Extremal bias coupling in magnetotellurics

Geophysics ◽  
1984 ◽  
Vol 49 (11) ◽  
pp. 1968-1978 ◽  
Author(s):  
L. B. Pedersen ◽  
M. Svennekjaer
Keyword(s):  
Magnetic Field ◽  
Linear System ◽  
Rotation Angle ◽  
Impedance Tensor ◽  
Vertical Magnetic Field ◽  
Electrical Field ◽  
Horizontal Magnetic Field ◽  
Single Station ◽  
Polarization Characteristics ◽  
Set Up

Bias effects due to uncorrelated noise on electrical and magnetic channels are cast into a quantitative framework which allows for an immediate conservative judgment of impedance tensor quality from a single station set up. The horizontal magnetic field is regarded as input to a linear system with the horizontal electrical field and the vertical magnetic field as outputs. The coherence between inputs, the predicted coherence between outputs and inputs, and the polarization characteristics of the inputs, determine the extremal bias effects. Exact expressions for impedance tensor elements and tipper elements are given. Moreover, rotation angle of principal impedance direction, skew, and ellipticity are calculated. Finally, we study some practical examples that show some of the characteristics predicted by our model.

scholarly journals Multi-point ground-based ULF magnetic field observations in Europe during seismic active periods in 2004 and 2005

2008 ◽  
Vol 8 (3) ◽  
pp. 501-507 ◽  
Author(s):  
G. Prattes ◽  
K. Schwingenschuh ◽  
H. U. Eichelberger ◽  
W. Magnes ◽  
M. Boudjada ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Field Intensity ◽  
Seismic Activity ◽  
Magnetic Field Intensity ◽  
Field Measurements ◽  
Fluxgate Magnetometer ◽  
Vertical Magnetic Field ◽  
Earthquake Epicenter ◽  
Electromagnetic Noise ◽  
Horizontal Magnetic Field

Abstract. We present the results of ground-based Ultra Low Frequency (ULF) magnetic field measurements observed from June to August 2004 during the Bovec earthquake on 12 July 2004. Further we give information about the seismic activity in the local observatory region for an extended time span 2004 and 2005. ULF magnetic field data are provided by the South European Geomagnetic Array (SEGMA) where the experience and heritage from the CHInese MAGnetometer (CHIMAG) fluxgate magnetometer comes to application. The intensities of the horizontal H and vertical Z magnetic field and the polarization ratio R of the vertical and horizontal magnetic field intensity are analyzed taking into consideration three SEGMA observatories located at different close distances and directions from the earthquake epicenter. We observed a significant increase of high polarization ratios during strong seismic activity at the observatory nearest to the Bovec earthquake epicenter. Apart from indirect ionospheric effects electromagnetic noise could be emitted in the lithosphere due to tectonic effects in the earthquake focus region causing anomalies of the vertical magnetic field intensity. Assuming that the measured vertical magnetic field intensities are of lithospheric origin, we roughly estimate the amplitude of electromagnetic noise in the Earths crust considering an average electrical conductivity of <σ>=10−3 S/m and a certain distance of the observatory to the earthquake epicenter.

Non-linear waves interactions in rotating shallow water magnetohydrodynamics

2020 ◽  
Author(s):  
Dmitry Klimachkov ◽  
Arakel Petrosyan
Keyword(s):  
Magnetic Field ◽  
Shallow Water ◽  
Shallow Water Equations ◽  
Plasma Layer ◽  
Vertical Component ◽  
Vertical Magnetic Field ◽  
Amplitude Equations ◽  
Horizontal Magnetic Field ◽  
Non Linear ◽  
Poincare Waves

