Discrimination and classification of buried unexploded ordnance using magnetometry

Magnetic surveys are quite successful at locating buried steel objects, including unexploded ordnance (UXO). However, many of the anomalies apparent in a magnetic image of a contaminated area are from metallic debris, shrapnel, and geological variations in ferromagnetic concentration. Observations are usually made in the far-field of the object so that, in most cases, we can only recover the dipole moment of a buried item. Due to self-demagnetization effects, the magnitude and direction of induced magnetism varies significantly with ordnance orientation. This results in an infinite number of ordnance-like objects that can reproduce a given dipole moment. To discriminate, we define a library of ordnance items expected to occur in the area and find how closely each recovered moment matches one of the UXOs in this library. We define the size of this mismatch as the remanent magnetization and produce a prioritized dig-list on the assumption that items with lower remanence are more likely to be UXO. Such a ranking scheme proves to be very effective when implemented at two sites in Montana. The analysis reveals that live-site and emplaced UXO have significantly different remanence and implies that previous tests of magnetic discrimination performance on seeded sites have been overly pessimistic.     

基于组合矩的激光雷达距离像目标识别

激光成像雷达距离像与目标表面物理结构特性密切相关,体现目标的本质特征,是目标识别的主要研究方向。采用组合矩的神经网络方法进行了相干激光雷达距离像目标识别仿真研究。用Hu不变矩和仿射不变矩两者的低阶矩组合表示距离像目标区域特征,利用反向传播(BP)神经网络识别不同方位角的车辆。当视场角不变时,训练10个目标,每个目标取3～19个样本,在不同载噪比(CNR)情况下,分析Hu不变矩、仿射不变矩和两者组合矩的识别率。理论分析和仿真实验表明利用组合不变矩进行距离像目标识别性能优于单独利用其中一种不变矩。     

Simple estimation of equivalent magnetic dipole moment tocharacterize ELF magnetic fields generated by electric appliancesincorporating harmonics

A simple method of quantifying the ELF (extremely low frequency) magnetic field distribution around electric appliances, which takes the harmonics into account, is newly proposed. The proposed method involves: (1) a simple estimation of the position of an equivalent magnetic dipole moment inside an appliance, using two magnetic field meters; (2) identification of the amplitude of the dipole moment magnetic-field measurements at certain points; and (3) calculation of the magnetic field distribution around the appliance using the estimated dipole moment. In this method, the dipole moment vector is assumed to be a similar value by allowing an uncertainty of 6 dB in the estimated magnetic field, which enables easy estimation of the dipole moment. In addition, the frequency characteristics of the magnetic field are taken into account by considering the harmonic components in the magnetic field waveform. The proposed method was applied to 13 types of appliances, and their equivalent magnetic dipole moments and harmonic components were determined. The results revealed that the proposed method is applicable to many electric appliances. The conditions required for the adoption of the method were also clarified     

RFID tags to aid detection of buried unexploded ordnance

The difficulty in reliably and efficiently detecting and locating buried unexploded ordnance (UXO) is a concern for the military. Tagging munitions and ordnance items before they are fired or launched would allow for a significantly more efficient means of locating buried UXO. Munition tagging can be accomplished by employing existing passive radio-frequency identification (RFID) tag technology, such as the Texas Instruments' solenoidal tags. This tagging approach would result in very low falsealarm rates compared to those currently experienced. The tags provide information on the munition's location and identity when the UXO tag interrogation module is brought nearby. This paper summarizes the analytical and experimental efforts Battelle pursued to reach these conclusions. Analytical efforts included modeling of the magnetic field's behavior to understand the requirements to transmit energy from the above-ground interrogator to the tag and from the tag back to the above-ground receiver. Laboratory work validated the modeling results and showed the feasibility of the technique. Field testing of a prototype system indicated the likely success of this approach to detecting buried UXO.     

