Anomalous Gravity Field Research Articles

This volume, Sedimentary Successions of the Arctic Region and their Hydrocarbon Prospectivity , developed around maps of the sedimentary successions of the Arctic Region, and contains a brief, but comprehensive compilation of geological and geophysical data characterizing all significant sedimentary successions in the Arctic, which cover 57% of the polar area north of 64° N. Its two main goals are to provide, based on present-day knowledge and data, a characterization of all Arctic sedimentary successions (or sedimentary accumulations) and to supply a snapshot of hydrocarbon-related exploration in the Arctic at the end of the first quarter of this century. To achieve these goals, we represent sedimentary successions as consisting of one or several ‘tectono-sedimentary elements’ (TSEs) based on the main tectonic regimes that formed accommodation space for accumulation of sediments. A TSE characterization template has been developed as an efficient method of organizing and presenting the most important information about the stratigraphy, structure and petroleum geology of a TSE, including the most significant exploration facts. This organizational architecture is the backbone of the volume and is a key feature that distinguishes it from other studies of Arctic sedimentary basins. The online volume includes six large-size foldout maps portraying the mapped TSEs in the Circum-Arctic context, including tectonic grain of the consolidated basement, anomalous gravity and magnetic fields, location of the Arctic sampling sites and seismic profiles.

The Cameroon Volcanic Line (CVL), which is around 1600 km long, the Adamawa Plateau, the northern sedimentary basins, the Central African Shear Zone, and the northern boundary between the Pan-African Mobile Belt and Congo Craton are the primary geological features of Cameroon. A good number of authors have attempted to comprehend the geology and gravity field along the CVL by using gravitational data from the EGM2008 model to analyze the gravity effects in areas around Cameroon and the CVL with a focus on its structures and subsurface characteristics. Despite the fact that many authors have written on the subject matter, more emphasis has to be placed on the determination of the gravity source and depth beneath the CVL. Experimental gravity field model XGM2019e_2159 and DTU21 data were used in this research to estimate the depth of the gravity source. Both DEXP (Depth from Extreme Points) and spectral analysis were carried out to complement the results and accuracy of the techniques. The main focus of this research is to investigate the gravity source depth of CVL using DEXP as the main approach to illustrate its application in solving geophysical and geologic problems and reveal details of volcanic structures beneath the CVL. In this work, we describe the steps taken to calculate the anomalous gravity field and regional and residual gravitational effects. We further performed application of the DEXP transformation of 3D gravity field distribution to produce a 3D model for the depth of gravity sources.

We used 2D gravity modeling to investigate the structural features of the Earth’s crust and upper mantle in the junction zone of the outer Ukrainian Carpathians and the East European Platform along the Khyriv―Rava-Ruska―Velyki Mosty line. The profile passes through a poorly studied and complex structure of the border between Ukraine and Poland: it crosses the Folded Carpathians, the Precarpathian Trough, the Rava-Ruska Zone, and ends in the outer zone of the Lviv Trough. As the initial model of the crust, a deep seismic-geological section along the eponymous traverse SG-1(66) was used. The modeling results confirmed the seismic-geological data on the depths of the Carpathian base and the basement surface along the traverse, identified the main tectonic blocks and deep faults of the Earth’s crust, and determined the densities of the sedimentary complex and basement rocks. The lower crust and upper mantle under the Folded Carpathians have higher density. The reflection in the anomalous gravity field of tectonic units of the Folded Carpathians, the predicted Turkivskyi Paraautochthonous Complex, as well as deep mafic magmatic formations between the Rava-Ruskyi and Velykomostivskyi Faults, was investigated. Under the regional negative gravity anomalies, a deepening of the Moho boundary was detected under the Folded Carpathians and the Teisseyre-Tornquist Zone up to 45 km and 50 km, respectively. The results obtained are consistent with foreign and domestic geophysical materials.

In the work the possibility of harmonic continuation (along the radius-vector) of anomalous potential values and its derivative from the Earth`s surface to satellite altitudes is investigated. It was performed by means of the Poisson integral, which allows solving the classical outer boundary Dirichlet problem for harmonic functions on the sphere in the form of Fourier series on spherical ones. The set of normalised harmonic coefficients of the global high-degree geopotential model EIGEN-6C4 of bounded one n = 2190 is used to compute the globular functions. The results of net anomalies of gravity force to outer space calculated at sea level and at the altitude of 500 kilometres in the form of cartograms and graphs are presented. The studies show that at distance from the Earth`s surface the characteristics of the anomalous gravity field differ in magnitude along a complex curve, and the images on the map seem to blur. Moreover, with reaching altitude up to 500 kilometres, the anomalies decreased compared to those of gravity calculated at the sea level, almost 6-7 times

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.

