Deep array electromagnetic sounding on the Baltic Shield: External excitation model and implications for upper mantle conductivity studies

Sokolova, E. Yu.; Варенцов, И.М.

doi:10.1016/j.tecto.2007.07.006

Cited by 18 publications

(8 citation statements)

References 26 publications

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“…The upper mantle conductance was estimated from the BEAR data during the EMMA project. It was shown that at depths of 150-300 km the conductance reaches 4-5 kS that may be expressed as "an asthenosphere conducting layer" (Sokolova and Varentsov 2007).…”

Section: Results Of Deep Soundingsmentioning

confidence: 99%

Results of Crust and Mantle Soundings in Central and Northern Europe in the 21st Century (Review)

Semenov

2015

Acta Geophys.

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The first decade of 21st century is characterized by the appearance of new approaches to deep induction soundings. The theory of magnetovariation and magnetotelluric soundings was generalised or corrected. Spatial derivatives of response functions (induction arrows) were obtained for the ultra-long periods. New phenomena have been detected by this method: secular variations of the Earth's apparent resistivity and the rapid changes of induction arrows over the last 50 years. The first one can be correlated with the number of earthquakes, and the second onewith geomagnetic jerks in Central Europe. The extensive studies of geoelectrical structure of the crust and mantle were realized in the frame of a series of international projects. New information about geoelectrical structures of the crust in Northern Europe and Ukraine was obtained by deep electromagnetic soundings involving controlled powerful sources. An influence of the crust magnetic permeability on the deep sounding results was confirmed.

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Section: Results Of Deep Soundingsmentioning

confidence: 99%

Results of Crust and Mantle Soundings in Central and Northern Europe in the 21st Century (Review)

Semenov

2015

Acta Geophys.

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“…A series of papers discussing the Baltic Electromagnetic Array Research-BEAR (e.g., Korja et al 2002;Varentsov et al 2002Varentsov et al , 2003aLahti et al 2005;Sokolova et al 2007) and Magnetotellurics in the Scandes-MaSca (e.g., Cherevatova et al 2015a, b) experiments in Fennoscandia concluded that non-planar source field distortion either was not present or effectively removed for periods less than ~ 10,000 s. Time-series data were independently processed using three different robust remote reference techniques (Egbert 1997;Smirnov 2003;Varentsov et al 2003a) for the BEAR experiment. The impedance response for all components of the amplitude and phase had little scatter when results from the differing processing schemes were compared to periods of 10,000 s (Lahti et al 2005), from which it was interpreted that the distortion attributable to a non-planar source in the MT response is suppressed.…”

Section: Source Fieldsmentioning

confidence: 99%

“…However, the assertion of Korja (2007) that there is a thinning of the lithosphere towards the Atlantic (westward) is not observed in the more recent higher-density data set (Cherevatova et al 2015a). As discussed previously, a series of papers discussing the validity and bounds of the plane wave assumption were also completed as part of the BEAR and MaSca experiments Varentsov et al 2002;2003a;Lahti et al 2005;Sokolova et al 2007;Cherevatova et al 2015a, b).…”

Section: Bear and Masca Array Mt Experiments Fennoscandiamentioning

confidence: 99%

“…Consideration of source effects is required when working in highlatitude warm polar regions, in the northern hemisphere ( Fig. 1) locations such as Alaska (e.g., Fisher et al 2004;Glen et al 2007), northern Canada (e.g., Jones and Spratt 2002;Evans et al 2005;Spratt et al 2014), Scandinavia (e.g., Korja et al 2002;Lahti et al 2005;Sokolova et al 2007;Cherevatova et al 2015a), Svalbard (e.g., Beka et al 2015Beka et al , 2017a, coastal Greenland (e.g., Heincke et al 2015;Hautot and Tarits 2016;Lauritsen et al 2016), Russia (e.g., Bubnov et al 2007), and in the southern hemisphere ( Fig. 1) for those working in the sub-Antarctic islands (e.g., Pedrera et al 2012) where during the summer months the ground is not covered in snow and ice as there is the potential for periods where the source field is a non-plane wave.…”

Section: Introductionmentioning

confidence: 99%

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On the Use of Electromagnetics for Earth Imaging of the Polar Regions

