Abnormal magnetic field. Regional magnetic anomalies

During the last years the abnormal magnetic field has been successfully used for the construction of non-uniformly scaled solid models, the geological environment, which is also an integral part of the estimation of perspectivity of geological structures on different minerals.

The magnetic or geomagnetic field, is a force field caused by electromagnetic processes in a kernel of the Earth (main, or a normal field), in the top layers of the ionosphere (a variation of a geomagnetic field) and magnetisation of earth crust materials. The former factor forms an abnormal magnetic field which displays presence of materials with different concentration of magnetic minerals in earth crust. The most abnormal magnetic field on the territory of Ukraine is represented on NAU map "the Abnormal magnetic field". It is received by an exception of the variations connected with ionospheric processes, and, a so-called, normal field from intensity of the general geomagnetic field. The normal field has no exact analytical image and consequently has several approximating models, one of which is represented on NAU map "the Normal magnetic field".

The abnormal magnetic field, which is found on the territory of Ukraine is differentiated greatly and consists of the regional and local components, which differ in the lateral dimensions of anomalies and the depth of their sources. Regional field component, represented on the map "Regional magnetic anomalies" is conditioned by the heterogeneity of structure of the lower part of earth crust and a relief of the magnetoactive layer bottom, which can be identified with an earth crust bottom (Мохоровічич section) or with an isothermal surface of Curie temperature magnetite as the main carrier of materials' magnetism. It displays regional features of the big geostructures, and particularly of separate blocks of the Ukrainian sheet and the imposed trenches of a platform part of territory, border of the East European platform and heterogeneity of earth crust within Mountainous Crimea and Carpathians. Local component of an abnormal magnetic field is influence of magnetised materials of the top part of the crust and displays its composition and a structure. Use of local magnetic anomalies in the search purposes has begun in Ukraine more than 100 years ago with opening and the further research of Krivorozhsky iron-ore deposit. Now this component of a magnetic field is used as one of the reliable information sources during geological mapping of materials, studying of fold and explosive tectonics, tectonic zoning, and together with the regional component and other geophysical data - for studying of correlations between near-surface and deep lithospheric structures. The former is an important factor in the development of searching criteria for the different types of minerals, including oil-and-gas. From this point of view special value acquires tracing according to abnormal magnetic field data of the so-called through faults and a trance of regional tectonic zones, which are often enough zones of activization and concentration of minerals.

During the last years the abnormal magnetic field has been successfully used for the construction of non-uniformly scaled solid models, the geological environment, which is also an integral part of the estimation of perspectivity of geological structures on different minerals.

Conclusions

General conclusions concerning conducted research.

1. The construction technique of the maps, displaying abnormal magnetic field (?Т)а, its regional (?Т)а.reg and local (?Т)а.loc. components and module T was developed.
2. The new estimation criterion of the terrestrial magnetic field storminess was offered, which can be used in the study of the geomagnetic field's secular course and its ecological aspect.
3. Maps of abnormal magnetic field (?Т)а, its regional (?Т)а.reg and local (?Т)а.loc. components, module T, storminess ?D and ecological storminess ?Decol foer the territory of Ukrain were worked out.
4. Maps of the geomagnetic field’s regional component (?Т)а.reg., storminess ?D and ecological storminess ?Decol for the East-European platform were created.

Natural conditions and natural resources. Geophysical fields

Physical fields that were caused or changed with planet natural environment called geophysical fields (GF). GF change and distribution in time and space depend on resources availability and power and on natural environment characteristics structure and its dynamics under the influence of natural and anthropogenic factors as well. It is important to know the present condition and change nature of GF for understanding of natural processes, that influence the people’s life and country economics, for long-timed socio-economic planning, rational earth usage, task-oriented minerals seeking and dangerous processes prediction.