An experimental investigation on the discrete Poisson’s upward continuation of anomalous gravity field data

Based on the analysis and interpretation of gravity and magnetic field anomalies, we studiedthe peculiarities of fault tectonics, structural-tectonic structure (including salt dome tectonics) of the Transcarpathian trough. We identified signs of the manifestation of deep faults and other large structural-tectonic elements in anomalies of gravity and magnetic fields. We then traced these structural-tectonic units by comparing the morphology, intensity, dimensions, and directions of the typical anomalous zones in the gravitational and magnetic fields with the tectonic structure of the region.
 We used digital maps of gravitational and magnetic fields; averaging transforms and relief-shadow images, we mapped local gravimagnetic anomalies. Analysis of the spatial structure of the original gravimagnetic fields and their transforms and structural-tectonic maps yielded a reflection of large tectonic elements of fault tectonics, anticlinal and salt dome structures in the gravimagnetic fields. Based on gravimagnetic data, the tectonic structure of the Transcarpathian trough was clarified, and the boundaries of tectonic zones and microplates were traced. The zone of the Transcarpathian deep fault is identified as a tectonic zone traced by a band of intense local positive anomalies of the gravity field along the Flysch Carpathians to the border of the Marmarosh massif. It is limited by high gradients from the southwest and northeast and is a reflection of the Pieniny and Marmarosh rock zones. The zone of the Transcarpathian deep fault is considered a suture zone of the Inner and Flysch Carpathians. In the structure of the anomalous gravity field of the Transcarpathian trough, a number of local anomalies associated with salt stocks, as well as individual anomalies, expected to be connected to salt-bearing deposits, werefound.
 We confirmed the effectiveness of gravimagnetic methods in the geological conditions of the Transcarpathian trough to detect anticlinal structures, basement protrusions, which create favorable conditions for oil and gas traps in sedimentary strata. Interpreting anomalous gravimagnetic fields in combination with geological and tectonic materials is an important condition for the integral process of studying the geological and tectonic structure of the Earth’s crust in the Transcarpathian region.

The Structural Zoning of the African–Antarctic Sector of the Southern Ocean Based on the Analysis of Anomalous Gravity and Magnetic Fields

The work is devoted to the construction and calculations of a three-dimensional density model of the sedimentary filling of the Carpathian-Pannonian region in order to obtain a more detailed map of the residual gravity field (stripped gravity map). This research was facilitated by and in-depth analysis of a large amount of data highlighting the density properties of Neogene-Quaternary deposits (the Pannonian Basin, the Transylvanian Depression, the Transcarpathian Trough), molasse deposits of the Carpathian Foredeep and flysch deposits of the Outer Carpathians in the Czech Republic, Slovakia, Poland, and Ukraine. Basic data for the construction of a three-dimensional density model of sedimentary deposits were obtained from laboratory studies of rock samples from drill core logging and deep exploratory wells, as well as rock samples taken from numerous outcrops in the research region. The average value of the density for molasse and flysch deposits of the Romanian part of the Carpathians was estimated based on the results of comparing the lithologic-stratigraphic complexes of these deposits in the adjacent areas of the eastern part of the Ukrainian Carpathians with similar ones in the Romanian Eastern Carpathians and the analysis of available data on the density of the Carpathian Foredeep and the Outer Flysch Carpathians for the Ukrainian part. The research method, which is a modification of geological reduction, has been applied in the work. Its essence consists of the sequential calculation and extraction of the three-dimensional gravity effect of sedimentary layers, the parameterization of which is better defined than those layers that lie deeper, from the anomalous gravity field. As a result, a residual gravity field is formed due to deep inhomogeneities associated with the consolidated part of the crust and the upper mantle. Calculations of gravity effects were carried out on a scale of 1:4,000,000 on a 10—10 km grid using the modern GMT-Auto. The detailed map of the residual (cleared of the effects of sedimentary layers) gravity field of the Carpathian-Pannonian region (stripped gravity map) is an effective tool in understanding the sources of the dominant gravity features of the studied region. Thus, the Pannonian Basin manifests itself as a general maximum with a number of local positive anomalies (more than 50 mGal), which are observed over small depressions filled with low-density thick sedimentary deposits: the Danube, Solnok, Makó, Békés Basins, and the Transcarpathian Trough. The phenomenon of positive and not negative values of the residual gravity field for these structures can be explained by the intrusion of the sedimentary cover of volcanic rocks, or the presence of high-density bodies with a special petrophysical composition (metamorphic complexes?) in the consolidated part of the crust. Another reason may be the effect of the regional background, which is due to the rise of Moho boundary in the Pannonian Basin to 24—26 km. The gravity minimum of the Western Carpathians, which on the map of the anomalous gravity field, consists of two parts (northern and southern), is reflected by one intense minimum, the southern one (-60 mGal). The northern part of this gravity minimum is practically leveled after calculations of the gravity effect of sedimentary filling, so it can be assumed that its source is low-density flysch and molasse deposits. The southern part of the gravity minimum of the Western Carpathians can be explained by the mass deficit in the consolidated part of the crust. Since the intensity of the gravity lows of the Eastern (-80 mGal) and Southern (-100 mGal) Carpathians remained high even after being cleared of the effect of the layer of sedimentary deposits, it can be assumed that they are due not only to the low values of the density of the sediments of the Outer Carpathians and the Carpathian Foredeep, but additionally also due to the gravity effect of deep inhomogeneities of the consolidated part of the crust (crustal root).

