Subsurface discrimination using electromagnetic induction sensors

This paper reviews the problem of subsurface discrimination using electromagnetic induction (EMI) sensors. Typically, discrimination is based on differences in the multiaxis magnetic polarizability between different objects. They review work on frequency and time domain systems, and their interrelationship. They present the results of comprehensive measurements of the multiaxis EMI response of a variety of inert ordnance items, ordnance fragments, and scrap metal pieces recovered from firing ranges. The extent to which the distributions of the eigenvalues of magnetic polarizability for the different classes of objects do not overlap establishes an upper bound on discrimination. For various reasons, the eigenvalues cannot always be accurately determined using data collected above a buried target. This tends to increase the overlap of the distributions, and hence degrade discrimination performance     

Enhanced discrimination among UXO-like targets using extremely low-frequency magnetic fields

We show that the ability to discriminate among unexploded ordnance (UXO) targets is enhanced by extending the measurement range of our continuous-wave (CW) electromagnetic induction (EMI) system down to extremely low frequencies (ELFs). In particular, we demonstrate that for a typical UXO target set, when data in the range 30 Hz to 24 kHz are extended downward toward 1-Hz discrimination performance improves by a factor of 1.5. Measurements and numerical simulations show that very low frequency magnetic fields penetrate deep into the target, enabling one to discriminate between thick- and thin-walled ferrous cylinders that are otherwise visually identical. Data acquisition at extremely low frequencies is a time-consuming process especially if data averaging is required to achieve acceptable signal-to-noise ratio. Therefore, in practice, discrimination improvement using ELF is probably best carried out using a "cued" approach. In a "cued" approach, target locations would be marked using another detection technique (e.g., a simple total-field magnetometer), and the low-frequency CW EMI system would be deployed to these known target locations for classification.     

半空间目标宽带电磁散射特性的快速插值方法

将幅相分离插值方法应用于半空间目标宽带电磁散射特性的分析中。针对半空间散射体表面感应电流的特性,将主要相位因子提出单独考虑,余下随频率变化相对平滑的剩余项运用最佳一致逼近快速求取其宽带特性,再结合半空间格林函数求解半空间背景下目标的电、磁矢量及标量位函数,进而求解其散射场,最终有效地分析了半空间目标的宽带电磁散射特性。数值结果证明了方法的有效性和准确性。     

Classification of landmine-like metal targets using widebandelectromagnetic induction

In their previous work, the authors have shown that the detectability of landmines can be improved dramatically by the careful application of signal detection theory to time-domain electromagnetic induction (EMI) data using a purely statistical approach. In this paper, classification of various metallic land-mine-like targets via signal detection theory is investigated using a prototype wideband frequency-domain EMI sensor. An algorithm that incorporates both a theoretical model of the response of such a sensor and the uncertainties regarding the target/sensor orientation is developed. This allows the algorithms to be trained without an extensive data collection. The performance of this approach is evaluated using both simulated and experimental data. The results show that this approach affords substantial classification performance gains over a standard approach, which utilizes the signature obtained when the sensor is centered over the target and located at the mean expected target/sensor distance, and thus ignores the uncertainties inherent in the problem. On the average, a 60% improvement is obtained     

Automated identification of buried landmines using normalized electromagnetic induction spectroscopy

Electromagnetic induction spectroscopy (EMIS) is used to identify a buried metallic object such as a landmine, based on its EMI spectrum. EMIS, however, depends on the object's electrical conductivity, magnetic permeability, shape, size, depth, and orientation. For a given mine made of specific metals, shape, and size, however, the only variables are the mine's burial depth and orientation. This paper describes a method of identifying a landmine using a normalized EMIS spectrum, which is independent of the orientation or depth. We assume that the target is small compared with its distance to or size of the sensor so that the source field at a target is uniform. In this case, the normalized spectrum will be range-independent and, therefore, the target identification is based on only spectral shapes. We have developed and tested an algorithm that matches a normalized EMIS spectrum to those of library targets. We applied the new process to 1) numerically simulated data, 2) experimental data from controlled sites using inert mines, mine simulants, and clutter items, and 3) finally extensive field data collected at a blind test site established by the U.S. Army. Test results show that targets are correctly identified with a misfit of less than 10%; they also show that 80% of clutter may be rejected, based on a misfit over 30%.     

