Transient Electromagnetic Field Research Articles

The optimal survey area of the semi-airborne TEM method

The semi-airborne transient electromagnetic method (TEM) has been widely used in resource exploration and other fields due to its high efficiency, strong adaptability to the complex environment and high resolution. However, the conventional semi-airborne electromagnetic method continues to use the survey area of the grounded-wire TEM method. Compared with the existing ground-based TEM method, the semi-airborne TEM method is more sensitive to signal intensity because observed data is easily affected by noise for receiving position in the air. Therefore, it's unreasonable to consider only signal intensity or sensitivity related to offset in survey area. To obtain more suitable area, we discuss the distribution of the transient electromagnetic field with electrical source as well as the attenuation characteristics of the signal at different flight altitude through the forward modeling. Then, we obtain the sensitive area and flight altitude applied by the error analysis method in typical models. Finally, we describe a method what is herein dubbed the fuzzy comprehensive evaluation, which greatly reflects the membership degree of the evaluation object from different single factors to each level of fuzzy subset. Therefore, we use this method to get the optimal survey area and flight altitude under different noise levels. In low noise level, the signal intensity and sensitivity account for 33.3% and 66.7% respectively in selecting the optimum area and altitude, resulting in that the optimal area and flight altitude are coincides with the sensitive area and altitude better. While in the area filled with noise, the most area of strongest signal intensity is considered the optimal area and altitude. A field example shows that the aforementioned method can be well resolved in identifying survey lines and improving data quality.

Plasma Transient Processes and Plane-Wave Formation in Simulations by FDTD Method

The processes of plane-wave formation in isotropic plasmas are both analytically and numerically described. An electromagnetic wave is launched into the plasma by a metal plate with a surface RF current. There is a transition time from the initially quiet plasma to the self-consistent oscillations of plasma particles in the electromagnetic field. The transient electromagnetic field in the plasma is written in exact and simplified formulas that are convenient for the calculation of the duration of the transient processes at any distance from the antenna. The latter is important for both analytical research and numerical research since it is the time in which the system reaches a steady state. The analytical results can also be used for validating the finite-difference time-domain (FDTD) codes for plasma waves.

Research on Multi-Physical Fields of High-Power PMSM/G Used for FESS During the Process of Controllable Charging and Uncontrollable Discharging

Flywheel energy storage system (FESS) has been widely used in many fields, benefiting from the characteristics of fast charging, high energy storage density, and clean energy. Uncontrollable discharging system can be used in some special occasions which doesn't have strict requirements on the power quality, such as electric heaters, emergency power supplies, etc. Taking a charging and discharging (C&D) system of 200 kW 15000 rpm high-speed Permanent Magnet Synchronous Motor/Generator (PMSM/G) used for FESS as an example, field-circuit coupling calculation model of PMSM/G during the process of controllable charging and uncontrollable discharging is established. And then the relationships between speed and current-voltage during charging process is calculated. A variable load calculation method based on transient electromagnetic field during the process of uncontrollable discharging is presented, and the eddy current field and transient temperature field of rotor sleeve and permanent magnets during the process of C&D under different loads are also calculated. The output characteristic curves of FESS under condition of constant loads during the process of uncontrollable discharging is drawn. The conjugate analysis method of rotor multi-physical fields is proposed to study the relationships between rotor sleeve stress and eddy current distribution, and the optimized sleeve thickness for high-speed PMSM/G is also presented. Also, a solution is presented to solve the temperature and sleeve fracture problems caused by eddy current under high-speed operation. This contents of the paper can provide references for efficient and reliable design of PMSM/G under extreme conditions used for FESS.

