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Crustal electrical structure across the Tangra-Yumco tectonic belt revealed by magnetotelluric data: new insights on the east-west extension mechanism of the Tibetan plateau

Chen Ning, Wang Xuben, Shao Changsheng, Zou Jun, Xu Zhengwei, Li Dewei, Wang Xiangpeng

Acta Geophysica

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2023

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Vol. 71, no. 2

783--794

EN

Geologic evolution of the Tibetan plateau is characterized by crustal extension and horizontal movement in the post-collision stage, during which, approximate north–south (N–S) trending tectonic belts typically represented by Tangra-Yumco rift are developed. The Tangra-Yumco tectonic belt is an ideal object to investigate the deep structure and mechanism of the crustal extension. The magnetotelluric (MT) method is effective in probing crustal structures, especially for high-conductivity bodies. A MT profile of east–west direction with dense stations has been carried out across the Tangra-Yumco tectonic belt. Resistivity models independently derived from two-dimensional and three-dimensional inversions provided more detailed geophysical constraints on the mechanism of crustal extension and deformation. A significant conductor with estimated melt fraction as 3.0–7.5% in mid-lower crust was revealed under the N–S tectonic belt, where the asthenospheric upwelling through the slab-tearing window might have induced partial melting of the lithospheric mantle and lower crust. Combined with previous studies, the upward migration of hot mantle materials and the expansion of the lower crust should be the primary mechanism driving east–west (E–W) extension of the brittle upper crust with high resistivity above the depth of 30 km. According to lateral electrical discontinuity in the upper crust, we inferred that there might exist three normal faults with the reference of topography and the trend of extension of the existing faults. The expansion and deformation of the conductor might have pulled the brittle upper crust and cause significant E–W extension, leading to the formation of the approximate N–S trending rift and normal faults.

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Magnetotelluric inversion of one and two dimensional synthetic data based on hybrid genetic algorithms

da Conceição Batista Joelson, Starteri Sampaio Edson Emanoel

Acta Geophysica

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2019

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Vol. 67, no. 5

1365--1377

EN

We applied the technique of the genetic algorithms and a local methodology integrating the Gauss–Newton and Conjugate Gradient (GNCG) techniques to test one-dimensional inverse modeling of synthetic magnetotelluric data. The result of this modeling applied to a homogeneous and isotropic five-layer model led to the development a hybrid algorithm (GAGNCG), combining the aforementioned techniques, for inverse modeling of one-dimensional magnetotelluric data. The GAGNCG modeling of the synthetic data performs more efciently than the local methodology in terms of both procedure and results. This showed that the hybridization procedure maximized the advantages of using the global search methodology and minimized the disadvantages of the local technique. Based on these results, we developed another hybrid methodology (GA2D), built from some characteristics of the genetic algorithm and the simulated annealing method, for the inverse modeling of two-dimensional magnetotelluric data. The results were satisfactory, and the GA2D algorithm was a good starting point for the inverse modeling of two-dimensional data.

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Integrated interpretation of seismic and magnetotelluric data on Shurab diapirs in Qom basin, Central Iran

Oskooi B., Moradi M., Smirnov M.

Acta Geophysica

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2019

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Vol. 67, no. 4

1071--1090

EN

In recent decades, diapirs are frequently used for CO2 and natural gas storage due to their extremely impermeable and nonreactive characteristics. Among various approaches, we use an integrated interpretation approach to resolve the diapir no. 4 belonging to the Shurab diapirs (SD). The SD is a group of diapirs that have pierced to the surface of the Qom basin of Central Iran, which is a candidate for natural gas storages. The complex geology of the SD is the main cause that previous 2D seismic surveys across the diapir could not provide required information to propose any location for any exploration borehole. Consequentially, 28 magnetotelluric (MT) and 1 audio-magnetotelluric station were measured along a SW-NE profle. Dimensionality and strike analysis for all stations is done by the use of phase tensor analysis. We used the nonlinear conjugate gradient algorithm to invert the TE- and TM-modes data simultaneously in 2D. The resistivity model was compared with the interpreted results of the post-stack depth migration model using seismic attributes. In order to extract the determinative geological information from the low-quality seismic section, envelope, variance, sweetness and instantaneous frequencies attributes were used. The integrated interpretation of the seismic and MT data resolves a precise geometry of the salt body, location of the dense part of the diapir as well as the tectonics around the diapir. The integrated interpretation of seismic and MT data of diapir no. 4 resulted in an exploration drilling program.

