Magnetotelluric Sounding Curves Research Articles

ІНТЕРПРЕТАЦІЯ 3D ГЕОЕЛЕКТРИЧНОЇ МОДЕЛІ РАЙОНУ ПЕРЕТИНУ ЗВІЗДАЛЬ-ЗАЛІСЬКОЇ ТА НЕМИРІВСЬКОЇ ЗОН РОЗЛОМІВ

The interpretation of the deep three-dimensional geoelectricalal model of the central part of the Zvizdal-Zaliska and Brusyliv fault zones of the Ukrainian shield, which was built with the involvement of data from experimental observations of the Earth's low-frequency electromagnetic field conducted in 2019 by the Institute of Geophysics named after S.I. Subotin of the National Academy of Sciences of Ukraine, within the Ordyntsi-Lobachiv and Zoziv-Stryzhavka profiles was done. Experimental materials were processed with the PRC_MTMV software package, tipper estimates were obtained for periods of geomagnetic variations from 50 to 3400 s and curves of apparent resistivity (amplitude values and impedance phases) from 10 to 10000 s. Based on the qualitative analysis of magnetovariational profiling parameters, magnetotelluric sounding curves and pseudo-sections, several local near-surface structures of complex orientation in space were identified, which are confined to the Pogrebyshchensky and Ogiivskyi faults, areas of separate parts of the Zvizdal-Zaliska, Brusyliv and Nemyriv fault zones, as well as their convergence , and outside the fault zones in the crystalline massive of the Ukrainian shield. The main result of the geological-geoelectrical interpretation of the 3D model is the discovery of new anomalies of low resistivity in the near-surface part of the earth's crust (from the surface to 0.5-2 km), which correspond to the structural and metallogenic features of the study area; confirmation and detailing of the anomalously heterogeneous distribution of resistivity in the tops of the upper mantle at depths from 70 to 120 km west of the Ukrainian shield. The area of intersection of the Zvizdal-Zaliska and Nemyrivs fault zones is characterized by both a galvanically connected spatially diverse system and local separate near-surface anomalies with low resistivity from 5 to 100 Ohm·m, most of which dip to 0.5-1 km. It is shown that there are connections between conductivity and structural features of the Zvizdal-Zaliska, Brusyliv and Nemyriv fault zones. In addition, some anomalies are found in the granite migmatites of the Ros' structural-formation zone, outside the tectono-metasomatic mineragenic zones. As a rule, anomalies are confined to zones of disintegration of crystalline basement rocks, elongated zones of metasomatization, and areas of distribution of graphitized rocks; moreover, some of the surface anomalies correspond to regions of weathering crusts. The conducted geological-geoelectrical analysis of the model showed that several areas require further study, namely the sub-latitude anomaly west of the Zoziv-Stryzhavka profile and the complex-shaped anomaly in the eastern part of the study area. As promising for the search for minerals, which meets the geoelectrical criteria, the area of the anomaly within the Brusyliv fault zone with a depth of up to 2 km is highlighted.

Study of a quantitative screening method of magnetotelluric sounding curves

An accurate tensor estimation function is a guarantee for acquiring a reliable underground electrical structure model. The past studies regarding transmission functions aim at obtaining smooth and continuous sounding curves through the time domain by data screening means such as least square, robust and remote reference processing methods as well as digital filtering techniques such as wavelet denoising, empirical mode decomposition and Hilbert–Huang transform denoising. As SNRs in different field measurement sites are different, if different data processing algorithms like single-site processing, remote reference processing, magnetic field correlation-based remote reference processing and magnetic channel sharing are used for data processing at the same measuring site, 3N-2 groups of sounding curves with different qualities will be obtained. Manual screening method has been mainly used in the past for screening of the abovementioned 3N-2 groups of sounding curves. If the quantity of sounding site is large, this manual screening method is of low efficiency without quantitative screening indexes. In this paper, discrete Frechet distance is mainly utilized to carry out quantitative screening of 3N-2 groups of sounding curves processed through the abovementioned four processing methods in order to obtain smooth, continuous and reliable sounding curves, thus avoiding subjectivity and low efficiency problems in the past manual screening. A quantitative sorting of all kinds of data processing results can be conducted, and the optimal sounding curve is finally picked out for data inversion. The above sounding curve selection result has improved processing accuracy of magnetotelluric data.