&lt;p&gt;This study is devoted to the development of the nonlinear theory of the magneto-Poincare waves and magnetostrophic waves in rotating layers of astrophysical and space plasma in the shallow-water approximation. These waves determine the large-scale dynamics of the various astrophysical and space objects such as solar tachocline, as well as&amp;#160; magnetoactive atmospheres of exoplanets trapped by tides of a carrier star, neutron stars atmospheres and the flows in accretion disks of neutron stars. For this purpose we derived magnetohydrodynamic shallow water equations with a rotation and presence of an external vertical magnetic field. The system is obtained from conventional magnetohydrodynamic equations for incompressible inviscid heavy plasma layer with free surface in an external vertical magnetic field. The pressure is assumed to be hydrostatic, and the height of the plasma layer is considered to be much smaller than horizontal scales of the flow. The magnetohydrodynamic equations in the shallow-water approximation play equally important role in the space and astrophysical plasma flows like classical shallow-water equations in the fluid dynamics of a neutral fluid. The magnetohydrodynamic shallow water equations with an external vertical magnetic field are modified by supplementing them with the equation for the vertical component of the magnetic field and divergence-free condition for magnetic field contains its vertical component. Thus the velocity field remains two-dimensional while the magnetic field becomes three-dimensional. It is shown that the presence of a vertical magnetic field significantly changes the dynamics of the wave processes in astrophysical plasma compared to the neutral fluid and plasma layer in a horizontal magnetic field. &amp;#160;We have investigated the interaction of Magneto-Poincare waves and magnetostrophic waves in the magnetohydrodynamic shallow water flows in external vertical magnetic field and in horizontal (toroidal and poloidal) magnetic field. In the absence of the horizontal magnetic field the dynamics of plasma appears to be similar to the neutral fluid dynamics and it is shown that there are four-waves interactions in this case. Using the asymptotic multiscale method we obtained the non-linear amplitude equations for the three interacting Magneto-Poincare waves and magnetostrophic waves. The analysis of the amplitude equations shows that there are two types of instabilities for four different types of three-waves configurations. These instabilities occur in both cases: in the external vertical magnetic field and in the horizontal magnetic field. For all types of instabilities the growth rates are found. In the absence of the vertical magnetic field we obtained the non-linear amplitude equations for the four interacting waves. It is shown that the resulting system of equations has the first integrals that describe the mechanism of energy transfer among interacting waves of small amplitude. This work was supported by the Russian Foundation for Basic Research (project no. 19-02-00016).&lt;/p&gt;

Three‐dimensional transient electromagnetic responses for a grounded source

Geophysics ◽  
1986 ◽  
Vol 51 (11) ◽  
pp. 2117-2130 ◽  
Author(s):  
Brian M. Gunderson ◽  
Gregory A. Newman ◽  
Gerald W. Hohmann
Keyword(s):  
Magnetic Field ◽  
Half Space ◽  
Time Derivative ◽  
Three Dimensional ◽  
The Body ◽  
Vertical Magnetic Field ◽  
Horizontal Magnetic Field ◽  
One Dimensional ◽  
Transient Electromagnetic ◽  
Electromagnetic Responses

When the current in a grounded wire is terminated abruptly, currents immediately flow in the Earth to preserve the magnetic field. Initially the current is concentrated near the wire, with a broad zone of return currents below. The electric field maximum broadens and moves downward with time. Currents are channeled into a conductive three‐dimensional body, resulting in anomalous magnetic fields. At early times, when the return currents are channeled into the body, the vertical magnetic field is less than the half‐space field on the far side of the body but is greater than the half‐space field between the source and the body. Later the current in the body reverses; the vertical field is enhanced on the far side of the body and decreased between the source and the body. The horizontal magnetic field has a well‐defined maximum directly over the body at late times, and is a better indicator of the position of the body. The vertical magnetic field and its time derivative change sign with time at receiver locations near the source if a three‐dimensional body is present. These sign reversals present serious problems for one‐dimensional inversion, because decay curves for a layered earth do not change sign. At positions away from the source, the decay curves exhibit no sign reversals—only decreases and enhancements relative to one‐dimensional decay curves. In such cases one‐dimensional inversions may provide useful information, but they are likely to result in fictitious layers and erroneous interpretations.