Field Measurements of Induced and Remanent Moments of Unexploded Ordnance and Shrapnel

Magnetometry is a widely used technique for clearance of areas contaminated with unexploded ordnance. The discrimination of hazardous ordnance from nonhazardous items is possible using apparent remanence if ordnance is shock demagnetized on impact with the ground. We developed a mobile device, the Magnetic Remanence Interrogation Platform (MRIP), for measuring the induced and remanent moments of steel samples. The MRIP comprises six three-component fluxgate magnetometers symmetrically distributed around a rotating sample holder. Samples are placed on the holder and are slowly spun through two complete rotations. The measurement is repeated after the sample is physically rotated by 90$^{circ}$ so that the previous up direction points east. The MRIP platform was deployed to two field sites. At the first site, 76- and 90-mm projectiles were measured. Most had relatively low remanent magnetization relative to the induced, suggesting that shock demagnetization occurs. Variations in the remanent moments of different specimens of 90-mm projectiles were attributed to variations in the type of steel used. There was a strong correlation of the direction of remanent and inducing field during burial, indicating evidence of shock magnetization and/or magnetic viscosity. At the second site, intact and partial 4.2-in mortars as well as shrapnel, base plates, and cultural debris were measured. Most of the base plates and intact mortars had low remanent magnetization, whereas the shrapnel and partial mortars tended to have higher remanent magnetization. The results indicate that there is an inherent risk in using apparent remanence for discrimination as not all ordnance is demagnetized on impact.      

Higher-Order Evaluation of Dipole Moments of a Small Circular Disk for Arbitrary Incident Fields (Correspondence)

In a recent note the induced electric and magnetic dipole moments P and M due to the diffraction of a plane wave on a small circular disk were given. The expression for the electric dipole moment holds, however, only if the electric field vector is parallel to the plane of incidence.     

Magnetic models of unexploded ordnance

Magnetometry is widely used for the characterization of areas contaminated by unexploded ordnance (UXO). To successfully discriminate hazardous UXO from nonhazardous clutter requires accurate models of the ordnance response. This paper develops an ordnance library with 15 different items using total-field magnetic data collected over a test stand. The induced and remanent magnetizations were obtained by varying the three-dimensional orientation of each item and measuring the magnetic field on a horizontal plane in the dipolar regime. Replicate measurements using multiple specimens of the same ordnance returned very similar induced magnetizations. The fitted moments were used to estimate the detection depths for different sensor noise floors. A prolate spheroid with a 3.5 aspect ratio was used to provide a good approximation to the detection depths for many of the ordnance items. Assuming a 1-nT noise floor, these orientation-dependent detection depths ranged from 10 to 17 times of the object's diameter.     

Investigation of a Lorentz force biomagnetometer

This work evaluates an approach to the noninvasive measurement of small ionic current flows by a technique of Lorentz force magnetometry. An instrument was constructed that is basically a very sensitive force-balance that can measure Lorentz forces experienced by ionic currents flowing in small objects when exposed to strong oscillating magnetic fields. For objects that can fit on a microscope slide, the system is sensitive to ion current dipole moments as low as 180 pA-m. Images were made of ionic currents flowing in thin profiles by a process of scanning a localized magnetic field over the object, measuring generated Lorentz forces, and using a computer to reconstruct images. It can be shown that this method of Lorentz magnetometry has an immunity to ambient magnetic noise and has system characteristics that might suggest its possible use in biomagnetometry of small thin specimens     

Environmental characterization with magnetics and STOLS

The application of techniques utilizing total field magnetic anomaly measurements can contribute significantly to the environmental characterization of lands contaminated with hazardous, toxic, or radioactive wastes. By addressing the correlated problem of detection and characterization of HTRW waste containers, predominantly made of ferromagnetic materials, high-density magnetics systems can be used to characterize a wide range of buried hazardous materials. Ferromagnetic materials such as iron and steel, induce an observable secondary magnetic field which can be measured and modeled to yield estimates of location, depth, and mass of the ferromagnetic objects. As a passive geophysical technique magnetics is logistically tractable, allowing for development of systems which acquire high density data over large areas. The STOL magnetics detection system was developed for this purpose, and is outfitted with seven total field magnetometers which are integrated with a differential GPS to provide 20 Hz magnetic-field measurements positioned with nominal absolute position accuracy of 30 cm. Processing of magnetic data requires the removal of the Earth's primary field and the interpolation onto a mesh. Image processing techniques are applied to enhance and segment the data for anomaly delineation. Isolated anomalies are modeled under the point dipole assumption to produce estimates of object position, depth, and mass     