Electromagnetic analysis of composite structures subjected to transient magnetic fields

When carbon composites are exposed to a transient electromagnetic field, a rapid temperature increase can be observed due to joule heating from magnetic induction. The electromagnetic induction heating and heat transfer in the composite are anisotropic and concentrated upon the carbon fiber orientation and distribution. In addition, the strength and frequency of transient electromagnetic fields have great influence on the final quality of the composite. A computational model has been developed by solving coupled Maxwell’s and heat transfer equations. The analysis accounts for the three-dimensional transient electromagnetic field and electrical conductivity of the composite material. This paper will illustrate the derived formulation and numerical solution based on finite element methods. The developed code is validated with a 2D closed-form solution. Numerical simulations of a cylinder and a flat laminated plate are conducted to illustrate the computational capability. The induction heating for composite manufacture is also discussed for current Army’s applications.

ПРИМЕНЕНИЕ ПСЕВДОВОЛНОВЫХ ТРАНСФОРМАНТ В ИМПУЛЬСНОЙ ЭЛЕКТРОРАЗВЕДКЕ ДЛЯ ВЫДЕЛЕНИЯ НАКЛОННЫХ ГЕОЛОГИЧЕСКИХ ГРАНИЦ

Актуальность. Метод нестационарных электромагнитных зондирований или зондирований становлением поля является востребованным инструментом при поиске и разведке месторождений различного типа полезных ископаемых. Современные задачи электроразведки требуют использования установок с большим числом источников и приёмников. Для подобного типа установок задача интерпретации полученных данных является весьма ресурсоемкой. Отсюда вытекает острая необходимость развития методов обработки и интерпретации данных, полученных в результате импульсных электромагнитных зондирований. Одним из возможных путей такого развития могут послужить трансформации данных зондирований становлением в пространство решений волнового уравнения. Цель: применить метод волновой трансформации к данным зондирований становлением, зарегистрированным над проводящими средами с наклонной границей в многоразносных установках, для получения новых качественных оценок исследуемого геоэлектрического разреза. Методика исследования базируется на новых численных алгоритмах для устойчивого преобразования данных зондирований становлением в волновую область. Исходные данные для отладки процедур трансформации (расчётные кривые становления поля) были получены с использованием программного комплекса MODEM3D, который предназначен для прямого моделирования переходных процессов диффузии электромагнитных полей. Результаты. Были получены псевдоволновые трансформанты сигналов нестационарных электромагнитных зондирований в средах с наклонной границей, и построены их годографы. Полученные наборы трасформаций позволяют выделить линию выхода наклонного контакта на дневную поверхность. Установлено, что перекрывающие наносы ослабляют максимумы псевдоволновых трансформаций и снижают возможность точной локализации линии выхода наклонной границы. Рассмотрено поведение тороидальных вихревых токов в зависимости от времени для модели геологической среды с наклонной границей.

Influence of Electric Shield Materials on Temperature Distribution in the End Region of a Large Water–Hydrogen–Hydrogen-Cooled Turbogenerator

To investigate the influence of different electric shield materials on the temperature distribution in the turbogenerator end region, a 330-MW water–hydrogen–hydrogen-cooled turbogenerator is considered in this paper. Mathematical and physical models of the three-dimensional (3-D) transient electromagnetic field in the turbogenerator end region are established. The magnetic density distribution and the losses of end parts are obtained with different electric shield materials. The loss values obtained from the 3-D transient electromagnetic field calculations with different electric shield materials are applied to the end parts as heat sources. Pressure and fluid velocity values from flow network calculations are applied to the end region as boundary conditions for the fluid and thermal coupling analysis. In addition, a 3-D fluid and thermal coupling model of the turbogenerator end region is established. The distributions of the surface heat transfer coefficient on the inner and outer surfaces of the electric shield are determined with different electric shield materials. Temperature distributions of the stator-end copper winding, finger plate, clamping plate, and electric shield in the turbogenerator end region are investigated with different electric shield materials. The calculated temperature results for the electric shield are compared with measured values, and the calculated results agree well with the measured values.

