Magnetotelluric Field Research Articles

ANALYSIS OF GEOPHYSICAL PARAMETERS VARIATIONS AND SEISMIC EVENTS AT THE POINT OF DEEP MAGNETOTELLURIC SOUNDING

The paper considers the results of processing and analysis of data from deep magnetotelluric soundings (DMT) performed in 2018. Comparison of variations in apparent resistivity, the endogenous component of the magnetotelluric field, lunar-solar tidal deformations and seismic events that were recorded during the research. The objective of the study is to detect the relationship between the appearance of variations in electromagnetic parameters of the data of magnetotelluric monitoring from the occurred earthquakes, its’ distance, energy class and position, relative to the DMT point.

ANALYSIS OF GEOPHYSICAL PARAMETERS VARIATIONS AND SEISMIC EVENTS AT THE POINT OF DEEP MAGNETOTELLURIC SOUNDING

The paper considers the results of processing and analysis of data from deep magnetotelluric soundings (DMT) performed in 2018. Comparison of variations in apparent resistivity, the endogenous component of the magnetotelluric field, lunar-solar tidal deformations and seismic events that were recorded during the research. The objective of the study is to detect the relationship between the appearance of variations in electromagnetic parameters of the data of magnetotelluric monitoring from the occurred earthquakes, their distance, energy class and position, relative to the DMT point.

Geophysical surveys in the area of the Krylatovskaya water well

Relevance and purpose of the work. Surface sources of clean water are increasingly depleted. In this regard, the role of underground sources of drinking and domestic water supply is increasing. Geophysical exploration methods can be an e ective tool for fnding groundwater. The tasks of the research included determining the possibilities of magnetic prospecting in the areal search for promising zones, and identifying the features of electromagnetic fields during audiomagnetotelluric and magnetovariational soundings in the area of an operating water well. Methodology. The magnetic field was surveyed with an MMP-203 proton magnetometer (Geologorazvedka plant, Leningrad). Electrical exploration included vertical electrical sounding (VES) with ERA equipment (GNPP Geologorazvedka, St. Petersburg) and recording of audio magnetotelluric fields with OMAR-2 equipment (IGF UB RAS, Ekaterinburg). Results. The result of the magnetic survey was a map of the anomalous magnetic field of the study area, where the water-abundant groundwater zone of the weathering crust can be distinguished by structural features. The zones of tectonic faults, promising for the presence of fractured-vein waters, are confedently distinguished by positive linear anomalies of the magnetic field. The characteristic features of electromagnetic fields during audiomagnetotelluric and magnetovariational soundings in the area of distribution of fractured waters have been clarified. The most revealing behavior is the behavior of the real and imaginary quadratures of the magnetovariational tipper, characterized by minimum values and zero crossing, respectively. According to the results of electromagnetic sounding, an increase in the thickness of the weathering crust with a decrease in resistance is recorded in the area of the water intake well, which is evidence of its water cut. Conclusions. The significance of magnetic prospecting for clarifying the structural and geological structure of the territory when searching for water-saturated zones has been determined. The high e£ciency of electromagnetic soundings on direct and alternating current was confirmed to determine the nature, depth and power of the identiŽed structures. Traditionally studied, in the search and exploration of groundwater, geophysical felds are replenished with new electromagnetic parameters. These include the module and quadratures of the audio range magnetovariational tipper. The research results indicate that these parameters are anomalous in relation to aquifers, which allows us to give reasonable recommendations on the location of drilling production wells.

Three-dimensional magnetotelluric modeling in general anisotropic media using nodal-based unstructured finite element method

Magnetotelluric sounding is an efficient and economical method for detecting Earth's deep structure because of its large surveying depth. We use a finite element approach to solve magnetotelluric fields in three-dimensional general anisotropic media based on the vector–scalar potentials and unstructured meshes. The implementation of a three-dimensional unstructured mesh generator TetGen with high-quality local mesh refinement can easily handle complicated models with high performance. The electromagnetic field equations are expressed in terms of Coulomb-gauged vector–scalar potentials, and the nodal-based finite element method can be used to compute the electromagnetic fields with high efficiency. The symmetric successive over-relaxation preconditioned conjugate gradient solver is used to solve the resulting linear equation system. The magnetotelluric responses in one-, two-, and three-dimensional medias are computed through our proposed approach. These results are then compared with the analytical solutions, two- and three-dimensional adaptive finite element solutions. Numerical examples demonstrate that these results are in good agreement. Because an unstructured mesh is incorporated in this approach, it has great potential for simulating the magnetotelluric responses in geometrically complicated situations.

