Geofizicheskiy Zhurnal

Induction arrow spatial and temporal variations

Igor Rokityansky — 2024-12-15

The induction arrow C is a characteristic of the variable geomagnetic field at one observation point (x, y) for a certain period T of geomagnetic variations B. It visually describes the magnitude and direction of the deviation of the geomagnetic field vector from the horizontal plane. The induction arrow is nonzero only when the vertical component Bz appears. Bz appears in three cases: 1) electrical conductivity’s horizontal gradients under the surface of the globe; 2) incomplete compensation of the vertical component of the primary field of the ionosphere-magnetosphere source by the secondary field induced in the horizontally layered Earth. The degree of compensation systematically changes during the day, year..., leading to temporal variations in the induction arrow (source effect); 3) the appearance of not-removed noise or lithosphere emission containing Bz. This article presents the spatial and temporal variations of the induction arrows. Real (in-phase) and imaginary (out-of-phase) induction arrows (vertical response functions or tippers) were obtained for every day with proper observations from 1991 to 2014 years for five intervals of periods: 150—300 s, 300—600 s, 600—1200 s, 1200—2400 s, 2400—3600 s, calculated from three components of geomagnetic field recorded at 137 observatories of the global network INTERMAGNET. To reduce the scatter, the daily values were recalculated to monthly values. Such global material from +87° to –88° of geomagnetic latitude was obtained for the first time and its analysis yielded new scientific results. The annual variations (with a period one year) are visible at about 2/3 of the observatories (at the other observatories, they are below the background level of shorter period variations and/or noise). Its amplitude strongly depends on the geomagnetic latitude and sometimes reaches such a high value as 0.4—0.5 (peak-to-peak) at high (>65°) latitudes and varies within 0.01—0.15 at middle and low latitudes. Previous studies at middle latitudes discovered that the annual variations in the northern component Au is positive everywhere (maximum in June, minimum in December) and proposed, as a global source model for the annual variations explanation, the ring current at the height 3―6 Earth’s radii. We discovered that at high latitudes, Au is usually negative. We propose to explain all the observed features of the induction arrow’s annual variations by variations of the ionosphere currents in the aurora zone. 11-year variations are found at ≈30 % of observatories located at all latitudes but more frequently in aurora zones. At a few observatories, trends (monotonous changes in arrows) were found. The largest trend of magnitude 0.2 for all periods was found in southern Greenland, where glaciers have been melting rapidly over the past 30 years, which makes it possible to associate both trends with global warming. In a few aurora observatories of North America trends of magnitude ≈0.1 were noticed. In these same years, the North Magnetic Pole unusually rapidly migrated for 1100 km in 23 years. It can be assumed that these trends are related to changes in the aurora oval position. At geomagnetic latitudes from –78° to +78°, the main harmonic (24 hours=86400 s) of daily variations of the geomagnetic field is always accompanied by at least one higher order harmonic 24/n (n=1÷7). At higher latitudes >78° of both hemispheres only the fundamental 24-hour harmonic is visible. Induction arrow temporal variations create difficulties in determining its constant component used to study the electrical conductivity of the Earth’s crust and upper mantle but can be very useful for geodynamic processes study

Magnitude probabilities for extreme earthquakes around the globe with Rank-Ordering

Gisela Domej — 2024-12-15

Earthquake likelihoods have occupied humankind ever since, and the estimation of potential magnitudes is crucial for a multitude of aspects of safety. In this work, we present a probabilistic analysis of extreme magnitudes in 16 regions across the globe characterized by different seismicity to invert the traditional question of «what probability is associated with certain magnitudes». We combine the Gutenberg-Richter Law and Rank-Ordering-Statistics in a methodological approach to estimate what magnitude ranges can be almost certainly (i.e., with 95%) expected, and what magnitudes become reasonably unlikely beyond those ranges.

This approach allows for estimating probabilities for maximal magnitudes per region and comparing thereto the maximal magnitudes (mr) that appeared in reality. The method explores the maximal magnitudes that could occur or be exceeded with a probability of 95%, if the respective mr are equal to or greater than these 95%-predictions, and how probable it is, that also these mr could be reproduced or exceeded. We suspect a lack of great magnitudes in the Alps and a surplus across the Atlantic Ocean from these statistical considerations.