Application of the method of auxiliary sources to the wide-bandelectromagnetic induction problem

The Method of Auxiliary Sources (MAS) is formulated and applied to solution of wide-band electromagnetic induction problems involving highly conducting and possibly permeable metallic objects. Improved remote sensing discrimination of buried unexploded ordnance (UXO) motivates the study. The method uses elementary auxiliary magnetic charges and magnetic current elements to produce the unknown field. Auxiliary sources are located on virtual surfaces that usually conform to but do not coincide with the real surface of the object. Once the source coefficients are determined, the-secondary field can easily be found. The method involves no confrontations with source or Green's function singularities. It is capable of treating penetrable as well as nonpenetrable objects. Because the solution is composed of fields that automatically satisfy the governing equations, by construction, all approximation resides only in the enforcement of boundary conditions at matching (collocation) points. Accuracy in satisfying the boundary conditions can be evaluated explicitly using noncollocation points over the surface. This in turn allows one to identify problem areas on the surface and make intelligent adjustments of the source distributions, to improve solutions at minimal cost. A general 3D formulation is presented, and a version specialized to treat bodies of revolution is applied in the specific test cases discussed     

Effects of the ground surface on polarimetric features of broadbandradar scattering from subsurface metallic objects

Throughout the world, the problem of buried unexploded ordnance (UXO) poses an enormous, persistent, and expensive problem. While UXO generally consists of sizable bodies of ferrous metal and can therefore be detected, with current technology it is extremely difficult to distinguish them reliably from typically widespread pieces of clutter. Thus the problem is one of subsurface discrimination. The authors previous modeling work on scattering of ground penetrating radar (GPR) from metallic objects surrounded by an infinite soil-like medium has suggested the utility of a number of key discriminants in broadband fully polarimetric sensing. In particular, resonance structure, induced field rotation and ellipticity, and bistatic observation of scattered signals were shown to offer key information about target shape and size. The authors investigate the effects on signature features of the proximity of a ground surface to the target, for the common case of shallow burial (<1 m). Overall, their analyses suggest that the key discriminants seen in scattering in an infinite medium survive the complex interactions with the ground surface. In some instances, these revealing signatures appear to be strengthened by the presence of a nearby surface. Multiposition backscatter also allows fundamental inferences about target elongation and symmetry when those cannot be obtained from single position viewing     

Mutual induction and the effect of host conductivity on the EM induction response of buried plate targets using 3-D finite-element analysis

A finite-element analysis of electromagnetic induction (EMI) in the presence of multiple buried metal targets is undertaken for the purpose of unexploded ordnance (UXO) detection and discrimination. The effects of mutual coupling between metal targets and the host conductivity are shown to be important. At high frequencies, mutual coupling is strong, and effects of host conductivity are relatively minor. At lower frequencies near the resistive limit, EMI responses are very small, but the effect of host conductivity becomes important. This is due to the galvanic current flow in the host medium that dissipates charge accumulations on the host/target interfaces. Qualitative analysis of induced current patterns in metal targets demonstrates that mutual coupling is strongly affected by target orientation and skin depth. Rigorous forward modeling of EMI responses is essential to understanding UXO sensor signatures so that discrimination between live UXO items and harmless fragments and clutter may become possible.     

Buried unexploded ordnance identification via complex naturalresonances

A classification technique focused on the identification of buried unexploded ordnance (UXO) using complex natural resonance (CNR) signature is considered. The total least square (TLS) Prony's (1795) method is used to extract CNRs from time-domain data, Full-scale UXO computational models and the body of revolution moment method (BORMM) code is used to obtain the backscattered fields, which are then used to give the theoretical free-space CNRs. Baum's (1993) transformation is used to relate a CNR in a lossy simple medium to the corresponding CNR in free space. A practical UXO classification yields an estimate of the UXO length from its CNR information. Successful UXO classification examples from actual measured data are presented     

Vehicle detection using a magnetic field sensor

The measurement of vehicle magnetic moments and the results from use of a fluxgate magnetic sensor to actuate a lighting system from the magnetic fields of passing vehicles is reported. A typical U.S. automobile has a magnetic moment of about 200 A-m²(Ampere-meters²), while for a school bus it is about 2000 A-m². When the vehicle is modeled as an ideal magnetic dipole with a moment of 200 A-m², the predicted results from an analysis of the sensor-vehicle geometry agree closely with observations of the system response to automobiles.     