Performance Analysis and Optimization of Liquid-Cooled and Flywheel-Type Eddy Current Retarder

Aiming at the heat fade of the continuous braking and low power density of traditional eddy current retarder (ECR) for heavy vehicles, a new liquid-cooled and flywheel-type eddy current retarder is proposed. The new ECR can keep a low working temperature because of the cooling type. It is assembled between the engine and the clutch, which can effectively improve the braking power density. The transient electromagnetic field of the ECR is solved by the finite-element method, and the braking torque characteristic curve is obtained. The thermal-fluid coupling field is analyzed using the computational fluid dynamics method. Finally, the structure parameters optimization of the rotor tooth is carried out. The braking torque increases by 14.8% and the weight of rotor tooth decreases by 9.5% after optimization. The test results show that the heat fade ratio of the new ECR braking torque is only 8.6%, but that of the traditional ECR is 46.7%.

The Electromagnetic Effects of Pulsed Horizontal Dipoles on a Thin Conductive Sheet: Time-Domain Analysis

The electromagnetic problem of calculating the pulsed radiation from a horizontal magnetic dipole in the presence of a planar thin conductive screen is solved analytically in the time domain via a modified Cagniard-de Hoop approach. The components of the relevant time-domain electric and magnetic dyadic Green's functions are expressed by simple one-dimensional integrals over a finite integration domain. Expressions for the transient electromagnetic fields can thus easily be obtained and numerical results are provided that illustrate the main time-domain shielding characteristics.

Influence of different underexcited operations on thermal field in the turbogenerator end region with magnetic shield structure

The losses and temperatures of end parts will increase obviously in the turbogenerator end region under underexcited operation. In order to study the influence of different underexcited operations on temperature of end parts in the turbogenerator end region with magnetic shield structure, a turbogenerator is considered as an example in this paper. Three-dimensional transient electromagnetic field of turbogenerator end region is given. The magnetic density distribution of clamping plate and magnetic shield is researched. The losses of end parts and stator end cores from three-dimensional transient electromagnetic field calculation are applied to end region as heat sources under different underexcited operations. Pressure value of fan outlet and velocity values of different inlets from flow network calculations are applied to end region as boundary conditions in the 3-D fluid and thermal coupling analysis model. Further, fluid field and thermal field are calculated under different underexcited operations. The distribution laws of fluid velocity and fluid temperature in the turbogenerator end region are determined. The temperature distribution laws of the stator-end copper windings, press finger, clamping plate, screen plate, magnetic shield, and stator end cores are studied under different underexcited operations. The accuracy of numerical calculation is validated by measured values.

Study on Effect of Transient Electromagnetic Field on Germination of Seed Plants

Study on Effect of Transient Electromagnetic Field on Germination of Seed Plants

A Kirchhoff migration imaging method based on grounded-source TEM virtual wave-fields and its applications

This research study was based on a Kirchhoff migration imaging method, and the aim of the study was to realize grounded-source transient electromagnetic virtual wave-field migration. Diffusion equations mainly characterize the induction characteristics of low-frequency electromagnetic waves which are not suitable for wave-fields. Wave equations mainly characterize wave reflections, refractions, and other propagation features, which are suitable for wave-field imaging processes. A unique mathematical transformation relationship exists between the diffusion fields of transient electromagnetic fields and seismic wave-fields. After the transformations from diffusion fields to virtual wave-fields are achieved, seismic interpretation methods can be used for the transient electromagnetic data interpretations. First, in order to realize Kirchhoff migration imaging based on transient electromagnetic wave-fields, precondition regularization methods are used for the wave-field inverse transformations. Then, after obtaining the wave-field data, dynamic correction methods are adopted to convert the grounded-source transient electromagnetic virtual wave-fields to quasi-seismic zero offset data. At that point, a Kirchhoff integral can be applied to realize the grounded-source transient electromagnetic quasi-seismic migration imaging. A theoretical model was used to test the method which was proposed in this study. The results showed that the Kirchhoff migration imaging was able to determine the geometric formations of underground dielectric interfaces, which fully demonstrated the applicability of the proposed method. Finally, migration imaging processing was performed on the data obtained from a mining area, and the imaging results were found to be consistent with the known geological data. Therefore, the feasibility of using a wave-field transform method and Kirchhoff migration imaging technology for grounded-source transient electromagnetic data interpretations was successfully confirmed.