Magnetotelluric measurements in La Malbaie area (Quebec): the anomalous vertical magnetic field

1985 ◽  
Vol 22 (10) ◽  
pp. 1530-1536 ◽  
Author(s):  
Michel Chouteau
Keyword(s):  
Magnetic Field ◽  
Impact Crater ◽  
Transfer Functions ◽  
Vertical Magnetic Field ◽  
Magnetotelluric Soundings ◽  
Vertical Transfer ◽  
Conductive Zone ◽  
Single Station ◽  
Induction Arrows ◽  
Characteristic Features

Magnetotelluric soundings were recorded in 1980–1981 at Ste-Mathilde near La Malbaie, Quebec for periods of 0.04–200 s. The present study deals only with single-station vertical transfer functions. Those functions show very characteristic features in their responses versus period. Of particular interest is a large in-phase (P) peak corresponding to a quadrature (Q) minimum between 0.1 and 0.2 s. For those periods, induction arrows indicate no obvious correlation between the anomalous vertical magnetic field (Z) and the conductive zone of Ste-Mathilde. Rather, they point toward the Malbaie River to the southwest of the survey zone. A possible model for the source of the vertical magnetic field is a shallow N130°E vertical dyke-type conductor, the top of which is some 1400 m under the Malbaie River valley. An interpretation could well be that it is one of the major graben faults related to the meteoritic impact crater of Charlevoix.

THE ELECTROMAGNETIC FIELDS OF A SUBTERRANEAN CYLINDRICAL INHOMOGENEITY EXCITED BY A LINE SOURCE

Geophysics ◽  
1972 ◽  
Vol 37 (6) ◽  
pp. 975-984 ◽  
Author(s):  
Allen Q. Howard
Keyword(s):  
Magnetic Field ◽  
Integral Equation ◽  
Line Source ◽  
Field Amplitude ◽  
Two Dimensional ◽  
Vertical Magnetic Field ◽  
Geophysical Prospecting ◽  
Field Phase ◽  
Horizontal Magnetic Field ◽  
Mine Rescue

The anomalous fields from a buried cylindrical inhomogeneity in an otherwise uniform half‐space are analyzed. The problem is rendered two‐dimensional by assuming that the uniform line source of current is parallel to the subsurface cylinder. The multipole scattered field coefficients are obtained from the numerical solution to the associated singular Fredholm integral equation of the second kind. The horizontal magnetic field amplitude, the vertical magnetic field phase, and the amplitude and phase of the ratio of horizontal to vertical magnetic fields are shown to be diagnostic of the location of the inhomogeneity. The results have possible applications to electromagnetic location in mine rescue operations and to geophysical prospecting.

Electrical Anisotropy of Thin Metal Films Growing on Dielectric Substrates

2002 ◽  
Vol 7 (2) ◽  
pp. 45-52
Author(s):  
L. Jakučionis ◽  
V. Kleiza
Keyword(s):  
Magnetic Field ◽  
Thin Films ◽  
Uniaxial Anisotropy ◽  
Metal Films ◽  
Thin Layers ◽  
Electrical Anisotropy ◽  
Electrical Field ◽  
Dielectric Substrates ◽  
Unequal Probability ◽  
Conductive Thin Films

Electrical properties of conductive thin films, that are produced by vacuum evaporation on the dielectric substrates, and which properties depend on their thickness, usually are anisotropic i.e. they have uniaxial anisotropy. If the condensate grow on dielectric substrates on which plane electrical field E is created the transverse voltage U⊥ appears on the boundary of the film in the direction perpendicular to E. Transverse voltage U⊥ depends on the angle γ between the applied magnetic field H and axis of light magnetisation. When electric field E is applied to continuous or grid layers, U⊥ and resistance R of layers are changed by changing γ. It means that value of U⊥ is the measure of anisotropy magnitude. Increasing voltage U0 , which is created by E, U⊥ increases to certain magnitude and later decreases. The anisotropy of continuous thin layers is excited by inequality of conductivity tensor components σ0 ≠ σ⊥. The reason of anisotropy is explained by the model which shows that properties of grain boundaries are defined by unequal probability of transient of charge carrier.

Two-dimensional oscillation of convection roll in a finite liquid metal layer under a horizontal magnetic field

2021 ◽  
Vol 911 ◽  
Author(s):  
Y. Tasaka ◽  
T. Yanagisawa ◽  
K. Fujita ◽  
T. Miyagoshi ◽  
A. Sakuraba
Keyword(s):  
Magnetic Field ◽  
Liquid Metal ◽  
Metal Layer ◽  
Two Dimensional ◽  
Horizontal Magnetic Field ◽  
Convection Roll

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