Transient Electromagnetic System Research Articles

Design and Development of A Semi-airborne Transient Electromagnetic System

The semi-airborne transient electromagnetic method (SATEM) is a geophysical method that combines the advantages of high-power ground-based transmitting and efficient airborne observation. In recent years, the threshold for SATEM application significantly reduced through the integration of the multi-rotor UAVs and the systems obtained by simple redevelopment of existing ground-based systems. However, it is still difficult to obtain good detection results with such systems for the following specific reasons: in terms of system design, there is a lack of method for estimating the key parameters of the system, resulting in the designed system being unable to effectively meet the requirements of the SATEM method and observation scenarios; in terms of technology, there is a lack of effective means of ensuring a high signal-to-noise ratio (high-current transmitting and effective suppression of major external noise) while meeting the demand for distortion-free observation. In response to these issues, a system bandwidth estimation method based on the analysis of information-carrying capacity is proposed. On this basis, the solutions for large current and fast turn-off transmitter and for overcoming major external noise. Through a survey located in a karst landform area, and the overall application performance of the system is discussed based on the inversion results.

An optimized and hybrid gating scheme for the suppression of very low-frequency radios in transient electromagnetic systems

Abstract. One of the most widely used approaches for measuring the earth's subsurface resistivity is the transient electromagnetic (TEM) method. However, noise and interference from different sources, e.g., radio communication, the instrument, the atmosphere, and power lines, severely taint these types of signals. In particular, radio transmission in the very low-frequency (VLF) range between 3 and 30 kHz is one of the most prominent sources of noise. Transient electromagnetic signals are normally gated to increase the signal-to-noise ratio. A precise selection of gate shapes is required to suppress undesired noise while allowing the TEM signal to pass unaltered. We employ the multi-objective particle swarm optimization technique to choose optimal gate shapes and placements by minimizing an objective function composed of standard error bars, the covariance between gates, and the distortion of the gated signal. The proposed method is applied to both fully sampled synthetic TEM data and to boxcar-gated field data. The best output from the search space of gate shapes was found to be a hybrid combination of boxcar and Hamming gates. The effectiveness of hybrid gating over traditional boxcar and semi-tapered gating is confirmed by an analysis of covariance matrices and error bars. The results show that the developed method effectively suppresses VLF noise in the middle gates, which are gates with center times spanning 30 to 200 µs , and in the late gates, which are gates with center times spanning 200 to 1130 µs. The analysis shows that the average improvement in standard errors obtained for the hybrid gating scheme over boxcar gating is 1.719 and 1.717 for synthetic and field data, respectively.

Power Source Converter Based on a Variable-Domain Fuzzy PI Control

The inadequacy of conventional control strategies for multi-phase interleaved parallel circuits in terms of adaptive adjustment makes it difficult to meet the power source design requirements for transient electromagnetic detection systems. This paper introduces a novel approach, the variable-domain fuzzy proportional–integral (PI) adaptive control strategy. This strategy dynamically adjusts the fuzzy domain in real time based on the input error magnitude, ensuring improved control effectiveness. By leveraging the benefits of both the functional scaling factor and the fuzzy reasoning scaling factor, we design scaling factors for the input and output domains to enhance control precision. The focus of this study is on a four-phase interleaved parallel converter, emphasizing the design of the variable-domain fuzzy PI control strategy for the voltage outer loop within the traditional dual closed-loop structure. An experimental prototype with a 220 Vac input, 380 V output, and a power rating of 1000 W is constructed. A comparative analysis between fuzzy control and variable-domain fuzzy PI control is conducted in the voltage outer loop of the dual closed-loop control. Results reveal that dual closed-loop control with variable-domain fuzzy PI control for the voltage outer loop significantly enhances system stability. The startup time to reach the steady state is reduced to 0.632 s, with an overshoot of 28.8 V. Transitioning from 25% load to full load takes only 0.096 s, resulting in a minimal drop of 21.4 V and an overshoot of 13.4 V. Similarly, switching from full load to 25% load in 0.167 s exhibits an overshoot of only 19.6 V. The adaptive regulation capability of the converter is markedly improved, showcasing smaller overshoots and higher-level controlling effectiveness.