Effect of Transmitter Current Waveform on Transient Electromagnetic Responses

Abstract —The theoretical studies and methodological guidelines for transient electromagnetic (TEM) surveys assume an ideal step turn-off of transmitter current. Yet, the pulse duration in the survey practice is nonzero: It depends on penetration depth and has economic or logistic limitations. The turn-off time, which cannot be too short for technical reasons, affects especially the late-time transient responses, while the transmitter waveform is more essential at early times. Therefore, forward calculations of voltage decay should take into account the real parameters of the turn-off ramp. Earlier it was suggested to use transmitter current of a special waveform to improve the resolution of TEM soundings, e.g., in the designed and successfully tested compensation method. In this study the problem is treated from a broader perspective, in terms of TE and TM polarization of the TEM field. The two components are excited by different sources and behave in different ways in responses of a two-layer earth with a resistive lower layer to transmitter current of different waveforms.

The Simulation of Transient Electromagnetic Based on Time-domain IP Model

When transient electromagnetic investigation methods are carried out in the field, the measured data often contain both the induced polarization (IP) effect and the electromagnetic effect. In order to study the IP effect in the transient electromagnetic response, many researchers first calculate the electromagnetic field which considers the IP effect by replacing traditional resistivity with complex resistivity of the Cole-Cole model in the frequency domain. After the forward modeling calculation of the electromagnetic field in the frequency domain that considers the IP effect, the transient electromagnetic field in time-domain is obtained by a time-frequency transform algorithm. In this paper, the resistivity is directly replaced by the time-variant resistivity expression of the Cole-Cole model by using digital filter algorithms when we simulate the transient electromagnetic fields in time- domain. The calculated result of the Cole-Cole model in time-domain and in frequency-domain are consistent with each other, as observed in the horizontal electric field and the vertical magnetic field comparisons, which indicates the correctness of the numerical computation method adopted in this paper. The research presented herein allows us to observe the influence of the IP effect on transient electromagnetic field as well as study the mechanisms of IP directly.

Time-Stepping FEM-Based Multi-Level Coupling of Magnetic Field–Electric Circuit and Mechanical Structural Deformation Models Dedicated to the Analysis of Electromagnetic Actuators

The present paper introduced a framework for multi-level coupling transient electromagnetic fields (EMF) and mechanical structural dynamics based on the finite element method (FEM). This framework was dedicated to predicting, with better accuracy, the wave magnetic force density for obtaining the mechanical deformation occurring in electromagnetic actuators (EMAs). The first-level EMF transient model coupling is related to the magnetic field equations that are strongly coupled with the electric circuit input voltage equations. This is done by considering the magnetic saturation through the Newton–Raphson (N–R) method. The time-stepping solution of the EMF model resulted in the magnetic force densities being computed from the Lorentz force (LZ) expressions, based on the space–time variation of the induced eddy current. For the second coupling level, the EMF model was sequentially coupled with the mechanical structural deformation equations (MDef) through the local magnetic force density to achieve minimal material dynamic displacement and deformation. The developed multi-physics EMF–MDef time-stepping (FEM) model tools were implemented using the Matlab software.

Transient Electromagnetic Pulse Emanation in Digital Systems in the Mode of Pulsed Excitation of the Printed Connector Elements

Specific features of the formation of transient electromagnetic pulse emanation fields induced by the pulsed excitation of printed elements are discussed. The field formation affects the electromagnetic compatibility of digital signal processing devices. It is shown that printed conductors in complex-layout boards with a specific geometrical configuration can work as electromagnetic emitters. It has been established that the video pulse length, clock frequency, and code sequence alternation phase determine the spectral behavior of a digital signal, which may coincide with the eigenfrequencies of printed emitters, in particular, forming a mode of excitation on the self-oscillations when decreasing video pulse length or front steepness. The results of investigations of potentially emitting printed element prototypes are reported.