Complex geophysical model of the Earth’s crust and upper mantle along the profile of RP-VI (Carpathians)

A complex geophysical model is presented along the regional profile of RP-VI, which is located near the outlet to the terrestrial surface of the eastern boundary of the Skiba zone at its contact with, mainly, the Borislav-Pokuta zone of the Precarpathian trough. For the interpretation of magnetic data, the results of an aeromagnetic survey of a scale of 1 : 200 000 at a flight altitude of 2000 m were used, for the gravitational field — state gravimetric maps of the territory of the USSR in the Bouguer reduction of scale 1 : 200 000. For geoelectrical data, the magnetotelluric sounding curves obtained over the last 30 years have been used. Interpretation of gravimetric and magnetometric data was performed using the computer technology of data analysis KOSKAD3D, which is intended for analysis of three-dimensional digital geoinformation by the methods of a probabilistic-statistical approach. Interpretation of geoelectrical data is performed using the two-dimensional modeling program (inverse problem) — using the REBOCC algorithm. For the analysis of seismic data, the results of both the last century and the last decade are involved. The results of interpretation of different geophysical methods are presented in the form of vertical sections of physical parameters and their complex models of the earth’s crust and upper mantle to a depth of 90 km. The results of the geological and geophysical data presented in this paper differ from those obtained earlier by a new approach to the complex interpretation of potential fields and their comparison with previously unknown geoelectrical models. On the basis of the interpretation of potential fields, a peculiar element of the structure of the Earth’s crust was revealed, ap-parently connected with a sharp change in the composition of the rocks along the profile. The roof of this object undulates from the outlet to the surface at the northwest end of the profile to 25 km in its southeastern part. The sole of the layer from the end of the profile to the point 150 remains constant at a depth of about 25 km, drastically changing from 150 to the depths of the M boundary. It is shown that a sharp change in this object is accompanied by the presence of a low-resistance object that extends from 25 to 80 km with a maximum width of about 40 km. It is suggested that the position of this heterogeneity is related to the Tyachiv-Nadvornyansko-Monastyrtsky fault. The possible connection of a part of the selected element with oil and gas content is considered.

Structure of the Zeya block of Toko Stanovik according to results of magnetotelluric sounding

The paper describes the results of magnetotelluric sounding (MTS) carried out in the Zeya block of the Stanovoi megablock, in the area of its junction with the Aldan Shield. The border between them runs along the Stanovoi fault. Based on the results of interpretation of MTS curves and the gravity, magnetic, and geological data, geological/geophysical sections have been constructed to a depth of 7 km along two ~20 km long profiles running across the Stanovoi fault. About 1 km thick and approximately 2 km long conductivity zone has been distinguished beneath the Okonon plateau of Quaternary basalts with electrical resistivity of <100 Ohmm. This anomaly is associated with ore mineralization in Early Proterozoic gabbro intrusion. A 3D density modeling was performed. High-density bodies of NW strike dip to the northeast to a depth of 25 km in the area of the Okonon basalt plateau.⁎Corresponding author.

Results of inversion of distorted magnetotelluric sounding curves (numerical experiment)

The paper presents the results of a joint inversion of magnetotelluric sounding (MTS) curves taken at several stations without preliminary selection and normalization and distorted by the presence of lateral electrical-conductivity inhomogeneities in the medium. In the calculations, we used synthetic MTS data for a three-dimensional model. Preparation and interpretation of data are carried out by the Trefftz method using a numerical model of the field and MTS curve distortions. To solve the inverse problem and optimize the subsurface model, we used a nonlinear least-squares method and an iterative process with calculation of the sensitivity matrix and its singular decomposition. The target functional is determined by the discrepancies between the model and synthetic experimental apparent-resistivity curves corresponding to the elements of the impedance tensors on the lateral diagonal. The reliability of the reconstructed subsurface model is characterized by the dispersion of the deviations of its parameters from the parameters of the known model used for the preparation of synthetic experimental data. The joint consideration of distorted apparent-resistivity curves at several stations increases the reliability of interpretation results. The obtained solution to the inverse problem is approximate and can be used as a starting model for more complex algorithms and programs.