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.     

并行高阶矩量法分析大型阵列天线的辐射特性

随着计算机多核技术和计算机集群技术的发展,并行高阶矩量法可以解决复杂大型目标的电磁特性。文中首 先对单个天线单元在高阶矩量法与有限元方法下的结果进行对比,验证了本文方法的正确性和可行性,然后给出了一 个36×12 单元的伞形印刷振子天线阵列的方向图结果,从而说明了本文方法可以处理实际工程中的大规模挑战性问 题。     

Nonvolatile Magnetoelectric Switching of Magnetic Tunnel Junctions with Dipole Interaction

The magnetoelectric effect is technologically appealing because of its ability to manipulate magnetism using an electric field rather than magnetic field or current, thus providing a promising solution for the development of energy-efficient spintronics. Although 180° magnetization switching is vital to spintronic devices, the achievement of 180° magnetization switching via magnetoelectric coupling is still a fundamental challenge. Herein, voltage-driven full resistance switching of a magnetic tunnel junction (MTJ) with dipole interaction on a ferroelectric substrate through switchable parallel/antiparallel magnetization alignment is demonstrated. Parallel magnetization alignment along the y direction is obtained under a bias magnetic field. By rotating the magnetic easy axis via strain-mediated magnetoelectric coupling, the parallel magnetizations in the MTJ reorient to the x axis with opposite paths because of dipole interaction, thus resulting in antiparallel alignment. Moreover, this voltage switching of MTJs is nonvolatile owing to variations in dipole interaction and can be well understood via phase field simulations. The results provide an avenue to realize electrical switching of MTJs and are significant for exploring energy-efficient spintronic devices.     

基于模拟退火算法的舰船磁场高精度建模方法

在舰船消磁与防护中,需要利用混合模型对舰船磁场的空间分布规律进行数学建模,而混合模型将磁偶极子阵列等距分布忽视了舰船结构的影响,且没有考虑测量数据的真实性,导致所建模型精度不高、稳定性较差.文章以混合模型系数矩阵条件数为目标函数,采用模拟退火算法（Simulated Annealing algorithm,SA）优化系数矩阵条件数,得到磁偶极子阵列的最优位置分布;然后通过考察单个点的拟合情况剔除误差大的测量点,优化测量数据;最后利用逐步回归求解模型方程,获得舰船磁场高精度稳定模型.实测数据建模表明：与磁偶极子等距分布的混合模型建模相比,本文方法所建模型精度可达96%,对测量数据误差的敏感性低,抗干扰能力强,具有较好的鲁棒性.本方法可用于复杂海况背景下的舰船磁场高精度建模,以及水面磁性目标探测与定位系统.     

Detection of Deeply Buried UXO Using CPT Magnetometers

The location and identification of unexploded ordnance (UXO) from magnetometer measurements have long been an area of active research interest. Ground surface methods, which are widely used for objects buried in shallow depths, become ineffective for objects deep in the soil because the object's anomalous field diminishes rapidly with distance, and interference by metal debris on the ground surface is significant. For the location and identification of deeply buried UXO, a total-field magnetometer equipped with a cone penetrometer technology (CPT) drilling system is employed in this paper. A novel inversion method is developed to extract information from individual and multiple CPT profiles. Compared with conventional inversion methods providing a single minimum as an estimate, this approach tests the likelihood distribution over the anticipated solution space, hence providing reliability. Real-world data are collected from a test site for the location and identification of a specific target. All experiments and simulations demonstrate the superior capability of this technique in solving real-world problems when only limited data points are available