Accounting for the effects of widespread discrete clutter in subsurface EMI remote sensing of metallic objects

In practice, most signal processing strategies for discrimination of buried objects are clutter limited. This applies even to discrimination of shallow sizable metallic objects, such as unexploded ordnance (UXO), which are to be found predominantly in the top meter of soil. The environment typically features widespread metallic clutter from detonated ordnance or other sources. Such fragments can be numerous and are often shallower than the objects of interest. Currently, the preeminent remote sensing mode for buried UXO is ultrawideband electromagnetic induction (EMI), operating over part or all of the band from some tens of hertz up to perhaps hundreds of kilohertz. Particularly because EMI fields fall off sharply with range, signals from shallow clutter may be relatively strong and can easily obscure essential scatterer signatures. To treat this, a rational theory of EMI scattering from widespread metallic clutter is formulated and tested. For dense, well-distributed clutter, analytical rules are derived for dependence of signal strength on sensor elevation, under various fundamental excitation types. For more erratic, sparse clutter distributions, signal statistics from Monte Carlo simulations show patterns like those from the analytical rules. The dependence of clutter signal magnitude on antenna elevation is determined for both thin surface layers and for volume layers of widespread small items, and for both dense and sparse clutter distributions. These are contrasted with the patterns expected from single, larger, discrete objects of interest, and the contrast is exploited in discrimination exercises for the screening problem. For sparse clutter distributions, results from inversion processing formulations that account for the patterns of clutter statistics are compared to simple least squares treatments.     

Eddy-current and current-channeling response to spheroidal anomalies

We derive the electromagnetic induction (EMI) response to conducting anomalies of spheroidal shape embedded in a weakly conducting host medium. As one application, prolate and oblate spheroids are convenient shapes for modeling buried unexploded ordnance, landmines, and naval mines. Under the assumptions of a perfectly conducting or nonconducting target and the assumption of a uniformly incident field, simple analytical solutions are obtained. Both the eddy-current and the current channeling responses are derived for prolate and oblate spheroids. For example, in a marine environment the current-channeling contribution can be quite large. As expected, these responses differ depending on the direction of the incident field relative to the spheroidal symmetry axis. The problem is solved separately for axial and transverse incidence, with the general case being a superposition of the two. These solutions could be used for fitting a spheroidal model to EMI data in which a small number of parameters, such as the location, size, shape, and orientation of the object, are to be determined.     

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     

Transient response computation of spheroidal objects using impedance boundary conditions

The transient response of imperfectly conducting and permeable spheroidal objects is determined using the impedance boundary conditions. Since the impedance boundary conditions are not known in the time domain, the frequency domain analysis is used to formulate the problem. The use of impedance boundary conditions relates conveniently the solution to the object's physical parameters and simplifies the computations. For highly conducting objects a simplified method is used to determine numerically the frequency domain data, which utilizes the numerical code for perfectly conducting objects. The excitation is assumed to be due to a periodic pulse train and generated by a small circular loop antenna. The procedure for the computation of the transient response, at very low excitation frequencies, is presented and the response forms for different object parameters and orientations are computed.     

On the Choice Between Transmission Line Equations and Full-Wave Maxwell's Equations for Transient Analysis of Buried Wires

In this paper, we evaluate the validity of transmission line (TL) solutions in the study of interaction of lightning transients with buried wires. The considered transients have frequencies between a few kilohertz to a few megahertz with risetimes 0.1-10 mus. Comparative simulations using TL equations and full- wave Maxwell's equations are carried out in the paper, and the solutions to both the equations are based on the finite-difference time-domain method. It is found that TL solutions are sufficiently accurate for lightning transient analysis of buried wires. It is also claimed that the TL approach remains valid for all transients having frequencies lower than those of lightning. TL solutions are computationally efficient, particularly when dealing with distributed power and railway systems. The TL approach is valid as long as the transverse electromagnetic mode (TEM) is dominant. However, other modes of propagation, classified as antenna modes, might be present depending upon the type of excitation source, its location, frequency, and the associated media. A possible approximate formula for the frequency above which the validity of TL solutions for buried systems is questionable is proposed based on the concept of penetration depth of fields into the soil. Discussions presented in the paper could motivate the application of TL solutions for electromagnetic transient analyses of the buried conductors of power, railway, and telecommunication systems.     

