Microseismic Sounding Research Articles

Model of a New Peripheral Near-Surface Magmanic Chamber of the Elbrus Volcanic Center

Intracrustal magmatic structures have pronounced resonant properties, due to which these structures are able to generate secondary seismic waves at their own frequencies. Based on strain data obtained with a 75-meter laser interferometer ‒ strainmeter, resonance modes are identified and parameters of magmatic structures of the Elbrus volcanic center are estimated. Such resonant modes are unique for each magmatic formation and determine the size and physical-mechanical properties of this magmatic structure. This paper analyzes the local formation, which manifested itself as a some compact area of numerous weak seismic pulses registered in the Elbrus region by the seismometers of the Geophysical Service of the Russian Academy of Sciences in 2011. The results of studies based on seismic and strain data, as well as the results of microseismic sounding, showed the presence of a new peripheral near-surface magmatic chamber in the Elbrus volcanic center. The characteristic size of this new magmatic chamber is 2.5‒3 km.

Karst detection by geophysical methods using microseismic sounding

Research aim is to test a technique for detecting buried karst using microseismic sounding based on spectral amplification of horizontal displacements. The main research tasks include the construction and analysis of geoelectric and microseismic sections. Methodology. The microseismic background was recorded using the OMAR-2s autonomous seismic station (Institute of Geophysics Ural Branch RAS, Ekaterinburg) in the frequency band 1–1000 Hz. Low-frequency horizontal seismic receivers from OYO Geospace Corporation were used as sensors. Vertical electrical sounding (VES) was carried out by a set of ERA-MAX equipment (SPC “ERA”, St. Petersburg) using a symmetrical Schlumberger installation at a frequency of 4.88 Hz, at currents of 5–50 mA. VES processing consisted in quantitative one-dimensional inversion according to the IPI2Win program (Moscow State University, Moscow). Results. Experimental and methodological work on the study of microseisms, accompanied by VES, was carried out on two karst sites of the Sverdlovsk region. Based on the results of processing, geoelectric and microseismic sections were constructed to determine the conditions of occurrence of bedrock and loose sediments covering them, as well as to identify local anomalies associated with karst manifestations. The results of electrical exploration and seismometry are in good agreement with each other and are confirmed by geological data. Conclusions. Vertical electrical sounding makes it possible to distinguish karst by the high contrast of the resistances of the host bedrock (limestone) and terrigenous filler material (clay, loam). Microseismic sounding makes it possible to delineate karst zones by high values of spectral amplification of weakly consolidated karst deposits, relatively low levels of microseisms in bedrock. Thus, on the basis of complex electrical and microseismic soundings, it is possible to confidently identify the areas of development of karst processes.

Изучение сейсмической реакции грунтов при микросейсмических зондированиях плотин

The basic fundamentals of spectral analysis of horizontal and vertical components of microseismic noise for studying the geological structure of dams are presented. The issues of constructing deep sections of three dynamic parameters of microseisms are considered. The aim of the work is to study the seismic reaction of soils during microseismic sounding of hydraulic structures. The main objectives of the research include the comparison of deep sections of soil reinforcement coefficients with sections of transmission ratios and data from other geophysical methods. As a result of microseismic sounding of three hydraulic structures, it was found out that the seismic reaction of the clay core of dams differs significantly from the surrounding soils. The amplification coefficients in the bedrock and dispersed soils of the aeration zone are characterized by minimal values. For water-saturated clays, these values are 5–10 times higher, which may indicate that the main reason for the increase in the level of microseismic noise is an increase in the overall porosity and moisture of soils. It is noted that the amplification coefficients of horizontal microseisms are more noise-resistant and detailed compared to the parameters of vertical microseisms. It is stated that deep sections of dynamic microseism parameters allow us to confidently distinguish the main elements of the structure of soil dams, as well as to identify probable zones of increased filtration.