The purpose of the work is the analysis and geological-tectonic interpretation of the anomalous gravity field of the Ukrainian Carpathians and adjacent territories, as well as the construction of a density model of the Earth's crust and upper mantle according to the international PANCAKE seismic profile. The need to build a density model along the PANCAKE profile is due to the significant interest of a number of geologists and geophysicists in the results of seismic research along this profile. It is also caused by certain discrepancies in the seismological models of different authors. The gravity modeling technique, used in the work, includes the analysis of geological-geophysical maps and models. They are related to the geological-tectonic structure of the research region, to the creation of the initial structural part of the model and to the determination of the densities of strata and blocks of the model. The geometry and densities of the model are refined by the selection method, which is based on the interactive solution of the direct problem of gravimetric and the analysis of the reasons for the inconsistency of the calculated gravity field and Bouguer anomalies. A qualitative correspondence of the density model to the tectonic interpretation of the seismic section along the PANCAKE profile was achieved by using the methods of gravity modeling. The modelling results confirm the four-layer structure of the Earth's crust: the sedimentary cover, the upper, middle, and lower parts of the crust, which differ significantly in density. There is also evidence of the difference of the ALCAPA lithospheric plate, Flysch Carpathians and Precambrian Craton in Earth's crust and upper mantle structure. The ALCAPA plate is characterized by a small thickness (up to 29 km) and a low density of the Earth's crust. The density of the ALCAPA upper mantle is lower (3.20-3.21×103 kg/m3) compared to the upper mantle under the Ukrainian Carpathians and the East European Craton (3.28-3.30×103 kg/m3). This may be related to a change of a mantle composition and increased heat flow under ALCAPA. The Ukrainian fragment of the East European craton in the PANCAKE profile zone is characterized by a typical thickness of the crust (~41-45 km). The upper part of the crystalline crust, in contrast to the middle (2.86-2.90×103 kg/m3) and the lower part (2.98-3.10×103 kg/m3), is characterized by a lower density and greater differentiation in horizontal direction and with depth (from 2.66×103 kg/m3 to 2.86×103 kg/m3). The complex transition zone (subduction zone, Carpathian Orogen) between the ALCAPA microplate and the East European Craton causes an intense negative Bouguer anomaly – the Carpathian gravity minimum, which reaches -90×10-5 m/s2. It has a complex nature: Neogene and Paleogene-Cretaceous flysch rocks low density (≤2.50×103 kg/m3) of the Boryslav-Pokuttia cover, the main huge Precarpathian sub-vertical fault (>4 km) on the extreme southwestern slope of the platform (relatively local factors) and significant deepening of the MOHO surface under the Carpathian structure (regional factor). According to our density model, the depth of the MOHO under the front of the Carpathian thrust reaches 56 km.

The paper reports on the deep geophysical studies performed by the Geological Survey of Russia (VSEGEI) under the international project – Deep Processes and Metallogeny of Northern, Central and Eastern Asia. A model of the deep crustal structure is represented by a set of crustal thickness maps and a 5400-km long geotransect across the major tectonic areas of Northeastern Eurasia. An area of 50000000 km2 is digitally mapped in the uniform projection. The maps show the Moho depths, thicknesses of the main crustal units (i.e. the sedimentary cover and the consolidated crust), anomalous gravity and magnetic fields (in a schematic zoning map of the study area), and types of the crust. The geotransect gives the vertical section of the crust and upper mantle at the passive margin of the Eurasian continent (including submarine uplifts and shelf areas of the Arctic Ocean) and the active eastern continental margin, as well as an area of the Pacific plate.

