一种用于准单站配置的步进频探地雷达建模方法

探地雷达是一种被广泛使用的无损检测技术。利用探地雷达对分层媒质进行全波反演时,构建精确的探地雷达正演模型具有十分重要的意义。该文提出一种可用于准单站配置的步进频探地雷达的建模方法。在该模型中,探地雷达系统及其与分层媒质间的相互作用被表示成线性方程,天线对雷达信号的影响被表示为只与频率有关的传输函数。为验证模型准确性,该文在实验室条件下搭建了准单站配置的步进频探地雷达系统,并对已知厚度的石膏板和木板的雷达测量信号进行全波反演。反演结果表明:石膏板和木板的厚度估计误差均不超过0.3 mm,验证了所提出的正演模型具有高准确度。利用石膏板和木板搭建分层模型,该文进一步比较了准单站配置和单站配置步进频探地雷达系统对介电常数差异较小的分层媒质的反演性能。实验结果表明:准单站配置探地雷达能获得更精确的反演参数。通过对分界面反射信号的信噪比估计可知,准单站配置比单站配置探地雷达系统能获得高出约10 dB的信噪比,因此具有更好的反演性能。      

Finite-difference time-domain simulation of scattering from objectsin continuous random media

A three-dimensional (3D) finite-difference time-domain (FDTD) scheme is introduced to model the scattering from objects in continuous random media. FDTD techniques have been previously applied to scattering from random rough surfaces and randomly placed objects in a homogeneous background, but little has been done to simulate continuous random media with embedded objects where volumetric scattering effects are important. In this work, Monte Carlo analysis is used in conjunction with FDTD to study the scattering from perfectly electrically conducting (PEC) objects embedded in continuous random media. The random medium models under consideration are chosen to be inhomogeneous soils with a spatially fluctuating random permittivities and prescribed correlation functions. The ability of frequency averaging techniques to discriminate objects in this scenarion is also briefly investigated. The simulation scheme described in this work can be adapted and used to help in interpreting the scattered field data from targets in random environments such as geophysical media, biological media, or atmospheric turbulence     

Linear tomographic inversion of stepped-frequency GPR data: experimental results on two test-sites

This paper deals with the tomographic inversion of experimental data acquired by a stepped-frequency ground penetrating radar (SFGPR).The experimental SFGPR has been designed for archaeological prospecting and makes it possible to exploit a multi-view/multi-static/multi-frequency configuration. The tomographic algorithm is based on a linear model of the electromagnetic scattering to reconstruct the shape of strongly scattering targets. It has been already validated against experimental data for objects in free-space and synthetic data for buried scatterers.Here, we present experimental results for buried objects starting from measurements collected at two test-sites. The first one has been a dedicated outdoor test facility which has allowed to test the experimental setup and the inversion algorithm in controlled conditions; the second one has been an archeological test-site.     

COMPLEX RAY ANALYSIS OF SCATTERING FROM RAM COATED TARGETS

With the extension of complex ray theory(CRT)from lossless media into lossy ones,the complex ray analysis in lossy media can be constructed and effectively used in the problemsof scattering analysis of objects coated with radar absorbing materials(RAM).This method hassome advantages,such as simple calculation procedure,clear physical meaning,wide applicationarea etc.As an example,the radar cross-section(RCS)of conducting flat plates coated withRAM is analyzed.The results show that the theoretical predictions agree very well with theexperimental results.     

MoM Analysis of Dipole Antennas in Crosshole Borehole Radar and Field Experiments

In this paper, we propose a method-of-moments (MoM) analysis that includes the borehole effects on crosshole borehole radar, and we verify that the MoM represents the crosshole borehole radar data correctly. We derive the far-field approximation of an electric field radiated by an electric point source. In this derivation, we assume that the point source is in a cylindrically layered medium, while the observed electric field is in another cylindrically layered medium. The equations that are derived are used as a transfer function to connect the two impedance matrices - one for the transmitting antenna and the other one for the receiving one. In these matrices, the influence of the scattered field by the borehole is included completely. We conducted field experiments in granite with a controlled borehole medium in order to investigate the influence of the borehole. In this paper, we estimated the relative amplitude and the group delay of the received signals between the water-filled borehole case and the air-filled borehole case. This paper showed that the estimated amplitude and delay depend on the frequency and that the proposed MoM could predict these frequency dependencies.     