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Potential Field, Geoelectrical and Reflection Seismic Investigations for Massive Sulphide Exploration in the Skellefte Mining District, Northern Sweden

Tavakoli S., Dehghannejad M., García Juanatey M. d. A., Bauer T. E., Weihed P., Elming S.-A.

Acta Geophysica

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2016

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Vol. 64, no. 6

2171--2199

EN

Multi-scale geophysical studies were conducted in the central Skellefte district (CSD) in order to delineate the geometry of the upper crust (down to maximum ~ 4.5 km depth) for prospecting volcanic massive sulphide (VMS) mineralization. These geophysical investigations include potential field, resistivity/induced polarization (IP), reflection seismic and magnetotelluric (MT) data which were collected between 2009 and 2010. The interpretations were divided in two scales: (i) shallow (~ 1.5 km) and (ii) deep (~ 4.5 km). Physical properties of the rocks, including density, magnetic susceptibility, resistivity and chargeability, were also used to improve interpretations. The study result delineates the geometry of the upper crust in the CSD and new models were suggested based on new and joint geophysical interpretation which can benefit VMS prospecting in the area. The result also indicates that a strongly conductive zone detected by resistivity/IP data may have been missed using other geophysical data.

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Evaluation of the feasibility of recovering the magma chamber's parameters by 3D Bayesian statistical inversion of synthetic MT data

Spichak V.

Acta Geophysica

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2012

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Vol. 60, no. 3

942-958

EN

Feasibility of recovering the magma chamber's parameters by 3D Bayesian statistical inversion of magnetotelluric data is estimated for the simplified conductivity model of the Vesuvios volcano. The results indicate that in the lack of prior information and data, the most efficient approach may consist in successive estimation of the geometry and the depth of the anomaly followed by estimation of the electric conductivity distribution in it. The horizontal boundaries of the target could be outlined by the high gradients of the impedance determinant phase pseudosections determined by the upward analytical continuation of the anomalous electromagnetic fields from the relief surface to the artificial reference plane located above the summit of the volcano. The vertical boundaries and the target extension as well as the electric conductivity could be estimated successively by means of 3D Bayesian statistical inversion of the collected magnetotelluric data carried out in the domain delimited by the estimated horizontal boundaries.

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Skonsolidowane podłoże regionu krakowskiego w świetle badań magnetotellurycznych – główne rysy tektoniczne

Ryłko W., Tomaś A.

Biuletyn Państwowego Instytutu Geologicznego

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2010

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nr 443

81--94

PL

W artykule przedstawiono główne rysy tektoniczne skonsolidowanego podłoża regionu krakowskiego. Autorzy do interpretacji wykorzystali osiem profili magnetotellurycznych, na których widoczne są ciała o zróżnicowanej, zazwyczaj dużej oporności, występujące na ogół na głębokościach większych niż 5 km. Ciała te powiązano z krami skonsolidowanego podłoża bloku górnośląskiego. Wyróżniono kry: Čadcy, cieszyńską, górnośląską oraz Rzeszotar. Kry te są rozdzielone rozłamami i strefami nieciągłości różnego wieku i o różnym stopniu znaczenia. Najprawdopodobniej rozłamy swoje najstarsze założenia mają w neoproterozoiku. Były one aktywne również w kilku młodszych cyklach orogenicznych, do alpejskiego włącznie.

EN

The study deals with the main tectonic features of consolidated basement in Kraków region. The authors used eight magnetotelluric profiles for the interpretation. The rock bodies of differentiated, usually high resistance, visible in the magnetotelluric profiles and occurring deeper than 5 km, were linked to the consolidated basement blocks of the Upper Silesian Block. Starting from the southeast the following blocks were identified: the Čadca and Cieszyn blocks, the Upper Silesian Block which encloses the former ones from the north and east, and finally the easternmost Rzeszotary Block. These slabs are separated by faults and discontinuity zones differing as to their significance and age. The dislocations likely developed on pre-existing Neoproterozoic faults. They were also subsequently active during a number of youngre orogenic cycles, including the Alpine one.