The paper considers the results of calculation of the three-dimensional density model of the Earth’s crust for the territory of the Republic of Niger in conditions of incomplete initial geological and geophysical information. A brief description of the geological structure of the research region is given and the task of the study is formulated. The initial data set of density modeling is described, including: the anomalous gravity field, the initial model of the medium, the constraints on the desired solution, and the weight functions of redistribution of field incompatibilities. Inversion of the anomalous gravity field was performed in a three-dimensional formulation for a regular grid with a 25×25 km spacing in the plan and 14 layers of irregular vertical grid. The density model of the crystalline crust obtained by solving the inverse problem was combined with a priori data on the density of the upper mantle layer and the previously constructed layered model of the sedimentary cover of the region. The main features of the density model of the Earth’s crust are considered and its density heterogeneities are compared with the regional geological and tectonic data. The leading role of young structures of the West African rift System and their relationship with density inhomogeneities in the lower and middle crust of the territory of the Republic of Niger was noted.

Study of the deep structure of the White Sea region is relevant to active geodynamics, manifestations of kimberlite magmatism, and the prospects of oil and gas searches. The aim of this work was to model the velocity and density structure of the earth’s crust in the White Sea region. Modelling was carried out using the known data of instrumental observations and the software complex “Integro”. With the help of 2D models based on deep seismic sounding (DSS) profiles and a digital map of the anomalous gravity field, the density structures of local areas of the region’s crust were refined. A 3D density model was built. Within the framework of this model, the positions of the density layers were determined. The relief of the Mohorovichich (Moho or M) discontinuity reflects the anomalies of the gravity field. Depression of the Moho boundary in the bottleneck of the White Sea indicates the vertical structure of the earth’s crust associated with manifestations of kimberlite magmatism.

Abstract—We consider an improved version of the block contrasting method for processing large and super large gravimetric and geomorphological data. The elements of the anomalous gravity and magnetic field are determined with the use of the modified S- and F-approximations. The results of the mathematical experiment are presented.

Abstract—The paper addresses issues related to constructing the analytical approximations of geopotential fields based on a modified method of S-approximations which approximates the initial field by a sum of potentials of a simple and a double layer distributed on a set of carriers of mass located below a given relief. Special attention is paid to the application of new highly efficient methods for solving the systems of linear algebraic equations (SLAE) of large and super-large dimensions to which the inverse geophysical and geodetic problems are reduced. An improved block contrasting method is proposed for solving these SLAEs. The underlying idea of the method is to divide the initial domain into the subdomains that are most contrasting by their properties and to subsequently deform the blocks. The results of mathematical experiments on constructing the analytical approximations of anomalous gravity and magnetic fields and separating the fields created by the different sets of the sources are presented.

The purpose of the work is development of a complex seismic-density-magnetic model of the geological cross-section for the geotraverse SG-1 (67) (Nadvirna―Otynia―Ivano-Frankivsk) and appreciation of oil and gas bearing perspectives in the Kolomyia Paleovalley. The research methodology is based on the analysis of anomalous gravity and magnetic fields and the creation of density and magnetic models of geological cross-section. Local anomalies are defined by methods of transformations. Modeling of the geological cross-section structure is performed by methods of solving direct and inverse problems of gravity and magnetic survey, which implemented the ideas of the criteria approach to creating optimal models of geological environments that are consistent with the observed geophysical fields and do not conflict with the data of drilling and seismic survey. The reliability of interpreting gravimagnetic data method is achieved by geological subordination: the condition of maximum application of seismic survey and geological and tectonic maps. The conceptual density model and magnetic model of the geological environment along the seism-geological profile of CG-I (67) was developed. The distribution of densities and magnetization of rocks in sedimentary cover and basement blocks to a depth of 20 km is detailed. As a result of modeling within certain stratigraphic complexes, zones with anomalous in density and magnetization of rocks were defined. The decompression zones, which are within positive structures, are identified as perspective ones. It is also confirmed that there is a high probability of deep intrusions that are predicted from materials of seismic survey within the limits of the hidden tectonic zone and reaching the bottom of sedimentary cover. The analysis of morphology of gravity and magnetic anomalies and gravimagnetic modeling allowed clarifying the geological structure of the Kolomyia Paleovalley cross-section and to appreciate oil and gas perspectives of its separate areas.

