| 1. |
- Autio, U.A., et al.
(författare)
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Magnetotelluric array in the central Finnish Lapland I : Extreme data characteristics
- 2020
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Ingår i: Tectonophysics. - : Elsevier. - 0040-1951 .- 1879-3266. ; 794
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Tidskriftsartikel (refereegranskat)abstract
- We present magnetotelluric (MT) data from the northern Fennoscandian shield, which show extreme behavior due to the complex geoelectric setting left behind by the tectonic reworking of the Archaean bedrock during the Palaeoproterozoic. The core of the MT data set from the central Finnish Lapland were acquired in 2014 in the context of the MaSca-project.Extreme data are defined by the condition where the determinant of the real and/or imaginary part of the impedance tensor becomes negative. This is related to, but is a more anomalous condition than out-of-quadrant off-diagonal impedance phases. Physically, these are associated to reversal of e.g. the electric field as compared to its direction for one-dimensional resistivity structure. So far, only few examples of extreme data have been explicitly reported in geophysical literature. Data previously classified as erroneous might belong to the class of extreme data instead and data quality analysis should take this into account. A resistivity contrast of at least ca. 1:1000 is required to observe extreme data, as indicated by synthetic modelling studies. Extreme data are challenging from a modelling point of view and thus also drive the need for new developments in forward and inverse codes for MT.In this study, behavior of the phase tensor parameterization in case of extreme data is clarified. The quadrants of the principal value phases, a piece of information inherently missing from the phase tensor, can be recovered if the parameters are followed as a function of period. The behavior of the determinant (average) of the impedance tensor is investigated, since out-of-quadrant determinant phases are sometimes observed in conjunction with extreme data. A special condition is demonstrated, which links out-of-quadrant determinant phases and extreme data to negative determinant of the galvanic distortion tensor.
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| 2. |
- Autio, U.A., et al.
(författare)
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Magnetotelluric array in the central Finnish Lapland II : 3-D inversion and tectonic implications
- 2020
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Ingår i: Tectonophysics. - : Elsevier. - 0040-1951 .- 1879-3266. ; 794
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Tidskriftsartikel (refereegranskat)abstract
- The northern part of the Fennoscandian shield records several Palaeoproterozoic phases of rifting, crustal shortening, metamorphism and structural reactivation. The geologic history has left behind a complex crustal geoelectric structure as evidenced by magnetotelluric (MT) data from the central Finnish Lapland acquired in the context of the MaSca-project in 2014. The data are characterized by strong 3-D effects such as high phase tensor skew values and anomalous induction vectors. Interestingly, however, at the same time a dominant E-W principal direction from the phase tensor data can be inferred. 3-D conductivity models derived using the ModEM code display, high crustal conductance (> 10,000 S) in the vicinity of the Central Lapland Greenstone Belt, the Peräpohja Belt and the Kuusamo Belt. Conductors in the northern and southern part of the study area are separated by a resistor coinciding with the Central Lapland Granitoid Complex. A remarkable feature is an arc-shaped conductor inside the northern part of the Central Lapland Granitoid Complex, which continues into the Central Lapland Greenstone Belt in the north. The conductor is associated with a major induction vector anomaly and also coincides with extreme responses (negative phase tensor principal values), which are discussed in an accompanying paper. The conductive structures in the models are interpreted as deeply buried graphite and sulphide bearing metasedimentary rocks or as reactivated Archaean shear zones. The revealed geoelectric structures also partly correlate with seismic reflection and other geophysical data from the area. A possible explanation for the observed pervasive E-W principal direction of the phase tensor data could be the aulacogen (failed rift) suggested in recent tectonic evolution models.
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| 3. |
- Cherevatova, M., et al.
(författare)
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A multi-resolution approach to electromagnetic modelling
- 2018
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Ingår i: Geophysical Journal International. - : Oxford University Press. - 0956-540X .- 1365-246X. ; 214:1, s. 656-671
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Tidskriftsartikel (refereegranskat)abstract
- We present a multi-resolution approach for 3-D magnetotelluric forward modelling. Our approach is motivated by the fact that fine-grid resolution is typically required at shallow levels to adequately represent near surface inhomogeneities, topography and bathymetry, while a much coarser grid may be adequate at depth where the diffusively propagating electromagnetic fields are much smoother. With a conventional structured finite difference grid, the fine discretization required to adequately represent rapid variations near the surface is continued to all depths, resulting in higher computational costs. Increasing the computational efficiency of the forward modelling is especially important for solving regularized inversion problems. We implement a multi-resolution finite difference scheme that allows us to decrease the horizontal grid resolution with depth, as is done with vertical discretization. In our implementation, the multi-resolution grid is represented as a vertical stack of subgrids, with each subgrid being a standard Cartesian tensor product staggered grid. Thus, our approach is similar to the octree discretization previously used for electromagnetic modelling, but simpler in that we allow refinement only with depth. The major difficulty arose in deriving the forward modelling operators on interfaces between adjacent subgrids. We considered three ways of handling the interface layers and suggest a preferable one, which results in similar accuracy as the staggered grid solution, while retaining the symmetry of coefficient matrix. A comparison between multi-resolution and staggered solvers for various models shows that multi-resolution approach improves on computational efficiency without compromising the accuracy of the solution.
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| 4. |
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| 5. |
- Savvaidis, Alexandros, et al.