A deep geoelectric model of the Bol’she-Bannyi hydrothermal system, Kamchatka

This paper reports the results of magnetotelluric sounding (MTS) in the area where the Bol’she-Bannyi hydrothermal springs are discharged. The MTS curves were inverted on the assumption of a twodimensional inhomogeneous model using longitudinal and transverse curves of apparent resistivity. It was found that the geoelectric section contains a nearly vertical anomaly of high electrical conductivity at depths of 5.5–8 km, which is the signature of a deep-seated fault. The resulting geoelectric section for the upper crust and data from regional magnetotelluric soundings were used as a basis for developing a conceptual deep model of the Bol’she-Bannyi hydrothermal system. We quote an approximate estimate of rock porosity. According to the model, deep fluids come from a crustal layer into the subvertical deep-seated fault then penetrate via fissures into the sedimentary–volcanogenic cover, and finally arrive at the ground surface in zones of high rock permeability. We provide a recommendation for drilling a deep well in order to determine the potential of the Bol’she-Bannyi hydrothermal field.

Inversion of magnetotelluric data in fault zones of Gorny Altai based on a three-dimensional model

Results of magnetotelluric sounding (MTS) in Gornyi Altai are interpreted on the basis of a numerical model of MTS curve distortions in a 3D earth. The distortions are modeled using the Trefftz method permitting the application of models of different degrees of similarity to the test medium (depending on available computational resources). The main advantages of this approach are demonstrated. There is no need to choose between different MTS curves (transverse and longitudinal, minimum and maximum, undistorted and distorted). Procedures of normalizing these curves become unnecessary. All recorded curves are fully used as input data for their inversion. Optimization of the model of the medium taking into account the distortions of MTS curves caused by surface and depth inhomogeneities improves the reliability of geoelectric sections.

The deep structure of the Olyutorskii earthquake rupture zone in the Koryak Upland: Geophysical evidence

This paper considers the data of magnetotelluric sounding (MTS) and gravity and magnetic surveys, as well as other geological information. We describe the methods that were used in making the observations and for interpretation of MTS data. We focus on incorporating the influence of the shift effect and the coast effect. Generalized geoelectric sections are based on mean longitudinal curves, which are less subject to distortion. Transverse curves were used for fault identification. The geoelectric sections include a sedimentary-volcanogenic layer, a high ohmic crust, and a conductive lithosphere layer. They contain faults that were detected from the divergences between oriented MTS curves and other data. The deep-seated faults in the Vyvenka basin have surface expression in the form of seismotectonic discontinuities that extend for 80 and 140 km. It was found that the hypocenters of the Olyutorskii earthquake and its aftershocks were in the crustal layer above the conductive lithosphere layer. Most aftershock hypocenters are typically situated in the area of the deep fault. We discuss a possible origin of the conductive lithosphere layer and of deep faults.

A Method of Correcting the Long Period Magnetotelluric Sounding Curve

During the collection of field data of the long period magnetotelluric sounding, apparent resistivity curve will be interrupted by noise, especially within the frequencies of dead frequency band, the distortion of the curve is serious, even the trend of the apparent resistivity curve cannot be determined. Based on the feature that impedance phase is hardly affected by random noise, this paper tried to correct the apparent resistivity distortion curve through impedance phase and applied the theory to data processing of Songpan-Hechuan LMT profile. Theoretical calculations and practical application show that the distorted apparent resistivity curve can be recovered segment by segment using impedance phase information influenced little by noise, having better practical value.

ГЕОДИНАМІКА

On the basis of interpretation of magnetotelluric sounding curves constructed geoelectric model of south-east of the Yana-Kolymskaya fold system. As a result of mapping of the deep geoelectrical parameters with data obtained by other methods, are identified generalized geological-geophysical criteria characterizing the gold units of the south-east of the Yana-Kolyma folded system.