ОПЫТ КОМПЛЕКСИРОВАНИЯ МИКРОСЕЙСМИЧЕСКОГО И МАГНИТОТЕЛЛУРИЧЕСКОГО ЗОНДИРОВАНИЯ НА УЧАСТКЕ СЕВЕРНОГО ФЛАНГА ЦЕНТРАЛЬНОГО СИХОТЭ-АЛИНСКОГО РАЗЛОМА

We present the results of a study of the crustal structure of the northern part of the Central Sikhote-Alin Fault (CSAF) by methods of microseismic sounding (MSS) and magnetotelluric sounding (MTS). A geoelectric section based on MTS data and a section of relative velocities of P-waves according to MSS data were constructed and interpreted at a depth of up to 9 km and a length of 42 km. The main blocks, their boundaries, fault zones and some anomaly zones identified by microseismic and magnetotelluric sounding practically coincide. The CSAF zone is expressed by a narrow subvertical zone between high resistivity blocks. The data obtained indicate that the fault zone in the study area is impermeable. A similar structure was identified 6 km northwest of the CSAF zone, which can be traced to twice the depth (up to 20 km). It is concluded that the combination of microseismic and magnetotelluric sounding methods is promising for studying the structure of the Earth's crust in fault zones.

Deep Structure of the Northwestern Termination of the Caucasus from New Geological and Geophysical Data

Abstract—In 2014–2019, integrated geological and geophysical studies were carried out in the largest flexural-rupture and fold-and-fault zones of the Northwest Caucasus (the Anapa, Akhtyr, Moldavanovka, Karabetov, Fanagoria and Azov anticlinal ridges and the Pshekha–Adler, Kerch–Tuzla, and Kerch Strait faults). Microseismic sounding method (MSM) was the main geophysical approach. The MSM studies identified the deep structural features of the Earth’s crust in the study region and made it possible to correlate them with the specific tectonic structures on the surface. The correlation was established by reconciling the results of the MSM studies, the parameters of sedimentary cover section, and crustal discontinuities revealed by drilling and previous studies of the converted waves from the earthquakes. It is found out that the Anapa flexure and longitudinal tectonic zones have the distinct deep roots beneath them. The trans-Caucasian Anapa flexural-rupture zone separates the pericline of the Northwest Caucasus from the region of the Taman Peninsula, whereas longitudinal flexures and fault structures divide the meganticlinorium from the subsided West Kuban and Azov blocks of the northern slope of the folded system and from the Kerch–Taman trough of the southern framing of the Northwest Caucasus. The faults in the study region are distinguished into deep faults penetrating into the lower crust and even reaching the upper mantle and near-surface faults localized within the sedimentary cover in the Earth’s interior. The seismogenic role of these tectonic deformations in the studied seismic region is determined.

EXPERIENCE OF USING ELECTRICAL TOMOGRAPHY, RADON SURVEY AND MICROSEISMIC SOUNDING IN SEARCH FOR PIPES CONTROLLED BY FAULT ZONES

The complex of geophysical methods was successfully applied in the Alakit-Markha kimberlite field of the Yakutsk diamondiferous province. A pipe was identified in the local forecast area specified within the field at the previous stage of prospecting. The studies using electric tomography (ET), radon survey (RS) and microseismic sounding (MS) covered an area of ≈1000⨯500 m. Based on the jointly processed ET and RS plan images and MS profiles, a tubular body was detected in the sedimentary cover. It is confined to a fault node and stands out among the host rocks by low electrical resistivity, increased soil radon concentrations and high spectral ratios of the horizontal and vertical components of microseisms. Its complex shape is manifested at the ground surface by two isometric structures (each being several hundred meters long), which jointly form a dike-like body at a depth of ≈40 m, as shown by the electrical tomography images. According to the MS data, its root part is detected to comprise one or two fractured narrow zones that are traceable to a depth of 2 km and below. The structure of the identified body and its chemical composition will be determined after exploratory drilling is complete. Today there are grounds to conclude that using ET, RS and MS methods jointly as a complex described in the article proved effective for identifying pipes controlled by fault zones in the sedimentary cover areas.

Influence of the Contribution of Body Waves to the Result of the Microseismic Sounding Method

The article presents a numerical study of the formation of the amplitude response in a free surface from simultaneous scattering of Rayleigh waves and vertically incident longitudinal waves by an embedded contrast velocity inclusion. It has been established that the significant presence of body waves in a microseismic field does not fundamentally change the result of the microseismic sounding method, which is based on the notion of the overwhelming contribution of the fundamental mode of a Rayleigh wave to the formation of the Earth’s microseismic field. Cases are considered when a microseismic signal at the same frequency is modeled only by the fundamental mode of the Rayleigh wave, only by a vertically incident longitudinal wave, and by both types of waves simultaneously. Variants of inhomogeneities with different dimensions and velocity properties are considered. The analysis was performed in a (λ, r)-space, in analogy with reconstruction of the structure of the geological setting in the microseismic sounding method, where λ is the wavelength of the fundamental Rayleigh mode and r is the coordinate on the Earth’s surface.

