Magnetotelluric Method Research Articles

Identification of Sub-Fault Zone Using Magnetotelluric Inversion (Case Study: Ketaun Fault, Lemeu Village, Lebong Regency)

Lemeu Village, Lebong Regency, borders the Bukit Barisan Mountain range and is crossed by the Ketaun Fault, which causes a high level of seismic activity, so it is necessary to conduct research on the potential existence of the Ketaun sub-fault as one of the efforts to mitigate natural disasters such as earthquakes. The Magnetotelluric method utilises the earth's natural electromagnetic field, which is used to determine the distribution of resistivity in the subsurface using the ADU-07e Magnetotelluric tool with two horizontal electrical sensors (Ex, Ey) and three horizontal (Hx, Hy) and vertical (Hz) magnetic sensors and uses seven research points with an interval of 1 km. Data processing uses MAPROS software to convert data from the time domain to the frequency domain and ZONDMT2D to obtain subsurface resistivity values. The results obtained from this study are 2D magnetotelluric cross sections showing a zone with low resistivity values between research points P4 and P5 which is thought to be a new fault zone with resistivity values ranging from 1.3 – 6.1 Ωm from a depth of 2.5 km to a depth of 10 km. The zone is assumed to be a new fault that is a branch of the Ketaun fault.

2D magnetotelluric imaging method based on visionary self-attention mechanism and data science

2D magnetotelluric (MT) imaging detects underground structures by measuring electromagnetic fields. This study tackles two issues in the field: traditional methods’ limitations due to insufficient forward modeling data, and the challenge of multiple solutions in complex scenarios. We introduce an enhanced 2D MT imaging approach with a novel self-attention mechanism, involving: 1. Generating diverse geophysical models and responses to increase data variety and volume. 2. Creating a Swin–Unet-based 2D MT Imaging network with self-attention for better modeling and relation capture, incorporating a MT sample generator using real data to lessen large-scale supervised training dependence, and refining the loss function for optimal validation. This method also includes eliminating MT background response to boost training efficiency and reduce training time. 3. Applying a transverse electric/transverse magnetic method for comprehensive 2D MT data response. Tests show that our method greatly improves 2D MT imaging’s accuracy and efficiency, with excellent generalization.

Study on the detectability of CSRMT in archaeological exploration: taking the ancient city wall remains as an example

Although electromagnetic methods have made significant advances in archaeological exploration, there remains a lack of a method that offers both high operating efficiency and high resolution. This study introduces a novel approach derived from the Controlled Source Radio Magnetotelluric (CSRMT) method. The CSRMT method combines the efficient configuration of the Controlled Source Audio-magnetotelluric (CSAMT) method with enhanced resolution capabilities. To evaluate the efficacy of the new method in archeological exploration, we designed a series of models based on existing references related to ancient city wall remains. Using the 3D contraction integral equation method, we numerically simulated the high-frequency component of the CSRMT method, and then on, the Laterally Constrained Inversion technique is used to invert the data. The effectiveness of the method was assessed qualitatively and quantitatively through analysing the electromagnetic field and apparent resistivity of models with varying widths, heights, top burial depths, and resistivity. Our analysis revealed that models with shallower burial depth and greater height of the underground city wall exhibited higher resolution. The findings demonstrate that the CSRMT method can efficiently and accurately identify buried remains, making it a highly promising tool for archaeological exploration and warranting further development.

Advancing Sustainable Geothermal Energy: A Case Study of Controlled Source Audio-Frequency Magnetotellurics Applications in Qihe, Shandong

Geothermal energy is a key part of sustainable and renewable energy strategies, especially for clean heating in northern regions. This study focuses on Qihe County in Shandong Province, applying a controlled source audio-frequency magnetotellurics (CSAMT) method to investigate deep karst geothermal reservoirs. This research addresses the complex geological conditions and electromagnetic interference in the region, aiming to improve sustainable geothermal resource development. The findings indicate that the geothermal reservoir in the study area primarily consists of Ordovician limestone, characterized by moderate burial depth, high water volume, and elevated water temperature. Integrating CSAMT with vertical electrical sounding (VES) and radiometric surveying has clearly defined the deep aquifer layers and major water-controlling fault structures. Drilling verification results demonstrate the significant effectiveness of the integrated geophysical methods employed, providing reliable technical support for deep geothermal exploration in similar regions. This study makes a significant contribution to the scientific and technical foundation necessary for the sustainable development and utilization of geothermal resources, supporting the broader goals of environmental sustainability and renewable energy.

