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1
Content available Tectonics of the northern Carpathians basement in the light of electromagnetic and gravity data analysis
100%
Stefaniuk M. , Cygal A.
Geotourism / Geoturystyka
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2023
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tom nr 1-2 (72-73)
67--67
EN
The northern part of the Carpathians covers the north-eastern area of the Western and north-western of the Eastern Carpathians. The basement of the Carpathians in this zone is of a transitional nature and is relatively poorly explored, which results from its deep burial, in particular under the so-called Outer Carpathians. The interpretation of the tectonics and geodynamics of the basement depends to a large extent on the analysis of large scale geophysical data. In this area, regional seismic surveys were carried out mainly using the so-called deep refraction and numerous geophysical works using gravity, magnetic, geomagnetic and magnetotelluric methods. The subject of the presented work is a review of the regional image of electromagnetic and gravity studies carried out in this area, with particular emphasis on the territory of Poland, within which the authors carried out numerous research works. Electromagnetic research allows for the construction of a regional model of basement resistivity distributions and the determination of general outlines of its geometry as well as the formulation or testing of the concept of its geodynamical interpretations. An auxiliary role in this aspect is played by gravity data allowing to recognize the density distribution of the basement and constituting a set of additional data for integrated interpretation. The area outside the territory of Poland was presented on the basis of literature data, creating an extensive regional background for the results of research related with the participation of the authors in Poland. Within the Polish Carpathians, there is a structural reconstruction of the Carpathian overthrust and its basement, as well as a clear change in the nature of geophysical fields, e.g. the system of gravity field anomalies. Due to the deep burial of the Carpathian overthrust in this area and the complex structure of the orogen, which hinder effective drilling penetration, its fragmentary and uncertain recognition is based mainly on geophysical surface studies. The complex structure of the orogen reduces the effectiveness of the use of the seismic reflection method, the participation of which is limited in practice to the recognition of the basement in the marginal zone of the Carpathian overthrust. In the remaining area, alternative methods of surface geophysics are used, i.e. the magnetotelluric and gravity method. An important role in recognizing the basement of the Eastern part of the Polish Carpathians was played by magnetotelluric soundings that cover the above mentioned area with a relatively dense network of several generations of measurement points. The results of the interpretation of the MT soundings were used to construct a resistivity model, which was verified by new results of regional processing of seismic data and magnetotelluric and gravity modelling. The visualization of resistivity distributions was presented through maps interpreted at selected depth levels and in the resistivity cross-sections form. Resistivity distributions are the basis for interpreting tectonic zones marked as resistivity contrasts. Forward modelling and inversion of gravity data were used to verify resistivity structural models.
2
Content available remote Estimation of electrical resistivity using artifcial neural networks: a case study from Lublin Basin, SE Poland
80%
Ważny J. , Stefaniuk M. , Cygal A.
Acta Geophysica
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2021
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tom Vol. 69, no. 2
631--642
EN
Artifcial neural networks method (ANNs) is a common estimation tool used for geophysical applications. Considering borehole data, when the need arises to supplement a missing well log interval or whole logging-ANNs provide a reliable solution. Supervised training of the network on a reliable set of borehole data values with further application of this network on unknown wells allows creation of synthetic values of missing geophysical parameters, e.g., resistivity. The main assumptions for boreholes are: representation of similar geological conditions and the use of similar techniques of well data collection. In the analyzed case, a set of Multilayer Perceptrons were trained on fve separate chronostratigraphic intervals of borehole, considered as training data. The task was to predict missing deep laterolog (LLD) logging in a borehole representing the same sequence of layers within the Lublin Basin area. Correlation between well logs data exceeded 0.8. Subsequently, magnetotelluric parametric soundings were modeled and inverted on both boreholes. Analysis showed that congenial Occam 1D models had better ftting of TM mode of MT data in each case. Ipso facto, synthetic LLD log could be considered as a basis for geophysical and geological interpretation. ANNs provided solution for supplementing datasets based on this analytical approach.
3
Content available remote Integrated magnetotelluric and seismic investigation of Cenozoic graben structure near Obrzycko, Poland
80%
Cygal A. , Stefaniuk M. , Kret A.
Acta Geophysica
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2021
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tom Vol. 69, no. 2
711--723
EN
This article presents the results of an integrated interpretation of measurements made using Audio-Magnetotellurics and Seismic Refection geophysical methods. The obtained results were used to build an integrated geophysical model of shal low subsurface cover consisting of Cenozoic deposits, which then formed the basis for a detailed lithological and tectonic interpretation of deeper Mesozoic sediments. Such shallow covers, consisting mainly of glacial Pleistocene deposits, are typical for central and northern Poland. This investigation concentrated on delineating the accurate geometry of Obrzycko Cenozoic graben structure flled with loose deposits, as it was of great importance to the acquisition, processing and interpretation of seismic data that was to reveal the tectonic structure of the Cretaceous and Jurassic sediments which underly the study area. Previously, some problems with estimation of seismic static corrections over similar grabens flled with more recent, low-velocity deposits were encountered. Therefore, a novel approach to estimating the exact thickness of such shallow cover consisting of low-velocity deposits was applied in the presented investigation. The study shows that some alternative geophysical data sets (such as magnetotellurics) can be used to signifcantly improve the imaging of geological structure in areas where seismic data are very distorted or too noisy to be used alone
4
Content available Reconstruction of the Miocene depositional architecture of the Carpathian Foredeep basin based on geophysical data
51%
Stefaniuk M. , Florek M. , Maćkowski T. , Hadro P. , Cygal A. , Pieniądz K. , Łapinkiewicz A. P. , Wachowicz-Pyzik A.
