Electromagnetic Exploration Methods Research Articles

Review of 3D electromagnetic modeling and inversion for mineral exploration

Many mineral deposits have low resistivity. This property makes the electromagnetic (EM) method a very useful tool for mineral exploration. In the past few decades, the application of EM exploration technologies has been reviewed by many researchers. However, most reviews focused on EM exploration methods, the development of equipment, or their applications. Three-dimensional forward modeling and inversions are high-accuracy EM interpretation techniques in which great progress has been made in recent years in mineral exploration. We review the development of EM technology for mineral exploration with a focus on 3D EM forward modeling, inversion technology, and its applications. We first briefly introduce the EM methods for mineral exploration. Next, we give a comprehensive review of 3D EM forward modeling, inversion, and data interpretation, with special attention paid to the development of EM theory and applications in mineral exploration. We hope this review can promote the application of 3D EM numerical simulation and inversion in mineral exploration.

Application of CSAMT Geophysical Method in Geothermal Exploration of Beijing

Abstract Geothermal resources are abundant in the southeast urban area of Beijing, but the complex geological structure and high geomagnetic interference in the urban area make it difficult to detect the distribution characteristics of geothermal resources by the electromagnetic exploration method from natural sources. This paper presents an integrated physical exploration method combining the controlled source acoustic magneto-telluric (CSAMT) and gravity exploration for middle-deep geothermal exploration in the southeast Beijing urban area. The method is verified by examining the recommended boreholes, which have produced water at temperatures of 62.5°C and 63°C. This is a highly beneficial and socially impactful outcome, and it serves as a valuable reference for future exploration and utilization of geothermal resources in this area. It also has significant implications for future exploration and utilization of geothermal resources not only in the southeastern urban area of Beijing but also in similar urban environments around the world. The methodologies and findings from this research offer a valuable blueprint for future geothermal energy prospecting and development, contributing to the global transition towards cleaner and more sustainable energy alternatives.

Research and Application of Deep Learning Selection Method for Magnetotelluric Power Spectrum Based on Rhoplus Correction

Magnetotelluric (MT) is an electromagnetic exploration method using natural field sources, which are unstable, irregular, and vulnerable to interference from electromagnetic signals. The total power spectrum of the magnetotelluric signal is superimposed by a sub-power spectrum of segmented time series. To reduce the influence of electromagnetic interference, the pre-processing of magnetotelluric data will delete the severely interfered sub-power spectrum. The selection of power spectrum often requires human-computer interaction, which is time-consuming and laborious, heavily dependent on personal experience, and has great randomness. This article innovation is put forward based on the classification of Rhoplus correction and deep learning automatic power spectrum method, first of all, the MT power spectrum impedance estimation results are Rhoplus one dimensional inversion and obtain fitting, using neural network learning fitting differential characteristics and classification, and then for the power spectrum of MT again after low-quality quantum power spectrum to eliminate overlay, Impedance estimation is performed, and the above process is repeated to find the optimal power spectrum combination. Through the comparison and verification of measured MT data, the automatic power spectrum selection method based on Rhoplus correction and deep learning classification achieve similar results to manual editing.

Controlled-source electromagnetic modeling using a versatile secondary electric-field formulation and efficient multigrid-based preconditioner

Frequency-domain controlled-source electromagnetic (CSEM) surveying is a widely used geophysical electromagnetic exploration method. To avoid the singularity near the transmitting sources and improve the accuracy of three-dimensional (3D) CSEM forward modeling, we adopt the vector finite element method and secondary electric field formulation combined with hexahedral grids to solve the frequency-domain Maxwell equations. Primary field computation applies the fast Hankel transform using digital linear filters to quickly calculate the electromagnetic field generated by different electromagnetic dipoles or loops in the layered medium. Therefore, our forward modeling algorithm is highly versatile and compatible with various electromagnetic sources and scenarios of CSEM application (e.g., airborne, offshore, or marine). To improve the convergence rate, we use an algebraic multigrid solver based on the established Hiptmair–Xu decomposition. Using direct solvers requires a vast amount of memory, while conventional iterative solvers usually fail or converge only very slowly. Our new iterative scheme does not require the divergence corrections commonly applied in finite difference or finite volume electromagnetic modeling to accelerate the convergence. We implement complete parallelism in our finite element programming to efficiently solve the CSEM forward problems on massively parallel clusters. We use different numerical methods to obtain high-precision comparison results for one-dimensional and typical 3D models to demonstrate our algorithm's effectiveness and correctness. Finally, a public model of 3 million cells is used to simulate the responses of marine CSEM surveying. The proposed new solver shows robust scalability when using nearly 1000 cores on the cluster, demonstrating that our algorithm is particularly suitable for large-scale 3D forward modeling.