Diffusion Law of Whole-Space Transient Electromagnetic Field Generated by the Underground Magnetic Source and Its Application

Mine water inrush stays as one of the major disasters in coalmine production and construction. As one of the principal methods for detecting hidden water-rich areas in coal mines, underground transient electromagnetic method (TEM) adopts the small loop of a magnetic source which generates a kind of whole-space transient electromagnetic field. To study the diffusion of whole-space transient electromagnetic field, a 3-D finite-difference time-domain (FDTD) is employed in simulating the diffusion pattern of whole-space transient electromagnetic field created by the magnetic source in any direction and the whole-space transient electromagnetic response of the 3-D low-resistance body. The simulation results indicate that the diffusion of whole-space transient electromagnetic field is different from ground half-space and that it does not conform to the “smoke ring effect” of half-space transient electromagnetic field, for the radius of the electric field's contour ring in whole space keeps expanding without moving upward or downward. The low-resistance body can significantly affect the diffusion of transient electromagnetic field. When the excitation direction is consistent with the bearing of the low-resistance body, the coupling between the transient electromagnetic field and the low-resistance body is optimal, and the abnormal response is most obvious. The bearing of the low-resistance body can be distinguished by comparing the response information of different excitation directions. Based on the results above, multi-directional sector detection technology is adapted to detect the water-rich areas, which can not only detect the target ahead of the roadway but also distinguish the bearing of the target. Both numerical simulation and practical application in underground indicate that the mining TEM can accurately reflect the location of water-rich areas.

Influence of Magnetic Permeability of the Press Plate on the Loss and Temperature of the End Part in the End Region of a Turbogenerator

Press plates are widely used in the end region of turbogenerators. However, due to the effect of magnetic field leakage from the end region, the eddy current losses of the copper screen and press plate will result in the copper screen and press plate having a high temperature under the rated load condition. In this paper, the influence of the magnetic permeability of the press plate on losses and temperature of the end region is researched in turbogenerators. Three-dimensional (3-D) fluid and thermal mathematic and physical models of the turbogenerator end region are established. Under different magnetic permeability values of the press plate, a 3-D transient electromagnetic field in the turbogenerator end region is calculated, and the obtained losses of the end parts are treated as heat sources. Fluid network equations are calculated, and the obtained fluid velocity and pressure values are applied to the end region as boundary conditions. After solving the fluid and thermal equations, fluid velocity around the surface of the end parts is obtained under the rated load condition. The temperature distribution of the end parts is determined for different magnetic permeability values of the press plate. The calculated temperature results agree well with the test data.

Proton imaging of relativistic laser-produced near-critical-density plasma

When ultrashort pulse laser interacts with near-critical-density plasma, extremely strong transient electromagnetic field will generate a great variety of nonlinear phenomena, such as efficient pulse absorption, magnetic self-channeling, nonlinear coherent structure, and electron and ion acceleration. It is of great significance to make a profound study of these physical processes for studying the laser-plasma interaction. Here in this work, we investigate the near-critical-density plasma structure and its temporal evolution by using proton radiography. The plasma is generated by the interaction of ultra-intense femtosecond laser (<i>I</i> <inline-formula><tex-math id="Z-20190727124236-1">\begin{document}$\sim $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20190610_Z-20190727124236-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20190610_Z-20190727124236-1.png"/></alternatives></inline-formula> 3.6 × 10<sup>18</sup> W/cm<sup>2</sup>) with high-density gas-jet target, which can produce plasma with electron density <i>n</i><sub>e</sub> <inline-formula><tex-math id="Z-20190727124238-2">\begin{document}$ \sim$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20190610_Z-20190727124238-2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20190610_Z-20190727124238-2.png"/></alternatives></inline-formula> 0.7<i>n</i><sub>c</sub> (here, <i>n</i><sub>c</sub> is the near-critical-density) for 800 nm laser. The proton beam is produced by the interaction of another ultra-intense femtosecond laser with stainless steel foil target. In the experiment, the proton beam is split into two asymmetric spots. On the one hand, the distance between two spots first increases rapidly and decreases slowly as time goes by. On the other hand, the size of proton beam spot on the right side is obviously lager than the one on the left side. The modification of proton beam profile indicates that a transient electric field with a maximum amplitude of 10<sup>9</sup> V/m is produced when ultrashort laser pulse interacts with the plasma. Besides, the electric field in the direction of laser propagation axis is stronger than that in the opposite direction. When the proton beam goes through the laser-plasma interaction area, most of the protons enter into the electric field in the direction of laser propagation axis, only a small number of protons enter into the electric field in the opposite direction, resulting in the fact that the proton beam is split into two asymmetric spots. The space-charge field in the plasma is induced by the laser ponderomotive force which expels the electrons piled up into a step-like profile. This field can be sustained for a long time, as the ions expand slowly because of the coulomb repulsion between ions, and the hot electrons continue to move forward with energy of a few MeV. At the end, these expanded ions gradually recombine with the reflowed electrons, causing the space-charge field to weaken until it disappears eventually. As a result, the deflection of the proton beam by the electric field in the plasma is also weakened, so the distance between proton beam splitting spots is correspondingly reduced. The hypothesis is justified by the particle-in-cell simulations. The results may have important implications in laser wake-field electron acceleration, ion acceleration and fast ignition scheme to inertial confinement fusion.