Turn-Off Current and TEM Field Based on Distributed Capacitance Model of Multiple Coils in Helicopter Transient Electromagnetic System

For compensating the primary field coupling from the transmitting coil and the sensing coil, the bucking coil is often used to achieve pure secondary field for the helicopter transient electromagnetic (HTEM) system. Traditionally, the distributed capacitance of the bucking coil and the transmitting coil is ignored and pay few attentions to study it. In fact, it has very important influence on the turn-off current of the transmitter and the transient electromagnetic (TEM) field of the system. In this paper, a novel distributed capacitance model of the HTEM coils is theoretically established at the first time, its effects on the turn-off current and the TEM field are analyzed in detail. It finds that the turn-off current in the transmitting coil and the bucking coil are not equal caused by the distributed capacitance although they are connected in series. The field experiments are completed and proved that the measured TEM fields agree very well with the simulated results based on the distributed capacitance model. This is very helpful to optimize the early-time TEM field and promote the shallow TEM data interpretation capability for the HTEM surveying.

Influence of Parasitic Capacitance on Turn-Off Current and Its Optimization Method for Transient Electromagnetic System

Influence of Parasitic Capacitance on Turn-Off Current and Its Optimization Method for Transient Electromagnetic System

Electromagnetic Detection System with Magnetic Dipole Source for Near-Surface Detection.

This paper proposes a nondestructive, separate transmitter-receiver (TX-RX) electromagnetic measurement system for near-surface detection. Different from the traditional dual-coil integrated design, the proposed transient electromagnetic (TEM) system performs shallow subsurface detection using independent TX coil and movable RX coils. This configuration requires a large primary field so that the far-away secondary field is able to generate reliably induced voltages. To achieve this goal, a bipolar current-pulsed power supply (BCPPS) with a late resonant charging strategy is designed to produce a sufficiently large magnetic moment for the exciting coil with low source interference. The magnetic dipole source (MDS) with a large proportion of weight is separated from the field observation device and does not need to be dragged or transported during the detection process. This setup lowers the weight of the scanning device to 3 kg and greatly improves the measurement efficiency. The results of the laboratory test verify the effectiveness of the separate MDS and RX module system. Field experimental detection further demonstrates that the proposed system can realize highly efficient and shallow surface detection within a 200 m range of the MDS device.

Real-time Detection System for Portable Transient Electromagnetic Unexploded Ordnance

Portable transient electromagnetic systems are not limited by terrain and can detect unexploded ordnance very flexibly. However, existing data processing methods are slow and cannot achieve real-time positioning of targets. In this paper, based on a laboratory-made portable transient electromagnetic detection system, the magnetic dipole model and tensor positioning algorithm are used to achieve fast and accurate target positioning. Using the magnetic gradient tensor algorithm for positioning reduces the calculation load, allowing it to run on embedded systems, and improving target positioning speed while ensuring the accuracy of measurements. Experimental results show that the vertical and horizontal positioning errors of the UXO are within 10 cm, and single-point positioning can be completed within 10 ms.

Simulation and Experimental Study on Coupling Effect of Fuze under Ultra-wide Spectrum Strong Electromagnetic Pulse

The lack of anti-strong electromagnetic interference ability of fuze has become an important aspect restricting the effectiveness of weapon system. Taking a certain type of fuze as an example, this paper analyzes the coupling mechanism of ultra-wide spectrum transient strong electromagnetic pulse to the fuze, analyzes the action mode of aperture coupling and field line coupling, establishes the mathematical model of space electromagnetic field, and then simulates the effect of aperture coupling and field line coupling of the fuze by using CST Microwave Studio software. It is concluded that the strongest coupling state is when the polarization direction of the electromagnetic field is parallel to the axis of the fuze. Combined with the theory and simulation, the ultra-wide spectrum transient strong electromagnetic pulse simulation system is used to conduct the strong electromagnetic pulse irradiation test, and the electromagnetic pulse effect threshold is obtained. It is concluded that the effect threshold leading to communication failure is about 27~40kV/m, which provides a certain theoretical and experimental basis for fuse strong electromagnetic protection.