Microseismic monitoring technique for hydraulic fracturing of a formation for hydrocarbon deposits

Serhii Kobrunov — 2024-12-15

For the oil and gas industry, the country's energy independence is primarily determined by the presence of hydrocarbon deposits in its subsoil and the correct assessment of their reserves. However, the reserves of most deposits of Ukraine, which have been in development for more than one year, are unfortunately not unlimited, and to increase oil and gas production, non-standard approaches are needed. At the same time, when extracting hydrocarbons of an unconventional type, as international practice shows, it is necessary to use not just drilling, but drilling with formation stimulation. Hydraulic fracturing (fracking) is an effective stimulation method. However, to control the result of fracturing, it is necessary to apply certain methods, among which microseismic monitoring can be distinguished. The purpose of the work is to analyze the modern basic methods of microseismic monitoring of hydraulic fracturing and to determine the most effective method for use in the geological and geophysical conditions of hydrocarbon deposits of Ukraine. The methods of microseismic fracturing monitoring are primarily distinguished by the deep signal registration system used: borehole or surface. The advantages and disadvantages of these systems, as well as modern equipment for microseismic monitoring of hydraulic fracturing, are considered. The relevance of the work is primarily related to the search for new approaches to the estimation of mining reserves and new technologies for the development of hydrocarbon deposits of Ukraine, in particular, unconventional types.

RomUkrSeis profile: a model of the deep structure of the lithosphere and its geological and geophysical interpretation. P. I. Density heterogeneity and electrical conductivity anomalies

I.B. Makarenko — 2024-12-15

For the first time, a 2D gravity model was calculated for the RomUkrSeis profile, and the lithosphere density heterogeneity was analyzed. The synthetic model of the geoelectric heterogeneities of the Earth’s crust and upper mantle was created. It was obtained from 2D―3D modeling of the Earth’s electromagnetic field. An overview of modern ideas about the geological structure of tectonic units along the profile is presented. We propose a deep position of crustal and crustal-mantle faults dividing the Earth’s crust and upper mantle into separate blocks according to the gravity model, taking into account the seismic model and geological-geophysical data. The southwest part of the profile is mainly characterized by a high fragmentation into blocks, while the northeast, by horizontal stratification. The lower densities (2.36―2.76 g/cm³) of the whole Earth’s crust up to 36 km depth in relation to the surrounding structures is confirmed in the Outer Carpathians. According to the gravity model, a low-density zone (2.55―2.60 g/cm³) in the upper crust, which covers the northeastern part of the Apuseni Mountains and partially the Transylvanian basin, is revealed. From the northeast, this zone is limited by the Bistrica-Pryde fault. We identified compaction zones in the lower crust and in two upper mantle blocks in the narrow keel of the Moho boundary; the blocks with the highest densities of 3.41 g/cm³ (to the southwest) and 3.42 g/cm³ (to the northeast) are separated by the Pre-Carpathian fault. The synthetic model of the resistivity distribution along the RomUkrSeis profile is a generalization of the results of geoelectrical models of various degrees of approximation of the geological environment, which were obtained from the experimental data of magnetotelluric sounding and magnetovariational profiling. Anomalies of high electrical conductivity in the Earth’s crust and upper mantle have a complex configuration, different intensity, and depth. They do not always correspond to surface geology. Electrical conductive objects are located at depths from 15 to 25―30 km and are characterized by anomalous resistivity from the first units of Ohm∙m in the Carpathian-Pannonian region to 10―20 Ohm∙m under the Volyn-Podilsk monocline and the western part of the Ukrainian Shield. As a result of the review of available geoelectrical data in the Pannonian-Carpathian region, the lithosphere-asthenosphere boundary depth is assumed to be 70―90 km with a total longitudinal conductivity of up to 6 kS. The descending of the asthenosphere top from 70 to 100 km in the transition zone between the Inner and Outer Eastern Carpathians and its rise to 70 km under the Carpathian Foredeep were revealed. Under the Volyn-Podilsk monocline and the western part of the Ukrainian shield, the total longitudinal conductivity of the asthenosphere does not exceed 1―2 kS.

Shear processes in anisotropic anticlinal geostructures under the gravity action

Michail Lubkov — 2024-12-15

Shear deformation and fracture of three-dimensional anisotropic anticlinal geostructures under gravitational loading was simulated to study the theoretical and practical aspects of natural and man-made gravitational shear deformations and fractures based on the variational finite element method for solving the elasticity problem for multilayer orthotropic shells of rotation taking into account shear stiffness. The shear deformation of anisotropic anticlinal geostructures under the action of gravity depends on the shapes, sizes, structure, and elastic properties of differently oriented rocks that make up these geostructures. The stiffer and more compact anticlinal geostructures are subjected to the smallest shear deformation. While maintaining the general shape and stiffness of anticlinal geostructures, the largest shear deformations are observed in the lower middle part of the anticlinal geostructure. To be resistant to gravitational failure, the internal bearing layers of an anticlinal geostructure cannot consist of rocks softer than semi-hard dispersed rocks. The most important elastic characteristics for maintaining the stability of geostructures are Young’s modulus in the longitudinal direction and Poisson’s ratios and shear moduli in arbitrary directions. A decrease in Young’s modulus in the longitudinal direction and shear moduli, as well as an increase in Poisson’s ratios, especially in the internal bearing layers, can lead to catastrophic changes and failures in anticlinal geostructures. A decrease in the geostructure’s outer layer’s elastic properties in different directions leads to noticeable quantitative and qualitative changes in the nature of shear deformation of anisotropic anticlinal geostructures under gravitational loading.