Exploring geothermal resources with the CSAMT and microtremor methods: a case study in Tangquan, Jiangsu Province, China

The development of geothermal energy has received extensive attention because of global energy scarcity and environmental pollution. The Tangquan area is in the north of the Yangtze fold belt, and the lack of magmatic rock development has resulted in a low-temperature type of geothermal reservoir. There are sporadic hot springs and cold springs exposed in the study area, but the uncertain deep geological structure, unknown geothermal reservoir formation mechanism, and lack of detailed exploration data seriously restrict the exploration and development of geothermal resources in the area. Filling underground faults with water can significantly reduce electrical resistivity, while traps filled with warm water can cause a decrease in S-wave velocity. Thus, a new integrated geophysical method, including the controlled source audio-frequency magnetotelluric (CSAMT) and microtremor methods, is applied for geothermal exploration in the region. The combination of CSAMT and microtremor methods can determine thermal-controlled and water-conducting structures more effectively and locate geothermal storage more accurately. The source, channel, storage, and cover of regional geothermal reservoir formation are analyzed using geophysical and geological data. That is, the regional NE- and NW-trending faults are explained as thermal-controlled and water-conducted structures, respectively. The deep Sinian Dengying Formation supplies a heat- and water-bearing space. The tectonic intersection area, especially the tensional fault zone, is found to be closely related to the existence of geothermal reservoirs, which is well verified by later drilling results. Finally, a geothermal reservoir model is established to comprehensively understand the distribution of geothermal energy in the region.

Exploration of Metallogenic Structure of Manganese Ore Using Magnetotelluric Method: A Case Study in Minle Region, Hunan Province, China

Exploration of Metallogenic Structure of Manganese Ore Using Magnetotelluric Method: A Case Study in Minle Region, Hunan Province, China

Dimensionality and Geoelectric Strike Analysis of 2D Magnetotelluric Modeling in Geothermal Area “XYZ”

Abstract Magnetotelluric (MT) method is crucial in geothermal exploration due to its capability to delineate geothermal prospect area by mapping subsurface resistivity distribution. However, in 2D Magnetotelluric modelling, galvanic distortion can distort the regional MT responses, leading to interpretation errors. This study employs phase tensor analysis to eliminate galvanic distortion. Results reveal the dimensionality of each station and three variations of the tipper strike, often referred to as the geoelectric strike. The tipper strike of each station showed varying values, while the tipper strike of all stations indicated N35°E. The tipper strike results required validation against additional geological information, such as faults, fractures, and changes in lithology, significantly influence the conductivity structure of subsurface, which indicated a geological strike of approximately N30°E. This study compares the MT data without rotation, rotation to tipper strike of each station, and tipper strike of all stations to assess effectiveness in eliminating galvanic distortion and improving data quality. The resulting 2D Magnetotelluric cross-section rotated to the tipper strike of all station, exhibits the lowest RMS Error of 2% and roughness of 654.6 among other variations, indicating improve quality. This interpretation identifies a geothermal system in the study area, closely aligning with actual subsurface geological conditions.

Three-dimensional geothermal reservoir model using the magnetotelluric method in medium and deep strata of Yishu fault zone, Rizhao Section

Three-dimensional geothermal reservoir model using the magnetotelluric method in medium and deep strata of Yishu fault zone, Rizhao Section

An integrated approach for sewage diversion: Case of the Huayuan mine, Hunan Province, China

Environment protection is a core priority for many governments in this century. Most environmental problems have diverse causes: emission of greenhouse gases from fossil fuels, resource depletion, or intense mining activities such as the Huayuan manganese mine. The positioning of mining factories and water treatment stations impacts the surrounding groundwater reservoir. As the mine expands, the environmental impact also increases, and the previous plan based on monitoring wastewater leakage has become inappropriate. Therefore, to solve this issue, a new study is required to understand the lateral resistivity distribution underground and to define a new station location for water treatment and divert the sewage to that station. In this study, the audio-frequency magnetotelluric method is used. Surveys of two long lines that cross the mining area to its boundaries are carried out. Data are robustly processed and inverted. Based on the inverted models, in addition to the geologic information, drilling inspections, and solid waste distributions map, the integrated interpretation develops two sites on the top of impermeable layers, which constitute a buffer point between the unsafe (high concentration of pollutants) and the safe zones in the northwestern part of the mine. From the resistivity distribution combined with the water quality analysis, a relationship between the fault structures reveals an interconnected conductive zone (CZ) in the southeastern part. As the main channels for water circulating underground, these CZs delineate the main groundwater reservoir with a clastic aquifer layer. However, close to factories, the water from faults contains solid wastes, thereby making the groundwater in that zone nonpotable, unlike the safety zone located in the northwestern part. This workflow can become a field guide to improve the environment of mines and the deployment of hydrogeologic drilling in a safe area.