An efficient 2.5‐D forward algorithm based on the spectral element method for airborne transient electromagnetics data

AbstractThe airborne transient electromagnetics method has been widely used in geophysical exploration in recent years, but it still faces challenges in balancing the accuracy and efficiency of electromagnetic data interpretation. As far as forward modelling is concerned, the higher the dimension of the geophysical model, the higher the accuracy of data interpretation, but, correspondingly, the more computing resources need to be consumed, which will greatly reduce investigation efficiency and practicability. In this paper, the spectral element method is first introduced for solving the 2.5‐dimensional forward modelling of the airborne transient electromagnetic system, which has a smaller computing scale than three‐dimensional modelling and is closer to the actual geological structure than either one‐ or two‐dimensional modelling. In the forward algorithm, a non‐uniform quadrilateral structured mesh is adopted to simplify the computing scale, and the Talbot algorithm rather than Gaver–Stefest algorithm is applied to the inverse Laplace transform to improve the numerical precision of this conversion. Moreover, we use parallel computing technology to improve the algorithm efficiency while keeping satisfactory accuracy. The study shows that, whether a low‐resistivity or high‐resistivity layered geophysical model, the numerical solutions of the proposed spectral element method forward algorithm agree well with the analytical solutions of the corresponding models; furthermore, the key factors affecting the accuracy of the numerical solution are analysed by experiments. Finally, we successfully applied it to the 2.5‐dimensional geoelectric model simulation.

Current oscillation elimination for the helicopter transient electromagnetic inverter based on pole configuration

In a helicopter transient electromagnetic system, the quality of the transmitting-current waveform will affect the geological exploration effect. In this paper, a helicopter TEM inverter, based on a single-clamp source and pulse width modulation technology, is designed and analyzed. Besides, it finds that there will be current oscillation in the early measuring stage. For this problem, first, the factors that cause the current oscillation are analyzed. Then, it is proposed to apply the RC snubber to eliminate this current oscillation. Since the imaginary part of the pole is the essence of oscillation, configuring the pole can eliminate the current oscillation. By establishing the early measuring stage system model, the characteristic equation of the load current with the snubber circuit is deduced. Next, the characteristic equation is solved by the exhaustive method and the root locus method to obtain the parametric region that eliminates the oscillation. Finally, through simulation and experimental verification, the proposed snubber circuit design method can be used to eliminate the early measuring stage current oscillation. Compared to the method of switching into the damping circuit, it can achieve the same performance, more important is that there is no switching action and it is easy to achieve.

Controls on flood managed aquifer recharge through a heterogeneous vadose zone: hydrologic modeling at a site characterized with surface geophysics

Abstract. In water-stressed regions of the world, managed aquifer recharge (MAR), the process of intentionally recharging depleted aquifers, is an essential tool for combating groundwater depletion. Many groundwater-dependent regions, including the Central Valley in California, USA, are underlain by thick unsaturated zones (ca. 10 to 40 m thick), nested within complex valley-fill deposits that can hinder or facilitate recharge. Within the saturated zone, interconnected deposits of coarse-grained material (sands and gravel) can act as preferential recharge pathways, while fine-textured facies (silts and clays) accommodate the majority of the long-term increase in aquifer storage. However, this relationship is more complex within the vadose zone. Coarse facies can act as capillary barriers that restrict flow, and contrasts in matric potential can draw water from coarse-grained flow paths into fine-grained, low-permeability zones. To determine the impact of unsaturated-zone stratigraphic heterogeneity on MAR effectiveness, we simulate recharge at a Central Valley almond orchard surveyed with a towed transient electromagnetic system. First, we identified three outcomes of interest for MAR sites: infiltration rate at the surface, residence time of water in the root zone and saturated-zone recharge efficiency, which is defined as the increase in saturated-zone storage induced by MAR. Next, we developed a geostatistical approach for parameterizing a 3D variably saturated groundwater flow model using geophysical data. We use the resulting workflow to evaluate the three outcomes of interest and perform Monte Carlo simulations to quantify their uncertainty as a function of model input parameters and spatial uncertainty. Model results show that coarse-grained facies accommodate rapid infiltration rates and that contiguous blocks of fine-grained sediments within the root zone are >20 % likely to remain saturated longer than almond trees can tolerate. Simulations also reveal that capillary-driven flow draws recharge water into unsaturated, fine-grained sediments, limiting saturated-zone recharge efficiency. Two years after inundation, fine-grained facies within the vadose zone retain an average of 37 % of recharge water across all simulations, where it is inaccessible to either plants or pumping wells. Global sensitivity analyses demonstrate that each outcome of interest is most sensitive to parameters that describe the fine facies, implying that future work to reduce MAR uncertainty should focus on characterizing fine-grained sediments.

