全极化探地雷达正演模拟及极化校准技术
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摘要
探地雷达是用高频无线电波来确定介质内部物质分布规律的一种探测方法，采用快速、连续、非接的探测方式，具有采集速度快、分辨率高的特点。探地雷达被广泛的应用于工程、环境、水文地质调查、考古、基础地质调查、矿产勘查、军事探测、星球探测等领域。但是探地雷达与其他探测方法一样，有着一定的局限性。存在穿透能力弱、多次波及其它杂波干扰严重、单极化的数据获取方式提供信息量有限、天线耦合的影响等缺点。传统的探地雷达的发射接收的都是同一性质的线性极化波，在进行等偏移距测量时，这两个天线以一定的天间距并排垂直于测线放置，工作时发射天线产生一个与天线长轴方向平行的线性极化波，由线性极化波的性质可知，接收天线对平行于天线长轴方向的电场极化成分最敏感。由于发射天线和接收天线的极化方向相同，所以在接收天线中只有平行于天线长轴方向的电场极化成分能够被接收到。
电磁波与具有多种电磁波视角的目标表面发生作用后，目标会使电磁波极化产生不同程度的旋转。传统的探地雷达天线只接收与发射天线相同极化方向的反射波。这就造成了目标体反射信息的缺失。在传统的探地雷达天线极化方式的基础上，保持发射天线极化方式不动而使接收天线旋转90°，定义与测线平行的方向为H极化，与测线垂直的方向为V极化，这样天线就有四种极化方式，分别是HH、、HV、 VH VV，其中HH和VV为共极化模式，HV和VH为交叉极化模式。基于以上的原理构建了全极化探地雷达系统，该系统是基于目标体反射信号的矢量特性，通过研究反射信号矢量分量之间关系，从而能够更好的获取地质体和地质表面的几何属性以及结构属性的信息。
本文利用时域有限差分方法编写了3D-FDTD正演模拟程序，通过获取不同方向的电磁场分量来模拟全极化测量，对典型目标体进行了正演模拟，获得了不同位置、不同旋转角度下目标体的极化响应特征。发现对于类似金属平板这类的具有单一电磁波视角的目标体极化响应不明显；而对于类似于三面角反射器之类的具有多电磁波视角的目标体极化响应强烈。
继续深入探讨了全极化测量模式下的随机表面模型和倾斜裂缝模型的极化响应。发现随机表面的粗糙程度能影响到电磁波的共极化响应和交叉极化响应，并且表面越粗糙，电磁波的交叉极化响应和共极化响应振幅越接近。而对于不同倾角的倾斜裂缝，不同方位角的极化测量的极化响应也不相同，通过获得的交叉极化响应和共极化响应信息可以对倾斜裂缝的方向角进行近似的求取。
全极化雷达系统在进行测量时将不可避免的带入诸如环境，系统本身的干扰。这时，如果不对极化雷达系统做一些相对振幅和相位的校准，所得到的测量结果将不能准确的反应目标体的响应。因此，校准的目的是确定测量极化散射矩阵过程中四个测量通道的增益（振幅和相位）(HH,HV,VH,VV)以及估计由发射和接收天线引起的交叉极化畸变的增益。一旦将这些校准系数确定下来，就可以确定并移除系统的影响。
本文提出了一种新的极化校准方法，利用0°和45°二面角反射器的理论散射矩阵和测量散射矩阵，通过将极化交调项从极化通道不平衡量和辐射校准中分离出来，避免了忽略高阶项而带来的校准误差，来获取较准确的极化校准系数。为了验证该方法的校准效果，利用全极化探地雷达系统进行了极化校准实验，分别测量了0°、22.5°、45°二面角反射器的测量散射矩阵，计算极化校准系数，分别对0°和45°二面角放射器的测量散射矩阵进行了自校准，同时也对22.5°二面角放射器进行了极化校准。无论从自校准还是极化校准的散射矩阵来看，都得到了很好的效果，特别是交叉极化项，较之以前的方法有了很大的改善。
总之，本论文的主要研究内容是利用时域有限差分（FDTD）方法对全极化探地雷达系统进行了正演模拟及提出了一种新的极化校准方法。目的是深入的研究全极化探地雷达系统在目标识别、方位判断、属性提取等的应用，提高探地雷达对地下目标体的识别能力。
Ground penetrating radar (GPR) is now a well-accepted geophysicaltechnique. The method uses radio waves to probe “the ground” which meansany low loss dielectric material. Use the way of rapid, continuous,non-detection and have fast acquisition speed and high resolution.Ground-penetrating radar is widely used in the field of engineering,environmental, hydrological, geological survey, archaeological,geological survey, mineral exploration, military exploration of theplanet detection. Like other detection method, ground penetrating radarhas some limitations. Such as the existence of weak penetrating powerseveral times spread to other serious clutter, uni-polar data acquisitionto provide a limited amount of information, the impact of the antennacoupling shortcomings.