Gravitational field of the Earth or field of gravity force is the field caused by gravity force and centrifugal force, that was entailed Earth axial rotation. It is conventionally divided into abnormal and normal. Abnormal gravitational field (AGF) represents Earth form and its subsoil structure characteristics. AGF changes in the wide range on the territory of Ukraine, thatia connected with characteristics of Earth crust material and upper mantle thickness. On the atlas map the AGF distribution is showed with the lines of the similar values of its quantity, that is called isoanomaly.

On the base of gravitational modeling of the schistose inhomogeneous three dimensional deep structures with the usage of data about abnormal gravitational field, deep seismic sounding data (DSS), experimental correlate dependences between specific thickness and seismic-wave propagation under the conditions of different temperatures and tenses considering material constitution amendments was created a map of abnormal Earth crust and upper mantle thickness. Due to the tendency to isostatic equilibrium of separate blocks and its complexes the bottom of the Earth crust immerses under heavier blocks and rises under lighter ones.

Thermal field also gives the important information about structure and dynamics of our planet. Thermal condition of the Earth crust is characterized by thermal flow thickness that comes from the Earth subsoil and dissipates from its surface and by temperature change depending on depth.

The main part in formation of the thermal flow plays: the long-living radioelements’ (uranium, thorium, potassium) break-up energy, the maximal concentration of which is observed in the Earth crust materials, primary Earth energy and the energy of the physicochemical processes that take place in its subsoil. The important part in positive geothermal anomalies formation plays also thermal conduction conditions that significantly change depending on depth and according to lateral. The leading part in positive geothermal anomalies formation plays active tectonic and magmatic processes that is attended by release of the great amount of heat.

Thermal flow thickness – is a quantity of heat that is released from subsoil to surface per time unit on area unit. It is measured in mW/m2 and is defined as the result of multiplication of the geothermal gradient in the definite depth interval and the material thermal conductivity of this interval. On the territory of Ukraine the thermal flow thickness changes from 25-30 mW/m2 to 100-110 mW/m2. Temperatures on the depth of 1 km changes from 20 to 70oC, and on the depth of 3 km – from 40 to 135oС. Thermal flow distribution is closely connected with geological development characteristics or the regions and its tectonics.

Deep thermal flow (DTF) is defined as observed thermal flow corrected considering numerous nearsurface influences: paleoclimate, groundwater move with vertical component, geological structures, that cause out-of-level bedding of the distribution surfaces with different thermal conductivity, young overthrusts, conglomeration of young sediments etc. DTF map shows the distribution of its background (35-50 mW/m2) and abnormal (60-130mW/m2) values on the territory of Ukraine.

Earth thermal energy is a geoenergy resource. The map given in the atlas (map of geoenergy resources thickness in tones of conditional fuel on square meter that can be mined with water geocirculate system with the carrier temperature not less than 60oC and its returning to the subsoil with T ~ 20oC) was made on the base of DTF data. General geoenergy resources of Ukraine (present time defined) approximately 20 times exceed all the reserves of fuel minerals on its territory. They reach 10 t c.f/m2 on some areas, that exceeds reserves that can be mined from the big oil or gas field. Geoenergy resources suitable for practical usage by steam getting (electric power) without additional heating are investigated only in Transcarpathia and not very limited territories of Crimea.
Magnetic or geomagnetic field is geophysical force field caused by electromagnetic processes in the Earth core (main or normal field), in upper ionosphere (geomagnetic field variation) and Earth crust material magnetization. Last factor forms abnormal magnetic field that shows availability of the materials with different concentration of magnetic minerals in the Earth crust. It is defined by excluding normal field intensity and its variations from general geomagnetic field intensity. Normal field does not have exact analytical image. Several approximate models are used for its description, one of which is in the atlas.

Abnormal magnetic field of the territory of Ukraine is very differentiated and consist of regional and local components which differ by diametrical anomaly sizes and source placing depth.

Regional field component showed on the map “Long waved magnetic anomalies” is provided by inhomogeneous structure of the Earth crust lower part and the relief of magnetoactive layer bottom, that can be equaled to Earth crust bottom (Mokhorovichich discontinuity) or to isothermal surface Curie temperature magnetite – the main carrier of materials’ magnetism. It reflects regional characteristics of the big geostructure.