Variational formulation of the thermal deformation problems of electrically conductive bodies in an electromagnetic field

The paper discusses issues concerning the thermal deformation of electrically conductive bodies under the action of the electromagnetic field. Similar problems arise, for example, in the analysis of induction heating processes. Transient electromagnetic field leads to heat release in electrically conductive bodies, and the change in temperature fields leads to a change of stress-strain state of a body. The creation of methods for the quantitative analysis of the stress-strain state of bodies under the action of an electromagnetic field is an urgent scientific problem because such an analysis allows us to evaluate the performance and durability of various structural elements. The modern approach dictates the need to consider three related problems: the problem of spatio-temporal distribution analysis of the electromagnetic field, transient heat-transfer problem and the problem of stress-strain analysis. The analysis of real technical and technological systems can only be done using appropriate numerical methods. In this case, the most universal is the finite element method, which has proven itself both in the analysis of the deformable bodies mechanics and in the analysis of various multiphysical problems. The usage of the finite element method requires an appropriate mathematical formulation of the problem. The mathematical problem formulation in variational form is considered in this article. Examples of corresponding functionals that allow finding solutions to a problem by finite element method are presented in the article. The functionals describing the transient distribution of the electromagnetic field are constructed based on the using of the concept of scalar electric and vector magnetic potentials. The influence of the electromagnetic field on the temperature distribution and the deformation process is taken into account by introducing distributed heat sources and distributed electromagnetic forces. The operation of varying the solution functions – potentials, temperature and displacements – makes it possible to obtain a system of resolving algebraic equations of the finite element method.

Filtering of noisy magnetotelluric signals by SOM neural networks

This work presents a systematic study for testing the effectiveness of Self-Organizing Map (SOM) neural networks in filtering magnetotelluric (MT) data affected by cultural noise. Although the MT method is widely used for geophysical investigation of the Earth’s interior, it is very sensitive to anthropogenic noise sources (e.g., power lines, electric railways, etc.), which can generate transient artificial electromagnetic fields disturbing the MT records. Thus, when not properly detected, man-made noises could lead to a distortion of the MT impedance tensors and consequently to wrong estimate of the resulting subsoil resistivity distribution. The choice to use SOM networks to filter noisy MT data comes from the expectation that the impedance tensors, estimated by Discrete Wavelet Transform analysis of MT time series, will cluster differently in presence of noise. This expectation is confirmed by the results of our extensive study on synthetic MT signals affected by temporally localized noise, which show that noisy and noise-free impedance tensor values distribute in well separate clusters. Moreover, as the SOM analysis provides a grid of weights (clusters), each one close to a particular subset of the input data, a criterion is proposed for selecting the cluster that gives the most reliable impedance tensor estimate. An application of the proposed SOM-based filtering procedure to actual MT data demonstrates its efficiency in denoising real MT signals.