Processing of noise contaminated magnetotelluric data using digital filters based on MATLAB

Magnetotelluric (MT) method is known as an effective geophysical method for imaging the condition of the subsurface resistivity distribution, especially in the geothermal system. However, this method is very sensitive to noise. The noise can occur due to mechanical activity such as vibration and electrical activity generated by powerline electric current near the point of measurement. To eliminate this noise is used digital filter consisting of despike filter and notch filter. Despike filter is used to eliminate irregular spike noise, while the notch filter is used to remove regular powerline noise. Furthermore, D+ can be applied to enhance the impedance data trend. Both of filters and D+ feature are built and then arranged into MTFilter program. The result of data processing using MTFilter program shows the improvement of MT data quality. Moreover, the inversion processing results of MT field data shows the resistivity distribution of structure which is more reliable with the characteristics of the geothermal system.

Response of cracking processes in variations of geophysical fields

This paper discusses a range of issues related to the interpretation of time variations of geophysical fields as a response of the geophysical environment to lunar-solar tidal influences and seismic events under conditions of submeridional compression. A model linking the mechanism of crack formation and generation of geophysical fields has been proposed. The concept of the energy characteristics of a seismic field of deep underground (endogenous) origin along the seismic wave field observed on the Earth's surface for three mutually perpendicular components has been introduced. The results of field studies of simultaneous recording of a seismic wave field by a three-component gradient system and a magnetotelluric field at monitoring sites of the Bishkek Geodynamic Proving Ground (BGPG in Northern Tien Shan) are presented. The test area is situated in an active intracontinental mountain Central Asian Orogenic Belt with the frequent seismicity. For the experiment purposes, we select the Ukok regime monitoring point in the Kochkor basin of Kyrgyz Tien Shan. Because the location is characterized by low-level of seismic noise and the technical-and-industrial influence. The applicability of such a joint approach in geophysics is discussed.According to the results of the experiments performed, we establish a possible causal relationship between lunar-solar tides and activation of the deep underground (endogenous) source. Based on the correlation analysis of gravitational tidal effects, and the results of magnetotelluric monitoring, the azimuthal dependence of the electrical conductivity of the medium was revealed. We detect characteristic changes in the studied electromagnetic parameters, which we identify as a result of the redistribution of fluids between the fracture systems, were. The joint analysis of data from regime magnetotelluric observations and seismicity distribution confirms the high sensitivity of the method to changes in the stress-strain state of the geological environment. The confinement of seismic events to zones with high gradients of variations of apparent electrical resistivity is shown.The work may be of interest to specialists in the field of Earth Sciences, dealing with the interpretation of experimental data from geophysical test sites.

Joint Inversion of Magnetotelluric Impedance and Tipper Data in 3D Axial Anisotropic Media

Magnetotelluric (MT) impedance and tipper data can be utilized to reflect underground structural information. However, when the axial anisotropic property of an abnormal body is marked, it will seriously influence MT measuring data. This study divides the MT field into primary and secondary components and replaces the conductivity parameter in a three-dimensional (3D) governing equation with axial anisotropic conductivity. Analysis of the influence of axial anisotropy on MT tipper data is presented, and a limited-memory Broyden-Fletcher-Goldfarb-Shanno method is then utilized to realize a 3D MT axial anisotropic inversion with impedance and tipper data. The tests presented in this paper show that the resolution of the inverting axial anisotropic model with impedance and tipper data is better than that of only using impedance data.

Imaging of groundwater contamination using 3D joint inversion of electrical resistivity tomography and radio magnetotelluric data: A case study from Northern India

ABSTRACTThe impact of untreated sewage irrigation and waste disposal practices on groundwater is investigated by 3D joint inversion of radio magnetotelluric and electrical resistivity tomography data. In this case study, electrical resistivity tomography and radio magnetotelluric field measurements were carried out on several profiles near a waste disposal site which was irrigated with untreated sewage water for agriculture purpose. In addition, radio magnetotelluric and electrical resistivity tomography measurements were carried out, far away from the waste site, to derive the uncontaminated geology. The data were analysed earlier using 2D inversion techniques. However, for the 2D inversion of the electrical resistivity tomography and radio magnetotelluric data, assumptions about the strike direction are required. As no clear strike direction is evident for the contamination, we considered the problem as 3D and interpreted the present data set using the 3D inversion algorithm ‘AP3DMT‐DC’. The inverted 3D resistivity model shows an unconfined aquifer of low resistivity which is overlain by an unsaturated slightly resistive near surface formation. With increasing distance from the waste sites, an increase in the resistivity of the shallow unconfined aquifer is observed. Furthest away from the waste site undisturbed geology is expected. We derived consistent and meaningful 3D resistivity models. The uncontaminated reference site indicates an increased resistivity for the aquifer layer. A synthetic 3D study was carried out to demonstrate and validate algorithm performance as well as convergence capabilities. The study demonstrates that the two methods, electrical resistivity tomography and radio magnetotelluric, complement each other. Besides, a better resolved inverted model is obtained through a 3D joint inversion, in comparison to individual 2D and 3D inversions.

