are partially performed with the grant support from the Russian Foundation for Basic Research (No. 20-05-00475). The subjects relating to the correlation of geophysical parameters with the average stress-strain behaviour of the geological environment are explored within the Russian State Governmental Task of the Research Station of the Russian Academy of Sciences (No. AAAA-A19-119020190063-2). We are also thankful to the reviewers for their comments and the Editor-in-Chief and all editors of the Journal of Earth Science enabling our research to be published. The final publication is available at Springer via https://doi.org/10.1007/s12583-020-1327-5.

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