Local abnormal field component is formed under the influence of magnetized materials of upper Earth crust part and represents its structure. This magnetic field component is used as one of the important information sources for geological material mapping, fold and disjunctive tectonics, tectonic zoning study and in complex with regional component and other geophysical data – for studying surface and deep lithosphere structure correlations, for making non-uniformly scaled three-dimensional models of geological environment, that are used when geologize geological structures to find different materials.

Magnetotelluric Earth field is a natural electromagnetic field, caused current system. This field is one of the important source of knowledge about present geological structure, tectonic processes, geodynamics and fluid rate of Earth crust and mantle. Material electrical properties are closely connected with temperature and fluid subsoil rates, mineral chemical composition, consisting of C, S, Fe and other metals, juvenile waters mineralization level, available melt of crust and mantle materials etc. On the base of observed magnetotelluric field analysis using the methods of film and two-dimensional finite difference electromagnetic fields modeling the given in atlas maps “Earth crust electroconductivity” and “Upper mantle electroconducivity” in the siemens (S) – units of conductivity were made. Electroconductivity anomalies are distinguished above “normal” generalized Eastern Europe geoelectrical section, that characterized by specific electric resistance (in ohm.m): 1000, 600, 250, 100, 50, 20, 10, 5, 1, 0.1 – in the geological environment layers with power (in km): 160, 40, 50, 70, 80, 100, 100, 160, 200, ?. Value of longitudinal electroconductivity of the environment film was taken as 10cm.

For studying deep Earth structure: geometry and main tectonic bounder location, physical parameters distribution of geological environment etc – another type of GF is used widely – seismic field. It is observed in the form of mechanical oscillations on the soil surface or in mines, caves, adits, boreholes. Oscillations are generated by seismic waves (longitudinal, diametrical, surface, channel), that propagate from the source fading, reflecting, refracting and reradiating in other types on geological environment inhomogeneities. Depending on source seismic field can be natural or anthropogenic.

Studying of anthropogenic seismic fields that were by special explosions or by Vibroseis generated and observed along geologic profiles, allowed getting unique data about Earth structure, that are used for minerals searching and solving other geological-geophysical problems. There are Earth crust and lithosphere sections through the main tectonic structures on the territory of Ukraine in the atlas.

1. Observations on the deep seismic sounding (DSS) geotraverse “Holovanivs’k – Kirovohrad - Taganrog”, that crosses in latitudinal way Eastern and Central parts of Ukrainian board, were made according to the uninterrupted profiling method using the mutually connected travel- time curves of the main waves. This model shows velocity properties and deep structure of the Archean-Proterozoic parts of the board.
2. Geotraverse “Black Sea – Baltic Sea” on the territory of Ukraine has the 900 km length. It crosses Paleozoic Skythian plate and Precambrian Ukrainian board. Velocity model was made on the base of two-dimensional figure wave field modeling, that was taken by DSS method.
3. DSS profile “Poltava- Sverdlovsk” goes along Dnipro-Donets’k avlakogen. DSS results was taken and interpreted by the big staff of Ukrainian geologists and geophysicians.
4. Deep seismic researches on the profile “Berehovo – Dolyna – Vyshnevets’ – Shepetivka - Chernigiv” showed the tectonic structure of different age and genesis: Transcarpathian Mesozoic – Palaeogene bending, Carpathia, that are one of the main European Alpine orogenes and Archean-earlyProterozoic Ukrainian board.
5. DSS profile “Putyvl’-Kryvyi Rih” join the ultradeep well UDW-8 and 9. In its Southern part it goes along Kryvoriz’ko-Kremenchutc’ka submedidianal early protherozoic protogeosyncline, and in the Northern – obliquely crosses Dnipro-Donets’k late proterozoic- Devon paleorift north-western extent.
6. On the territory of Ukraine great amount of seismic profiling were done (over 10 th km) for studying Earth crust and lithosphere structure. A map of Mokhorovichich surface was made in the atlas according to these data. The word “surface” is conventional, as actually it is powerful transition zone, that divide Earth crust from the upper mantle, that is characterized by complicated structure, alternation of thin layers with higher and lower seismic waves velocities.