Modelling of Terrain Effect from the Magnetotelluric (MT) Field Data

Magnetotelluric (MT) data were recorded over highly undulating terrain in Himalayan region from Roorkee to Gangotri section in period 0.001-1000 second. In the presence of topographic distortion the interpretation may become misleading. A simple scheme based on finite difference method for the simulation of the topographic distortion in magnetotelluric response is presented. The finite difference based, forward response computation algorithm, has been extended for undulating topography. The distortion coefficients, representing the topographic effect, are designed for correcting the observed distorted impedance tensor recorded in the vicinity of topographic features. The accuracy of the scheme is checked by comparing the computed responses with the finite element, Rayleigh scattering and transmission surface results for transverse electric (TE-mode) and transverse magnetic (TM-mode) responses. The modified algorithm is used to model the terrain effect on MT data recorded from Himalayan terrain.

Using the Magnetotelluric Method for Detecting Aquifer Failure Characteristics under High-Intensity Mining of Thick Coal Seams

In the ecologically fragile mining area of northwest China, high-intensity mining has seriously affected the aquifer and surface eco-environment. In order to better implement water-preserved mining in ecologically fragile areas, the aquifer failure characteristics should be first detected accurately; therefore, it is necessary to find a convenient and fast detection method. Based on the analysis of the basic principles and influencing factors of the magnetotelluric (MT) method, the feasibility of using the MT method to detect aquifer failure is verified by testing the mined area with MT detection and field borehole measurement. Subsequently, the failure characteristics of overburden and unconsolidated aquifers under high-intensity mining are studied by MT detection and physical simulation. By comparing the physical simulation with the field measurement from the aspects of the maximum surface subsidence, interval of periodic weighting and step cracks, the reliability of the height of the water flowing fracture zone and caving zone obtained from physical simulation is verified. The analysis from MT detection and physical simulation shows that the results of the two methods are in accord with each other, which further confirms that the MT method can be used to detect the failure of overburdened structures and aquifers. The penetrating fractures are the main channel for the downward seepage of water resources, which is caused by the “two-zone” of overburden model and located in the “dimple” shape in the apparent resistivity (AR) isogram. It can provide a reference and technical support for the corresponding new water-preserved mining technology and the construction of digital mines.

Development of a Three-Axis Gradient System for Seismoacoustic Data Acquisition in Geodynamically Active Regions

For geodynamically active regions of the Earth, the task of geophysical monitoring is very urgent; geodynamic processes act on geophysical properties and fields. A successful solution to the problem of separating a magnetotelluric field into endogenic and exogenic components allows detection of the moment of fissure initiation, but so far without localization of the active area. For such localization (as a first approximation, direction finding), it is possible to monitor the seismoacoustic field, which is also associated with fissuring. The article describes the development and testing of a three-axis gradient system for recording the seismoacoustic field. Its technical support and requirements are considered in detail. The article presents the results of testing the system at fixed stationary points of the Scientific Station of the Russian Academy of Sciences in Bishkek. Such a gradient system gives grounds to separate sources by the direction to them owing to the presence of a vertical and two transverse components.

Variations of Geomagnetic Response Functions before the 2011 Tohoku Earthquake

Induction vectors have been extensively calculated using data from 19 Japanese observatories for a dozen years preceding the huge 2011 Tohoku earthquake (EQ). At 6 observatories anomalous variations of induction vectors were separated in the years of 2008-2010 that can be identified as middle-term precursors. These observatories are located not at the shortest distance from the EQ epicenter, that is in agreement with the widely known phenomenon of spatial selectivity of EQ precursors. The analysis of horizontal tensors reveals a conductivity anomaly under the central part of the Boso peninsula (at 30 km from Tokyo) with a WNW-ESE strike coinciding both with the Sagami trough strike and the strike of well conducting 3 km thick sediments. A joint analysis of geoelectric and tectonic data leads to a preliminary conclusion that the Boso conductivity anomaly connects two large scale conductors: Pacific sea water and a deep magma reservoir beneath a volcanic belt. Between two so different conductors an unstable transition zone can be expected which should be sensitive to changes of stress. Applying our original processing including two steps analysis and elimination of annual and monthly periods, a short-term two-month-long precursor of bay-like form was successfully separated at the observatory of Kanozan, KNZ (over the Boso anomaly) despite its strong noise. All the results were obtained with advanced multi-windows multi-rr (remote reference) robust programs with a coherency control. Dependence of the results of induction vector calculation on geomagnetic activity was carefully studied, and this dependence is relatively strong when the magnetotelluric field and noise have approximately the same magnitude. But even in this case we could identify the precursor field.