Quasi-magnetostatic solution for a conducting and permeablespheroid with arbitrary excitation

Broad-band electromagnetic induction (EMI) methods are promising in the detection and discrimination of subsurface metallic targets. In this paper, the quasi-magnetostatic solution for a conducting and permeable prolate spheroid under arbitrary excitation by a time-harmonic primary field is obtained by using the separation of variables method with vector spheroidal wave functions. Numerical results for the induced dipole moments are presented for uniform axial and transverse excitations, where the primary field is oriented along the major and minor axis of the prolate spheroid, respectively. They show that the EMI frequency responses are sensitive to the orientation and permeability of the spheroid. An approximation is also developed that aims to extend the exact solution to higher frequencies by assuming slight penetration of the primary field into the spheroid. Under this approximation, a system of equations that refers only to the external field expansions is derived. It is shown that, for spheroids with high relative permeability, this approximation is in fact capable of yielding an accurate broad-band response even for highly elongated spheroids     

Support Vector Machine and Neural Network Classification of Metallic Objects Using Coefficients of the Spheroidal MQS Response Modes

Two different supervised learning algorithms, support vector machine (SVM) and neural networks (NN), are applied in classifying metallic objects according to size using the expansion coefficients of their magneto-quasistatic response in the spheroidal coordinate system. The classified objects include homogeneous spheroids and composite metallic assemblages meant to resemble unexploded ordnance. An analytical model is used to generate the necessary training data for each learning method. SVM and NN are shown to be successful in classifying three different types of objects on the basis of size. They are capable of fast classification, making them suitable for real-time application. Furthermore, both methods are robust and have a good tolerance of 20-dB SNR additive Gaussian noise. SVM shows promise in dealing with noise due to uncertainty in the object's position and orientation.     

Electromagnetic induction spectroscopy for clearing landmines

An estimated 110 million landmines, mostly antipersonnel mines laid in over 60 countries, kill or maim over 26000 people a year. One of the dilemmas for removing landmines is the amount of false alarms in a typical minefield. Broadband electromagnetic induction spectroscopy (EMIS) is a promising technology that can both detect and identify buried objects as landmines. By reducing the number of false alarms, this approach significantly reduces costs associated with landmine removal. Combining the EMIS technology and a broadband EMI sensor, the scientific phenomenology that has potential applications for identifying landmines, unexploded ordnance, and hidden weapons at security checkpoints can now be explored     

Optimization of pulse radiation from dipole arrays for maximum energy in a specified time interval

The optimum solution for maximized radiated energy in a specified time interval from anN-element dipole array at a specified farfield position is presented. The solution is obtained in terms of time-domain dipole terminal voltages which are constrained in bandwidth and total input energy, with the currents on the dipoles satisfying Pocklington's integral equation. The orthogonality of characteristic terminal modes is used in the derivation of the optimum solution, and the far-zone field is expanded as a finite sum of characteristic modal fields with unknown mode coefficients. The optimum mode coefficients are found in terms of prolate spheroidal wave functions. An additional constraint can be used to find the optimum solution with a reduced sidelobe level. The effects of signal bandwidth and time interval specification on the peak field intensity and energy density in the direction of optimization are shown and limiting cases are found to agree with previous results.     

Modal analysis for a bounded-wave EMP Simulator - part II: radiation leakage and mode suppression