(författare)
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The seismically active Atalanti fault in Central Greece : A steeply dipping fault zone imaged from magnetotelluric data
- 2012
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Ingår i: Tectonophysics. - : Elsevier BV. - 0040-1951 .- 1879-3266. ; 554, s. 105-113
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Tidskriftsartikel (refereegranskat)abstract
- The WNW-ESE striking Atalanti fault is one of the large fault segments of the ca. 110-km-long WNW-ESE trending Sperchios-Chalkis Fault System that subsided along the NE slopes of the Kalidromon and Chlomo Mts. in Central Greece forming an extensive graben similar to the Corinthiakos Gulf. Although, the fault is characterized by recent seismic activity, it has up to now not been investigated by means of geophysical methods that could define the geometrical features of the structure in depth. Magnetotelluric measurements performed along three 14-km-long profiles A, B, and C and across the fault with a total of 28 stations indicate that its geoelectric strike is WNW-ESE. Two-dimensional bimodal and the determinant of the impedance tensor inversions were applied to the data. The best data fit was achieved using the determinant data. The resulting 2D models were further interpreted both separately and all together in order to define the geometry of the structure down to a depth of several kilometers. The derived resistivity model defines that the Atalanti fault strikes WNW-ESE and forms a damage zone that contains two high-angle fault branches that dip at angles of 60 degrees and 80 degrees respectively. Thus, the interpreted geometry of the Atalanti fault and consequently the Sperchios-Chalkis Fault System differs from previous interpretation of a low-angle extensional crustal structure. In addition, the electrical resistivity model indicates the subsidence of the overthrust of the Internal Hellenides over the Parnassos zone toward the NNE from a depth of 2 km to deeper crustal levels due to the Atalanti fault.
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| 6. |
- Selway, Kate, et al.
(författare)
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Magnetotelluric constraints on the temperature, composition, partial melt content and viscosity of the upper mantle beneath Svalbard
- 2020
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Ingår i: Geochemistry Geophysics Geosystems. - : American Geophysical Union (AGU). - 1525-2027. ; 21:5
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Tidskriftsartikel (refereegranskat)abstract
- Long‐period magnetotelluric (MT) data can be used to interpret upper mantle temperature, hydrogen content, and the presence of partial melt, all of which strongly influence mantle viscosity. We have collected the first long‐period MT data in Svalbard and have combined them with pre‐existing broadband MT data to produce a model of the electrical resistivity of Svalbard's upper mantle. Asthenospheric resistivities are low compared to stable continental settings but more comparable to young oceanic asthenosphere, suggesting that the physical state of Svalbard's upper mantle is controlled by its proximity to the Mid‐Atlantic Ridge. Interpretation of the MT model using a petrologically‐constrained genetic algorithm approach shows that partial melt is present in the uppermost asthenosphere beneath Svalbard. This is the first direct evidence of partial melt in Svalbard's asthenosphere from deep geophysical soundings. Viscosities calculated from the geophysical data show a low viscosity layer (~1018 Pa s) coincident with the partial melt layer, underlain by a higher viscosity layer (~1020 Pa s) extending to the transition zone. Viscosities calculated from glacial isostatic adjustment (GIA) data in Svalbard show a considerable range due mainly to uncertainties in past ice sheet models. Improved constraints on Svalbard's mantle viscosity from geophysical data may help to improve these GIA models.
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| 7. |
- Smirnov, Maxim Yu., et al.
(författare)
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Magnetotelluric measurements across the Sorgenfrei-Tornquist Zone in southern Sweden and Denmark
- 2009
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Ingår i: Geophysical Journal International. - 0956-540X .- 1365-246X. ; 176:2, s. 443-456
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Tidskriftsartikel (refereegranskat)abstract
- The results of a magnetotelluric experiment across the Sorgenfrei-Tornquist-Zone (STZ) in the southwestern part of the Baltic Shield are presented. The STZ marks the border between the Proterozoic intact part of the shield in Sweden to the north and the reactivated part including the Danish basin and the Ringkobing-Fyn High (RFH) to the south. We deployed 69 magnetotelluric sites along two profiles, each 350 km long. High quality data were collected in the period range 0.003-20 000 s. Data were processed using a robust multiremote-reference technique. Strike and dimensionality analysis as well as directions of induction arrows support the general 2-D character of the conductivity distribution in the area, allowing us to model the data in 2-D. Satisfactory agreement can be obtained between the measured and calculated transfer functions projected onto the profile direction. In order to ensure that the final model is stable we have performed a sensitivity analysis using synthetic models with different hypotheses about mantle conductivity and including a priori models for the sedimentary basin. The crustal part of the model compares well with that of Thybo which is derived from seismic, gravity and magnetic data. In the Danish basin we resolve a thick sedimentary cover that extends to the southwest. At the STZ a resistive body surrounded by more conductive material coincides with a zone of high seismic velocities and densities may be interpreted as a large magmatic intrusion. The STZ manifests itself electrically very clearly in the lower crust and upper lithospheric mantle as a narrow zone of high conductivity. The thickness of the electric lithosphere shows a gradual thinning from The Baltic Shield in Sweden (about 300 km) across the STZ into the Danish basin (about 100 km).
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| 8. |
- Smirnov, Maxim Yu, et al.
(författare)
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Processing of Electromagnetic Arrays with Missing Data - Source Field Structure and Source Free Transfer Functions
- 2016
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Ingår i: EAGE/DGG Workshop on Deep Mineral Exploration 2016: Chasing Both Land and Sea Deposits. - Netherlands : European Association of Geoscientists and Engineers, EAGE. ; , s. 54-57
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- New approach for multivariate analysis of EM data MsDEMPCA (Missing data EM Principal Components Analysis) is applied to several large arrays: the Baltic Electromagnetic Array Research (BEAR), Electromagnetic Mini Array (EMMA), Magnetotellurics in Scandes (MASCA), IMAGE obesrvatory data and various EarthScope MT arrays, with the goal of demonstrating how the MV approach can clarify signal and noise characteristics, improve estimates of TFs, and, perhaps most importantly, quantify biases in these estimates.
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