Traditional ground-penetrating radar transmitting and receiving arethe same linear polarized wave. In progress of the same offset distancemeasurement, the two antennas are placed side by side on the measured line.Transmitting antenna generate a linear polarized wave along antenna axisdirection. From the nature of the linear polarized wave, we can see, thereceiving antenna is most sensitive to the electric field parallel alongthe axis of antenna polarization. Since transmitting antenna andreceiving antenna have the same polarization direction, only in thereceiving antenna parallel along the axis of antenna electric fieldpolarization components can be received.
Electromagnetic radiation to the target surface, the target causesthe polarization of electromagnetic wave to produce different degrees ofrotation. Traditional ground-penetrating radar antennas only transmitand receive the same polarization direction of the reflection wave. Itwill cause the lack of information of the target reflectance. On the basisof the traditional ground-penetrating radar antenna, keep transmittingantenna polarization is fixed leaving the receiving antenna rotated90°. We define H polarization which parallel with measurement lines, Vpolarization which perpendicular to measurement lines. So, there are fourkinds of polarization antennas. They are HH, HV, VH, VV, where HH and VVcalled co-polarization mode, HV and VH called cross-polarization mode.
Based on the above principles, we developed full-polarimetric groundpenetrating radar system, it base on the vector characteristics of thereflected signal of the target. From studying the signal vector components,we can get better access to information of the geological bodies andgeometric. As encountered in our lives, an amount of information containedin the color photographs included in the amount of information is muchexcess of a black and white picture.
In this paper, we use the finite difference time domain method to writethe3D-FDTD forward modeling program. Forward modeling some typicaltargets and obtained different location’s and different target rotationangle’s polarization responses. Found that such goal with theperspective of a single electromagnetic target like mental platepolarization response is not obvious, with multiple electromagneticperspective of the target like the trihedral reflector polarizationresponse strongly.
Continue to discuss the polarization response of thefull-polarization measurements on the model of random surfaces andinclined strikes. Found that the surface roughness can affect theelectromagnetic co-polarization response and cross-polarizationresponse. For different inclination of the inclined strikes, the azimuthof polarization measurement of the polarization response is not the same.Through the cross-polarization response and the co-polarization responseinformation can approximate solve the direction of inclined strikes’angle.
Full polarimetric GPR system will inevitably brought someinterference, such as the environment, the system itself during themeasurement. At this time, if not do some of the relative amplitude andphase calibration of full polarimetric GPR system, the measurementresults will not accurately reflect the response of the target. So, thepurpose of calibration is to determine the measurement of the polarizationscattering matrix in the four measurement channel gain and the estimatedgain of the cross-polarization distortion caused by transmit and receive antennas. Once these calibration coefficients are determined, the systemeffect can identify and remove.
A calibration procedure for a polarimetric GPR system has beendescribed in this paper. This calibration algorithm provides a simplemethodology, both in taking calibration measurements and processingcalibration parameters. The procedure effectively separates thecrosstalk from the channel imbalance and radiometric calibration andremoves the crosstalk errors caused by higher-order terms. We used thestandard calibration targets consisting of a0°and a45°orienteddihedral corner reflectors. From the theoretical values and measured dataof the two calibration targets, the calibration parameters were derived.Measured scattering matrices of targets could then be calibrated usingthese calibration parameters. Comparison of the calibrated scatteringmatrix of a22.5°oriented dihedral corner reflector and its theoreticalscattering matrix has indicated satisfactory calibration accuracy,especially for the crosstalk. The experiments results have demonstratedthe feasibility of this proposed calibration technique.