Hill

2019

Surv Geophys

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The polar regions are host to fundamental unresolved challenges in Earth studies. The nature of these regions necessitates the use of geophysics to address these issues, with electromagnetic and, in particular, magnetotelluric studies finding favour and being applied over a number of different scales. The unique geography and climatic conditions of the polar regions means collecting magnetotelluric data at high latitudes, which presents challenges not typically encountered and may result in significant measurement errors. (1) The very high contact resistance between electrodes and the surficial snow and ice cover (commonly MΩ) can interfere with the electric field measurement. This is overcome by using custom-designed amplifiers placed at the active electrodes to buffer their high impedance contacts.(2) The proximity to the geomagnetic poles requires verification of the fundamental assumption in magnetotellurics that the magnetic source field is a vertically propagating, horizontally polarised plane wave. Behaviour of the polar electro-jet must be assessed to identify increased activity (high energy periods) that create strong current systems and may generate non-planar contributions. (3) The generation of 'blizstatic', localised random electric fields caused by the spin drift of moving charged snow and ice particles that produce significant noise in the electric fields during periods of strong winds. At wind speeds above ~ 10 m s −1 , the effect of the distortion created by the moving snow is broad-band. Station occupation times need to be of sufficient length to ensure data are collected when wind speed is low. (4) Working on glaciated terrain introduces additional safety challenges, e.g., weather, crevasse hazards, etc. Inclusion of a mountaineer in the team, both during the site location planning and onsite operations, allows these hazards to be properly managed. Examples spanning studies covering development and application of novel electromagnetic approaches for the polar regions as well as results from studies addressing a variety of differing geologic questions are presented. Electromagnetic studies focusing on near-surface hydrologic systems, glacial and ice sheet dynamics, as well as large-scale volcanic and tectonic problems are discussed providing an overview of the use of electromagnetic methods to investigate fundamental questions in solid earth studies that have both been completed and are currently ongoing in polar regions.

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“…Possible violations of the plane wave assumption resulting in so-called source effects have caused a lot of discussion in connection with geoelectromagnetic induction and magnetotelluric studies of the Earth (e.g. Wait, 1954;Mareschal, 1986;Osipova et al, 1989;Viljanen et al, 1993;Varentsov et al, 2003;Sokolova et al, 2007). In general, the farther away the primary currents the better the plane wave assumption is satisfied, which means in practice that the source effect problem becomes significant in auroral and equatorial electrojet regions.…”

Section: Plane Wave Casementioning

confidence: 99%

Ground effects of space weather investigated by the surface impedance

2009

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The objective of this paper is to provide a discussion of the surface impedance applicable in connection with studies of geomagnetically induced currents (GIC) in technological systems. This viewpoint means that the surface impedance is regarded as a tool to determine the horizontal (geo)electric field at the Earth's surface, which is the key quantity for GIC. Thus the approach is different from the traditional magnetotelluric viewpoint. The definition of the surface impedance usually involves wavenumber-frequency-domain fields, so inverse Fourier transforming the expression of the electric field in terms of the surface impedance and the geomagnetic field results in convolution integrals in the time and space domains. The frequency-dependent surface impedance has a high-pass filter character whereas the corresponding transfer function between the electric field and the time derivative of the magnetic field is of a low-pass filter type. The relative change of the latter transfer function with frequency is usually smaller than that of the surface impedance, which indicates that the geoelectric field is closer to the time derivative than to the magnetic field itself. An investigation of the surface impedance defined by the space-domain electric and magnetic components indicates that the largest electric fields are not always achieved by the plane wave assumption, which is sometimes regarded as an extreme case for GIC. It is also concluded in this paper that it is often possible to apply the plane wave relation locally between the surface electric and magnetic fields. The absolute value of the surface impedance decreases with an increasing wavenumber although the maximum may also be at a non-zero value of the wavenumber. The imaginary part of the surface impedance usually much exceeds the real part.

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Deep array electromagnetic sounding on the Baltic Shield: External excitation model and implications for upper mantle conductivity studies

Cited by 18 publications

References 26 publications

Results of Crust and Mantle Soundings in Central and Northern Europe in the 21st Century (Review)

Results of Crust and Mantle Soundings in Central and Northern Europe in the 21st Century (Review)

On the Use of Electromagnetics for Earth Imaging of the Polar Regions

Ground effects of space weather investigated by the surface impedance

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