【作者】 董航；

【导师】 刘四新；

【作者基本信息】 吉林大学 ， 地球探测与信息技术， 2010， 博士

【摘要】 在我国经济高速发展的背景下,对于资源环境及地下地质构造的探测需求越来越大,这些需求带动了各种探测技术的发展。作为探地雷达家族中的钻孔雷达,具有特别的应用方式和特有的深层探测优势,对于一些特殊地点也可进行针对性探测。这使得钻孔雷达系统具有独特的应用领域。由于钻孔雷达是探地雷达中一个比较特别的成员,相对于地面用普通探地雷达,目前国内外钻孔雷达的应用还不是很广泛,对其完整的系统化研究还比较缺乏。而在雷达设备领域中,时域脉冲体制的雷达系统占据主要地位,对步进频率体制系统的全面研究就更少。本文在国家863专题项目的支持下,以电子学角度,从系统方面整体分析和研究雷达信号的发射、接收以及传输过程和影响雷达性能的主要因素;以此为基础根据课题需要,开发完成了国内第一套基于VNA的SFCW(步进频率连续波)钻孔雷达原型实验系统以及第一套完整的野外实用系统,并进行了与课题相关的野外实验。在探测理论方面,本文研究和分析了电磁波的主要特性参数和目标信息获取机制。电磁波的波速和衰减特性受到媒质的电特性参数介电常数和电导率制约,在不同的目标媒质中有不同的变化,电磁波的波速和衰减常数也发生变化,最后反映在雷达电信号上,接收系统中回波信号的相位和幅度与发射信号相比就具有了变化,所接收的雷达图像中的测距反射率廓线就是雷达目标特性的反映。为便于分析和提取钻孔雷达系统工作的特性参数,作者将雷达探测系统分为外部和内部两个系统。外部系统是电磁波对环境的探测系统;内部系统是硬件设备构成的电信号系统;两个系统的分界面是钻孔雷达的收、发天线。针对外部雷达波信号探测的研究是决定硬件设计的基础,本文建立了钻孔雷达在地下介质中电磁波的传输通道模型,通过建立雷达方程的方式研究和分析了电磁波在传输过程中和在目标处(也就是两不同媒质交界面)的特性。给出了影响雷达测量的三类主要参数,即①与硬件收发系统相关的雷达功率指标和雷达波工作频率;②钻孔雷达系统的天线增益、效率;③目标界面处,雷达散射截面参数中的两媒质的介电常数和目标尺度大小。探讨了钻孔雷达的最大可探测范围的制约因素---发射功率和探测信号范围。同时也就提出了外部系统对内部系统的设计要求。针对信号系统的要求是较大的信号探测动态范围和较高的系统灵敏度,针对天线设计的要求是在较大工作带宽上,具有较高的、稳定的方向性增益和较高的天线效率。外部系统的探测需求决定了钻孔雷达的内部硬件系统的设计方向,为实际研发实用化钻孔雷达,作者通过构建基于VNA的SFCW体制原型系统的方式,应用相关器件组建了钻孔雷达的原型实验系统。这套钻孔雷达系统也就是钻孔雷达探测的内部系统。通过对于这套原型系统的信号噪声分析,提出了钻孔雷达设计的关键参数--动态范围和接收系统的灵敏度。同时分析了雷达系统中的关键器件如放大器组件和天线系统的参数对系统设计的影响。为本文后期研发野外探测用的实用钻孔雷达系统进行了前期准备。针对钻孔雷达探测环境中周围介质的复杂性,为减少钻孔雷达研发过程中对于野外实验的依赖,作者提出了利用水的高衰减特性,借助水槽构建钻孔雷达实际勘探环境的模拟实验场的新方法。在此基础上,开展钻孔雷达探测的实验研究,有助于分析和探索雷达系统所遇到的实际问题,并以水槽实验为基础进行野外实验的经验积累。这种方法有利于解决钻孔雷达实验场地难以获取的问题,同时还减小了实际的探测空间,可在相对野外较小的水槽水体内完成很多实验内容,有一定推广的价值。在本课题的水槽实验中,本文通过大量测量数据和相关分析,总结了这套钻孔雷达原型实验系统的性能,这种限制较少的原型系统的动态范围和灵敏度,可以认为是SFCW体制下,实际雷达系统所能达到的设计指标上限。为实现钻孔雷达实际探测需要研制专用钻孔雷达天线,钻孔雷达天线处于内部硬件系统和电磁波探测信号系统的连接端口,收发天线对探测信号和接收信号的能量转换的效果和效率是整个系统探测有效实施的主要凭籍。设计完成能在宽频带内工作的雷达天线是基于VNA的SFCW体制雷达的要求。本文通过雷达探测的经验总结并根据钻孔雷达的探测特点,提出在保证天线匹配特性的条件下,将雷达天线的实在增益这个包含了天线真实效率的参数作为天线带宽的主要设计目标参数。通过针对圆柱型对称振子天线的主要参数特性的分析,为拓展雷达天线的带宽,对圆柱型对称振子天线进行了改进设计和仿真研究。设计和加工了天线核心件,并制作了两种尺度易于组装的通用钻孔雷达天线。针对钻孔雷达天线的宽频带内增益稳定的要求,进行了对天线方向性参数的仿真研究和天线匹配性能实验测试,得到了一些有对实际探测有意义的结论,并从结果中讨论了所设计和加工天线的性能。实际的钻孔雷达探测相比地面探地雷达,其优势在于可进行深入地下的深层地下构造的电特性探测。但由此需要比较长的信号传输线,原型系统中的相应较短的同轴传输线就无法使用。本文针对同轴传输线的特点进行了分析,由于同轴电缆单位长度对信号的衰减较大,并且在不同频率下的衰减量也不同。在实际钻孔雷达野外测量中,长距离同轴电缆已不适合使用模拟信号的钻孔雷达探测的需要。本文利用光纤作为实用钻孔雷达的传输线缆,利用光电转换器件及单模光纤构建了钻孔雷达的模拟光纤传输系统,本文对光纤传输系统的原理和关键器件及主要参数进行了分析,对于光纤传输系统的架构和作用进行了研究和分析。设计开发了相关器件。其间针对某光电转换器件给出了其线性动态范围的公式,对于系统指标设计和器件选择有关键性的作用。利用这套实用钻孔雷达系统进行了初步的野外实地探测,对于探测结果和系统性能进行了讨论。钻孔雷达的研发中,对于地下器件尺度的限制和要求较高。所有器件如放大器、光电转换器,收发天线系统都受限于钻孔横截面。在系统研发过程中,需要保证设计指标的前提下,在尺度限制内完成整体设计和开发相关组件。这也是钻孔雷达开发的最基本的要求。另外钻孔雷达辅助系统的设计包括探管设计和地下电源的设计和加工,由于本课题中钻孔雷达的探测目标是至少在地下500米深,探管及接头的密封和抗压设计也是保证课题任务完成的关键。在目前阶段完成的天线系统、光纤传输系统、地下探管和地下设备电源系统都满足设计指标。新型玻璃钢探管的设计加工也已完成。通过对钻孔雷达系统的初步开发,给我们提供了一个开展钻孔雷达应用研究和进一步的系统开发的平台。摆脱了虚拟的系统仿真的研究限制,在此基础上可以拓展钻孔雷达的研究层面,更深入和细致的研究成果值得期待。更多还原

