地下埋藏目标与分层粗糙面复合散射探地雷达回波特性研究

探地雷达（ground penetrating radar,GPR）是一种重要的无损检测工具,被广泛应用于道路缺陷检测、路面以下物体检测以及非结构化地形感知等领域. 针对基于深度学习的介电常数图反演方法特征提取不全、准确度不高的局限性,本文引入注意力机制, 提出了一种GPR图像智能反演算法,设计了适用于GPR图像反演的神经网络InvNet以完成从GPR图像到介电常数图的非线性映射. 该算法首先利用时间维度卷积和全局特征残差编码器提取GPR时间维度和全局特征,接着采用改进的注意力机制作为注意力语义提取模块提取特征图的全局语义信息,最终通过介电常数解码器完成GPR介电常数图重构. 在基于时域有限差分法构建的GPR反演数据集上,InvNet反演结果的结构相似性指数（structural similarity index measure,SSIM）达到了97.14%,均方误差（mean square error, MSE）降为0.003 0,平均绝对误差（mean absolute error,MAE）降为0.015 7;相比于最新GPR反演网络算法GPRInvNet和PINet,InvNet的SSIM分别提高了1.06%和0.86%,MSE分别降低了0.24%和0.25%,MAE分别降低了1.41%和1.02%. 实验表明本文方法能够根据GPR图像有效反演出对应的介电常数模型图,并且与现有方法的对比结果展示了其优越的反演性能.

     

探地雷达中蝶形振子天线的改进

探地雷达是进行地下环境无损探测的一种有效工具,用来探测和识别地下目标,比如水管、电力线和通信线路、考古方面的保护等.然而,传统的探地雷达系统在可靠性、分辨率、探测深度等方面存在不足.因此,改善被测信号特性,设计和开发更高性能的天线就显得很有必要.本文研制了一种贴片电阻加载的蝶形振子天线,用时域有限差分方法对其进行了分析,实际应用结果表明天线具有良好的波形保真度,良好的辐射特性和屏蔽效果.可以广泛地应用于探地雷达和其它超宽带系统.     

水体的探地雷达成像数值模拟与实验比较研究

对中心频率为1.6 GHz的探地雷达(GPR)进行水箱管线成像研究,获得了金属和聚氯乙烯(PVC)管线在不同深度、不同管径组合以及不同介质损耗等情况下的雷达剖面。建立了二维时域有限差分程序对实验进行数值模拟对比研究,该程序能对薄壁管线进行高精度建模。相同模型下的数值模拟结果与实验符合较好。从图像解释的角度重点讨论了实测与数值模拟的雷达剖面的异同,分析了电磁波多次反射造成的一系列双曲线现象,并探讨了高损耗介质的GPR数值模拟与实测结果略有不同的原因及解决对策。     

探地雷达阻性加载天线的应用研究

阻性加载技术可以有效拓展天线带宽、降低时域振铃.该文以双纽线构成平面印制天线的辐射臂, 分析阻性加载对其频域和时域特性的影响.通过数值仿真, 讨论了天线在两种阻性加载方式下阻抗带宽和时域波形的变化.设计了一款指数型微带转平行双线的巴伦, 并对巴伦和天线进行了加工及测试.测量结果表明, 天线时域波形拖尾振铃小于10%;在0.8~6 GHz频带内, 天线单元回波损耗小于-10 dB.将天线与雷达收发系统集成后, 进行地下目标探测实验.雷达探测结果成像清晰、浅层目标分辨率较高.该天线具有超宽带、低振铃、紧支撑等特点, 可满足便携式探地雷达的工程化应用需求.     

介质粗糙面与组合目标宽带电磁散射特性研究

文中对介质粗糙面和组合目标的宽带复合电磁散射特性进行研究,通过数值计算得到了复合散射系数频率响应曲线。分析结果显示,后向复合散射系数在宽频带范围内随着频率的变化而产生振荡,粗糙面的高度均方根、介电常数、组合目标尺度、目标介电常数等因素对复合散射系数有显著的影响,且影响规律较为复杂。相比之下,粗糙面的相关长度对复合散射系数的影响较为微弱。当目标是理想导体时,频率响应曲线近似呈现出准周期性的振荡形态。     

相位调制表面雷达回波特性研究

作为一种新型的人工周期结构材料,相位调制表面(PSS)在雷达目标隐身方面有良好的应用。文中以线性调频信号为例,建立了回波模型并详细分析了其回波特性。首先,阐述了相位调制表面的基本工作原理,建立了雷达回波模型;然后,应用雷达信号模糊函数理论研究了回波的匹配滤波输出,分析了输出峰值点的幅度、相位及空间分布特性,推导了关键因素对滤波输出的影响;最后,进行了相应的仿真验证。结果表明,就对雷达系统的影响而言,相位调制表面具有与间歇采样转发干扰相似的效应,但同时它还具备一些有源方式难以比拟的优点,这使其可望在雷达对抗、目标特性控制等方面有良好的应用前景。     

