Geomagnetic depth sounding and magnetotelluric results from a seismically active region northeast of Quebec City

1977 ◽  
Vol 14 (2) ◽  
pp. 256-267 ◽  
Author(s):  
Y. Honkura ◽  
R. D. Kurtz ◽  
E. R. Niblett
Keyword(s):  
Active Region ◽  
Transfer Functions ◽  
Impedance Tensor ◽  
Quebec City ◽  
Electric Currents ◽  
Magnetotelluric Data ◽  
Geomagnetic Induction ◽  
Tensor Methods ◽  
Depth Sounding ◽  
Lateral Variations

Observations of geomagnetic and telluric field variations have been made at four stations near La Malbaie, Quebec, in a seismically active region northeast of Quebec City. The data were analysed by transfer function and impedance tensor methods to study the nature of geomagnetic induction in the region and its structural implications. Transfer functions and telluric fields were influenced by several factors including the nearby St. Lawrence River, lateral variations in conductivity caused by geological complexities, and conduction of electric currents probably originating through induction in the ocean.It is demonstrated that it is possible to distinguish between induction by the vertical geomagnetic field and induction by the horizontal components. Anomalous vertical fields caused by vertical field induction are observed in the area and the electric currents associated with them have a flow pattern which is similar to that of currents induced by the horizontal field. Further, the Z-induced currents changed impedance tensor elements by as much as 20% though the effect was usually smaller than this. Nonetheless, the magnetotelluric data confirmed the presence of a conductivity anomaly in the Laurentides Park region of Quebec.

scholarly journals On the relevance of source effects in geomagnetic pulsations for induction soundings

2018 ◽  
Vol 36 (2) ◽  
pp. 337-347 ◽  
Author(s):  
Anne Neska ◽  
Jan Tadeusz Reda ◽  
Mariusz Leszek Neska ◽  
Yuri Petrovich Sumaruk
Keyword(s):  
Field Line ◽  
Electromagnetic Induction ◽  
Transfer Functions ◽  
Equatorial Region ◽  
Impedance Tensor ◽  
Geomagnetic Pulsations ◽  
Correlated Noise ◽  
Field Line Resonances ◽  
Induction Arrows ◽  
Depth Sounding

Abstract. This study is an attempt to close a gap between recent research on geomagnetic pulsations and their usage as source signals in electromagnetic induction soundings (i.e., magnetotellurics, geomagnetic depth sounding, and magnetovariational sounding). The plane-wave assumption as a precondition for the proper performance of these methods is partly violated by the local nature of field line resonances which cause a considerable portion of pulsations at mid latitudes. It is demonstrated that and explained why in spite of this, the application of remote reference stations in quasi-global distances for the suppression of local correlated-noise effects in induction arrows is possible in the geomagnetic pulsation range. The important role of upstream waves and of the magnetic equatorial region for such applications is emphasized. Furthermore, the principal difference between application of reference stations for local transfer functions (which result in sounding curves and induction arrows) and for inter-station transfer functions is considered. The preconditions for the latter are much stricter than for the former. Hence a failure to estimate an inter-station transfer function to be interpreted in terms of electromagnetic induction, e.g., because of field line resonances, does not necessarily prohibit use of the station pair for a remote reference estimation of the impedance tensor.

A hybrid regularization scheme for the inversion of magnetotelluric data from natural and controlled sources to layer and distortion parameters

Geophysics ◽  
2012 ◽  
Vol 77 (4) ◽  
pp. E301-E315 ◽  
Author(s):  
Thomas Kalscheuer ◽  
Juliane Hübert ◽  
Alexey Kuvshinov ◽  
Tobias Lochbühler ◽  
Laust B. Pedersen
Keyword(s):  
Transfer Functions ◽  
Homogeneous Distribution ◽  
Stable Configuration ◽  
Inversion Algorithm ◽  
Magnetotelluric Data ◽  
Data Set ◽  
Near Surface ◽  
Minimum Solution ◽  
Magnetic Transfer ◽  
Distortion Parameters

Magnetotelluric (MT), radiomagnetotelluric (RMT), and, in particular, controlled-source audiomagnetotelluric (CSAMT) data are often heavily distorted by near-surface inhomogeneities. We developed a novel scheme to invert MT, RMT, and CSAMT data in the form of scalar or tensorial impedances and vertical magnetic transfer functions simultaneously for layer resistivities and electric and magnetic galvanic distortion parameters. The inversion scheme uses smoothness constraints to regularize layer resistivities and either Marquardt-Levenberg damping or the minimum-solution length criterion to regularize distortion parameters. A depth of investigation range is estimated by comparing layered model sections derived from first- and second-order smoothness constraints. Synthetic examples demonstrate that earth models are reconstructed properly for distorted and undistorted tensorial CSAMT data. In the inversion of scalar CSAMT data, such as the determinant impedance or individual tensor elements, the reduced number of transfer functions inevitably leads to increased ambiguity for distortion parameters. As a consequence of this ambiguity for scalar data, distortion parameters often grow over the iterations to unrealistic absolute values when regularized with the Marquardt-Levenberg scheme. Essentially, compensating relationships between terms containing electric and/or magnetic distortion are used in this growth. In a regularization with the minimum solution length criterion, the distortion parameters converge into a stable configuration after several iterations and attain reasonable values. The inversion algorithm was applied to a CSAMT field data set collected along a profile over a tunnel construction site at Hallandsåsen, Sweden. To avoid erroneous inverse models from strong anthropogenic effects on the data, two scalar transfer functions (one scalar impedance and one scalar vertical magnetic transfer function) were selected for inversion. Compared with a regularization of distortion parameters with the Marquardt-Levenberg method, the minimum-solution length criterion yielded smaller absolute values of distortion parameters and a horizontally more homogeneous distribution of electrical conductivity.