Investigation of broadband electromagnetic induction scattering by highly conductive, permeable, arbitrarily shaped 3-D objects

Operating as low as tens of hertz and as high as hundreds of kilohertz, new broadband electromagnetic induction (EMI) sensors have shown promise for classification of unseen buried metallic objects. The three-dimensional (3-D) and bodies-of-revolution (BOR) numerical studies reported here are designed to explain key scattering sensitivities that may either be useful in or may limit object classification capability. The target is excited either by a spatially uniform oscillating primary magnetic field or by the oscillating field from a loop antenna. The problem is formulated in terms of Poison's equation for scalar potential outside the object, where conductivity and electric field values are low and consequent conduction currents are generally negligible. The Helmholtz equation for vector potential applies inside the highly conducting and permeable object. In both regions, the electromagnetic phenomena of interest are magneto-quasi-static (MQS). The simulation algorithm uses the method of auxiliary sources (MAS), with auxiliary magnetic charges and auxiliary magnetic current elements distributed on auxiliary surfaces. These surfaces generally conform to but do not coincide with physical surfaces, providing extraordinarily efficient and accurate 3-D solutions. Comparisons to available analytical solutions and experimental data validate the solutions. The simulations and data illuminate broadband MQS scattering phenomenology for both magnetic and nonmagnetic metallic objects. Distinctive sensitivities are shown and signature effects analyzed relative to the scatterer's shape and aspect ratio, orientation, sharp points and edges, finite wall thickness in hollow bodies, and compound structure in which a geometrically complex body consists of a number of distinct sections, e.g., fins.     

Electric field sensors in electromagnetic sounding

A staked and an insulated dipole antenna were used to monitor transmissions from a distant very-low-frequency transmitter. The field strength recorded from the staked antenna was found to be much larger than that recorded from the insulated antenna. It is shown that this can be adequately described as magnetic field induction. It is postulated that an insulated antenna is a far better electric field probe for surface impedance investigations including audio magnetotelluric (AMT), controlled source audio magnetotelluric (CSAMT), and VLF sounding techniques     

Multiple targets characterization of electromagnetic vulnerability

We use statistical selection methodology to characterize multiple targets in electromagnetic fields, which result from ground, weather, and sea clutter or internal electromagnetic interference due to the antenna subsystems. This electromagnetic interference occurs at the same frequencies as the target returns yet a specific statistical distribution in amplitude emerges over time. Holland and St. John [Statistical electromagnetics, Technical Report AFRL-DE-PS-TR-1998-1025, Air Force Research Laboratory, Direct Energy Directorate, Air Force Material Command, Kirtland Air Force Base, 1988] concluded that the observed EM field power fluxes and cable powers follow either a chi-square distribution or a log-normal distribution, which implies either distribution in all measurements due to internal electromagnetic interference (EMI) problems. That is such an EM field can be characterized by either an exponential distribution or a log-normal distribution. These cases exist also in a far field as a result of what the aircraft returns, while the distribution exists due to multiple reflecting surfaces instead of internal EM fields from radar subsystems previously mentioned. Clutter also produces these same distributions in the far field. For the exponential distribution case, we propose subset selection procedure to identify EM fields whose reference parameters are those of candidate targets rather than the interfering EM sources. We discuss the properties of our proposed procedure by numerical examples and give simulation examples to illustrate the procedure.     

Static induction transistor image sensors

New image sensors, based on the operational principle of static induction transistor (SIT), are described in this paper. Two operational modes of SIT image sensors are described here. One is the electron-accumulation mode in which electrons are stored in the floating-cell region and another is the electron-depletion mode in which electrons are removed from the floating-cell region in response to optical input. The electron-depletion mode is superior to the electron-accumulation mode in the charge retention characteristics, its temperature dependence, and the operational tolerancy. The described SIT image sensors, which utilize the vertically configured SITM structure, are very promising for application in very-large-area image converters due to high-speed high-packing density, and wide dynamic range, and especially due to extremely low power dissipation.     