Microseismic Sounding and Active Faults of the Kerch-Taman Region

The results of integrated geological and geophysical studies of active faults and deep structure of the Kerch-Taman region are presented. Data on the deep structure of potential sources of strong earthquakes determining the level of seismic hazard are obtained. Examples of the active faults, well studied on the surface, show that these structures are clearly spotted by microseismic sounding method as the displacements of contrasting layers in the section at a depth of a few km or as narrow low-velocity bodies. In the zones of the Tobechik, South Azov, and Phanagoria faults at depths of 7–12 km, high-velocity isometric inclusions are established. These segments are especially important for making ther repeated MSM measurements for seismic forecasting purposes.

Deep structure of the Anapa flexural-rupture zone, Western Caucasus

In the period from 2007 to 2017 complex geological and geophysical studies were carried out in the three largest flexural-rupture fault zones in the North-West Caucasus (Anapa, Akhtyrka and Moldavan). The micro-seismic sounding (MSM) was used as the main geophysical method. Studies with the help of MSM allowed us to identify the features of the deep structure of the earth’s crust in the study area and to associate them with specific tectonic structures on the surface.The binding was carried out by harmonizing the results of the MSM and the parameters of the section of the sedimentary cover and crustal boundaries according to the drilling data and the work previously performed by the reflected wave method (MOVZ). It was found that the Anapa flexure and longitudinal tectonic zones have clear deep roots, and also separate the pericline of the North-Western Caucasus from the Taman Peninsula and from the lowered blocks of the Northern slope of the folded system.Faults in the study area are divided into: (1) deep faults of the Caucasian stretch, penetrating into the lower crust and even to the upper mantle, and (2) near-surface faults, do not extend to the depths beyond the thickness of the sedimentary cover. The seismogenic role of these tectonic disturbances in the studied seismically active region has been determined.

Reconstruction of Inhomogeneities of a Medium During Microseismic Sounding Along a Curvilinear Surface

The paper presents the results of numerical two-dimensional modeling of Rayleigh wave propagation along the curvilinear boundary of a solid ideal medium with a contrasting inclusion. It is shown that ignoring the influence of both a convex and concave relief when interpreting microseismic sounding data can lead to errors for a wide range of reliefs and contrast between an inclusion and the surrounding medium; additional corrections during data processing make it possible to account for or even eliminate these errors in the entire considered parameter range.

Deep Structure of the Anapa Flexural–Rupture Zone, Western Caucasus

In the period from 2007 to 2017 complex geological and geophysical studies were carried out in the three largest flexural-rupture fault zones in the North-West Caucasus (Anapa, Akhtyrka and Moldavan). The micro-seismic sounding (MSM) was used as the main geophysical method. Studies with the help of MSM allowed us to identify the features of the deep structure of the earth’s crust in the study area and to associate them with specific tectonic structures on the surface.The binding was carried out by harmonizing the results of the MSM and the parameters of the section of the sedimentary cover and crustal boundaries according to the drilling data and the work previously performed by the reflected wave method (MOVZ). It was found that the Anapa flexure and longitudinal tectonic zones have clear deep roots, and also separate the pericline of the North-Western Caucasus from the Taman Peninsula and from the lowered blocks of the Northern slope of the folded system.Faults in the study area are divided into: (1) deep faults of the Caucasian stretch, penetrating into the lower crust and even to the upper mantle, and (2) near-surface faults, do not extend to the depths beyond the thickness of the sedimentary cover. The seismogenic role of these tectonic disturbances in the studied seismically active region has been determined.