Advances in Marine Intelligent Electromagnetic Detection System, Technology, and Applications: A Review

This paper introduces the current state of the art in marine intelligent electromagnetic detection sensors, systems and platforms for the detection and health monitoring of offshore structures, especially for subsea cables and pipelines. The presented survey is by no means exhaustive, but introduce some notable results and relevant techniques in the detection and inspection of marine structure electromagnetic monitoring fields. Particularly, this paper presents a review of the main research works focusing on electromagnetic detection techniques and intelligent marine vehicles for subsea cables. The marine electromagnetic exploration techniques are elaborated as active and passive detections based on different electromagnetic field generation and sensing principles. The pulse induction-based detection method for subsea ferromagnetic lines detection, transient and controlled-source electromagnetic systems in marine geological fields under the category of active detection approach, are reviewed. According to the classical applications on subsea cable-like target inspection, the passive detection approach is detailed classified as crisscrossing detection and along-tracking detection, which are characterised by different trajectories of detection vehicles. Finally, the paper discusses the marine electromagnetic detection techniques in general and highlights challenges that need to be addressed in developing detection vehicles with lower electromagnetic noises and advanced autonomy on carrying out detection missions.

A Nine-Level Inverter with Adjustable Turn-Off Time for Helicopter Transient Electromagnetic Detection.

The current inverter is the core component of the helicopter transient electromagnetic (HTEM) detection system. It should meet the concerns of low loss, high power, and fast turn-OFF time. This article proposes a new circuit topology based on nine-level inverter technology to overcome the drawbacks of typical PWM (pulse width modulation) inverters, such as switching losses and harmonics. This circuit topology overcomes the shortcomings of the traditional single constant voltage clamp circuit in which the turn-OFF time is not adjustable. Using an inverter with the proposed topology is able to avoid the complex PWM control method and switching loss. In this way, the current rising edge and falling edge of this inverter are also improved effectively. The proposed inverter has adjustable turn-ON-time and turn-OFF time, which is significantly different from the conventional single-clamp inverter. Through subsequent experiments, the inverter proved to have the capability of generating trapezoidal current waveforms. Moreover, by modifying the FPGA (Field Programmable Gate Array) control program, three different turn-OFF times are achieved. The nine-level inverter has a peak current of 1.5 A with an adjustable turn-OFF time from 129 μs to 162 μs. Moreover, the switching frequency of the inverter is reduced from 10 kHz to below 100 Hz. The experimental results further demonstrate that it achieves lower switching losses and more flexible transmission. Our work in this article provides an efficient way to improve the performance of HTEM detection systems.