分布式电磁矢量传感器阵的二维测向算法

采用由分布式电磁矢量传感器组成的稀疏线阵,针对相干信号提出了一种扩展孔径2维波达角(Two Dimensional DirectionofArrival,2D DOA)估计算法.通过构造增强矩阵来消除信号的相干性,利用稀疏线阵包含的向量特性、x轴和y轴空间相位因子获得高精度无模糊估计.与传统算法相比,算法不但保留了传感器的向量特性,而且增大传感器各分量之间的间距以及传感器之间的间距来获得2D孔径扩展,具有更高的测向精度.仿真验证了本文算法的有效性.     

Moto Kanda and I on electromagnetic sensors

This paper summarizes, with a short bibliography of major references, the work of Dr. Motohisa (Moto) Kanda and Carl Baum on electromagnetic sensors. This topic of common interest was the subject of many of our discussions over the years     

New sensors for measuring very short electromagnetic pulses

In order to detect very short electromagnetic pulses under 100 ps in width, a wide-band receiving antenna with a bandwidth from 0 to 14 GHz is needed. The usual short dipoles or monopoles do not provide both the required fidelity (bandwidth) and sensitivity. A novel type of electromagnetic pulse (EMP) sensor has been designed, analyzed, and tested. It is simple, inexpensive, and can detect the electromagnetic pulse with both high fidelity and high sensitivity. An approximate analysis is given for better understanding and optimum design of the proposed EMP antenna     

Efficient 2D DOA estimation of coherent signals in spatially correlated noise using electromagnetic vector sensors

This paper considers a new azimuth-elevation DOA estimation algorithm for multiple signals using electromagnetic vector sensor array. We firstly exploit the planar-plus-an-isolated sensor array geometry (Li et al. in IEE Proc Radar Sonar Navig 143(5):295–299, 1996) to define a full rank cross-covariance matrix. Then we develop an efficient ESPRIT-like algorithm using the so-called propagator to estimate the steering vectors of electromagnetic vector sensor, without performing eigen-decomposition into signal subspaces. Finally, we compute the vector cross product to obtain the closed-form azimuth-elevation angle estimates. The new algorithm does not require 2D iterative searching, and is applicable to coherent (fully correlated) signals and spatially correlated noise. In addition, the proposed algorithm offers enhanced estimation precision by sparse array aperture extension, but suffers no DOA cyclical ambiguity. Monte-Carlo simulations are presented to verify the effectiveness of the proposed algorithm.     

A comparison of algorithms for subsurface target detection andidentification using time-domain electromagnetic induction data

The performance of subsurface target identification algorithms using data from time-domain electromagnetic induction (EMI) sensors is investigated. The response of time-domain EMI sensors to the presence of a conducting object may be modeled as a weighted sum of decaying exponential signals. Although the weights associated with each of the modes are dependent on the target/sensor orientation, the decay rates are a function of the target's composition and geometry and therefore are intrinsic to the target. Since the decay rates are not dependent on target/sensor orientation or other unobservable parameters, decay rate estimation has previously been proposed as a viable method for target identification. The performance attained with Bayesian target identification algorithms operating on the entire time-domain signal and decay rate estimates is compared through both numerical simulations and application to experimental data. The decay rate estimates utilized in the numerical simulations are assumed to achieve the Cramer-Rao lower bound (CRLB), which provides a lower bound on the variance of an unbiased parameter estimate. The simulations as well as results obtained with experimental data show that processing the entire time-domain signal provides better target identification and discrimination performance than processing decay rate estimates     