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Ocena istniejących danych geofizycznych i geologicznych z obszaru pola badawczego Skierniewice (Niecka Warszawska) w aspekcie konstrukcji modelu strukturalnego głównych zbiorników wód geotermalnych rejonu Łowicza

Pussak M., Bujakowski W.

Technika Poszukiwań Geologicznych

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2007

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R. 46, nr 2

141-147

PL

Przedstawiona analiza związana jest z pracami wykonywanymi w ramach projektu realizowanego w ramach 6 Programu Ramowego UE pt. „Integrated Geophysical Exploration Technologies for deep fractured geotermal systems” (akronim I-GET), w którym IGSMiE PAN jest jednym z 11 partnerów. Głównym celem projektu jest opracowanie i przetestowanie innowacyjnej metodyki badań systemów i złóż geotermalnych. Podstawą opracowania niniejszej analizy jest pakiet zadaniowy nr 3 – „Geofizyczne prace polowe, przetwarzanie danych”. W ramach ww. pakietu przewiduje się wykonanie badań sejsmicznych i magnetotellurycznych o specjalnie dobranej metodyce prac polowych oraz przetwarzania danych w miejscach reprezentujących różny typ zbiorników geotermalnych. Polami testowymi w projekcie I-GET są: Gross Schoenebeck (Niemcy), Travalle (Włochy), Hengill (Islandia) oraz obszar pola badawczego Skierniewice (Polska). Wyniki tych badań będą testem decydującym o możliwości i przydatności zastosowania takiego kompletu metod geofizycznych do rozpoznawania innych potencjalnych złóż geotermalnych występujących w analogicznych warunkach geologiczno-strukturalnych umożliwiających jednocześnie pozyskanie wysokotemperaturowych mediów do celów produkcji energii elektrycznej i cieplnej.

EN

Presented analysis is connected with project elaborated within the frame of UE 6th Framework Programme titled „Integrated Geophysical Exploration Technologies for deep fractured geotermal systems” (acronym I-GET), where PAS MEERI is one of the consortium partners. Main aim of the project is to elaborate and test innovative methodology of the geothermal system and aquifer exploration. Background of that analysis is workpackage No 3 (WP3) – Geophysical field works and data processing. Within that workpackage seismic and magnetotelluric measurements based on original methodology of data acquisition and data processing are planned to carry out in test fields represented different geothermal condition. That test place are: Metamorfic - Travalle (Italy), Volcanic - Hengill (Iceland),deep sediments - Gross Schoenebeck (Germany) and shallow sediments - Skierniewice (Poland). Results of the research will be key-point of applicable this kind of geophysical methods (seismic and magnetotelluric) to recognize another geothermal energy source occurred in similar geological condition allows to exploitation of high-temperature mediums for heat and power production.

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Electrical conductivity anomalies in the crust and upper mantle of the Ukraine

Kulik S., Burakhovich T., Khazan Y.

Acta Geophysica Polonica

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2002

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Vol. 50, nr 4

547-565

EN

A wide range of tectonic structures exists on the territory of the Ukraine. Observations of geomagnetic induction vectors revealed anomalously high conductivity of many of them. A quasi-3D geoelectrical model of the Earth's crust and upper mantle of the Ukraine is based upon a thin-sheet approximation inside a regional cross-section of the electric conductivity. As the initial data we used induction vectors measured over the whole Ukrainian territory (about 2500 measurement points) for periods of geomagnetic variations ranging from 150 up to 2000 seconds. After removing the influence of the known distribution of the subsurface conductance (sediments), the residual vectors have been analyzed as being caused by the Earth's crust and upper mantle sources. The model demonstrates that values of the local conductance reduced to thin-sheets range from 0.5 to 20 kS in the Earth's crust and from 2 to 6 kS in the upper mantle. Both, the nature and the origin, of these high conductivity zones are different for different regions. Some possibilities including a contribution of the percolation process in two-phase media are discussed.