Magnetotelluric data analysis: removal of bias

Geophysics ◽  
1978 ◽  
Vol 43 (6) ◽  
pp. 1157-1166 ◽  
Author(s):  
Wolfgang M. Goubau ◽  
Thomas D. Gamble ◽  
John Clarke
Keyword(s):  
Magnetic Fields ◽  
Reference Signal ◽  
Impedance Tensor ◽  
Magnetotelluric Data ◽  
New Techniques ◽  
One Dimensional ◽  
Strike Direction ◽  
Electric And Magnetic Fields ◽  
A Site ◽  
Correlated Noises

Two new techniques for analyzing 4‐channel magnetotelluric (MT) data are described. These techniques produce estimates of the elements [Formula: see text] of the impedance tensor that are unbiased by noise in the autopowers of the electric and magnetic fields. Effectively, each technique uses one field channel as a reference signal that can be correlated with the other three channels. Method 1 obtains estimates for the [Formula: see text] in terms of crosspowers of the Fourier components of the electric and magnetic fields [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text]. Method 2 is a generalization of method 1, and obtains estimates for [Formula: see text] in terms of weighted crosspowers. Both methods fail when the geology is one‐dimensional, or two‐dimensional with one electrode oriented along the strike direction. To obtain results that are stable for any geology and that are unbiased by autopower noise, at least five channels of data are required. To also minimize bias by correlated noises, one needs six channels of data, two channels of which are for fields measured at a site that is remote from the base MT station. The analysis of MT data using a remote magnetometer as a reference is discussed.

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].

Quantisation of the conductance in units of e2/2h in a ballistic quasi-one-dimensional channel, produced by strong electric and magnetic fields

1992 ◽  
Vol 11 (2) ◽  
pp. 233-235 ◽  
Author(s):  
N.K. Patel ◽  
L. Martín-Moreno ◽  
J.T. Nicholls ◽  
M. Pepper ◽  
J.E.F. Frost ◽  
...  
Keyword(s):  
Magnetic Fields ◽  
One Dimensional ◽  
Electric And Magnetic Fields

scholarly journals Magnetotellurics with a remote magnetic reference

Geophysics ◽  
1979 ◽  
Vol 44 (1) ◽  
pp. 53-68 ◽  
Author(s):  
T. D. Gamble ◽  
W. M. Goubau ◽  
J. Clarke
Keyword(s):  
Magnetic Field ◽  
Standard Deviation ◽  
Magnetic Fields ◽  
Apparent Resistivity ◽  
The Other ◽  
Impedance Tensor ◽  
Free Data ◽  
Electric And Magnetic Fields ◽  
Conventional Methods ◽  
The Magnetic Field

Magnetotelluric measurements were performed simultaneously at two sites 4.8 km apart near Hollister, California. SQUID magnetometers were used to measure fluctuations in two orthogonal horizontal components of the magnetic field. The data obtained at each site were analyzed using the magnetic fields at the other site as a remote reference. In this technique, one multiplies the equations relating the Fourier components of the electric and magnetic fields by a component of magnetic field from the remote reference. By averaging the various crossproducts, estimates of the impedance tensor not biased by noise are obtained, provided there are no correlations between the noises in the remote channels and noises in the local channels. For some data, conventional methods of analysis yielded estimates of apparent resistivities that were biased by noise by as much as two orders of magnitude. Nevertheless, estimates of the apparent resistivity obtained from these same data, using the remote reference technique, were consistent with apparent resistivities calculated from relatively noise‐free data at adjacent periods. The estimated standard deviation for periods shorter than 3 sec was less than 5 percent, and for 87 percent of the data, was less than 2 percent. Where data bands overlapped between periods of 0.33 sec and 1 sec, the average discrepancy between the apparent resistivities was 1.8 percent.

Routine 2D inversion of magnetotelluric data using the determinant of the impedance tensor

Geophysics ◽  
2005 ◽  
Vol 70 (2) ◽  
pp. G33-G41 ◽  
Author(s):  
L. B. Pedersen ◽  
M. Engels
Keyword(s):  
Inverse Modeling ◽  
Sedimentary Cover ◽  
A Priori ◽  
Synthetic Data ◽  
Impedance Tensor ◽  
Quality Of Data ◽  
Magnetotelluric Data ◽  
Strike Direction ◽  
Recent Developments ◽  
3D Elements

Recent developments in the speed and quality of data acquisition using the radiomagnetotelluric (RMT) method, whereby large amounts of broadband RMT data can be collected along profiles, have prompted us to develop a strategy for routine inverse modeling using 2D models. We build a rather complicated numerical model containing both 2D and 3D elements believed to be representative for shallow conductors in crystalline basement overlain by a thin sedimentary cover. We then invert the corresponding synthetic data on selected profiles, using both traditional MT approaches, as well as the proposed approach, which is based on the determinant of the MT impedance tensor. We compare the estimated resistivity models with the true models along the selected profiles and find that the traditional approaches often lead to strongly biased models and bad data fit, in contrast to those using the determinant. In this case, much of the bias is removed and the data fit is improved. The determinant of the impedance tensor is independent of the chosen strike direction, and once the a priori model is set, the best fitting model is found to be practically independent of the starting model used. We conclude that the determinant of the impedance tensor is a useful tool for routine inverse modeling.

scholarly journals Eindimensionale Plasmaströmung in gekreuzten elektrischen und magnetischen Feldern

1965 ◽  
Vol 20 (8) ◽  
pp. 1019-1026 ◽  
Author(s):  
E. Rebhan
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Field Strength ◽  
Shock Front ◽  
Magnetic Field Strength ◽  
Plasma Flow ◽  
One Dimensional ◽  
Electric And Magnetic Fields ◽  
The Magnetic Field ◽  
Steady Shock

An investigation was made of a steady, one-dimensional plasma flow in crossed electric and magnetic fields. The interaction between the flow and the fields causes various flow types. In general, the flow is either supersonic or subsonic in the entire channel. Under certain circumstances, however, a transsonic flow may develop. Finally, flows exist with a steady shock front, the position and strength of which depend on the magnetic field strength and the pressure at the end of the tube.

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