【作者】 傅磊；

【导师】 刘四新；

【作者基本信息】 吉林大学 ， 地球探测与信息技术， 2014， 博士

【摘要】 探地雷达是一种利用高频电磁波进行无损探测的浅层地球物理技术，被广泛应用于各种近地表探测中，它具有高分辨率、无损性、高效性、抗干扰能力强、探测目标广泛等优点。然而对于植被严重覆盖的区域、或者地形起伏较大的区域、或人类无法到达的危险区域如战场、雷场等的大面积浅层探测，地面探地雷达显得无能为力，而机载探地雷达却会是一种有效的探测手段。机载探地雷达天线必须悬挂在具有一定高度并且高速运动的飞行平台上，发射天线发出的电磁波要经过空气中的几何扩散传播，然后经地面耦合后发射到地下，遇到地层界面后发生反射、折射、散射等，部分能量向上传播到达机载雷达的接收天线后，被接收天线接收并记录下来，根据接收到的雷达回波，可以推断地下地质体。基于麦克斯韦方程组推导了时间域有限差分三维及二维离散表达式，同时给出了同轴各向异性完全匹配（UPML）吸收边界条件在不同区域的离散表达式。通过数值计算表明，UPML边界条件的吸收效果好于GPML的效果，在后文的时间域有限差分数值模拟中均采用UPML边界条件。理论上，时间域有限差分法要求一个波长内至少有10个网格，对于大尺度的机载探地雷达数值模拟，尤其是大尺度的三维时间域有限差分数值模拟，时间域有限差分对内存要求非常高。为了提高机载探地雷达数值模拟的计算效率，本文借鉴多区域模拟技术，将模拟区域分为两个区域，空中区域和地下区域，在空中区域采用解析解求解空气中波场的传播，而在地下介质中采用时间域有限差分方法来求解电磁波在地下介质中的传播过程，将二者结合起来即称之为多区域-时间域有限差分模拟技术。通过模拟算例表明，相比于全区域时间域有限差分算法，多区域-时间域有限差分模拟技术在内存开销和计算时间方面大大减少，同时多区域模拟技术能够达到全区域的模拟精度。机载探地雷达探测中，天线极化方向、天线高度以及粗糙地表均会对最终接收到的电磁波信号产生影响，故而在机载探地雷达的实际应用中，尽量确保天线的极化方向与目标体走向垂直，同时尽可能降低天线的飞行高度。天线是机载探地雷达系统中重要的组成部分之一，文中首先对三种典型的雷达天线进行数值仿真，结果表明轴向螺旋天线具有方向性好，增益高，宽频带的特点，然而该天线的电尺寸较大，容易受风阻的影响，且其制作复杂；蝶形天线具有宽频带，制作简单的优点，然而该天线电尺寸较大，此外两臂为三角形，同样容易受风阻影响；而偶极天线具有电尺寸小，结构简单的优点，受风阻的影响较小，当采用加载技术及巴伦时，可以获得较高的频带宽度。基于阿特舒勒加载原理，本文制作了阿特舒勒电阻加载平面偶极天线，测试结果表明该电阻加载平面偶极天线具有较宽的频带，子波形态比较“干净”，可以作为收发天线。基于手持式矢量网络分析仪（FieldFox-N9925a）和阿特舒勒电阻加载平面偶极天线，本文构建了步进频率机载探地雷达原型系统。矢量网络分析仪实现射频信号的发射和接收功能，加载平面偶极天线实现电磁波的辐射和接收。同时采用可视化编程环境Agilent Vee编写了步进频率机载探地雷达数据采集软件，该软件实现对网络分析仪的参数设置以及扫频触发，并将数据回传并保存至本地计算机硬盘。同时采用无线Wifi技术，实现在地面无线遥控步进频率机载探地雷达系统。为了检验机载探地雷达原型系统实际探测效果，开展了相关实验。第一个实验借助于绳索将收发天线悬于空中完成，实测剖面中可以清晰的识别出来自地面及地下目标体信号；第二个实验借助于硬质木框架将机载雷达系统悬于空中，通过点测完成了剖面测量，通过与100兆赫兹地面探地雷达资料比较可以发现，来自地表干扰物的反射波能量较强，同时可以识别出来自地下目标体界面的反射信号。实验结果表明基于FieldFox-N9925a和Wifi的步进频率机载探地雷达原型系统具有探测地下目标体的能力。机载探地雷达数值模拟表明，天线的飞行高度变化会带来雷达剖面中反射波形态畸变，本文提出了一种基于实测GPS数据的飞行高度校正方法，经过校正后的雷达剖面与实际情况相符。起伏的地表会带来地表散射杂波，地表起伏度越大，地表散射杂波能量越强，强散射杂波有可能覆盖浅层目标体信号，不利于雷达资料解释。为了消除地表杂波的影响，本文借鉴勘探地震中的逆时偏移成像技术，采用二维时间域有限差分模拟电磁波场的正向和反向传播，并应用互相关成像条件获得成像空间的图像。为了验证该方法的有效性，首先构建了不同地表粗糙度模型，并合成了共偏移距机载探地雷达数据，对合成数据分别采用传统的克希霍夫偏移和逆时偏移技术进行偏移处理，结果表明克希霍夫偏移成像在一定程度上能使散射杂波能量聚焦，但是偏移剖面中仍然存在地表散射杂波的影响；而逆时偏移成像技术却能消除地表散射杂波的影响，偏移结果中来自起伏地表的散射波能量得到聚焦，偏移剖面中起伏地表的形态及空间位置与真实模型一致，逆时偏移成像效果明显好于克希霍夫偏移成像结果。更多还原