电磁感应式无线充电发热与电磁辐射仿真研究

目前大多数对于无线充电电磁环境安全的研究都局限在电动汽车磁共振式无线充电,然而电磁感应式无线充电装置更接近人体,使用时间更长,本文针对电磁感应式无线充电导体异物易感应出涡流引起发热以及人体电磁辐射问题进行了仿真研究. 无线充电系统结构参考Qi标准,首先利用参数电路对负载电阻、线圈间距及偏移距离对传输效率和输出功率的影响进行仿真;随后使用确定的参数仿真研究不同材质的导体异物在30 min内的温升. 仿真结果表明:当导体异物正对线圈时温升明显,金银等材料的温度均可达100 °C以上;当导体异物不正对线圈时,温升在可接受范围内. 最后使用CST仿真得到电磁场强度、人体温度以及CEM43 °C热剂量值,以此来体现电磁感应式无线充电对人体电磁辐射的非热效应及热效应,电场最大值0.642 7 V/m,磁场最大值2.694 0 A/m,人体大脑温度最高37.3 °C,CEM43 °C热剂量值0.009 5,结果基本在正常参考值范围内. 本文研究有利于电磁感应式无线充电装置的推广应用.     

APPLICATION OF TRANSIENT ELECTROMAGNETIC PULSE IN DETECTING SUBSURFACE TARGETS

A basic scheme for detecting subsurface targets with nanosecond EM pulse and anespecially developed high quality travelling-wave antenna are described.The antenna is a sortof sector antenna with structure of three layers,which possesses higher radiation efficiency andbetter travelling-wave properties.A fine resolution graphic system and a high speed display areemployed in terminal processing.Metal pipes buried about 1 m under the earth can be detectedand clearly displayed.High resolution and short processing time of the system,compared withother similar devices,make it suitable for engineering use.     

一维FSS电磁散射宽带特性的快速计算

基于渐近波形估计（AWE）技术和矩量法（MOM），快速分析了一维频率选择表面（FSS）的宽带电磁散射特性，首先采用MOM法将平面波照射下FSS的电场积分方程（EFIE）转化为关于感应电流的矩阵方程，并由该方程确定频率导数矩阵方程（MEFD）；再在所考虑的频带内的某一给定频率处求解MEFD，得到给定频率处的频率导数感应电流；最后根据Pade逼近理论由给定频率处的频率导数感应电流确定周期性结构在任意频率入射波照射下的感应电流，根据FSS上的感应电流及谱域Floquet谐波模计算FSS的电磁散射宽带特性，计算结果表明，AWE能有效逼近MOM逐点扫描计算的结果，同时在计算速度上可加快十几倍。     

Miniaturised electromagnetic bandgap structures for broadband switching noise suppression in PCBs

A novel concept for ultra-wideband suppression of switching noise in high-speed printed circuit boards (PCBs) is proposed, implemented and tested. This concept consists of using non-symmetrical, embedded electromagnetic bandgap (EEBG) structures in conjunction with material with high dielectric constants. The proposed design modifies the classical EEBG structures to achieve a high degree of miniaturisation and an unprecedented broadband suppression of switching noise.     

Computer-aided design tool for electromagnetic sensors - [instrumentationnotes]

Numerical simulation is an invaluable tool for the designer of any device. By providing a model of the sensor within the design parameters, simulation can substantially speed up and simplify the design process and give a reliable estimation. The use of optimization algorithms can lead to much better sensor characteristics and reduce sensor costs and operation.     

Characterization of highly dispersive materials using compositecoaxial cells: electromagnetic analysis and wideband measurement

In this work, theoretical modeling and experimental characterization of composite coaxial-line cells are made for accurate dielectric measurement. The coaxial cells are designed such that highly dispersive materials can be characterized over a wide band of frequencies. In addition, a transmission line matrix (TLM) algorithm with a new type of node is proposed for electromagnetic analysis of the complex composite geometry of the measurement cells. Such a hybrid-mode simulation allows the electrical performance of the proposed cells to be determined for a variety of complex permittivity measurements. The proposed node in the cylindrical coordinate not only accommodates cylindrical coaxial discontinuities in the cell, but also describes the frequency-dependent properties of material. Experiments on a network analyzer were done with a number of APC-7 coaxial cells and various liquid samples to verify the wideband characteristics of the technique. It is found both theoretically and experimentally that the sample volume can be adjusted to ensure a wideband sensibility of the S-parameters. A sensitivity factor is also defined to characterize the frequency response of the coaxial cells. Such an electromagnetic analysis (direct problem) can be used in extracting unknown dielectric characteristics of a material (inverse problem) based on a recently proposed generic approach