【Abstract】 Accompanying the rapid development of our nation’s economics, there has been a growing need for the exploration of natural resource, environment, and subsurface structure, which motivates the development of various detection techniques. The borehole radar, as a type of the Ground Penetrating Radar (GPR) and because of its unique application mode and advantage of detection in depth, can perform detection pertaining to some particular locations, therefore the borehole radar has its specific application fields.As a special type of GPR, borehole radar currently has not been widely used domestic and abroad compared with general GPR, and is lacking in the systematic research involving it. When considering the area of radar equipment, time-domain pulse radar system is dominant, while stepped frequency system receives much less attention. Supported by the national 863 project, this thesis analyzes and studies the transmitting, receiving and transfer procedure of the radar signal, and the main factors that affect the radar performance are also considered. According to the requirement of this project, the national first set of borehole radar prototype experimental system and the first set of field practical SFCW (stepped frequency continuous wave) system have been implemented based on VNA, field tests pertaining to the project has been performed.In the area of detection theory, this thesis studies and analyzes the major characteristic parameters and the mechanism for the extraction of the target information. The variation in the wavespeed and attenuation characteristic of the electromagnetic wave is confined by the electrical characteristic parameters of the different target medium such as dielectric constant and electric conductivity, and characterized by the change of the phase and amplitude of return signal in the receiving system. The range reflectivity profile in the received radar image represents the radar target feature.To facilitate the analysis and extraction of operation properties of the borehole radar system, the radar detecting system in this thesis is divided into external and internal subsystems; the former is the environment detecting subsystem using electromagnetic wave, the latter the electrical signal subsystem composed of hardware equipment, and the ports between the two subsystems are the transmitting and receiving antenna. The research for the detection of the outside radar signal is the basis of hardware design. In the thesis, a transmission channel model of the electromagnetic wave in the subsurface media is developed, and the properties of the electromagnetic wave, during transmission process and on the target (the interface of two different types of media), are also discussed through establishing radar equations. Three types of parameters presented affecting radar measurement include radar power and radar operating frequency at the ports of transmitting and receiving antenna relevant to hardware, antenna gain and efficiency of the borehole radar system, and dielectric constants of the two different media on the target interface and