A. V. Gorbatikov

Microseismic field affected by local geological heterogeneities and microseismic sounding of the medium

Experiments and numerical model studies have shown that heterogeneities of the Earth’s crust distort the spectrum of the low frequency microseismic field, decreasing spectral amplitudes of a specific frequency f at the Earth’s surface over high velocity heterogeneities and increasing them above low velocity heterogeneities. The frequency f is connected with the depth of a heterogeneity H and the velocity of the fundamental mode of Rayleigh waves VR(f) through the relation H = 0.5 VR(f)/f. The low frequency microseismic field is considered as the superposition of trains of Rayleigh fundamental modes with different frequency spectra. The paper proposes an experimentally tested technology enabling the determination of the deep structure of complex geological objects using data on the microseismic background field.

Simulation of the Rayleigh waves in the proximity of the scattering velocity heterogeneities. Exploring the capabilities of the microseismic sounding method

The interaction between the fundamental mode surface Rayleigh waves and the buried heterogeneities with various sizes and different velocity contrasts was studied on base numerical simulation. The field of surface oscillations in the proximity of the scattering heterogeneities was computed as a function of frequency. The synthetic seismograms were used for numerical simulation of the microseismic sounding technology proposed earlier, implying that the solution of the inverse problem for the structure of the medium containing inclusions can be derived from the information contained in the ambient microseismic field. It is assumed that the depth of the layer to be reconstructed is linked with the frequency of the microseisms by a simple relation with the help of a numerical coefficient equal to 0.4–0.5. The combined results of the simulation of a direct problem together with the simple inverse problem solution show that the microseismic sounding technique ensures adequate estimation of the medium structure. Previously, the technology was based on the experimental data only and was phenomenological in character. Some relations between the velocity parameters of the original model heterogeneities and their reconstructed images were also studied.

Deep structure of the Mt. Karabetov Mud volcano

The results of multidisciplinary geological‐geophysical investigations of mud volcanism in the Taman mud-volcanic province are presented. Geophysical data on the internal structure of Mt. Karabetov mud volcano were obtained for the first time, and pathways of fluid migration down to a depth of 15‐25 km were determined. Mud volcanism is a surprising and quite rare natural phenomenon, whose mechanisms had not been fully studied. At present, we can consider that the correlation of mud volcanism with the dynamics of deep fluids and hydrocarbon pools has been established [2, 9]. Intense mud-volcanic activity is observed in the territory of Russia, first of all, on the Taman Peninsula, which is an integral part of the modern evolution of fluid-magmatic systems in the Northern Caucasus [9]. During the last few years, Mt. Karabetov, which is one of the most active mud volcanoes in the Taman region, has been the object of multidisciplinary geological‐geophysical and geochemical investigations [3, 4]. This volcano is characterized by explosive type of eruptions with periodic manifestation of the whole power of this seemingly harmless natural phenomenon. During the field works of the Institute of Physics of the Earth (IPE RAS) in August‐September 2007, a detailed geological-geomorphological mapping of Mt. Karabetov mud volcano was carried out and supplemented by remote sounding data. As a result, it became possible to trace the tectonic deformations of young forms of topography and various manifestations of exogenous geological processes in the study region. Simultaneously, profile geophysical measurements were carried out using the microseismic sounding method [5, 6]. It was shown experimentally and using numerical models that inhomogeneities of the Earth’s crust specifically distort the spectrum of the low-frequency microseismic field. In particular, spectral amplitudes of specific frequency f decrease above highvelocity anomalies at the Earth’s surface and increase above low-velocity anomalies. Frequency f is related to the depth of the anomaly H location and velocity of the fundamental mode of Rayleigh wave V R ( f ) as H = . The low-frequency microseismic field is considered as a superposition of wave packets of fundamental Rayleigh modes with different frequency compositions. The method was used as a principally new technology of microseismic sounding of near-surface (0‐0.5 km) and deep (up to 50 km) structures of the Earth’s crust. The technology was successfully tested in practice on different geological objects in terms of scale and genesis [14].

Statistical characteristics and stationarity properties of low-frequency seismic signals

Statistical properties of microseismic signals are studied in the frequency range 0.03–15 Hz at various points of the Earth near and far from sources of microseisms. It is found that various frequency ranges differ in the property of stationarity. The minimum interval of stationarity of microseisms in the range 0.12–1.1 Hz is estimated at 1–1.5 h. A conclusion is drawn that the measurement accuracy of the spectral density of microseisms cannot be improved above a certain limit by increasing the time of signal stacking.