Characterization of Surface–Borehole Transient Electromagnetic Response in Electrical Anisotropic Media

When encountering sedimentary rocks with obvious laminations or fracture development zones, the conductivity of the conductive medium in different directions will change significantly, and the subsurface medium will exhibit macroscopic conductivity anisotropy. To analyze the impact of electrical anisotropy on the surface–borehole transient electromagnetic exploration method, we used the finite element method to investigate the electrical anisotropy surface–borehole transient electromagnetic three-dimensional (3D) forward algorithm, in which we used a tetrahedral mesh to spatially discretize the time–domain Maxwell equation. Then, we discretized it using the second-order backward Eulerian difference method, and we obtained the fields through the PARDISO solver. The validity and correctness of the algorithm were verified through comparison of a one-dimensional (1D) anisotropic model, a complex three-dimensional (3D) isotropic model, and a three-dimensional (3D) anisotropic half-space model. A typical anisotropic geological model was constructed to analyze the effects of anisotropic strata and anomalies in the different principal axis directions on the surface–borehole transient electromagnetic response. The results show that the response of the anisotropic medium is related to the direction of the transmitting source, and the response pattern is complex and volatile. The electrical anisotropic anomaly does affect the amplitude, which should be given special attention when performing surface–borehole transient electromagnetic inversion interpretation.

The application of a novel perfectly matched layer in magnetotelluric simulations

Truncation boundaries are needed when simulating a region in magnetotelluric (MT) modeling. As an efficient alternative truncation boundary, perfectly matched layers (PMLs) have been widely applied in many high-frequency wavefield simulations. However, the governing equation of most electromagnetic exploration methods is for the diffusion field, in which the conduction current is significantly greater than the displacement current. Because the wave and diffusion fields have completely different relations for the frequency and constitutive parameters, conventional PMLs, which are mainly designed for the wavefield, are not a good choice for the diffusion field. For this reason, we propose a formula for a PML that covers the entire frequency band, including the wave and the diffusion fields. It is based on a uniaxial PML. Moreover, we derive a simplified form for the diffusion field. To check the feasibility and application potential of the proposed formula for a PML in MT simulations, we have implemented PMLs using a finite-element method and compared our results with those for a conventional long-distance extended grid. The results of the 1D simulation demonstrate that PMLs can achieve high accuracy and stable performance and have a broad application range. In 2D and 3D models, the air layer is an obstacle in the application of a PML. By selecting appropriate parameters for the PML, 2D and 3D models can achieve satisfactory performance. Therefore, the proposed PML is useful for MT simulations and can achieve satisfactory truncation performance.

Geosciences in Central South University: A state-of-the-art review

In recent decades, colleagues working in the Discipline of Geological Resources and Geological Engineering at Central South University made significant progress in theoretic study and application of geophysics, ore deposit and shale gas geology, 3D predictive modeling of concealed resources, and geological engineering. In geophysics, world-class progress was achieved in the development, data processing, equipment, and scientific survey of electromagnetic method in onshore and offshore environments and the tectonic evolution of the Tibetan Plateau. Especially, advanced wide-field electromagnetic exploration method and equipment as a highlight technique won the first prize of National Science and Technology Invention of China. In ore geology, progressive and complex characteristics of most nonferrous ore deposits and the geodynamic relationship between crust-mantle reaction and mineralization in south China were revealed. Progressive metallogenic models of certain typical ore deposits were established based on the study of fluid inclusion and geochemistry. According to characteristics of complex metallogenic system of polygenetic ore deposit, key ore-controlling factors were found and summarized. The investigation on unconventional resources advanced China???s shale gas resource evaluation system. In 3D predictive modelling of metallogenic, a large-scale location prediction model was established for exploration of crisis mines and concealed ore deposits. Our developed 3D predictive modeling techniques for concealed orebodies were widely used to explore deep mineral resources in China. In geological engineering area, the key technologies for deep drilling into complex formations were developed. Especially the drilling fluid and tools were utilized in the fieldwork. The rock and soil mechanics analysis method and anchor technologies were also established and applied to engineering practice.