Results of gravity modeling of the central part of the Korsun-Novomyrhorod pluton (Ukrainian Shield)

T.L. Mikheeva — 2024-12-15

Three-dimensional gravity modeling of the Gorodishchen and Smilyan gabbro-anorthosite massifs, located in the central part of the Korsun’-Novomirgorod pluton (Ukrainian shield), was performed. A three-dimensional model of the upper crust of the research area was developed using maps of the anomalous gravity field of 1:200 000 scale, taking into account the results of detailed seismic studies by the methods of the RWM (reflected wave method) and CDP (common depth point). Differences in the structure of the intrusive complex of the anorthosite-rapakivi granite formation and the gneisses of the Ingulo-Ingulets series surrounding it were reflected in the seismic wave fields, which made it possible to determine the boundaries of the entire intrusive. For separating the basic rocks and rapakivi granites that differ in their density, three-dimensional gravity modeling was performed using computer technology of automated interpretation of geophysical data based on the trial and error method. For the geological objects parameterization, an approximation model is proposed, which is represented by a set of three-dimensional rod bodies.

In the process of solving the inverse problem, various criteria for local optimization of gravitaty field sources were implemented. Three different functionalswere calculated during the iterative process. It is proved that the joint use of the functionals allows to reduce various types of noise in the observed gravity data. During the solving the inverse problem we found out that using of various types of functionals in the algorithms of trial and error methods is quite appropriate.

Three-dimensional gravity modeling made it possible to identify and outline gabbro-anorthosite bodies in the upper part of the section with maximum thickness of up to 4—5 km, to clarify the shape and dimensions of the rapakivi granites, and to study the contacts of the intrusive complex with the gneisses surrounding it. The obtained model, which takes into account all the available information on density and geometric parameters of the anomaly forming objects, could be used to obtain additional reliable geological information about the structure of the Gorodishchensk and Smilyan gabbro-anorthosite massifs.

The reliability of this algorithm for three-dimensional trial and error gravity method, using an approximation model in the form of a three-dimensional rods construction, allows us to recommend it for the study of similar gabbro-anorthosite massifs of the Ukrainian Shield, primarily the Korosten pluton.

On the nature of the earth’s magnetic field

Vadim Gordienko — 2024-12-15

The block of information analyzed in the article includes the currently accepted ideas about the main source of the magnetic field as the axial dipole, the action of which is supplemented by a long series of regional sources. The dipole exists long enough to ignore the influence of the process of its formation, moves in the body of the planet and changes orientation to the opposite one. The duration of the polarity conservation periods varies arbitrarily by two orders of magnitude. There is no physical justification for this strange mechanism; the assumed vehicle is thermal convection in the liquid core of the Earth. It is unknown what energy sustains the heat and mass transfer and how it leads to the listed exotic properties of the magnetic field source. As an alternative, it is proposed to consider convective flows in the global asthenosphere, caused by the influence of active processes in the tectonosphere. Its volume is comparable to the volume of the outer (liquid) core. The global asthenosphere is a layer of partial melting beneath the entire Earth’s surface at depths of about 800―1100 km. It was discovered by analyzing the thermal history of the planet. Energy sources can be radioactive decay and polymorphic transformations of matter. Several convective cells can be located within the global asthenosphere; the superposition of their effects presumably forms the main part of the Earth’s magnetic field. The preliminary joint solutions of the electromagnetic and hydrodynamic problems available in the literature (for the outer core) indicate, in the author’s opinion, that the problem deserves a more substantive study.

Regarding the discussion about the banded magnetic anomalies of the oceans

O.B. Gintov — 2024-12-15

The author supports the theory of plate tectonics with a plume component, pointing out the weak aspects of the arguments regarding the banded magnetic anomalies (BMA) of the ocean floor, which are the subject of discussions between supporters and opponents of this concept. Noting V.V. Gordienko's critical approach to the BMA, the author acknowledges the shortcomings of the justifications, but emphasizes the weight of other evidence for the existence of plate tectonics, such as seismotomography, tectonophysics, paleoclimate, etc.