Geohydroacoustic Noise Monitoring of Under-Ice Water Areas of Northern Seas

The paper presents the results of theoretical and experimental research into the structure of geohydroacoustic wave fields generated in continuous ice-covered northern seas. A simplified mathematical model is constructed that takes into account experimental data demonstrating that the generation of different types of geohydroacoustic waves in the lithosphere–hydrosphere–ice cover system are primarily influenced by the water layer with the ice cover. The seafloor structure mainly affects the characteristics of propagating waves rather than the generation of new modes. Mathematical modeling results have laid the basis for new technologies to localize inhomogeneities in ice-covered water areas. The main distinguishing feature of this novel technology for monitoring a medium under ice-covered marine conditions is the possibility to measure noise signal parameters without active geohydroacoustic emission sources. Methods that measure the characteristics of surface-type waves are the most promising for use in northern sea conditions, in particular, microseismic sounding and noise tomography. Integration of these methods combines the recent achievements of passive geophysics and takes into account the particularities of underwater acoustics. To obtain information on the wave propagation medium, both the wave field amplitude and phase characteristics are used. To detect particular types of waves in records, spatiotemporal signal processing methods are used with the appropriate choice of frequency range. The authors describe their new-generation seismohydroacoustic information-measuring modules (embedded buoys), which are equipped with vector and molecular-electronic primary transducers. The information-measuring modules are designed for combined use with distributed ice-class arrays capable of monitoring continuous ice-covered northern seas year round. Studies of how ice-embedded information-measuring systems function, as well as verification of the obtained theoretical results, were carried out during field tests in February 2017. At each measurement point, the receiver system consisted of three reference devices that took measurements on the seafloor, in the water column, and on the ice surface. Mockups of the tested geohydroacoustic buoys were embedded at points offset by 1 km. Dropped 32 kg weights were used as the sources. Controlled perturbations in the ice experiments made it possible to obtain qualitative spectrograms of geohydroacoustic perturbations in layered structures and analyze the dispersion curves. When the fundamental bottom modes were studied, the signal source consisted of an underwater charge at a depth of 10 m. The embedded seismohydroaoustic information-measuring modules successfully passed the ice-based tests in field conditions at low temperatures, demonstrating the reliability of the obtained seismohydroacoustic information. The experimental data agree very well with theoretical estimates obtained with the created model of a layered geological medium. These studies demonstrated that natural sea noise contains useful information reflecting the internal structure of the seafloor and the water layer and led to development of the instrumental and methodological foundations of a noise technology for localizing inhomogeneities in the aquatic environment and layered bottom structures of northern seas by means of passive microseismic noise monitoring.

Taking into Account the Influence of Relief in the Method of Microseismic Sounding

The traveling of surface waves in a model geophysical medium with a nonsmooth boundary is studied. It is shown that relief significantly distorts the microseismic noise field recorded at the day surface. This result should be taken into consideration when interpreting the data of passive surface-wave geophysical studies of deep crustal inhomogeneities.

On the Issue of Taking into Account the Influence of the Relief in the Method of Microseismic Sounding

On the Issue of Taking into Account the Influence of the Relief in the Method of Microseismic Sounding

Deep Structure of the Zone of Tolbachik Fissure Eruptions (Kamchatka, Klyuchevskoy Volcano Group): Evidence from a Complex of Geological and Geophysical Data

With the use of the method of low-frequency microseismic sounding, the configuration of the magmatic feeding system of the Tolbachinsky Dol—a regional zone of areal basaltic volcanism in the southern part of the Klyuchevskoy volcano group in Kamchatka—is studied. The initial data are obtained by a stepby-step recording of the background microseismic noise in 2010–2015 within a thoroughly marked-out survey area covering the zones of fissure eruptions in 1975–1976 and 2012–2013 and, partly, the edifice of the Ploskii (flat) Tolbachik volcano. The depth sections reflecting the distributions of the relative velocities of seismic waves in the Earth’s crust are constructed. For a more reliable interpretation of the revealed deep anomalies, the results of independent geological and geophysical studies are used. The ascertained low-velocity structures are closely correlated to the manifestations of present-day volcanism. It is shown that the feeding structure of the Tolbachinsky Dol is spatially heterogeneous, incorporating subvertical and lateral pipeshaped magma conduits, closely spaced magma feeding channels, and shallow magma reservoirs. A longlived local transcrustal magma conducting zone is revealed, and regularities in the deep structure of the feeding systems of fissure eruptions are identified. The configuration of the established subvertical magma conduits permits basalts moving to rise to the surface by different paths, which, inter alia, explains the contrasting magma compositions observed during a single eruption. Thus, based on the instrumental data, it is shown that the magmatic feeding structure of the Tolbachinsky Dol has a number of specific peculiarities and is significantly more complicated than has been previously thought about the areal volcanic fields.

Model of the Deep Structure of the Elbrus Volcano in the Light of New Geophysical Data

A geophysical section by the method of microseismic sounding is obtained along the profile that completely crosses the Elbrus volcano in the area of its eastern summit. The peculiarities of the deep structure and edifice of the Elbrus volcano are revealed.