Analysis of electrojet-distorted magnetotelluric sounding curves

South America presents several unique geomagnetic features, one of which is the Equatorial Electrojet (EEJ), a current system which extends itself east to west in Northeastern Brazil for almost 3500 km. Considering the fact that the influence of this phenomenon at low magnetic latitudes can be detected at great distances from its central axis, it is important to simulate its effect in magnetotelluric (MT) exploration. To accomplish this and by the use of an integral equation scheme, we have modeled the MT response of confined three-dimensional (3D) bodies (dykes in an homogeneous half-space) and deep 3D structures (horst and graben located at Marajo Basin in Northern Brazil). As the inductive source, we replace the classical plane wave source by a common line of current, besides gaussian and undulatory electrojets. The results of the modelling indicate that the studied effect is prominent in both one-dimensional (1D) and 3D media for periods ( T) greater than 10 s. It decreases with distance, but it is detectable as far as 3000 km from the center of the EEJ. It is also observed that for T greater than 10 s, the computed soundings can be strongly distorted, mainly by EEJ effects in the host medium which, in turn, cause changes in the final branch of the 3D soundings. For structures in the Marajo Basin, our results reveal that the 3D galvanic effect prevails in the interval 0.1< T<10 s, while source influence occurs mainly for T>10 s. On the other hand, automatic inversion of these 3D data shows that, in equatorial regions having complex geology, some errors arise in conventional 1D interpretation of the MT soundings. This is due to the superposition of the host medium response, the galvanic effect of 3D structures, and the EEJ influence.

The time-domain magnetotelluric response over three-layer earth models

SUMMARY Magnetotelluric data observed in frequency and time domains are expressed as apparent resistivity. The apparent resistivity is a weighted spatial average of the subsurface resistivity distribution. In this paper, we develop analytical expressions to compute the apparent resistivity in the time domain for various three-layer earth models. The present approach to computing the magnetotelluric response in the time domain is found to overcome the problems encountered by the method of images. The magnetotellurk response in the time domain for various three-level models have been computed and shown graphically. The time-domain responses show a characteristic behaviour with a small change in layered parameters (resistivity and thickness of the layers), whereas frequency-domain responses do not show such behaviour. This characteristic behaviour of time-domain magnetotelluric sounding curves will be useful in the qualitative interpretation of field data.

Distortions of magnetotelluric sounding curves due to slope

Distortions of magnetotelluric sounding curves due to slope

Distortions of magnetotelluric sounding curves due to a slope

This study considers a particular case of 2D modeling, namely the topographic effect on MT data. A simple way to remove topographic effects is to incorporate surface elevations, or depressions, into the forward model. In cases where a 2D approximation is valid, any 2D algorithm may be used for the calculations. Such a scheme allows model results and observations to be compared along a profile. This approach, however, does not necessarily provide useful answers since the topography may, and in fact nearly always does, couple with the geoelectric structure and/or varies on a more rapid scale than the observations. In order to understand the effect of topography on MT data the degree and characteristics of the distortion introduced by a simple slope are considered. The slope model constitutes a general result with distortion characteristics that can be extended to other features such as a depression or a hill. The introduction of additional features such as a superimposed geoelectric structure would greatly complicate the problem due to electromagnetic coupling effects which would necessarily be a function of frequency. The single slope, or half-graben, is probably the simplest class of general near-surface distortions that affect MT data. The period range used is from 0.02 s (50 Hz) to 100 s (0.01 Hz) i.e. both audio and magnetotelluric observations are considered. The elevation of the slope is fixed at a “throw” of 1 km and slope angles range from 0° to 90°. The spatial and frequency distortions due to the slope are considered in detail. At a particular location the distorting effect of the slope introduces a false gradient into an otherwise 1D sounding curve. The topographic distortion introduced is investigated by applying a 1D inverse procedure to the obtained results. The way in which false layers may be introduced into the geoelectric section is illustrated. An analytical solution to a purely DC topographic problem is also considered. The analytical results are compared with the corresponding low frequency results obtained with the numerical models of the present study.