Groundwater exploration using combined controlled-source and radiomagnetotelluric techniques

Geophysics ◽  
2005 ◽  
Vol 70 (1) ◽  
pp. G8-G15 ◽  
Author(s):  
Laust B. Pedersen ◽  
M. Bastani ◽  
L. Dynesius
Keyword(s):  
Transfer Functions ◽  
Three Dimensional ◽  
Groundwater Exploration ◽  
Impedance Tensor ◽  
Two Dimensional ◽  
Controlled Source ◽  
Reflection Seismic ◽  
Seismic Sections ◽  
Clay Layers ◽  
Clay Lenses

Radiomagnetotelluric (RMT) (14–250 kHz) combined with controlled-source magnetotelluric (CSMT) (1–12 kHz) measurements were applied to the exploration of groundwater located in sandy formations at depths as great as 20 m below thick clay lenses. A combination of approximately 30 radio frequencies and controlled-source frequencies is essential for penetrating the thick clay layers. The electromagnetic transfer functions of impedance tensor and tipper vectors point toward a structure that is largely two-dimensional, although clear three-dimensional effects can be observed where the sandy formation is close to the surface. The determinant of the impedance tensor was chosen for inversion using two-dimensional models. The final two-dimensional model fits the data to within twice the estimated standard errors, which is considered quite satisfactory, given that typical errors are on the level of 1% on the impedance elements. Comparison with bore-hole results and shallow-reflection seismic sections show that the information delivered by the electromagnetic data largely agrees with the former and provides useful information for interpreting the latter by identifying lithological boundaries between the clay and sand and between the sand and crystalline basement.

DIAGRAMS FOR MAGNETOTELLURIC DATA

Geophysics ◽  
1976 ◽  
Vol 41 (4) ◽  
pp. 766-770 ◽  
Author(s):  
F. E. M. Lilley
Keyword(s):  
Impedance Tensor ◽  
Two Dimensional ◽  
Magnetotelluric Data ◽  
One Dimensional ◽  
Magnetic Components ◽  
Strike Direction ◽  
Electric And Magnetic Fields ◽  
The Relationship ◽  
Special Case ◽  
Phase Differences

Observed magnetotelluric data are often transformed to the frequency domain and expressed as the relationship [Formula: see text]where [Formula: see text] [Formula: see text] and [Formula: see text] [Formula: see text] represent electric and magnetic components measured along two orthogonal axes (in this paper, for simplicity, to be north and east, respectively). The elements [Formula: see text] comprise the magnetotelluric impedance tensor, and they are generally complex due to phase differences between the electric and magnetic fields. All quantities in equation (1) are frequency dependent. For the special case of “two‐dimensional” geology (where structure can be described as having a certain strike direction along which it does not vary), [Formula: see text] with [Formula: see text]. For the special case of “one‐dimensional” geology (where structure varies with depth only, as if horizontally layered), [Formula: see text] and [Formula: see text].

A Transfer Function Analysis of a Geomagnetic Depth Sounding-Profile across Central British Columbia

1973 ◽  
Vol 10 (7) ◽  
pp. 1089-1098 ◽  
Author(s):  
H. Dragert
Keyword(s):  
British Columbia ◽  
Transfer Functions ◽  
Function Analysis ◽  
Three Dimensional ◽  
Vertical Component ◽  
Rocky Mountain ◽  
Three Dimensional Model ◽  
Transfer Function Analysis ◽  
Single Station ◽  
Depth Sounding

Time variations of the geomagnetic field observed across British Columbia at a mean latitude of 54 °N are analyzed using 'single-station' and 'paired-station' optimum transfer functions. The frequency and spatial dependence of both coastal and inland geomagnetic anomalies are estimated with the following results. (1) The normal coast effect is strongly perturbed by lateral conductivity inhomogeneities both north and south of the profile. (2) A simple, single NW–SE striking conductivity contrast between the Cordillera and plains cannot account for the total geomagnetic anomaly in the area of the Rocky Mountain Trench; a three-dimensional model is required, incorporating (i) a lateral inhomogeneity striking east–west and located to the south of the profile, (ii) the effect of induction by the vertical component of source or secondary fields.

Electric currents and coronal heating in NOAA active region 6952

1994 ◽  
Vol 428 ◽  
pp. 860 ◽  
Author(s):  
T. R. Metcalf ◽  
R. C. Canfield ◽  
H. S. Hudson ◽  
D. L. Mickey ◽  
J.-P. Wulser ◽  
...  
Keyword(s):  
Active Region ◽  
Coronal Heating ◽  
Electric Currents ◽  
Noaa Active Region

scholarly journals Electric Currents Connected With the Proton Flares of 7 July and 2 September, 1966

1971 ◽  
Vol 43 ◽  
pp. 417-421
Author(s):  
A. B. Severny
Keyword(s):  
Active Region ◽  
Field Strength ◽  
Magnetic Flux ◽  
Electric Field ◽  
Electric Field Strength ◽  
Electric Conductivity ◽  
Electric Current ◽  
Current Density ◽  
High Energy ◽  
Electric Currents

It is observed that the change of the net magnetic flux associated with flares can exceed 1017 Mx/s, which corresponds according to Maxwell's equation to the e.m.f. ∼ 109 V which is specific for the high energy protons generated in flares. It is shown that this value of e.m.f. can hardly be compensated by e.m.f. of inductance which should appear due to the actually measured motions in a flare generating active region. The values of electric field strength thus found, together with measured values of electric current density (from rotH), leads to an electric conductivity which is 103 times smaller than usually adopted.

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