Comparison of response characteristics between the electromagnetic method of “Earth-ionosphere” mode and traditional magnetotellurics

ABSTRACTAs an emerging electromagnetic exploration method, the electromagnetic method of “Earth-ionosphere” mode utilises a fixed high-power long-source antenna to emit extremely low frequency electromagnetic waves. Signals can be received with a large source–receiver offset to perform resource exploration. As a novel exploration method, it is necessary to compare the results with traditional electromagnetic exploration methods to verify its feasibility and analyse its response characteristics. We first introduce the basic principle of the electromagnetic method of “Earth-ionosphere” mode. Then, one-dimensional forward simulations are developed to determine the similarities and differences between the response of this electromagnetic method in the near, far, and waveguide zones with that from the magnetotellurics. The responses of the three-dimensional model are simulated using the edge-based vector finite element method. The forward simulation responses show that the electromagnetic method of “Earth-ionosphere” mode has a significant abnormal response to geological targets, which is similar to other electromagnetic exploration methods. At large source–receiver offsets, the tensor response of the proposed electromagnetic method is consistent with the magnetotellurics. At small source–receiver offsets, there is a near-field response that is similar to the controlled source audio-frequency magnetotellurics. There are some differences between the scalar response and the tensor response when the antennas have certain azimuth angles. Finally, we present the field measured data for the electromagnetic method of “Earth-ionosphere” mode and compare the field measured data with the magnetotelluric data. We deepen our understanding of the electromagnetic method of “Earth-ionosphere” mode by comparing the results with the traditional magnetotellurics, which provides guidance for further development.

A Study on Transient Electromagnetic Interpretation Method Based on the Seismic Wave Impedance Inversion Model

Abstract A comprehensive transient electromagnetic interpretation method based on seismic wave impedance inversion has been proposed according to the advantages of seismic and transient electromagnetic exploration methods to mitigate of hidden water inrush disasters in coalmines. Combined seismic wave impedance inversion results and resistivity logging data, the method was used to establish a geo-electric model. The stratigraphic horizon and the stratigraphic electrical characteristics were determined by the wave impedance inversion data and the resistivity logging data respectively. Furthermore, the structure and water-bearing property in the stratum were estimated by the analysis of the difference between the measured transient electromagnetic data and the calculated data from the forward model. The numerical calculation of the fault-containing model shows that the comprehensive interpretation method could determine the water-bearing capability of the fault as well as it tendency. An advantage of this new method is the effective avoidance of the influence of low-impedance overburden on the data interpretation. The practical application can accurately explain the location and the water-bearing property of the disaster-causing factors of the hidden water inrush in the coalmines.

Approximate semianalytical solutions for the electromagnetic response of a dipping-sphere interacting with conductive overburden

Electromagnetic exploration methods have important applications for geologic mapping and mineral exploration in igneous and metamorphic terranes. In such cases, the earth is often largely resistive and the most important interaction is between a conductor of interest and a shallow, thin, horizontal sheet representing glacial tills and clays or the conductive weathering products of the basement rocks (both of which are here termed the “conductive overburden”). To this end, we have developed a theory from which the step and impulse responses of a sphere interacting with conductive overburden can be quickly and efficiently approximated. The sphere model can also be extended to restrict the currents to flow in a specific orientation (termed the dipping-sphere model). The resulting expressions are called semianalytical because all relevant relations are developed analytically, with the exception of the time-convolution integrals. The overburden is assumed to not be touching the sphere, so there is no galvanic interactions between the bodies. We make use of the dipole sphere in a uniform field and thin sheet approximations; however, expressions could be obtained for a sphere in a dipolar (or nondipolar) field using a similar methodology. We have found that there is no term related to the first zero of the relevant Bessel function in the response of the sphere alone. However, there are terms for all other zeros. A test on a synthetic model shows that the combined sphere-overburden response can be reasonably approximated using the first-order perturbation of the overburden field. Minor discrepancies between the approximate and more elaborate numerical responses are believed to be the result of numerical errors. This means that in practice, the proposed approach consists of evaluating one convolution integral over a sum of exponentials multiplied by a polynomial function. This results in an extremely simple algorithmic implementation that is simple to program and easy to run. The proposed approach also provides a simple method that can be used to validate more complex algorithms. A test on field data obtained at the Reid Mahaffy site in Northern Ontario shows that our approximate method is useful for interpreting electromagnetic data even when the background is thick. We use our approach to obtain a better estimate of the geometry and physical properties of the conductor and evaluate the conductance of the overburden.