探地雷达超宽带背腔蝶形天线设计与实现

设计实现了一种中心频率为400 MHz的吸波材料填充式背腔蝶形天线,并将此天线应用于超宽带探地雷达系统,组装完成了一套400 MHz无线控制探地雷达系统样机。背腔式设计的探地雷达收发天线可以克服传统蝶形天线在H面全向辐射所带来的缺点,从而提高雷达系统的信噪比及收发天线之间的隔离度。对背腔蝶形天线的设计进行了规律性研究,总结了吸波材料填充式背腔蝶形天线系统在工程化实现时的设计经验。实际路测数据验证了所设计背腔天线在此套探地雷达系统中性能表现稳定良好。     

脉冲探地雷达的模拟计算

本文在给出Debye型色散媒质中2．5维时域有限差分法（2．5D－FDTD法）迭代公式的基础上，对无载频脉冲波在不同色散媒质中的传播特性进行了计算，分析了脉冲产生畸变的原因，并提出对部分畸变脉冲进行整形的方法。分别对地下单体目标和群体目标的雷达回波电平图进行了模拟计算，并与实际无载频脉冲探地雷达的探测结果进行比较，二者有较好的一致性，证实了本文所给计算公式的正确性。另外，还分析了土壤参数对雷达探测深度和分辨率的影响。     

基于探地模型的目标散射特性研究

以不同情况下的地埋目标散射特性为前提,能够准确地从探地雷达图像中提取目标体的结构信息。使用CST仿真软件上的时域求解器构建频率范围在0.5～3.5 GHz的脉冲体制探地雷达系统的目标散射仿真模型,并对不同传播介质、不同目标体与不同埋藏深度的地埋目标散射特性进行电磁仿真模拟,预演了多种情况下探地雷达系统的探测过程。通过比较不同情况下探地模型的目标回波信号,得出结论：土壤含水会使地面反射增强,目标回波衰减不易识别;目标的尺寸改变对回波幅度有影响,对波形影响不大;当目标材质的电参数改变时,目标体回波波形变化明显;当目标与传播介质的电参数相差较大时,目标回波明显;随着目标埋深增加,目标体回波在时域上向后延迟,可以根据回波时间差计算目标体埋深。通过对仿真结果的分析证明该方法可靠有效,并且仿真过程简便,适用于不同情况下探地雷达的目标散射模拟。     

平坦/粗糙地表下目标散射特性的MPSTD算法分析

将MPSTD算法与特征变量-物理边界(CV-PB)匹配条件相结合来模拟探地雷达(GPR)问题,对平坦地表/粗糙地表下不同电磁参数、不同形状目标的散射特性进行了分析。由于这类电磁问题中不仅存在多种类型的子域分界面,而且子域分界面两侧存在多种媒质参数,所以它的准确分析不仅进一步验证了CV-PB匹配条件的有效性和准确性,而且也为电大尺寸空间复杂地表、复杂媒质中曲面形状目标体散射特性的快速、准确分析提供了一种思路。     

Time-domain sensing of targets buried under a Gaussian,exponential, or fractal rough interface

The authors numerically examine subsurface sensing via an ultrawideband ground penetrating radar (GPR) system. The target is assumed to reside under a randomly rough air-ground interface and is illuminated by a pulsed plane wave. The underlying wave physics is addressed through application of the multiresolution time-domain (MRTD) algorithm. The scattered time-domain fields are parametrized as a random process and an optimal detection scheme is formulated, accounting for the clutter and target signature statistics. Detector performance is evaluated via receiver operating characteristics (ROCs) for variable sensor parameters (polarization and incident angle) and for several rough-surface statistical models     

Time-domain sensing of targets buried under a rough air-groundinterface

We consider plane wave time-domain scattering from a fixed target in the presence of a rough (random) surface with application to ground penetrating radar. The time-domain scattering data are computed via a two-dimensional (2-D) finite-difference time-domain (FDTD) algorithm. In addition to examining the statistics of the time-domain fields scattered from such a surface, we investigate subsurface target detection by employing a (commonly used) matched-filter detector. The results of such a detector are characterized by their receiver operating characteristic (ROC), which quantifies the probability of detection and probability of false alarm. Such ROC studies allow us to investigate fundamental assumptions in the matched-filter detector: that the target response is deterministic and the clutter signal stochastic, with the two signals treated as additive and independent     

Electromagnetic scattering from a buried cylinder in layered media with rough interfaces

A solution to scattering from a cylinder buried arbitrarily in layered media with rough interfaces based on extended boundary condition method (EBCM) and scattering matrix technique is developed. The reflection and transmission matrices of arbitrary rough interfaces as well as an isolated single cylinder are constructed using EBCM and recursive T-matrix algorithm, respectively. The cylinder/rough surface interactions are taken into account by applying the generalized scattering matrix technique. The scattering matrix technique is used to cascade reflection and transmission matrices from individual systems (i.e., rough surfaces or cylinders) in order to obtain the scattering pattern from the overall system. Bistatic scattering coefficients are then obtained by incoherently averaging the power computed from the resulting Floquet modes of the overall system. In numerical simulations, the bistatic scattering coefficients are first validated by comparing the simulation results with the existing solutions which are the limiting cases including scattering from two-interface rough surfaces without any buried object and from a buried cylinder beneath a single rough surface. Subsequently, the numerical simulations of scattering from a buried cylinder in layered rough surfaces are performed to investigate the relative importance and sensitivity of various physical parameters of layered rough surfaces to incoherent scattering coefficients. Results show layered rough interfaces can significantly alter the scattering behaviors of a buried cylinder.     