Radiation leakage from a high-power bounded-wave electromagnetic pulse simulator can cause severe electromagnetic interference (EMI) with surrounding equipment. In Part I, we analyzed the electromagnetic mode structure inside a simulator. In Part II, we examine the relative contribution of these modes to radiation leakage. We have calculated the temporal cross correlation between the electromagnetic modes inside the simulator and the temporal waveform of the forward-radiated far field. The TM1 and TM2 modes show strong cross correlation over a range of switching times. For the TEM mode, on the other hand, this holds true only for larger values of the switching time. Hence, for a given mode amplitude, higher order TM modes tend to produce more leakage than the TEM mode. However, since TEM is far stronger than TM throughout the simulator, it may still contribute more in absolute terms. Increasing the angle between simulator plates excites more higher order modes and increases their strength, thereby enhancing leakage. Pulse compression with respect to time increases the relative strength of higher order modes. Placing a spatial mode filter inside the simulator significantly reduces TM/sub 1/ without modifying the desired TEM mode. However, TM/sub 2/ remains largely unchanged. The reduction in TM/sub 1/ depends sensitively upon the parameters of the filter. These observations illustrate a new method of understanding and improving simulator design from the point of view of EMI.     

Spherical Vector Wave Expansion of Gaussian Electromagnetic Fields for Antenna-Channel Interaction Analysis

In this paper, we introduce an approach to analyze the interaction between antennas and the propagation channel. We study both the antennas and the propagation channel by means of the spherical vector wave mode expansion of the electromagnetic field. Then we use the expansion coefficients to study some properties of general antennas in those fields by means of the antenna scattering matrix. The focus is on the spatio-polar characterization of antennas, channels and their interactions. We provide closed form expressions for the covariance of the field multimodes as function of the power angle spectrum (PAS) and the channel cross-polarization ratio (XPR). A new interpretation of the mean effective gains (MEG) of antennas is also provided. The maximum MEG is obtained by conjugate mode matching between the antennas and the channel; we also prove the (intuitive) results that the optimum decorrelation of the antenna signals is obtained by the excitation of orthogonal spherical vector modes.      

Single-mode operation of 1.3 µm InGaAsP/InP buried crescent lasers using a short external optical cavity

Highly single longitudinal mode operation of 1.3 μm InGaAsP/InP buried crescent lasers has been obtained using a60-600 mum long external cavity defined by a spherical mirror. The intensity of the neighboring modes was suppressed by a factor of 1200 for dc excitation and by a factor of 800 for combined dc and 250 MHz current modulation.     

Transient coupling analysis using the singularity expansion method

The problem of electromagnetic coupling between objects has been extensively studied within the context of continuous-wave (CW) signals. The problem of coupling between objects for transient excitation has not received as much attention. This paper addresses the problem of transient coupling using the singularity expansion method (SEM). Specifically, the proper physical interpretation of the SEM modes of a system of objects is discussed within the context of observed transient surface currents and scattered fields. It is seen that the SEM modes of the system of objects are global quantities and hence have no clear physical interpretation prior to times when global modes can be established. It is also apparent that the early-time response of the system can be expressed using the SEM modes of the individual objects for some time periods. These observations are important for the effective use of target identification schemes based on natural resonance phenomena in multiple target situations as well as the proper use of SEM in EMC coupling analyses     

Network-theoretical model of the gyrotron oscillator part II: Empty cavity oscillation modes

A network-theoretical model of the gyrotron has been elaborated which is both conspicious and rigorous. The electromagnetic field in the presence of the electron beam, to be determined in a self-consistent calculation, might be regarded as an abstract vector in infinite-dimensionalHilbert space. In the first part of this paper, a method was described to determine the oscillation modes of empty gyrotron cavities. Once these oscillation modes are known, their fields can be used as the “coordinate system” for the representation of this “vector”. In this representation,Maxwell's equations reduce to a system of ordinary differential equations — the so-called “resonator equations”, describing the linear dependence of the field amplitudes on the excitation coefficients. They are readily solved in the frequency domain. The solution can be represented as a parallel resonance circuit, if the mode amplitudes are identified with equivalent voltages, and the exciation coefficients with equivalent currents. The nonlinear dependence of the excitation coefficients on the mode amplitudes, on the other hand, is then identified with the device (or electron beam) admittance. The latter is determined numerically, for a number of points in the amplitude-frequency plane, and subsequently represented in the frequency domain. In order to efficiently compute the device admittance, the coordinates are transformed into the electron beam frame, and afterwards an averaging method is applied. The interaction of these admittances determines the behaviour of the gyrotron.     