Investigation on Time Domain Coded Electromagnetic Exploration Method

To suppress airwave interference, a time-domain coded electromagnetic exploration method is proposed with Pseudo-Random Binary Sequences (PRBS) code as the source signal. Based on the PRBS code and receiving voltages, which are recorded simultaneously, the impulse response of the earth can be obtained by the time-domain deconvolution signal recovery method, and the target information is effectively contained in the impulse response. The selection methods of clock frequency and code length were analyzed and field experiments were carried out, which demonstrated that the signal quality of the EM data could be improved by decreasing the clock frequency, or increasing the code length. The proposed method could greatly suppress the airwave interference and effectively identify underground anomalies.

Inverse problem of magnetometric resistivity response from exponentially varying conductive ground

Abstract Magnetometric resistivity is an electromagnetic exploration method that has been used successfully to investigate electrical conductivity structures within the earth. We derive an analytical solution of the steady state magnetic field due to a direct current semi-infinite source on a multilayered earth with a layer having exponentially varying conductivity. Our variation in conductivity is realistic and can be generalized to all cases of exponential profiles. The Hankel transform is introduced to our problem and analytical result is obtained. Our solution is achieved by solving a boundary value problem in the wave number domain and then transforming the solution back to the spatial domain. An inverse problem via the use of the Levenberg-Marquardt optimization technique is introduced for finding the conductivity parameters of the ground. The optimal result of our model is close to the true value with percentage errors of our two conductivity parameters less than 2.6% and 3.6% after using only 3 iterations.

Electromagnetic Methods for Shale Gas Exploration in Southern China

The shale gas is becoming a new strategic energy source in the world. To improving the efficiency of exploration and production of such a non-conventional resource has been a global choice of oil and gas powers. Many previous researches have revealed that China has equivalent amount of shale gas reserves as USA. China-based research has shown that the southern China is one of the most potential areas for shale gas exploration. However, the complex topography with carbonate covers and sophisticated geological structures in southern China is a main difficulty for seismic exploration to get reliable and high-resolution data. This paper alternatively present electromagnetic exploration methods that are applicable to shale gas exploration in such rugged area.

The application of the high-frequency electromagnetic sounding method in the exploration of underground thermal water

High-frequency electromagnetic sounding is an electromagnetic exploration method using the natural high-frequency electromagnetic field as a field source. It has higher resolution and greater depth penetration than the direct current method and is especially fit for geothermal energy exploration and low-and mid-level groundwater detection. We introduce a successful application of high-frequency electromagnetic sounding for evaluating geothermal water resources. The high frequency electromagnetic system (MT-U5A with a frequency range from 10 KHz to 1 Hz) is first applied to sample field data from China. A remote reference station is used to assure sampled data quality. We then perform 2D inversion image processing with the electrical method data. The results basically indicate the spatial distribution of underground geothermal water and provide favorable clues to finding the sources of the subsurface geothermal water in this area.

Inversion Study of Spectral Induced Polarization Based on Improved Genetic Algorithm

As a kind of electromagnetic exploration method in frequency domain, spectral induced polarization (SIP) technique is a branch of electromagnetic exploration methods in geophysics exploration, which is widely used in environmental and engineering geophysical prospecting, as well as mineral exploration, oil & gas and coal exploration. In the theory of SIP data interpretation, inversion of three spectral induced polarization parameters (m, τ and c) in Cole-Cole model is a basic job. Due to their respective special character in the function expression, the inversion of m, τ and c from frequency spectral response data based on linear inversion theory is usually instable and even sometimes not convergent when data have a little errors. In this paper, we adopt a kind of improved genetic algorithm to implement inversion. Inversion on one Cole-Cole model shows that the algorithm converges fast, has very good stability and high precision, and even can permits the observed data random error up to 15%. Inversion on two Cole-Cole models, which simulates the fact that the effects of electromagnetic and induced polarization exist simultaneously, show the algorithm is still fast convergence and high precision, and permits a few observed data error (say, 5% random error). Only when random errors add up to 10%, the results of inversion begin to become obvious error.