Robust inversion of 1D magnetotelluric data using the Huber loss function

In this study, a robust 1D inversion was performed on magnetotelluric (MT) data by utilizing the Huber loss function to avoid misleading interpretation of results, which is caused by the presence of outliers in the data. The MT method utilizes the ratio between the electric field (E) and the magnetic field (H), which are perpendicular to each other, to obtain impedance values (Z). These values are used to extract subsurface information in the form of electrical resistivity variations with depth. In the study, forward modeling of the 1D MT responses was conducted by recursively calculating Z values. Robust inversion was performed using the Huber loss function as an objective function to minimize the difference between the calculated and observed data. The Huber loss function combines the squared loss and the absolute loss to anticipate the presence of outliers in MT observation data. The robust inversion was performed on synthetic data with additional noise and field data. The inversion process utilized 50 layers with a thickness of 500 m, a hyperparameter δ=5×10-2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\varvec{\\delta }= 5\ imes {10}^{-2}$$\\end{document}, λ=0.01\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\lambda =0.01$$\\end{document}, and λt=100\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\lambda }_{t}=100$$\\end{document}. The number of iterations used was 500 for inversions that used synthetic data and 3000 for inversions that used field data. The robust inversion scheme using the Huber loss function successfully overcame the presence of outliers and accurately estimated the actual model parameters, both for synthetic data and field data that contained outliers. The misfit was relatively small and close to zero, indicating that the inversion code works well.

Modelling of the terrain effect in magnetotelluric data from the Garhwal Himalaya region

Abstract. The magnetotelluric (MT) method is a passive geophysical technique based on using time variations in the geoelectric and geomagnetic field to measure the electrical resistivity of the surface layer. It is one of the most effective geophysical techniques to study the deep structure of the Earth's crust, particularly in steep terrain like the Garhwal Himalaya region. MT responses are distorted as a result of undulating/rugged terrain. Such responses, if not corrected, can lead to the misinterpretation of MT data with respect to geoelectrical structures. In this study, two different correction procedures were used to compute the topography distortion for a synthetic model of the Garhwal Himalaya region from the Roorkee to the Gangotri section. A finite-difference algorithm was used to compute the MT responses (apparent resistivity and phase) for irregular terrain. The accuracy of the terrain correction procedures was checked using the results of different topography models for various periods from the literature. The relative errors between two terrain correction procedures were calculated with respect to the flat earth surface and were almost equal to zero for most of the sites along the Roorkee–Gangotri profile except at the foothill, where the error was high for shorter periods. The similar topography procedures of two terrain-corrected responses (TCR1 and TCR2) showed that there is no need for topography correction along the Roorkee–Gangotri profile because the slope angle is less than 1°.

A de-noising method of magnetotelluric signals based on the generalized S-transform

The generalized S-transform has excellent time-frequency resolution capability. It can effectively demonstrate the non-stationary characteristics of signals and the dynamic changes in energy. This provides the conditions for denoising in the time-frequency domain. The magnetotelluric (MT) survey method is often affected by various artificial noises, and these noises are very prominent in certain frequency bands. To address this problem, a method based on the generalized S transform is proposed to de-noise MT data. The generalized S transform is used to improve the time-frequency localization of the different signal components, meanwhile the electromagnetic noise can be conveniently suppressed in the time-frequency domain. Simulated signals and measured MT signals contaminated by common sinusoidal and triangular wave interference are processed. The results were compared with those of the Wavelet threshold method. Results show that the presented method is more conducive to the identification and suppression of electromagnetic noise and shows better de-noising results than the Wavelet threshold method. After de-noising, the useful information of the MT signal affected by strong interference is highlighted, which provides favorable conditions for power spectrum and impedance estimation in the future. Further, the response parameter curve calculated from the de-noised data becomes stable. The proposed method provides a new alternative for de-noising of MT signals, especially those with low signal-to-noise ratios.