In short, the main contents of this paper is use the finite differencetime domain (FDTD) to forward modeling polarimetric ground penetratingradar system and give a new polarization calibration technique. Thepurposes of studying polarimetric ground penetrating radar system are toidentify the target body, direction judgments and attribute extractionapplications. Improve the ability of ground penetrating radar to identifyunderground targets.
电磁波与具有多种电磁波视角的目标表面发生作用后，目标会使电磁波极化产生不同程度的旋转。传统的探地雷达天线只接收与发射天线相同极化方向的反射波。这就造成了目标体反射信息的缺失。在传统的探地雷达天线极化方式的基础上，保持发射天线极化方式不动而使接收天线旋转90°，定义与测线平行的方向为H极化，与测线垂直的方向为V极化，这样天线就有四种极化方式，分别是HH、、HV、 VH VV，其中HH和VV为共极化模式，HV和VH为交叉极化模式。基于以上的原理构建了全极化探地雷达系统，该系统是基于目标体反射信号的矢量特性，通过研究反射信号矢量分量之间关系，从而能够更好的获取地质体和地质表面的几何属性以及结构属性的信息。
本文利用时域有限差分方法编写了3D-FDTD正演模拟程序，通过获取不同方向的电磁场分量来模拟全极化测量，对典型目标体进行了正演模拟，获得了不同位置、不同旋转角度下目标体的极化响应特征。发现对于类似金属平板这类的具有单一电磁波视角的目标体极化响应不明显；而对于类似于三面角反射器之类的具有多电磁波视角的目标体极化响应强烈。
继续深入探讨了全极化测量模式下的随机表面模型和倾斜裂缝模型的极化响应。发现随机表面的粗糙程度能影响到电磁波的共极化响应和交叉极化响应，并且表面越粗糙，电磁波的交叉极化响应和共极化响应振幅越接近。而对于不同倾角的倾斜裂缝，不同方位角的极化测量的极化响应也不相同，通过获得的交叉极化响应和共极化响应信息可以对倾斜裂缝的方向角进行近似的求取。
全极化雷达系统在进行测量时将不可避免的带入诸如环境，系统本身的干扰。这时，如果不对极化雷达系统做一些相对振幅和相位的校准，所得到的测量结果将不能准确的反应目标体的响应。因此，校准的目的是确定测量极化散射矩阵过程中四个测量通道的增益（振幅和相位）(HH,HV,VH,VV)以及估计由发射和接收天线引起的交叉极化畸变的增益。一旦将这些校准系数确定下来，就可以确定并移除系统的影响。
本文提出了一种新的极化校准方法，利用0°和45°二面角反射器的理论散射矩阵和测量散射矩阵，通过将极化交调项从极化通道不平衡量和辐射校准中分离出来，避免了忽略高阶项而带来的校准误差，来获取较准确的极化校准系数。为了验证该方法的校准效果，利用全极化探地雷达系统进行了极化校准实验，分别测量了0°、22.5°、45°二面角反射器的测量散射矩阵，计算极化校准系数，分别对0°和45°二面角放射器的测量散射矩阵进行了自校准，同时也对22.5°二面角放射器进行了极化校准。无论从自校准还是极化校准的散射矩阵来看，都得到了很好的效果，特别是交叉极化项，较之以前的方法有了很大的改善。
总之，本论文的主要研究内容是利用时域有限差分（FDTD）方法对全极化探地雷达系统进行了正演模拟及提出了一种新的极化校准方法。目的是深入的研究全极化探地雷达系统在目标识别、方位判断、属性提取等的应用，提高探地雷达对地下目标体的识别能力。
Ground penetrating radar (GPR) is now a well-accepted geophysicaltechnique. The method uses radio waves to probe “the ground” which meansany low loss dielectric material. Use the way of rapid, continuous,non-detection and have fast acquisition speed and high resolution.Ground-penetrating radar is widely used in the field of engineering,environmental, hydrological, geological survey, archaeological,geological survey, mineral exploration, military exploration of theplanet detection. Like other detection method, ground penetrating radarhas some limitations. Such as the existence of weak penetrating powerseveral times spread to other serious clutter, uni-polar data acquisitionto provide a limited amount of information, the impact of the antennacoupling shortcomings.