On the territory of Ukraine Earth crust thickness changes in wide ranges from 25 to 65 km. Maximal crust thickness fixes under the Carpathia (65 km), Mountain Crimea (up to 60 km), jn the Ukrainian board (Odessa-Yadliv, Kryvyi Rih-Krupets’k, Orikhovo-Pavlohrad early Protherozoic geosyncline zones – 50-60 km). Minimal Earth crust thickness is observed in the Transcarpathian bending region (25 km), under Dnipro-Donets’k avlacogene (30-35 km), on the Ukrainian board, in the Zaporizhzhya middle massif region (25-30 km), Kirovohrad protoplatform block region (35 km) and on the water area of Black Sea depression (25-30 km).

Natural seismic fields caused by the local and strong distant earthquake foci, must be taken into account because of their dangerousness when accommodations, important buildings, objects at ecological and anthropogeneous risk are built. The cause of earthquakes is the present geological structure tectonic activity. Distribution of different magnitude earthquakes on the given territory in time and dimension is called seismicity.

On the territory of Ukraine the high seismicity level is observed mainly on the territory or Carpathia and Crimea-black Sea region.

Seismicity of the Carpathia region depends on the earthquakes with epicenters in Transcarpathia, Carpathia and on the adjacent territory of Poland, Slovakia, Hungary, Romania. The most active is Transcarpathia.
On the Western regions territory (from 18th century and to the present time) earthquakes are characterized mainly by foci depth (h) 2-10 km and magnitudes (M)< 5,5. due to the small depth these earthquakes caused local effects on the soil surface with intensity up to 7-8 points on the MSK-64 scale. Similar fluctuations are felt on the Transcarpathia from deeper (h=35 km) and bigger (M=6.8) earthquakes with epicenters in Romania at the ~60 km from Ukrainian bounder. the biggest at the adjacent territories of the Precarpathia earthquake was in the 1875 year in the Lvivs’ka region. It has magnitude M=5.3, focus depth h=19 km and was felt in the epicenter with the intensity 6 points. In Chernivtsi intensity reached 3 points.

The territory of Ukraine is influenced by the subcutaneous earthquakes from Vrancea zone in Romania. Earthquake foci are placed in the mantle on the depths from 80 to 190 km. Maximal magnitudes reached 7.6. Due to big depths of the foci and magnitudes earthquakes from the Vrancea are felt on the huge territory: from Greece on the south to Finland on the north. There are earthquake foci from the Vrancea zone from 11th century with magnitudes higher than 3.5 on the epicenter map. For the last two centuries isoseims for the strongest earthquakes were defined.

Seismicity of the Crimea-Black sea region is defined by the earthquake epicenters, placed in the Black Sea water area near the Southern Coast of Crimea. They are characterized by the highest in the Ukraine magnitudes (M=6.8). On the epicenter map earthquake are represented with M>2 from the first century to the present time. On the lower part of Crimea and Sea of Azov waters area earthquake foci with M>1 are shown.

On the platform part of Ukraine only several local earthquakes are known. Their foci were in the Earth crust range, and to it, seismic effect had local character. Seismic swing intensity in the epicenter reached 6-7 point. Earthquake with intensity of 6 points on the MSK-64 scale that took place on January 3, 2002, near the village Mykulyntsi Ternopils’ka region and aftershock trail are the evidence of seismic activity availability of the platform tectonic structures on the territory of Ukraine.

