Magnetotellurics (MT) in mountains—noise, topographic and crustal inhomogeneity effects

doi:10.1016/0031-9201(86)90089-0

Physics of the Earth and Planetary Interiors

Volume 42, Issue 3, 28 February 1986, Pages 165-177

https://doi.org/10.1016/0031-9201(86)90089-0 Get rights and content

Abstract

In a mountainous region the aim of magnetotelluric sounding is to detect deep fractures, indicated by electric conducting zones in the crust.

Nevertheless, some factors, such as the EM noise of different character, the rugged topography, the electric inhomogeneities, are increased in a mountainous region, and to determine or overcome them special measuring and data processing techniques should be applied.

This paper reviews these factors or effects illustrating them with examples from the Eastern Alps and the Hungarian Central Mountains.

References (11)

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Statistische Zusammenhänge zwischen elektrischer Leitfähigkeitsverteilung und Bruchtektonik in Transdanubien (Westungarn)
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Fractures as conducting dykes and corresponding two-dimensional models
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Physical and mathematical modeling of crustal conductivity anomalies in the Pannonian Basin
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There are more references available in the full text version of this article.

Cited by (17)

Smooth magnetotelluric impedance estimation by optimization
2023, Journal of Applied Geophysics
In urban and industrialised areas, magnetotelluric (MT) recordings often contain a significant amount of high-power near-field noise which, constituting non-Gaussian noise for most of the recording length, may lead to the failure of the common transfer functions (TF) estimation methods. The most evident consequence of this is the appearance of sharp points in the TF curves versus frequency, deteriorating their inherent smoothness. The TF's smoothness is in fact one of the main criteria adopted by MT practitioners to evaluate the data quality and effectiveness of the estimation procedure; when not reached by the estimation, it may be a-posteriori introduced by splines or filtering procedures, but these approaches lack physical soundness. We propose a new heuristic algorithm aimed at obtaining the maximum TF versus frequency smoothness, through an optimization scheme rejecting EM events in frequency-domain. The algorithm searches for frequency-dependent power thresholds to be applied to the events, rejecting those making the TF sharp. The TF's smoothness is the objective function to be minimized, and the model space is constituted by the infinite set of threshold vectors for each analysed frequency. After the process, the Ordinaly Least Squares (OLS) residual distribution is much closer to a Rayleigh one, allowing for an unbiased estimation. We applied the procedure both to synthetic and real MT data. Measured data has been collected for a geothermal exploration project in the East Africa Rift, where strong coherent noise affected the recordings. The combination of the technique with the remote-reference technique effectively reduced the bias due to the artificial signal. The physical consistency of the resulting TF has been tested by dispersion relationships.
Electrical resistivity structure of the Northwestern Anatolia and its tectonic implications for the Sakarya and Bornova zones
2001, Physics of the Earth and Planetary Interiors
The crustal structure of the northwestern Anatolia was studied by magnetotelluric measurements with periods up to 500 s at 20 locations along a line (line KS) crossed Istanbul, Sakarya and Bornova Zones to investigate deep electrical resistivity patterns that contribute to the understanding of tectonic setting of the region. The results obtained from the two-dimensional (2D) inversion of magnetotelluric data for the modes transverse electric (TE) and transverse magnetic (TM) show that the tectonic structure up to 5 km depth was comparatively complex and that the structural pattern was generally disturbed by the grabens and Neogene basins of low resistivity feature. The low resistivity (<80 Ω m) zones stretched from the Istanbul Zone towards the northern part of the Sakarya Zone and from the southern border of the Bornova Zone towards the Gediz Graben in south characterize sedimentary sequences for this depth range. Based on the more detailed near-surface 2D geoelectrical model constructed using dc Schlumberger resistivity data the basin-fill deposits is about 1.5 km and 750 m thick in the Gediz and Görede Grabens, respectively. Other smaller regions of low resistivity were also interpreted as the small-scale sedimentary Neogene basins within the Sakarya and Bornova Zones. Below about 5 km depth geoelectric models except a zone beneath Kazdağ range show more resistive structure underlying these sediments. The resistive structure was correlated with the Precambrien crystalline rocks and gneiss schist basement of Sakarya and Bornova Zones, respectively. A major zone with relatively conductive (∼10 Ω m) occurred within highly resistive mid crust beneath Kazdağ range represents an electrically macro-anisotropic structure which could be followed from magnetotelluric sounding data in both TE and TM modes. It was discussed that the origin of this zone is associated with a coeval plutonism and metamorphism of latest Oligocene age, which yields a considerable low conductivity in the crystalline rocks. Of special interests was the zone of electrical conductivity, as it seems to exist a correlation between conductive structure and geotectonic features, which may support ideas about the high-grade metamorphism and accordingly north–south extending in the northwestern Anatolia.