Distributed wide-field electromagnetic method for coal mining goaf detection in a complex urban environment: A case study in Jinan, China

Urban expansion has resulted in many proposed projects located over coal mining zones, which has highlighted the importance of detecting the spatial scope and water abundance of goafs before the commencement of construction work. Although electromagnetic (EM) methods have proven effective for goaf detection, their applications in intensely noisy and urbanized environments remain limited. To address this challenge, we evaluate an investigation of a coal mining goaf in Jinan, China, using the distributed wide-field EM (DWFEM) method. A third-order [Formula: see text] sequence pseudorandom signal with 39 survey frequencies is transmitted to achieve long-time data acquisition at each station. Unlike the controlled-source audio-frequency magnetotellurics method, the DWFEM records only the electric field [Formula: see text] component. The synthetic model tests and field data demonstrate the consistency of the [Formula: see text] apparent resistivity and the Cagniard resistivity in the far field. The long-time acquisition and 1C recording greatly improve data quality and exploration efficiency. We also use an all-angle resistivity calculation formula and an electrode layout method parallel to the wire source to obtain electrical connections in different directions. The DWFEM inversion results are obtained using the 1D Gauss-Newton iterative method under a plane-wave assumption. By interpolating the data from different measurement stations, we image resistivity depth profiles and obtain 3D subsurface electrical data for the subsurface from 0 to 1000 m. We interpret the obtained profiles with geologic and mining information, revealing two significant water-enriched goaf areas. Validation is performed using seismic data and drill cores. The results significantly enhance our understanding of the characteristics of the coal mine under our project and highlight the applicability of the DWFEM for detecting goafs in complex urban environments.

Application of integrated geophysical techniques in geothermal exploration in Binhai County, Jiangsu Province

AbstractIntegrated geophysical technology is a necessary and effective means for geothermal exploration. However, integration of geophysical technology for large‐scale surveys with those for geothermal reservoir localization is still in development. This study used the controlled source audio‐frequency magnetotelluric method technology for large‐scale exploration to obtain underground electrical structure information and micromotion detection technology to obtain underground wave velocity structure information. The combination of two detection technologies was used for local identification of geothermal reservoirs. Further, auxiliary correction and inversion constraint were implemented through the audio magnetotelluric sounding technology for maximum authenticity restoration of the near‐ and transition‐field data. Through these technology improvements, a geothermal geological model was established for the Binhai County of Jiangsu Province in China and potential geothermal well locations were identified. On this basis, a geothermal well was drilled nearly 3000 m deep, with a daily water volume of over 2000 m3/day and a geothermal water temperature of 51°C at the well head. It is found that predictions using the above integrated geophysical exploration technology are in good agreement with the well geological formation data. This integrated geophysical technology can be effectively applied for geothermal exploration with high precision and reliability.

Identification of shale gas existence based on 2-dimensional magnetotelluric data in the Kutai basin, East Kalimantan, Indonesia

Research on unconventional methods in the shale gas exploration process has begun to be developed to meet the demand for oil and natural gas sustainability. The research area is located in the Kutai Basin, East Kalimantan, Indonesia. The current study from the Geological Survey Center shows that the Kutai Basin has 46.79 TCF of shale gas potential. The magnetotelluric (MT) method was applied in this study to describe the distribution of subsurface resistivity values. The inversion schemes performed in the MT method are 1-dimensional and 2-dimensional inversions. In this study, there are 8 measurement stations in one line. The presence of low resistivity values or conductive zones identifies the potential for black shale layers. The potential for shale gas is suspected to be detected at station KT29, which is included in the Pamaluan Formation, and at stations KT12, KT31, KT13, and KT15, which are included in the Pulaubalang Formation, because it is associated with source rock from petroleum in the Kutai Basin system, as supported by previous research data.

Resistivity Distribution of Lembang Fault Based on Magnetotelluric Data

The Lembang fault located 15 Km north of Bandung City is one of the active faults situated on Java Island, Indonesia. The Lembang fault is an extension of the Cimandiri fault that can be one of the potential sources of earthquakes in Indonesia. In this study, the Magnetotelluric method was used to model the subsurface resistivity at the Lembang Fault area. The data acquisition was conducted in 8 points of measurement forming a line perpendicular to the Lembang Fault. The analysis was performed using 1D inversion of apparent resistivity and phase with the frequency range of 1 Hz to 320 Hz. The contrast apparent resistivity has observed in two sites around the fault. The inversion results show resistivity contrasts around the LMB02 point that is suspected to be the Lembang Fault. These results are consistent with geological data in the Lembang Fault area. Aside from the LMB02 point, there are also resistivity contrasts around the LMB05 point. However, from a geological perspective, it is not yet known whether the area around the LMB05 point represents a fault.