Abstract This contribution is focused on a common utilization of microgravimetry (very precise and detailed gravimetry) and geoeletrical methods (ground penetrating radar and electric resistivity tomography) in the detection of subsurface cavities in non-destructive archaeological prospection. Both methods can separately detect such kind of subsurface objects, but their complementary and at the same time an eliminating aspect can be very helpful in the interpretation of archaeogeophysical datasets. These properties were shown in various published case-studies. Here we present some more typical examples. Beside this, we present here for a first time an application of the electric resistivity tomography in the interior of a building (a church) in Slovakia. We also demonstrate an example with an extremely small acquisition step in microgravity as a trial for the detection of cavities with very small dimensions – in this case small separated spaces for coffins as a part of the detected crypt (so called columbarium). Unfortunately, these cavities were too small to be reliably detected by the microgravity method. We have tried the well-known 3D Euler deconvolution method to obtain usable depth estimates from the acquired anomalous gravity field. Results from this method were in the majority of cases plausible (sometimes little bit too shallow), when compared with the results from the ground penetrating radar. In one selected example, the 3D Euler solutions were too deep and in the present stage of study we cannot well explain this situation. In general, all presented results support an important role of common combination of several geophysical methods, when searching for subsurface cavities in non-destructive archaeological prospection.

Research subject. The deep structure of the Yugan-Koltogor zone located in West Siberia was investigated with the purpose of detecting prospective oil and gas bearing areas.Materials and methods. The methods of lineament extraction and the computer modelling of rock density were employed. Lineament extraction was conducted on the basis of geophysical data, including detailed (1 : 200 000) maps of anomalous magnetic and gravity fields. In order to detect faults, telemetering methods were used, along with the results of studies conducted to investigate core materials from wells in the region. The modelling of the deep structure of the pre-Jurassic basement was performed on the basis of its geological map by solving a direct problem while fitting geological body densities.Results. Six largest faults of the Yugan-Koltogor zone identified on the geological map by a special sign “deep faults and regional schistosity zones” are of particular interest as possible oil bearing areas.Conclusions. The modelling of the deep structure of the pre-Jurassic basement of the West Siberian Platform has shown the granite decompaction areas of the Yugan zone to be highly promising in terms of oil and gas deposits.

The Purpose of the paper is to study the possibilities of regression and variance analyses under quantitative interpretation of gravitational anomalies. Methods. On example of the Ulanbaatar and Nalaikh intermountain depressions (Mongolia) the application possibilities of the principal component method and the pseudo-inverse algorithm under the transformation and inversion of gravitational fields are considered. Results. The quantitative characteristics of the thickness of loose deposits of the structures under investigation are obtained through regression and variance analyses. The maximum thickness of the deposits of the Ulanbaatar depression is 100-200 m whereas the Nalaikh depression is 600 m. The obtained estimates are confirmed by the results of investigated depression gravitational field inversion by other methods. Conclusions. The transformation and inversion results of the gravitational field of sedimentary deposits of low thickness (100-200 m) when replacing infinite length rectangular prisms by the parallelepipeds limited along the strike vary insignificantly and remain within the initial limits (while maintaining the magnitude of the lack of density). The depression filled with the sediments of great thickness features a significant increase in this characteristic (from 600 m to 900 m) while maintaining the lack of sediment density. Such variations in the sedimentary thickness of the studied objects can be explained by the magnitude of the visibility angle of individual geometric bodies approximating precipitation lenses. It is found that the application of the method of principal components of the variance analysis in the identification of the regional and local (residual) components of the gravitational field and the inversion of the anomalous gravity field using a pseudo-inverse algorithm of the regression research method are quite possible and effective.

The concept of curvatures of equipotential surfaces is of theoretical and practical importance in gravity gradiometry because curvatures describe the shape of equipotential surfaces, which can yield information about the shape of the source. Although the fundamentals of curvatures are well-established, their connection to modern gravity gradiometry and the associated applications in exploration geophysics remain areas of active research. In particular, there is a misunderstanding in the calculation of the said curvatures directly from measured gravity gradient data that are now widely used in exploration geophysics. The error stems from the incorrect use of the formulas in a fixed user coordinate system that are only valid in a rotated coordinate system. We demonstrate that the gravity gradient tensor must be rotated to a local coordinate system whose vertical axis is aligned with the local anomalous gravity field direction so that the curvatures of the anomalous equipotential surface can be calculated correctly using these classic formulas. To facilitate practical application, we present theoretical and practical aspects related to coordinate systems and rotations of the gravity gradient tensor. We have also developed an approach for estimating local gravity for use in the curvature calculation by wavenumber-domain conversion from gradient tensors. The procedure may form a basis for developing new interpretation techniques in gravity gradient gradiometry based on curvatures.