分层粗糙面下方介质目标散射的快速算法

为快速获取分层粗糙面与下方介质目标的复合电磁散射特性,提出了一种基于前后向迭代算法(FBM)和双共轭梯度法(Bi-CG)的快速互耦迭代算法。推导了一维分层粗糙面与下方介质目标(二维散射问题)的耦合边界积分方程组,用FBM求解分层粗糙面的表面积分方程,而用Bi-CG求解目标的表面积分方程,目标和粗糙面的相互耦合作用通过更新两方程的激励项来迭代求解。应用该算法计算了下方存在介质目标时双层介质粗糙面的双站散射系数,与传统矩量法得到的结果相吻合,验证了该算法的正确性;分析了不同极化波入射时该算法的收敛性,讨论了目标尺寸和位置变化对双站散射系数的影响。     

GPR phase-based techniques for profiling rough surfaces and detecting small, low-contrast landmines under flat ground

We present a new technique whereby phase variation signatures are used to profile two-dimensional (2-D) rough surfaces and to discern shallowly buried, small, low-contrast landmines under a flat ground. The method has been tested using data measured over a composite surface containing two rough dielectric surface patches, and over a flat ground under which small, low-contrast antipersonnel landmines are shallowly buried. The results show that the phase-based technique is capable of profiling rough surfaces and of detecting small, low-contrast landmines with different internal structures buried underneath a flat ground.     

On the scattering of electromagnetic waves by bodies buried in a half-space with locally rough interface

A method for the scattering of electromagnetic waves from cylindrical bodies of arbitrary materials and cross sections buried beneath a rough interface is presented. The problem is first reduced to the solution of a Fredholm integral equation of the second kind through the Green's function of the background medium. The integral equation is treated here by an application of the method of moments (MoM). The Green's function of the two-part space with rough interface is obtained by a novel approach which is based on the assumption that the perturbations of the rough surface from a planar one are objects located at both sides of the planar boundary. Such an approach allows one to formulate the problem as a scattering of cylindrical waves from buried cylindrical bodies which is solved by means of MoM. The method is effective for surfaces having a localized and arbitrary roughness. Numerical simulations are carried out to validate the results and to show the effects of some parameters on the total field. The present formulation permits one to get the near and far field expression of the scattered wave.     

基于GPR的机场跑道钢筋回波检测与抑制

钢筋强反射回波及其多次波严重影响探地雷达在机场跑道灾害目标检测与识别的应用.本文研究机场跑道钢筋回波检测及抑制的方法.首先利用Hyp-curvelet变换将GPR时空二维回波信号投影到尺度空间,目标回波的能量将聚集且与其他目标分离,进而基于尺度空间中的局部峰值进行目标检测.然后结合目标回波的初始相位及时频特征,在尺度空间中消除钢筋回波分量.最后将数据反变换回时空域,得到只含有灾害目标回波的数据.仿真实验表明,本文所提算法在低信噪比的情况下对钢筋回波检测与抑制能取得很好的效果,重构的结果中灾害目标回波保存完整且钢筋回波罕有残余.     

Electromagnetic detection of buried dielectric targets

Scattering from subsurface objects is studied using the T-matrix method. The scatterer is represented by a dielectric ellipsoid, and the source is assumed to be an electric dipole in a borehole. This approach accommodates rotations of the scatterer relative to the source and also permits relaxation of the two-dimensional constraint inherent in some models. In addition, it provides a framework within which a large number of subsurface scattering problems can be studied. To illustrate how this approach can be applied, the fields scattered from highly eccentric ellipsoids are found. In addition, polarization results demonstrate that a method based on the polarization of the scattered field may provide a more robust diagnostic of scatterer location than methods using only a single field component. An experiment based on a scale model of a subsurface tunnel is described. Results from the analytic model compare favorably with those obtained from the experiment     

Scattering from multiple objects buried beneath two-dimensional random rough surface using the steepest descent fast multipole method

Generalized formulations are presented to analyze the electric field scattered from multiple penetrable shallow objects buried beneath two-dimensional random rough surfaces. These objects could have different materials, shapes, or orientations. In addition, their separation distance may range from a fraction of a wavelength to several wavelengths. The fast algorithm, steepest descent fast multipole method (SDFMM), is used to compute the unknown electric and magnetic surface currents on the rough ground surface and on the buried objects. Parametric investigations are presented to study the effect of the objects proximity, orientations, materials, shapes, the incident waves polarization, and the ground roughness on the scattered fields. A significant interference is observed between the objects when they are separated by less than one free space wavelength. Even when the clutter due to the rough ground is removed, the return from the second object, can be dominating causing a possible false alarm in detecting the target. The results show that the distortion in target signature significantly increases with the increase of both the proximity to a clutter item and the ground roughness.