Magnetotelluric imaging of the creeping segment of the San Andreas Fault near Hollister

Fluids may play a significant role in the earthquake rupture process. Since fluids can dramatically change the electrical resistivity of crustal rocks, electromagnetic exploration methods can image in situ fault‐zone fluids. Magnetotelluric (MT) data has been collected on segments of the San Andreas Fault (SAF) in Central California exhibiting a range of seismic behavior from locked to creeping. This paper describes MT data from the creeping segment of the SAF near Hollister, California. Inversion models image a zone of enhanced conductivity beneath the surface trace of the SAF that has similar geometry and greater depth extent than the fault zone conductor at Parkfield. Comparison with geologic and seismic tomographic studies show this zone is bounded by the San Andreas and Calaveras Faults, and coincident with a zone of low seismic P‐wave velocities. Combined with previous MT studies, this indicates a correlation between seismicity and the presence of fault‐zone fluids.

EM target response in complex hosts

Electromagnetic exploration methods are only useful if they give the user an enhanced likelihood of finding economic targets or at least decrease the probability of drilling false targets. Given reasonably clean data, many schemes exist to identify simple targets in uniform hosts. However, the EM responses of potential economic targets are often obscured by those from heterogeneities in the host as well as a number of structural factors like topography and unconformities. The AEM response is further complicated by system factors such as airframe currents, plus variations in bird position and flight path. This paper examines the changes in response of a simple target as its environment becomes increasingly more complicated. We start with a 1km long conducting dipping dyke and then add overburden, topography, a paleochannel, heterogeneities, faults and then put it at the boundary between two rock units. We also look at system factors such as transmitter angle, airframe currents and non-level flight as well as the effect of different flight paths. Although CDIs do not yield an accurate picture of the subsurface geoelectric structure, they nonetheless represent a useful transform on the data that can be used for pattern recognition studies.

Random scattering in magnetotellurics

Typical well logs show substantial variations of formation electrical resistivity over small spatial scales, down to the resolution of the logging tool. Using a plane stratified earth model, we examine the effects of this fine‐scale microstructure on scattering of the naturally occurring electromagnetic (EM) waves used in magnetotellurics. We show how 1-D magnetotelluric (MT) data may be viewed as arising statistically from a smoothed effective medium version of the resistivity‐depth profile. The difference between the data produced by the true medium and the effective medium is attributable to random scattering noise. This noise is fundamental to magnetotellurics and other diffusive‐wave EM exploration methods since it arises from the very small spatial scales that are usually ignored. The noise has unique statistical properties, which we characterize. We show that if scattering is the dominant noise source, a thin layer of increased resistivity at depth can be reliably detected only if the noise statistics are incorporated properly into the detection algorithm. This sets a new fundamental limit on the vertical detection capability of MT data. The theory agrees well with Monte Carlo simulations of MT responses from random resistivity microlayers.

Case histories of LOTEM surveys in hydrocarbon prospective area

During the past two decades electromagnetic exploration methods have been used where reflection seismology has had problems, notably in are as of crystalline overthrust and volcanic cover. We have applied the long-offset transient electromagnetic (LOTEM) method in hydrocarbon prospective areas during test surveys in Europe and Australia. Using both electric and magnetic field transient data, the resistive and conductive parts of the geoelectric section and some structural features were resolved.

A wide‐band electromagnetic exploration method—Some theoretical and experimental results

Theoretical electromagnetic (EM) responses of a conductive circular cylinder in a conductive half‐space are obtained for a frequency range between 10 Hz and 30 kHz. The results show that the spectral characteristics can be used to determine the subsurface configuration of a target body, particularly in terms of its depth. Laboratory experiments using a wide‐band sweep‐frequency source provide support for the theoretical prediction of the spectral behavior. The experiment employed a harmonic EM source continuously sweeping logarithmically from 4 kHz to 4 MHz over a graphite slab submerged in a conductive solution. The results suggest that a wide‐band EM spectral profile displayed in a frequency‐distance space may provide many intuitive interpretation schemes which may not be possible with a single or discrete frequency profile.