Geotourism / Geoturystyka
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2023
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tom nr 1-2 (72-73)
68--69
EN
The Carpathian Foredeep in Poland is divided into two parts, eastern and western, with different tectonic frameworks and conditions of the Neogene sedimentary fill. The boundary is the so-called Krakow Ridge associated with the contact of two regional tectonic units: Upper Silesian and Malopolska blocks. The width of the Foredeep varies regionally and significantly differs in the western and eastern parts. It was developed within the epi-Variscan platform. Two zones can be distinguished in the Foredeep: the inner (older) zone and the outer zone. The subject of the presented work is the eastern part of the outer zone of Foredeep located in front of the head of the Carpathian thrust and filled mainly by autochthonous Miocene formations. The meridional width of this zone of the basin varies from about 10 km in the vicinity of Krakow to nearly 100 km in the central part. Its tectonic framework is defined from the south and south-west by marginal structures of the Carpathian overthrust and tectonic units of the folded Miocene. From the north-east and north-west, the border is marked by a system of faults in the foreground of Roztocze Upland and the Holy Cross Mountains. In the Sub-Cenozoic basement, a set of large faults of NW-SE length, with different times of formation and activity, is marked. These faults locally define horst structures and tectonic grabens. Some of the faults continue under the Carpathians, under which there is also a system of transversal faults in relation to the main axis of the orogeny. The influence of the tectonic structures of the basement is noticeable within the Miocene cover by faults disappearing towards the surface and continuous deformations of the adaptive type. The outer foreland basin is filled with marine molasse type deposits of the unfolded autochthonous Middle Miocene with a thickness of up to approx. 3.500 m. The complex of Miocene formations is formed, in the lower, south-western and central part, by strongly differentiated submarine fan deposits accompanied by basin plain formations and gravitational flow deposits, including turbidite deposits characteristic of flysch sedimentation. The outer part of the fans smoothly transitions into the zone of fine-clastic sedimentation of the basin plain. Above the complex of submarine fan sediments, there are thick complexes of sediments of deltaic origin, which are also intensively variable facies, creating a set of channel (coarse-grained) and extra-channel (finegrained) facies. The highest, relatively thin part of the sediments is formed by shallow coastal shelf formations. Submarine fans and river deltas developed mainly in the zone of the south-western and southern coasts of the Miocene reservoir, surrounded by river mouths providing an abundant supply of material from the rising and eroded Carpathians. In the north-eastern and locally even in the middle part of the basin, sediments may appear, for which the feeding area was located in the hinterland of the northern and north-eastern coast of the Miocene Sea. The limited scope of extraction of drill cores resulting from the exploratory and exploitation nature of drilling makes it necessary to use borehole and surface geophysical data to reconstruct the depositional architecture. Processing and interpretation of geophysical data for a complex of Miocene sediments with such characteristics are problematic and ambiguous. Numerous sources of sedimentary material supply in the form of river mouths and submarine channels cause a significant diversification of the depositional architecture of the Miocene basin, making it difficult to trace uniform stratigraphic and lithological and facies boundaries. Sedimentary conditions cause, on the one hand, a certain monotony of the sediments, dominated by clastic formations, enriched by evaporate sediments horizons, and on the other hand, great lateral and depth facies differentiation. Geophysical well-logging data allows to recognize the lithological and facies variability of sediments and to determine the sequence of changes along the borehole trajectory. Seismic reflection data was used to track lateral variability. For the seismic reflective method, the reflective boundaries, characterized by a significant, abrupt change in acoustic impedance, are of primary importance. Within the Miocene basin, numerous reflective boundaries with high lateral variability and non-obvious stratigraphic identification are observed. In a complex of siliciclastic deposits, seismic wave reflections are recorded from the boundaries separating fine-grained lithofacies and medium- and coarse-grained facies. Due to the dominance of deltaic sediments and submarine fans in the depositional architecture of the Miocene complex, the regional continuity of such boundaries is problematic, and their unambiguous stratigraphic identification is practically impossible. To sum up, intense lithological and facies variability of clastic deposits, both lateral and vertical, should be expected within the Miocene complex. The sediments of individual fragments of submarine fans and deltas overlap each other, and there may also be overlaps with the sediments of neighboring fans. Such characteristics of the complex translate into a variable seismic pattern with numerous reflective boundaries and intense lateral variability of the seismic signal characteristics
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