【Abstract】 Ground penetrating radar (GPR) is a kind of shallow geophysical technology fornondestructive detection by using electromagnetic wave, which is widely used in manykinds of near surface detection. The advantages of GPR include high resolution,non-invasive, high efficiency, and strong anti-interference ability and so on. But forlarge-scale problems or areas that are heavily vegetated, or dangerours areas such asbattlefield and minefield, conventional land surface GPR is inadequate. While the airborneGPR-the GPR system is mounted on a flying aircraft, is used as a potentially effectivealternative tool to conduct survey at these areas. The airborne GPR antennas usually shouldbe hanged in a high-speed flying platform with a certain distance from the ground. Theelectromagnetic (EM) wave from the transmitting antenna propagates in the air bygeometric spreading, and penetrates into the ground. Part of the EM wave reflects backwhen it encounters the discontinuous interface, the reflecting wave propagates back into theair; it could be recored by the receiving antenna. The recorded airborne GPR data could beused to infer the underground structure.Based on Maxwell equations, we have derived the discrete formula of finite differencetime domain in3D and2D domain, besides, the discrete expressions of FDTD in differentboundary area with uniaxial perfect matched layer (UPML) is derived. The absorptioneffect of UPML boundary condition is better than that of GPML, and UPML boundarycondition will be used in the numerical simulation of finite difference time domain in mythesis. In theory, FDTD requires10grids in a wavelength at least. As for airborne groundpenetrating radar simulation, especially for the3D numerical simulation of large scaleproblem, the FDTD method in the full domain is very memory consuming.In order to improve the computational efficiency of numerical simulation for airborneGPR survey, we use multi region technology. The modeling area is divided into two regions,the freespace region and the underground region. The analytical solution is used solve the EM wave propagation in the freespace region, while the FDTD solution is used to solve theEM propagation in underground medium, these two solutions are combined for solving theEM wave propagation, which is called multi-region finite difference time domain(MR-FDTD) technology. Numerical simulations show that RM-FDTD needs less memoryand calculating time compared with full FDTD, in addition, MR-FDTD could achievepretty nice precision. As for the airborne ground penetrating radar survey, antennapolarization, antenna height and the rougheness of ground surface will affect theelectromagnetic wave signal. The simulation results show that it is important to make theantenna polarization direction perpendicular to the target underground, make the flyingheight as lower as possible.Antenna is one of the most important parts for airborne ground penetrating radarsystem. We have simulated three typical radar antenna including axial helical antenna,bowtie antenna and dipole antenna. The results show that the axial helical antenna has gooddirectivity, high gain and broadband characteristic; however the geometry size for thehelical antenna is large, which will be affected by the wind. The bowtie antenna hasbroadband frequency range, and its manufacturing process is simple, however, its geometrysize is larger, and the shape of arms is the triangle which will be affected by the wind aswell.The electrical size of dipole antenna is small and usually equals half wavelength; thestructure of dipole atenna is simple which is suitable for the airborne GRP. When theresistivily loading technique and balun is employed to dipole antenna, we can obtainbroadband frequency range.Based on the Altshuler resistivily loading principle, we havemanufactured the resistor loaded planar dipole antenna, the test results show that thefrequency band of the manufactured antenna is pretty wide, besides the wavelet of theantenna is very clean, which could be used for transmitting and receiving antenna.We have setup a step frequency airborne ground penetrating radar prototype systembased on the handheld vector network analyzer (FieldFox-N9925a) and the resistor loadedplanar dipole antenna. Vector network analyzer is used to transmit and receive RFsignal,the loaded planar dipole antennas are used to emit and receive electromagnetic wave. At thesame time a data acquisition softwave for the step frequency airborne GPR is programed based on the visual programming environment Agilent VEE. The parameters setting andsweep trigger could be control by the software, acquired data could be saved to local disk.The wireless Wifi technology is introduced in the prototype system, which make it possibleto control the step frequency airborne ground penetrating radar system through wireless.In order to test effectiveness of the system, we have carried out two experiments. Theantennas are hanging in the air by rope for the first experiment, the reflecting waves fromthe ground and the underground could be clearly identified from the measured GPR profile;as for the second experiment, the prototype system including vector network analyzer,antennas, Wifi and so on are putted on the roof of a hard wooden framework, comparedwith the GPR data measured on the ground using100MHz antenna, we could found thereflecting signal from the underground targets. The experimental results show the airborneprototype system based on the FieldFox-N9925a and Wifi has the ability to detectunderground targets.Numerical simulations show that antenna flying height variation will bring thereflecting wave distortion in radar profile, we present a height correction method based onreal GPS data of flying antenna, the corrected airborne radar profile coincidents with theactual situation.The roughness ground surface would bring surface scattering clutter; strong scatteringclutter may cover the shallow target signal, which will make the radar data interpretationdifficult. In order to eliminate the influence of ground clutter, reverse time migration (RTM)technique which is widely used in exploration seismic is introduced to process airborneGPR data. We employ the2D TM model FDTD to model the EM wave forward andbackward propagation, and the cross-correlation imaging condition is used to get theimaging in the migration space.In order to test the proposed RTM method for EM wave,four2D model with different roughness surface of the ground are constructed, and thecommon offset airborne GPR data are synthesized.The traditional Kirchhoff migration andRTM technology are used to process the synthetic airborne GPR data. The migration resultsshow that the Kirchhoff migration method could focus some energy from scattering clutter,but the surface scattering clutters still exist in the migration results; as for the RTM imagingresults, the energy from scattering clutters are well focused, besides the shape and position of the roughenss ground surface match well with the real model. The migration results formRTM algorithm is better than of Kirchhoff migration method.更多还原