ГЛУБИННАЯ ГЕОЭЛЕКТРИЧЕСКАЯ МОДЕЛЬ АВАЧИНСКО-КОРЯКСКОЙ ГРУППЫ ВУЛКАНОВ НА КАМЧАТКЕ

The article presents the methods and results of the magnetotelluric sounding within the Avacha-Koryaksky group of volcanoes. Geoelectrical section was studied within the period range from 0.0001 to 1000 seconds and above. The authors performed a numerical two-dimensional modeling. Initially, we used test models for possible distortions of curves. The analysis of the magnetotelluric parametres allowed us to characterize the geoelectrical inhomogenuities. Curves along the strike and across the strike were used as main curves. Since longitudinal curves are less prone to coast effect, they were used with transverse curves in order to create a geoelectrical model based on a 2D magnetotelluric field numerical modeling. The created geolectrical model has a conductive bed in the upper part of the section that is connected with an igneous-sedimentary cover. The deep part of the model includes near-vertical conductive zones, which denote a zone with deep faults. The paper describes possible nature of the revealed anomalies and provides rough estimation of rock porosity in the conductive zones.

ECloud - Magnetotelluric Webapp

Geophysical web applications are highly underrated. There are many potential benefits for transitioning to a cloud based web applications including compatibility improvement, low on-going operating costs and better access to existing third-party code and tools for facilitating rapid development. For users, time spent on installation, general IT maintenance such license management and upgrading is minimized. While web applications for business style applications are deeply established, scientific applications on the web are only just emerging. This research builds upon our previous feasibility study (Pethick and Harris 2015), which was the development of a 1D MT web application. This is to be redevelop into a commercial grade cloud hosted geophysical inversion web app that can cooperatively invert seismic and magnetotelluric (MT) data. Our software, ECloud, is designed to be user friendly where geoscientists and drillers can upload MT field data directly from their laptop, tablet or mobile to obtain subsurface geo-electrical distributions quickly with minimal input. This application is designed to be scalable, suiting cloud environments and is currently hosted on an Amazon EC2 instance. The preconceived notion that web applications are slow will be challenged. The purposefully designed MT algorithm and software structure will hopefully result in lower computation times while minimizing restrictions based on hardware requirements (i.e., primary memory).

Direct solutions of 3-D magnetotelluric fields using edge-based finite element

This paper utilizes the 3-array variation of the compressed sparse row (CSR) format to store upper triangular elements of coefficient matrix, and applies the Intel MKL PARDISO solver to solve large sparse linear systems of equations, when calculating the 3-D magnetotelluric (MT) fields using edge-based finite element (FE). By default, Intel MKL PARDISO stores matrices in RAM, namely in-core (IC) mode, however, it is quite memory consumption. One can specify that Intel MKL PARDISO stores matrices on disk, it is referred to as out-of-core (OOC) mode. But the OOC mode consumes longer time than the IC mode. We compare both time and memory consumption of IC and OOC mode via COMMEMI 3D-1A and 3D-2A models. As shown in the examples, the OOC mode can solve very large problems since the amount of main memory required is significantly reduced, in the acceptable computing time, which makes frequency loop parallelization of 3-D MT modeling possible on a small memory PC.

3D forward modeling of magnetotelluric fields in general anisotropic media and its numerical implementation in Julia

We have developed a finite volume (FV) algorithm for magnetotelluric (MT) forward modeling in 3D conductivity structures with general anisotropy. The electric and magnetic fields are discretized on a conventional staggered grid, which cannot directly address the full-tensor conductivity. To overcome this difficulty, an interpolation scheme is used to average different components of the electric field to the same position. We formulate the algorithm in pure matrix form and implement it in a new language, Julia, making the programming process highly efficient and leading to a code with excellent readability, maintainability, and extendability. The validity of the FV Julia code is demonstrated using a layered 1D anisotropic model. For this model, the FV code provides accurate results, and the computational cost is reasonable. Being preconditioned with the electromagnetic potential ([Formula: see text]) system, the iterative solvers including quasi-minimal residual and biconjugate gradient stabilized exhibit a good convergence rate for a wide range of periods. The direct solvers MUMPS and PARDISO are highly efficient for small model sizes. For a relatively large model size with 2.18 millions unknowns, the linear system of one period can be solved by MUMPS within 360 s with multiple threads involved in the computation, and the memory usage is only 11.6 GB in the “out-of-core” mode. We further calculated MT responses of a 3D model with dipping and horizontal anisotropy, respectively. The results suggest that the electrical anisotropy can have significant influence on the MT response.