Three-dimensional magnetotelluric modeling with nontrivial anisotropy by a regularization approach

The regularization approach has been successfully applied to remove spurious solutions in the magnetotelluric (MT) forward problems of isotropic Earth media. However, spurious modes are more likely to occur in numerical solutions of anisotropic media, as electrical anisotropy introduces many more complications to electromagnetic (EM) induction in such media. This study focuses on developing the regularization approach to 3D MT forward problems of anisotropic media, especially those of nontrivial anisotropy. The governing equation is now derived with a conductivity tensor, and an accordingly adapted form of a scaled grad-div term is augmented to regularize the solutions and constrain the divergence-free condition. A new scaling scheme is proposed to cope with the complicated distribution of current densities in nontrivial anisotropy media, and an effective conductivity is approximated by the diagonal elements of the conductivity tensor to formulate the scaling factor. Numerical tests show that, for various models of electrical anisotropy, the regularization approach can effectively enforce the divergence condition and successfully suppress spurious solutions. Therefore, for nontrivial anisotropy media, this approach can also improve the efficiency of the iterative solvers while retaining the accuracy of the solutions. The derivation of the governing equation is based on the MT method. However, this strategy should be generally applicable to other frequency-domain EM methods.

MT2DInv-Unet: A 2D magnetotelluric inversion method based on deep-learning technology

Traditional gradient-based inversion methods usually suffer from the problems of falling into local minima and relying heavily on initial guesses. Deep-learning methods have received increasing attention due to their excellent nonlinear fitting ability. However, given the recent application of deep-learning methods in the field of magnetotelluric (MT) inversion, there are currently challenges associated with achieving high inversion resolution and extracting sufficient features. We develop a neural network model (called MT2DInv-Unet) based on the deformable convolution for 2D MT inversion to approximate the nonlinear mapping from the MT response data to the resistivity model. The deformable convolution is achieved by adding an offset to each sample point of the conventional convolution operation, which extracts hidden relationships and allows the flexible adjustment of the size and shape of the feature region. Meanwhile, we design the network structure with multiscale residual blocks, which effectively extract the multiscale features of the MT response data. This design not only enhances the network performance but also alleviates issues such as vanishing gradients and network degradation. The results of synthetic models indicate that our network inversion method has stable convergence, good robustness, and generalization performance, and it performs better than the fully convolutional neural network and U-Net network. Finally, the inversion results of field data show that MT2DInv-Unet can effectively obtain a reliable underground resistivity structure and has a good application prospect in MT inversion.

Research on 3D Time-Lapse Electric Field Inversion Algorithm for Controlled Source Audio-Frequency Magnetotelluric Method

The controlled source audio-frequency magnetotelluric method (CSAMT) stands out for its economic efficiency and widespread application in geophysical monitoring. However, the separate inversion of time-lapse monitoring data encounters challenges in comparing and identifying abnormal changes due to variations in data fitting. Furthermore, the utilization of a method akin to Cagniard apparent resistivity for inversion necessitates the simultaneous observation of at least two components of the electromagnetic field, making it unsuitable for extensive three-dimensional observations. This paper proposes a 3D time-lapse electric field inversion algorithm for CSAMT, addressing the complexities in geophysical monitoring. The algorithm introduces two regularization factors and defines an objective function with both temporal and spatial constraints. Synthetic testing reveals the stability of the 3D time-lapse electric field inversion algorithm, demonstrating its effectiveness in delineating underground variations. This solution resolves the challenges posed by the independent inversion of time-lapse monitoring data.

Natural source electromagnetic survey for geothermal application in industrial area with strong electromagnetic noise

The magnetotelluric (MT) method is a widely used geophysical technique for deep structure exploration. However, rapid industrial development has resulted in significant anthropogenic interference for practical applications of this method in recent years. To carry out MT survey in regions with strong electromagnetic (EM) noise, we develop a strategy, in which part of MT sites are set for night time period and a newly proposed automatic clustering method based on MT signal patterns is applied to process measured data from Yingde area in Guangdong. The MT sites are employed in night time to avoid human activity and measure the quiet time MT signal to make sure that we can capture portion of MT signal. Based on cosine similarity, we can use clustering algorithms to separate the signals from the noise. The application of this strategy for geothermal exploration in Yingde area in Guangdong, indicates the validity of our proposed strategy for real geothermal detection.