Combining the Wiener Filter With Calibration Device to Improve the Accuracy of the Helicopter Transient Electromagnetic System

The transfer characteristic of the receiver sensor (TCRS) in a helicopter transient electromagnetic (HTEM) system can severely distort the measured data, especially early in the off-time. Removing TCRS is essential for an accurate and quantitative description of HTEM data. Precise Measurement and suppression of the TCRS are challenging due to measurement errors and noise in the HTEM system. We propose to use a calibration device to train the Wiener filter and then construct a TCRS deconvolution function to remove TCRS and enhance the HTEM system’s shallow detection capabilities. First, we design a calibration device and get its ideal and actual response in an existing HTEM system. Then the Wiener filter is trained using the measured and ideal response of the calibration device. Finally, we use a Wiener filter to obtain a recorded signal free of TCRS. Field tests show that the Wiener filter effectively suppresses the influence of TCRS on the recorded signal. After processing the observed signal with the Wiener filter, the blind time is reduced from 88.54 μs to 67.71 μs. The blind spot is reduced by 14.90 m for 100 Ω∙m homogeneous earth and 33.32 m for 500 Ω∙m homogeneous earth. The accuracy of the processed signals in channel two is improved by 16.93%. Accuracy for other channels is enhanced by 1.5%.

Multiwaveform Current Transmitter for Induced Polarization Effects Surveying Based on the SQUID-TEM System

The induced polarization effects magnetic field measurement method based on the superconductive quantum interfere device (SQUID) and transient electromagnetic (TEM) system can improve the exploration accuracy of metal minerals. As the core component of the SQUID-TEM system based on magnetic source, the current transmitter should reach consensus in three key aspects: high power, controllable turn-off time and combined current waveforms. In order to solve the problem of single transmitting current waveform and uncontrollable turn-off time in traditional current transmitter, a topology structure of current transmitter with multiple waveform combinations in time domain is proposed in this paper. By using the passive multi-group clamp technology and RLC series resonance technology, the falling edge turn-off time of trapezoidal and triangular transmitting current can be controlled, and the pulse width of semi-sinusoidal transmitting current can be adjusted. The laboratory and field experiments are implemented to test the performance of the current transmitter. Compared with the commercial PROTEM67 transmitter, the test results show that it can successfully transmit three current waveforms and their combination waveforms, the current amplitude is more than 100A, and the transmitting current turn-off time is controllable. The field measurement response results show that the combined multi-waveform transmit system can effectively measure the induction field and the polarization field generated by the Cole-Cole polarization ring, which provides technical support for the high-precision detection of induction-polarization effects based on the SQUID-TEM system.

Performance of light fixed‐wing airborne time‐domain electromagnetic system for mapping the near‐surface cover layer in an alluvial plain context: A numerical study

AbstractThe mapping of the vertical and lateral variations in the physical properties of the few‐meter cover layer over near‐surface aquifers is important for hydrogeological modelling, particularly for the quantification of the recharge of groundwater systems. The first ground‐based time‐domain electromagnetic survey over a small catchment (Avesnelles, France) of the watershed of Orgeval (Seine basin) was carried out to determine discontinuities in the first silt layer as well as in the Brie multilevel aquifer limestone horizon. The results highlighted the following: (1) a good sensitivity of the time‐domain electromagnetic survey to the presence of multi‐decametric resistive sand lenses, particularly in a location where they were previously identified and (2) the interest in conducting a survey at a fine sampling step but extending to the meso‐scale. To overcome the sampling issue over a watershed of several hundred square kilometres, we proposed numerically assessing the use of a prototype of low‐cost airborne transient electromagnetic systems towed by light fixed‐wing airplanes (with transmitting and receiving loops in the same plane). The present numerical analysis, in 1D for the vertical (i.e. thickness) variation and in 3D for the lateral extensions of localized sandy and resistive units, showed that a conductive few‐meter cover can be mapped even with a system flying at 50 m with, however, the need of a priori constraint on the resistivity of the first layer to estimate its thickness variation as accurately as possible. Even if it did not bring more sensitivity to the layer thickness and despite the severe difficulty of practical implementation with a decametric emission loop, the vertical co‐planar configuration potentially offered better near‐surface lateral resolution (down to ∼40 m) to delineate the sandy units (discontinuities) within the silt layer (if units are at least 50 m in size) and provided better spatial constraints compared to the classical horizontal co‐planar geometry used in the time‐domain electromagnetic. Even if not aerodynamically in the plane of the emission loop, the measurement of the Hx component with a vertical dipole emission loop (PERPxz geometry for perpendicular) improved the lateral resolution (down to ∼20 m; still with at least 50 m size sand units) and confirmed that a geometry different from the classical horizontal co‐planar configuration could be valuable.