Generalized analytic solutions and response characteristics of magnetotelluric fields on anisotropic infinite faults

In order to understand directly and analyze accurately the detected magnetotelluric (MT) data on anisotropic infinite faults, two-dimensional partial differential equations of MT fields are used to establish a model of anisotropic infinite faults using the Fourier transform method. A multi-fault model is developed to expand the one-fault model. The transverse electric mode and transverse magnetic mode analytic solutions are derived using two-infinite-fault models. The infinite integral terms of the quasi-analytic solutions are discussed. The dual-fault model is computed using the finite element method to verify the correctness of the solutions. The MT responses of isotropic and anisotropic media are calculated to analyze the response functions by different anisotropic conductivity structures. The thickness and conductivity of the media, influencing MT responses, are discussed. The analytic principles are also given. The analysis results are significant to how MT responses are perceived and to the data interpretation of the complex anisotropic infinite faults.

Adaptive finite element modelling of three-dimensional magnetotelluric fields in general anisotropic media

We present a goal-oriented adaptive finite element (FE) modelling algorithm for 3-D magnetotelluric fields in generally anisotropic conductivity media. The model consists of a background layered structure, containing anisotropic blocks. Each block and layer might be anisotropic by assigning to them 3×3 conductivity tensors. The second-order partial differential equations are solved using the adaptive finite element method (FEM). The computational domain is subdivided into unstructured tetrahedral elements, which allow for complex geometries including bathymetry and dipping interfaces. The grid refinement process is guided by a global posteriori error estimator and is performed iteratively.The system of linear FE equations for electric field E is solved with a direct solver MUMPS. Then the magnetic field H can be found, in which the required derivatives are computed numerically using cubic spline interpolation. The 3-D FE algorithm has been validated by comparisons with both the 3-D finite-difference solution and 2-D FE results. Two model types are used to demonstrate the effects of anisotropy upon 3-D magnetotelluric responses: horizontal and dipping anisotropy. Finally, a 3D sea hill model is modelled to study the effect of oblique interfaces and the dipping anisotropy.

Electromagnetic Spider Web Placement in the Longmen Mountain Fault Zone

The design of the electromagnetic spider web deployed in the Longmen Mountain fault zone, Sichuan province, is described. Instrument every year to replace the battery life can reach 20 years. In seismically active areas, the principle for deployment is placing multiple groups of wireless transmitting modules. These spider webs of multiple modules are capable of detecting about 30 square kilometers within the scope of magnetotelluric field changes. Each module is usually equipped with an electronic compass for detection of the magnetic anomaly; using a special timer it opens four times per minute for about 5 to 15 s. If the signal is found to be abnormal, it can repeatedly scan many signals per hour. The signal is assessed by the GPRS network, which detects the seismic anomaly in the signal a few days before the earthquake as an early warning. Here, we focus on using ten tactics of ancient China as models for spatially deploying the electromagnetic spider web for further analysis of the quake epicenter. The equipment has low cost and low power consumption, so it is a good method for earthquake prediction.

Two‐dimensional magnetotelluric inversion using reflection seismic data as constraints and application in the COSC project

AbstractWe present a novel 2‐D magnetotelluric (MT) inversion scheme, in which the local weights of the regularizing smoothness constraints are based on the envelope attribute of a reflection seismic image. The weights resemble those of a previously published seismic modification of the minimum gradient support method. We measure the directional gradients of the seismic envelope to modify the horizontal and vertical smoothness constraints separately. Successful application of the inversion to MT field data of the Collisional Orogeny in the Scandinavian Caledonides (COSC) project using the envelope attribute of the COSC reflection seismic profile helped to reduce the uncertainty of the interpretation of the main décollement by demonstrating that the associated alum shales may be much thinner than suggested by a previous inversion model. Thus, the new model supports the proposed location of a future borehole COSC‐2 which is hoped to penetrate the main décollement and the underlying Precambrian basement.