The microseismic sounding method: Application for the study of the buried diatreme structure

1222 The recent standard approach to exploration of kimberlite bodies in prospecting areas includes aero� magnetic survey with subsequent definition of charac� teristic magnetic field anomalies based on route records and maps of ∆Т isodynams. Subsequently, the anomaly is verified by the ground magnetic explora� tion survey and the results of the latter are, in turn, used for outlining verification drilling sites. The important stage in planning drilling works is the con� firmation of the kimberlite body occurrence, outlining its configuration, and determining occurrence depth, which usually need alternative research methods. On the other hand, the practical activity of the dia�

Deep structure of the North Vent Area, Great Tolbachik Fissure Eruption of 1975–1976, Kamchatka: Evidence from low-frequency microseismic sounding

Studies were conducted to improve our knowledge of the deep structure of the magmatic system and the plumbing system for the North Vent, Great Tolbachik Fissure Eruption of 1975–1976 based on recordings of background microseismic emission by broadband digital instruments along two parallel lines running through eruptive centers of various ages across the main magma-conducting fault. The method of low-frequency microseismic sounding was used for constructing deep sections down to a depth of 20 km, showing the shear-velocity distributions along these lines. Elements of the magmatic system were revealed beneath both vents in the form of low-velocity anomalies. We identified regions of magma chambers at different depths together with the channelways that connect these. It was found that magma might come to shallow chambers from different deep-seated sources along spatially isolated magma conduits, which is one of the possible causes of the variation in the basalt composition during the eruptions. For the zone of areal volcanism we are the first to demonstrate a change in magma-conducting conduits in the transition from the crystalline basement to the volcanogenic sedimentary rock sequence, with subvertical channels being replaced by inclined forms. It was shown that the elements of the magmatic system beneath both eruptive centers studied here are similar. It is hypothesized that there is a regularity in the configuration of plumbing systems in the middle part of the Tolbachik regional zone of areal volcanism.

The structure and contemporary activity of the Intramoesian fault in northeastern Bulgaria obtained through a complex of new geological-geophysical methods

The structure of the Intramoesian fault is studied for the purpose of estimating its contemporary activity. The fault is known in the territory of Romania and Bulgaria, but it is insufficiently studied both from the geological point of view on the surface and from the geophysical point of view, from which the pattern of its deep structure can be inferred. The fault zone is the key structure for the solution of the problem of estimating the seismic hazard of the region, since the latest studies of this territory indicate the existence of traces of relatively young tectonic processes. According to some concepts the Intramoesian fault sets bounds to the tectonic plate, which is subducted under the Carpathian fold system in the region of the Vrancea Mountains. The paper under consideration presents the results of the field study of the southeastern, the Bulgarian, part of the fault with the application of a complex of geological-geomorphological and geophysical methods. On this basis, the structural segmentation of the fault is carried out and the specific features of its intersection with the disjunctives of another structural orientation are inferred. The data, which determine the degree of its geological and seismic activity, are also discussed.

Development of the model of the deep structure of Akhtyr flexure-fracture zone and Shugo mud volcano

Most mud volcanoes are located along large tectonic zones within the Alpine and Central Asia folded regions, Pacific mobile belt, and rift zones of the Atlantic and Indian oceans. One of the main conditions of the functioning of mud volcanic processes in mobile belts of the Earth’s crust is dislocation and folding of sedimentary rocks. Precisely such geological conditions exist in the Taman mud volcanic province, where the well-known Shugo Volcano is located.

Microseismic sounding method: Implications of anomalous Poisson ratio and evaluation of nonlinear distortions

The implications of the Poisson ratio of a heterogeneity for the microseismic image of the latter reconstructed by the microseismic sounding method are studied in the numerical experiments. In particular, the cases are considered with anomalous effective Poisson ratios, which may probably occur if the microseismic waves propagate through fractured zones where the crack opening is commensurate with or larger than the characteristic amplitude of the vibrations. Based on the numerical simulations, the nonlinear distortions in the microseismic sounding method, which arise due to the perturbation of the microseismic (probing) signal by introducing the heterogeneities, are estimated.

Deep structure of the Moscow Aulacogene in the western part of Moscow

Geological and geomorphological studies of the Moscow Aulacogene in the western part of Moscow suburbs have been conducted. This deep structure has been studied by microseismic sounding. The resulting section presents the faults which frame the Teplostanskii Graben in the south (Ramenskii or Butovskii, expressed on the surface as a ledge in the relief) and in the north (Pavlovo-Posadskii, being traced on the surface as a series of lineaments and the valley of Setun’ River). The position of the surface Archean-Lower Proterozoic crystalline basement within the limits of the graben and within the limits of buried elevated blocks (Krasnogorskii and Tumsk-Shaturskii) frame it in the north and south, respectively. Additionally, another fault has been identified in the central part of the graben: the Solntsevskii fault, which has a north-western course and which separates the deflection of the basement in two blocks that are sunken in a slightly different degree. The low-velocity horizons of the Riphean-Vendian complex which make up the graben at depths of 2 to 4.5 km have been found. Down to depths of 15 km, as a component of the upper crust, the graben is underlain by a high-velocity material which also forms the upper part of the section of the crystalline basement in the neighboring elevated block. A low-velocity block of the lithosphere is located in the larger (northern) part of the graben deeper (down to 40 km) beneath the zones of Pavlovo-Posadskii and Solntsevskii faults; in the southern part there is a high-velocity block. In the fault zones framing the graben in the north and south, the surface layer and soil displays a flow of juvenile hydrogen and helium which exceeds several tenfold the background values. According to the collected data, the Teplostanskii Graben has roots traceable through the entire crust and penetrating into the upper mantle.