Magmatic plumbing systems of the monogenetic volcanic fields: A case study of Tolbachinsky Dol, Kamchatka

Clusters of small-volume volcanoes that individually may be defined as monogenetic, but have interlinked and interconnected plumbing systems, are used to be categorized as monogenetic volcanic fields (MVF). We argue that such volcanic clusters should be distinguished as separate type of volcanism, intermediate between monogenetic and polygenetic. The magma plumbing system structure of the MVF (its complexity and polymagmatic character) is the key argument for the potential separation of them in a classification. To avoid confusion caused by genetic meaning of the used words we suggest using a term “areal volcanism” or “areal volcanic fields” (AVF instead of MVF) as defining this special type of volcanic activity. Here we provide a review of the main characteristic features of one of the largest Holocene AVF, which is active now – the Tolbachik field of cinder cones in the southern part of Klyuchevskaya volcano group (Kamchatka), known in the literature as Tolbachinsky Dol. This paper is focused on the research of magma plumbing system. We consider structural, morphological, geological, geochemical and petrological data on the erupted basalts and their genesis. Specially planned seismic experiments made in 2010–2015 (seismic tomography and microseismic sounding) allowed modeling of the principal elements of the magma plumbing system of Tolbachik AVF. Analysis of the investigations made in this area shows that Tolbachik AVF has a complex, dynamic, variable magmatic feeding system, which can be visualized as a superposition of subvertical and sublateral magma conduits. The contrast composition of the erupted rocks is caused by their different, although genetically connected, magma sources and mixing processes. One of the long-lived eruptive centers of Tolbachik AVF is Plosky Tolbachik stratovolcano, which lost its independent activity and was captured by Tolbachik AVF in Holocene. The AVF formed rejuvenated volcanism using the feeding system of the stratovolcano like an “old anthill”. The magma plumbing system characteristics of Tolbachinsky Dol strongly support the idea of separation of AVF from monogenetic volcanism type in the classification.

Influence of Embedded Inhomogeneities on the Spectral Ratio of the Horizontal Components of a Random Field of Rayleigh Waves

Study of the statistical parameters of the Earth’s random microseismic field makes it possible to obtain estimates of the properties and structure of the Earth’s crust and upper mantle. Different approaches are used to observe and process the microseismic records, which are divided into several groups of passive seismology methods. Among them are the well-known methods of surface-wave tomography, the spectral H/V ratio of the components in the surface wave, and microseismic sounding, currently under development, which uses the spectral ratio V/V0 of the vertical components between pairs of spatially separated stations. In the course of previous experiments, it became clear that these ratios are stable statistical parameters of the random field that do not depend on the properties of microseism sources. This paper proposes to expand the mentioned approach and study the possibilities for using the ratio of the horizontal components H1/H2 of the microseismic field. Numerical simulation was used to study the influence of an embedded velocity inhomogeneity on the spectral ratio of the horizontal components of the random field of fundamental Rayleigh modes, based on the concept that the Earth’s microseismic field is represented by these waves in a significant part of the frequency spectrum.

Studying the Deep Structure of Elbrus Volcano by Microseismic Sounding

This paper presents results from a study of the deep structure of the Elbrus edifice and adjacent areas using geophysical techniques. We confirmed the existence of a shallow magma chamber, derived a more accurate location of the chamber in the host rocks and its characteristic dimensions, and compared new results with known studies. More accurate estimates have been obtained for the temperature at the top of the magma chamber and new evidence is adduced concerning the deep structure of the fluid magmatic systems in the Elbrus volcanic area.

Main Sections of the Earth Crust and Upper Mantle of the Greater Caucasus by New Technologies. 2. Deep Seismic Sounding, Micro Seismic Sounding

The work reviews the possibility of application of the seismic tomography method for processing the materials of deep seismic sounding (DSS). It presents comparative results by various softwares as GeoGiga Seismic PRO 7.0 (Canada), XTomo-LM (St. Petersburg). The work shows the possibility of using tomography together with the data of the micro seismic sounding (MSS) method. The basic section of the Earth's crust and upper mantle by tomographic sections. It is determined that the cover of the foundation is strongly indented, i.e., all the foundation is fragmented into blocks, which are enclosed by the net of faults formed according to the gradient zones of speeds and fragmentation of speed inhomogeneity in the tomography section. The thickness of the earth crust under the Greater Caucasus increases to 60—70 km. It reveals the speed image obtained as a result of seismic tomography, earlier constructed speed models of the DSS profiles of Bakuriani—Stepnoye and Nakhchivan—Volgograd reinterpreted by Pavlenkova G. A, also correlative analysis of the method of deep seismic sounding (DSS) and the method of micro seismic sounding (MSS) of the profile of Tskhinval—Ardon.