A simple sensitivity analysis of time‐domain and frequency‐domain electromagnetic measurements

Electromagnetic responses in both time and frequency domains can be looked upon as weighted spatial averages of the nearby conductivity distribution. Using this concept, I analyze the simplest possible situation: a uniform half‐space in the presence of uniform current sources. Both transient and harmonic measurements are equally effective in providing conductivity values for the half‐space; while the conductivity values are the same, the averaging processes are not. To different types of measurements, even within the same domain, there correspond different types of sensitivity or weighting functions. Some of these functions emphasize the near‐surface conductivity, while others emphasize the conductivity at depth. These and other features of the weighting functions explain the behavior of magnetotelluric sounding curves.

Distortion of magnetotelluric sounding curves by three‐dimensional structures

Distortions of magnetotelluric fields caused by three‐dimensional (3‐D) structures can be severe and are not predictable using one‐dimensional or two‐dimensional modeling. I used a 3-D modeling algorithm based upon an extension of a generalized thin sheet analysis due to Ranganayaki and Madden (1980) to examine field distortions in crustal environments. Three major physical mechanisms cause these distortions. These mechanisms are resistive coupling between the electrical mantle and upper crust, resistive coupling between conductive features within the upper crust, and local induction of current loops within good conductors. Each mechanism produces different spatial and frequency effects upon the background field, so identification of the dominant mechanism can be used as an interpretational aid. I finally use this analysis to identify distortion mechanisms seen in magnetotelluric data from Beowawe, Nevada to aid in an interpretation of that area.

Effects of three‐dimensional structure on magnetotelluric sounding curves

Significant errors may result when applying one‐dimensional (1-D) or two‐dimensional (2-D) interpretation methods to magnetotelluric (MT) data collected in three‐dimensional (3-D) environments. Both depths and resistivities can be grossly incorrect if 1-D or 2-D methods are applied in a 3-D setting. We present examples of MT sounding curves generated using a 3-D modeling program (Madden and Park, 1982) and illustrate some interpretation pitfalls if 3-D effects are not considered. The 3-D effects discussed herein are attributed to a surface layer heterogeneity, and can be readily identified in MT data from a well‐designed MT survey. The MT survey must include stations chosen to yield information about regional structure. Alternatively, carefull examination of a geologic map will help the intepreter estimate the regional effects present in the data.

Inverse Magnetotelluric Problem for Sounding Curves from Czechoslovak Localities

In fitting geoelectrical models to sounding curves from certain localities in Czechoslovakia, we applied “the Hedgehog” method. In the first step it uses the Monte Carlo procedure to find a successful model. Then, it goes systematically through the values of both the electrical conductivities and the layer thicknesses varying within given limits in the neighbourhood of the successful model previously found.The inversion was effected for different types of magnetotelluric sounding curves corresponding to specific geological structures. From a large family of models tested by the Hedgehog procedure the best fitting and the most frequently occurring models were chosen. The direct problem was solved for their parameters and the resulting theoretical sounding curves were compared with the experimental ones.

CAN TRANSITION IN THE ELECTRICAL PROPERTIES OF THE EARTH BE DETECTED?*

AbstractThe interface phenomena for low periods, i.e. the initial oscillations, and the cut‐off periods are two characteristic features in Magnetotelluric Sounding curves. A method is proposed to distinguish between transitional and homogeneous layered earth by means of these two features.

THE INVERSE PROBLEM OF MAGNETOTELLURIC SOUNDING

Based on the model of a flat layered earth, a nonlinear, least‐squares method is used to invert magnetotelluric sounding curves to obtain the layer resistivities and thicknesses. Partial derivatives of the apparent resistivity with respect to layer parameters show the manner in which the layer parameters are contrbuting to the apparent resistivities. Uniqueness of the inversion is not guaranteed, but when the partial derivatives are linearly independent and the relative magnitudes of the layer resistivities of the initial guess are not too far from the correct ones, the convergence of the method to the correct values seems to be ensured.