The use of data-adaptive filtering for noise removal on magnetotelluric data
1989, Physics of the Earth and Planetary Interiors
A new method is described which can be used to reduce the bias error as well as the random error of the magnetotelluric impedance tensor. This reduction in noise can be done in a single stack, thus making averaging unnecessary or keeping it to a minimum. This can speed up a magnetotelluric sounding considerably. The method is based on a data-adaptive filtering technique. A unique feature of the method is that no a priori information of the signal or noise characteristic is necessary for its operation. Another advantage is that the method adapts itself to changing signal and/or noise characteristics. It is shown how random noise, spikes and steps can be removed by this method. A description of the noise problems in MT is given. The data-adaptive method is described and its properties are illustrated by applying it to severely distorted synthetic signals. Its effectiveness in reducing MT tensor bias and random noise is shown through the use of the unit impedance tensor.
Natural and man-made EM variations in the Komló coalfield
1989, Physics of the Earth and Planetary Interiors
The strongly tectonized structure of the Komló coalfield (Southern Hungary) was studied by magnetotelluric (MT) deep soundings. An experiment was also made to use the man-made electromagnetic noise very frequently appearing on MT records to obtain some information concerning the coalfield's structure. Comparison of the two different data sets highlights the limits of the application of EM noise in geophysical exploration.
A long-term MT study between 1982 and 1985 showed some MT resistivity variations probably associated with mining at the place of the greatest soil subsidence.
Processing of noisy magnetotelluric data using digital filters and additional data selection criteria
1988, Physics of the Earth and Planetary Interiors
Although the magnetotelluric (MT) method is known to be effective and fast in probing the electrical conductivity structure of the Earth at crustal depths, the results are often degraded by industrial and cultural noise. To obtain reliable processed results for modelling, it is first necessary to extract or select the natural signals from the contaminated time series.
Various noise-reduction techniques based on digital filters are discussed with special reference to persistent noise signals, e.g. from power lines, DC-operated railways and electrical fences. Both previously suggested techniques (delay-line and notch filtering) and two other procedures (maximum entropy extension and deconvolution filtering) are applied to both synthetic data and to field observations from southern Scotland and the Italian Alps. Better quality data sets and more geophysically acceptable Earth models are shown to result.
Noise of a more intermittent nature has recently been observed in MT observations near the development site of the geothermal power station on Milos, Greece. Large highly coherent electromagnetic field signals were observed to coincide with the opening and closure of the valves on the test wells. In this case, meaningful apparent resistivity curves could be obtained from an undisturbed subset of the previously accepted data, which had been selected mainly on the basis of signal power.
Delay-line filtering is shown to be superior to notch filtering in eliminating non-sinusoidal noise, while both the MEM extension and the window deconvolution techniques are found to be useful in spike removal.
These studies illustrate that use of an automatic data selection procedure should only be undertaken with great care in areas where the cultural noise is high. In such cases, continuous time-domain monitoring of the MT signals is recommended. The appropriate techniques of noise reduction can then be applied.
A geothermal aquifer in the dilation zones on the southern margin of the Dublin Basin
2020, Geophysical Journal International

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Magnetotellurics (MT) in mountains—noise, topographic and crustal inhomogeneity effects

Abstract

Statistische Zusammenhänge zwischen elektrischer Leitfähigkeitsverteilung und Bruchtektonik in Transdanubien (Westungarn)

Acta Geod. Geoph. Mont. Hung.

Fractures as conducting dykes and corresponding two-dimensional models

Geophys. Prospect.

The results of electro-magnetic soundings in the transitional zone between the Pannonian Basin and the Eastern Alps

Magyar. Geofizika

Magnetotelluric sounding in the transitional zone between the Eastern Alps and Pannonian Basin

J. Geophys.

Physical and mathematical modeling of crustal conductivity anomalies in the Pannonian Basin

Acta Geod. Geoph. Mont. Hung.