吸波涂层目标散射特性的复射线分析

将复射线理论推广到媒质有耗的情形,可以建立起损耗媒质中的复射线方法并用来分析吸波涂层目标的散射特性。该方法具有计算简便、物理概念明确、适用范围较广等特点。本文以涂层金属平板为例所进行的目标雷达截面分析结果表明,理论计算与实验结果能够很好地吻合。     

全波形机载激光雷达数据处理技术的研究进展

全波形机载激光雷达可以直接快速获取地面特征点的3维空间位置,相比于其它测绘方法,全波形机载激光雷达在一定程度上性能更优。首先介绍了全波形机载激光雷达激光测距的工作原理,随后讨论了全波形机载激光雷达数据处理的一般性方法,并总结了其数据处理技术国内外研究现状的主要进展,最后归纳了全波形激光雷达数据处理研究中的关键问题,在此基础上对其数据处理的研究前景进行了展望。     

A curve evolution approach to object-based tomographic reconstruction

We develop a new approach to tomographic reconstruction problems based on geometric curve evolution techniques. We use a small set of texture coefficients to represent the object and background inhomogeneities and a contour to represent the boundary of multiple connected or unconnected objects. Instead of reconstructing pixel values on a fixed rectangular grid, we then find a reconstruction by jointly estimating these unknown contours and texture coefficients of the object and background. By designing a new "tomographic flow", the resulting problem is recast into a curve evolution problem and an efficient algorithm based on level set techniques is developed. The performance of the curve evolution method is demonstrated using examples with noisy limited-view Radon transformed data and noisy ground-penetrating radar data. The reconstruction results and computational cost are compared with those of conventional, pixel-based regularization methods. The results indicate that the curve evolution methods achieve improved shape reconstruction and have potential computation and memory advantages over conventional regularized inversion methods.     

Analysis and applications of backscattered frequency correlationfunction

The application of the radar backscatter frequency correlation for classification and inversion of physical parameters of terrestrial targets is investigated. Traditionally, in radar remote sensing, the backscattering coefficients and the backscatter phase difference statistics of a distributed target are considered for estimating the biophysical parameters of interest. Because of the complex nature of random media scattering problems, however, target classification and parameter inversion algorithms are very convoluted. One obvious way of enhancing the success and accuracy of an inversion algorithm is to expand the dimension of the input vector space. Depending on the radar parameters, such as footprint (pixel) size, incidence angle, and the target attributes (physical parameters), the backscatter signal decorrelates as function of frequency. In this paper, analytical and experimental procedures are developed to establish a relationship between the complex frequency correlation function (FCF) of the backscatter and the radar and target attributes. Specifically, two classes of distributed targets are considered: 1) rough surfaces and 2) random media. Analytical expressions for the frequency correlation function are derived and it is shown that the effect of radar parameters can be expressed explicitly and thus removed from the measured correlation functions. The University of Michigan wideband polarimetric scatterometer systems are used to verify the theoretical models and inversion algorithms developed in this study     

UWB Tomographic Radar Imaging of Penetrable and Impenetrable Objects

In this paper, the capability of ultra-wide-band (UWB) sensor arrays for tomographic radar of electrically large objects is presented. The major concern when imaging is extended to real objects is to achieve a correct reconstruction of the object shape and its electric properties. A general framework based on a UWB bifocusing operator (UWB-BF) with good tomographic imaging capabilities is presented. This general approach provides a comprehensive understanding of the basic tradeoffs with regard to sensing geometry and image quality parameters. Through numerical simulations and measurements applied to canonical as well as to complex objects, basic design criteria are assessed and the potential of UWB tomographic radar imaging is presented.      