Design of a Differential Low-Noise Amplifier Using the JFET IF3602 to Improve TEM Receiver.

The observed data of transient electromagnetic (TEM) systems is often contaminated by various noises. Even after stacking averages or applying various denoising algorithms, the interference of the system noise floor cannot be eliminated fundamentally, which limits the survey capability and detection efficiency of TEM. To improve the noise performance of the TEM receiver, we have designed a low-noise amplifier using the current source long-tail differential structure and JFET IF3602 through analyzing the power spectrum characteristics of the TEM forward response. By the designed circuit structure, the JFET operating point is easy to set up. The adverse effect on the JFET differential structure by JFET performance differences is also weakened. After establishing the noise model and optimizing the parameters, the designed low-noise differential amplifier has a noise level of 0.60nV/Hz, which increases the number of effective data 2.6 times compared with the LT1028 amplifier.

Research on De-Noising Method of Grounded Electrical Source Airborne Transient Electromagnetic Data Based on Singular Spectrum Analysis

The grounded electrical source airborne transient electromagnetic (GREATEM) system is widely used in groundwater resources detection, geothermal resource detection, geological structure detection, and other fields due to its wide detection range, high detection efficiency, and high resolution. The field data received by the GREATEM system is easily affected by various noises, such as instrument system noise, power frequency noise, sferics noise, and other noise, which reduce the data signal-to-noise ratio (SNR) and affects the data interpretation accuracy. This paper proposes a singular spectrum analysis (SSA) for the GREATEM data de-noising in response to this problem. First, we calculate the electromagnetic response of a uniform half-space using a GREATEM system with an electrical source to verify the effectiveness of the SSA algorithm for GREATEM data de-noising. To determine the appropriate parameters for SSA, we propose a particle swarm optimization algorithm to choose the window length. Later, SSA is used to decompose a synthetic quasi-two-dimensional earth model of GREATEM data. After SSA, the SNR of the reconstructed signal increased by 36 dB, and the RMSE does not exceed 4.9 × 10−6, which verifies the feasibility of the SSA for de-noising GREATEM data. Finally, through field measurement data processing, the effectiveness of the method is further confirmed.

Research on valley current prediction control algorithm based on Buck converter

Helicopter transient electromagnetic (HTEM) launch systems require increasingly high current response rates, a valley current control algorithm based on Buck topology circuit is proposed in this paper. By sampling inductor current valley value, output voltage and the input voltage in the current cycle, the valley value of inductor current in the next cycle is predicted, and thus the duty cycle is derived so that the inductor current can track the reference value in one switching response cycle, which improves the response speed of inductor current and analyzes the stability and speed of the closed-loop system through the Byrd diagram. The valley current algorithm is not only simple, but also easy to implement on digital signal processors (DSPs). Finally, the valley current algorithm is verified by software simulation and experiments.

Underground transient electromagnetic real-time imaging system for coal mine water disasters

Due to advantageous sensitivity to low resistance and noncomplicated operation, transient electromagnetic methods are the most widely used geophysical methods for advanced detection of coal-mining-disaster water bodies. To realize real-time imaging, we developed a wireless antenna attitude angle measuring instrument based on an electronic compass, which measures antenna attitude angle real-time. According to attitude angle information sent through WiFi to the host of transient electromagnetic real-time imager instruments, current transmission, signal acquisition, and antenna rotation angle information are achieved for real-time imaging. We studied the direct algorithm of the apparent resistivity of the whole-space transient electromagnetic field in overlapping loop mode based on double-logarithmic-transform piecewise least-squares fitting, which not only ensures accuracy but also accelerates calculation. On-site detection was carried out in a coal mine, and the detection results are generally consistent with the drilling results.