Danger level, which can be caused by earthquakes, is displayed on the maps of general seismic zoning (GSZ) in the numbers of MSK-64 macroseismic scale. These maps are used in long-term social-economic planning, rational land use, acceptance of administrative and technical decisions concerning maintenance of existing constructions' stable operation and placing of the new ones (HPS, APS, pipelines, etc.). In Ukrainian seismic areas design-survey and construction works are regulated through three probabilistic ЗСР-2004 maps, marked as A, B and C. They represent intensity values of seismic shakings, which can become evident once in 500, 1000 and 5000 years accordingly, or, in other words, may be exceeded with probability of 10 %, 5 % and 1 % in the next 50 years.
Maps of seismic zoning (SZ) display a predicted increase in seismic numbers on different territory lots, according to the one, presented on GSZ map. Increases may be positive or negative, depending on the local soil conditions, relief and presence of tectonic failures. Engineering-geological researches data, macroseismic inspections of earthquakes consequences data, tool observation over the earthquakes' seismic fields, explosions, natural and technogenic microseisms are used while creating SZ maps. SZ maps are used for planning of population aggregates' development, maintenance of existing constructions' stable operation and designing of the new ones.
The maps of geophysical fields represented in the atlas and the maps and schemes, created on the basis of their interpretation, which display depth of the lithosphere structure, dynamics of tectonic structures, the danger, connected with earthquakes, creep movements, shifts, subsidences, etc., is an important tool for the preception of a deep planet structure, purposeful search of minerals, protection of the population, habitation and important constructions from dangerous endogenous processes and the secondary engineering-geological phenomena, connected with them.

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Nonmagnetic Deep Blocks of Earth Crust in connection with the Nodes of Traversal of Transemegablock Crustmantle Magmoactive zones

Edges of the deep magnetic blocks of the earth crust in association with section knots of transmegablock crust-mantle structurally-deformational (magmoactive) zones are perspective in kimberlite magmatism.

Non-magnetic deep blocks of the earth crust in association with section knots of transmegablock crust-mantle structurally-deformational (magmoactive) zones are perspective in kimberlite-lamproite magmatism.

Genetic and structural-genetic connections between abnormal magnetic field of the earth and its oil-and-gas content are substantiated.

On the basis of the developed maps of regional and local magnetic anomalies of the oil-and-gas basins of Ukraine, locations of the basic deep oil-and-gas monitoring fractures and fractures and oil-and-gas providing channels are substantiated, and the ways of hydrocarbons migration can be connected with it.

Petromagnetic Types of Earth Crust

By the magmatism type, size of magnetisation and yhe lithosphere structure, four petromagnetic earth crust types are singled out.

The most widespread mafic and sialmafic types, which are characterised by the basic magmatism and high magnetisation values, are considered to be the structures of extension modes. Areas of primordial continental crust consolidations, rifts (paleorifts) and zones of oceanic and continental lithosphere articulation can be analogues of such structures.

Initial magmatism of the basic structure and formation of the magnetic sources is considered to date back to early stages of the large tectonomagmatic cycles, which coincide in the time relation with the periods of maximum dislocations of the Ukrainian board in width or its maximum turns.

Regions' perspectivity in minerals of magmatic, hydrothermal and pegmatite types is defined by the through lithospere structures together with corresponding conditions in earth crust.

Results of 3D Analysis of Magnetic Model of Earth Crust

3D Earth crust magnetic models with other geological-geophysical data in connection with tectonics, magmatism, metallogenic specialisation and minerals prognostication were analysed.

Magmatic formations of the basic and ultrabasic structure of pre-platform development stage of Ukrainian board, which are drawn off on the surface of crystal base, are situated mainly over deep blocks (or their edges) of the Earth crust with the raised magnetisation values;

Reduction of average magnetisation value of the materials with basic and ultrabasic structure along with the reduction of their formation age is discovered, and the maximum magnetisation of crust bottoms of archaic Podolsky, Buzhsky, and partially Serednyoprydniprovsky megablocks and the West Priazovsky block, its smaller intensity in the Volynsky megablock and practically not magnetic bottom crust of the platform activization areas is connected with it.