Scale Model Experimental Validation and Calibration of the Half-Space Green's Function Born Approximation Model Applied to Cross-Well Radar Sensing

Efficient forward models that describe the physical nature of the geophysical problem are desired for subsurface sensing and reconstruction of a contrasting contaminant pool volume. An analytical model to approximate sensing with radar is developed and implemented in the frequency domain in terms of the half-space lossy dyadic Green's function. The Born approximation is employed as a linear forward model, which will eventually be used for tomographic inversion for object detection. The forward model is compared with measurements generated by a cross-well radar (CWR) experiment in a controlled soil test tank using broadband borehole antennas. Soil parameter (dielectric constant and loss tangent) variance with frequency is represented by a quadratic polynomial. Calibration for soil parameters is performed via CWR data using an iterative nonlinear parameterized inversion technique. With the appropriate calibration, good agreement is obtained with wideband experimental measurements for several different borehole antenna placements, confirming the accuracy of the model.     

Subsurface cavity imaging by crosshole borehole radar measurements

Travel time tomography uses direct waves of crosshole radar data. A great deal of information concerning the media is contained in later arrivals, such as reflections and diffractions. More reliable results can be obtained by incorporating both direct waves and later arrivals. In this paper, the combination of travel time tomography and migration is introduced. A smoothed velocity model obtained by travel time tomography is used as the basis of migration. Then, a reverse-time migration technique is applied to direct-wave-suppressed crosshole data. The result of tomography indicates that there is an abnormal area, and the result of migration gives a more accurate position of the abnormal area, which is a cavity. The real example shows that the cooperation of tomography and migration is an effective technique for subsurface-cavity imaging. The synthesis of all information, including parallel crosshole measurement and forward simulation, assists in the interpretation of the results of tomography and migration.     

Simulation of near-surface detection of objects in layered media by the BCGS-FFT method

Near-surface electromagnetic characterization of objects buried in multilayered earth is important for the detection and identification of landmines, unexploded ordnance, and underground structures. However, so far little progress has been made in the development of fast algorithms for inhomogeneous objects in a layered medium. We report an iterative technique, the stabilized biconjugate gradient fast Fourier transform (BCGS-FFT) method, that simulates near-surface detection of three-dimensional, inhomogeneous objects buried in multilayered media. The CPU time and memory cost of the BCGS-FFT method is O(NlogN) and O(N), respectively, where N is the number of unknowns. This method is significantly more efficient than method of moments. It is capable of solving large-scale electromagnetic scattering problems with an arbitrary inhomogeneous object embedded in a layered medium with an arbitrary number of layers. Examples in subsurface detection of large buried objects are shown to demonstrate the efficacy of this method. At present, the object must be located completely within one single layer in this multilayer medium, but efforts are underway to remove this limitation.     

基于遗传算法的探地雷达层状介质参数反演算法

探地雷达因其无损性与高效性,逐渐成为道路检测、地下勘探等领域的有效探测手段,而探地雷达的传统厚度反演算法存在无法有效检测分层介质厚度、相对介电常数等参数的问题,具有较大的多层介质参数反演误差。因此,本文提出了一种基于遗传算法的层状介质参数反演算法。算法从时域角度出发,结合共中心点方法,设计了基于遗传算法的优化模型,反演地下层状介质结构的参数信息。通过实验仿真,本文所提方法可以应用于地下多层介质,较为准确的反演出层状介质参数信息。      

分层媒质中浅二维目标的探测

本文探讨利用时域散射场信息探测分层媒质中法二维目标、分层媒质中目标的探测具有明显的应用价值．但相对自由空间而言，这种探测要困难得多。本文给出一种基于体等效原理的探测分层媒质中钱二维目标的方法。这是一种选代方法．从一阶玻恩近似开始迭代．每次迭代过程中正问题和逆问题各计算一欢，正问题采用FDTD方法。利用体等效原理，得到反映时域散射场和目标介电常数分布之间关系的体积分方程，经傅立叶变换并离散化后得到反演方程。为了克服问题的病态．采用了伪求逆技术、文中列举了几个具有代表性的例子，最后讨论了有关的几个问题。     

分析凋落波场传播的三维复射线理论

将二维复射线理论推广到三维空间，讨论无界均匀媒质中三维复射线场的基本特点，以及它在曲界面上的反射特性和在分层媒质中的传播特性。由于采用了复射线近轴近似和集合复射线技术，使分析过程显著简化。本文给出的这种理论分析方法可应用于激光射束在光学系统中的传播，雷达波束在天线罩中的传播，以及目标散射特性分析等。     