Radio waves in an irregular spheroidal model of the earth-ionosphere waveguide

Describes a program to compute the amplitude and phase anomalies of the electromagnetic fields in a perturbed spheroidal model of the Earth-ionosphere waveguide characterized by an effective height that varies along the propagation path, and also to trace the loci of the mode numbers in the irregular region of the spheroidal waveguide and to compute the transmission and reflection scattering coefficients. The language used is Fortran IV     

Improved Pseudospectral Mode Solver by Prolate Spheroidal Wave Functions for Optical Waveguides With Step-Index

We propose a set of more efficient basis functions prolate spheroidal wave functions of order zero, comparing with the conventionally used Chebyshev polynomials expanding the optical fields in interior subdomains, to improve significantly the convergence rate of pseudospectral mode solver for solving dielectric optical waveguides with step-index. The bandwidth parameter, which influences strongly the computational accuracy, in prolate spheroidal wave functions is also optimized in this work. First, the numerical examples of two-dimensional waveguides show that the new basis functions achieve faster convergence than Chebyshev polynomials. Furthermore, a 3-D rib waveguide based on the full-vectorial formulations demonstrates that the proposed approach reduces 23% of computational time and 25% of memory storage for obtaining the convergent values of the effective index for the tenth order quasi-TE and quasi-TM modes.     

Theoretical analysis and range of validity of TSA formulation for application to UXO discrimination

Operating in the magnetoquasistatic regime (a few hertz to perhaps a few 100 kHz), electromagnetic induction (EMI) sensing has recently emerged as one of the most promising avenues for discrimination of subsurface metallic objects, e.g., unexploded ordnance. The technique of thin-skin approximation (TSA) was devised to deal with numerical problems caused by the rapid decay of fields beneath the scatterer's surface. The rather nonintuitively broad applicability and specific error patterns of the TSA formulation are explained here by theoretical analysis based on analytical solutions and approximate Monte Carlo simulation. In the limiting case of infinitesimal skin depth (EMI perfect reflection), the scatterer aspect ratio (AR) is inferred without regard to metal type. Alternatively, the AR of some homogeneous magnetic objects is inferred from the pattern of transverse to axial response ratio over the entire EMI ultrawideband. Use of the method in inversions for electromagnetic parameters reveals fundamental nonuniqueness problems and shows their basis, which is not dependent on the method of forward solution.     

面向光纤多模选择性激发的电磁超表面设计

针对光纤中多个模式的选择性激发问题,提出利用介质超表面实现光纤多模的频率选择性激发方案. 将超表面结构分为四个象限区,对前三象限超表面结构采用便利搜索法设计,对第四象限结构采用深度学习方法进行反向设计,实现了频率控制两个模式的模分复用. 该方案为实现光纤多模复用提供了一种新的思路,且选择性激发的模式个数具有可拓展性;在通信领域具有潜在的应用前景.     

副瓣对消技术在抑制雷达间电磁干扰中的应用

为了更好地抑制地面雷达之间的电磁干扰,通过与随队干扰的干扰样式进行对比,得出了雷达间电磁干扰信号主要通过天线副瓣耦合进入接收机的特点,并在此基础上将副瓣对消技术引入到抑制雷达间电磁干扰中,分析了副瓣对消的基本原理,并采用相关器和增益放大器来自动调整权系数.最后对副瓣对消技术的抗干扰效能进行了验证,实验结果表明:采用副瓣对消技术后的雷达抗干扰效能提高了20.1 dB,可以有效地抑制雷达间电磁干扰.     

两媒质半空间复杂形体电磁散射

本文利用矢量波函数变换方法讨论了两媒质半空间的电磁散射问题，从Ｍａｘｗｅｌｌ方程出发，讨论了单矩法在三维复杂形体散射问题上的实施。并在数学球面上将内部区域的有限元解与外部区域矢量波函数变换的结果相匹配，从而得到复杂埋入体的电磁散射特性。作为检验和示例，本文计算了在平面波照射下自由空间导体球，埋入导体球，埋入介质覆盖钝锥等的散射场，其中一些结果与可供比较的经典解或其它算法的结果进行了比较，吻合较好。