Traditional ground-penetrating radar transmitting and receiving arethe same linear polarized wave. In progress of the same offset distancemeasurement, the two antennas are placed side by side on the measured line.Transmitting antenna generate a linear polarized wave along antenna axisdirection. From the nature of the linear polarized wave, we can see, thereceiving antenna is most sensitive to the electric field parallel alongthe axis of antenna polarization. Since transmitting antenna andreceiving antenna have the same polarization direction, only in thereceiving antenna parallel along the axis of antenna electric fieldpolarization components can be received.
Electromagnetic radiation to the target surface, the target causesthe polarization of electromagnetic wave to produce different degrees ofrotation. Traditional ground-penetrating radar antennas only transmitand receive the same polarization direction of the reflection wave. Itwill cause the lack of information of the target reflectance. On the basisof the traditional ground-penetrating radar antenna, keep transmittingantenna polarization is fixed leaving the receiving antenna rotated90°. We define H polarization which parallel with measurement lines, Vpolarization which perpendicular to measurement lines. So, there are fourkinds of polarization antennas. They are HH, HV, VH, VV, where HH and VVcalled co-polarization mode, HV and VH called cross-polarization mode.
Based on the above principles, we developed full-polarimetric groundpenetrating radar system, it base on the vector characteristics of thereflected signal of the target. From studying the signal vector components,we can get better access to information of the geological bodies andgeometric. As encountered in our lives, an amount of information containedin the color photographs included in the amount of information is muchexcess of a black and white picture.
In this paper, we use the finite difference time domain method to writethe3D-FDTD forward modeling program. Forward modeling some typicaltargets and obtained different location’s and different target rotationangle’s polarization responses. Found that such goal with theperspective of a single electromagnetic target like mental platepolarization response is not obvious, with multiple electromagneticperspective of the target like the trihedral reflector polarizationresponse strongly.
Continue to discuss the polarization response of thefull-polarization measurements on the model of random surfaces andinclined strikes. Found that the surface roughness can affect theelectromagnetic co-polarization response and cross-polarizationresponse. For different inclination of the inclined strikes, the azimuthof polarization measurement of the polarization response is not the same.Through the cross-polarization response and the co-polarization responseinformation can approximate solve the direction of inclined strikes’angle.
Full polarimetric GPR system will inevitably brought someinterference, such as the environment, the system itself during themeasurement. At this time, if not do some of the relative amplitude andphase calibration of full polarimetric GPR system, the measurementresults will not accurately reflect the response of the target. So, thepurpose of calibration is to determine the measurement of the polarizationscattering matrix in the four measurement channel gain and the estimatedgain of the cross-polarization distortion caused by transmit and receive antennas. Once these calibration coefficients are determined, the systemeffect can identify and remove.
A calibration procedure for a polarimetric GPR system has beendescribed in this paper. This calibration algorithm provides a simplemethodology, both in taking calibration measurements and processingcalibration parameters. The procedure effectively separates thecrosstalk from the channel imbalance and radiometric calibration andremoves the crosstalk errors caused by higher-order terms. We used thestandard calibration targets consisting of a0°and a45°orienteddihedral corner reflectors. From the theoretical values and measured dataof the two calibration targets, the calibration parameters were derived.Measured scattering matrices of targets could then be calibrated usingthese calibration parameters. Comparison of the calibrated scatteringmatrix of a22.5°oriented dihedral corner reflector and its theoreticalscattering matrix has indicated satisfactory calibration accuracy,especially for the crosstalk. The experiments results have demonstratedthe feasibility of this proposed calibration technique.
In short, the main contents of this paper is use the finite differencetime domain (FDTD) to forward modeling polarimetric ground penetratingradar system and give a new polarization calibration technique. Thepurposes of studying polarimetric ground penetrating radar system are toidentify the target body, direction judgments and attribute extractionapplications. Improve the ability of ground penetrating radar to identifyunderground targets.
引文
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