Leaky-wave analysis of transient fields due to sources in planarly layered media

The transient field due to canonical sources in planarly layered media is obtained using a leaky-wave analysis. By proper choice of integration paths in both the complex frequency and complex wavenumber planes, transient fields are obtained exactly as a temporal inversion integral over a discrete sum of residues. The residues include both proper and improper surface-wave modes, analytically continued into the complex frequency plane. The method is applicable for all times of interest, although for certain source-receiver locations an "early-time" period is identified which encompasses the specular reflection from the nearest interface, and during which time the residue series requires special treatment. The presented analysis leads to a computationally simple and efficient method for obtaining transient fields due to sources in layered media. Results are shown for the transient potential due to line and point sources over a grounded dielectric slab, although the technique is applicable to multiple planar layers.     

机载正下视SAR探测极地冰川的回波数值模拟

建立了大尺度分层粗糙面散射物理模型, 基于Kirchhoff近似、几何光学和射线追踪方法, 推导给出粗糙面散射场的计算公式, 提出了一种快速模拟机载正下视合成孔径雷达(Synthetic Aperture Radar, SAR)探测冰川回波的数值仿真方法, 以帮助分析实测数据.对SAR系统设计、雷达回波信号处理算法研究都有很大帮助, 可以应用到星体地下结构探测回波的模拟.理论公式及数值仿真结果验证了此数值仿真方法的正确性.在不同的仿真场景和雷达系统参数下, 数值仿真模拟了冰川散射回波, 定量分析了冰层表面粗糙度、次表面粗糙度、山体倾斜度、雷达系统参数等对天底区域和非天底区域散射回波的影响.本文的仿真方法可以快速计算任意大尺度特定地形所对应的雷达探测仪回波数据.     

On the Diffusion of Electromagnetic Waves and Applicability of Diffusion Equation to Multipath Random Media

The diffusion behavior of electromagnetic (EM) waves in two dimensional (2-D) multipath media is studied through integral equation based full wave Monte Carlo simulations. The influences of some physical factors are explored, among which the area density of the embedded obstacles manifests itself to be the most important one in determining wave diffusion. A lossy system starts to behave diffusively when the area density approximately exceeds 5%, and the diffusion equations are generally applicable for predicting power decay. At low densities, the power-distance relation of the waves appears to follow power laws. The sizes and shapes of the obstacles have a secondary effect on the diffusion of waves. Whenever a system contains small objects or objects with reflecting sides, the waves therein are more diffusive and the diffusion equation approximates the reality more accurately. Absorption loss decreases wave diffusion in general, but our results show that the diffusion equation for a system with very lossy but small obstacles can work very well for predicting power decay.     

A time-domain differential solver for electromagnetic scatteringproblems

The authors' objective is to extend computational fluid dynamics (CFD) based upwind schemes to solve numerically the Maxwell equations for scattering from objects with layered non-metallic sections. After a discussion on the character of the Maxwell equations it is shown that they represent a linearly degenerate set of hyperbolic equations. To show the feasibility of applying CFD-based algorithms, first the transverse magnetic (TM) and the transverse electric (TE) waveforms of the Maxwell equations are considered. A finite-volume scheme is developed with appropriate representations for the electric and magnetic fluxes at a cell interface, accounting for variations in material properties in both space and time. This process involves a characteristic subpath integration known as the `Riemann solver'. An explicit-Lax-Wendroff upwind scheme, which is second-order accurate in both space and time, is employed to solve the TM and TE equations. A body-fitted coordinate transformation is introduced to treat arbitrary cross-sectioned bodies with computational grids generated using an elliptic grid solver procedure. For treatment of layered media, a multizonal representation is employed satisfying appropriate zonal boundary conditions in terms of flux conservation. The computational solution extending from the object to a far-field boundary located a few wavelengths away constitutes the near-field solution. A Green's function based near-field-to-far-field transformation is employed to obtain the bistatic radar cross section (RCS) information