target scaling included in the radar scattering section parameters. The limitations of maximum detectable range for the borehole radar such as the transmitted power and detected signal range are considered. Then the design requirement of the external subsystem for the internal subsystem is proposed. As far as the signal and system requirements are concerned, wide signal detecting dynamic range and high system sensitivity are needed. The design of antenna requires stable and high directional gain and high antenna efficiency within wide working bandwidth.The detection requirement of the external system determines the design mode of the internal hardware system of borehole radar. To implement practical borehole radar, the author constructs a borehole radar prototype experimental system using relevant devices by constructing prototype system employing SFCW mechanism based on VNA (Vector Network Analyzer). This borehole radar system is the internal subsystem for borehole radar detection. Basing on the signal and noise analysis of this system, the key parameters relative to the borehole radar design are proposed, which are dynamic range and sensitivity of the receiving system. The key device such as amplifier in the radar system and the effect of the antenna system parameters on system design are also analyzed. All the above work lays a solid foundation for the later research and development of practical borehole radar system for field detection.Owing to the complexity of the surrounding media in the detected environment for borehole radar, and to reduce the dependency on field test during the research and development of borehole radar, the author proposes a novel method of constructing a test field that simulates the exploration environment in practice for borehole radar by using gutter and the high attenuation characteristic of water. The experimental research of borehole radar detection that is carried out helps to analyze and explore the encountered problem in practice for radar system, and the field tests performed based on the gutter experiment help accumulate relevant experiences. The benefit of this method is that it solves the problem of the scarcity of borehole radar test field, and that the practical detecting space may be reduced to the smaller water body of a gutter in contrast with the field, thus it is a method worthy of expansion. Performances of the borehole radar prototype experimental system are summarized through the large amount of measured data and relevant analysis, and dynamic range and sensitivity of prototype system with little limitation can be considered as the upper bound of the design specification that the practical SFCW radar system can achieve.Specific borehole radar antenna need be developed to accomplish the practical detection. The borehole radar antenna is at the linking port between the internal hardware system and the electromagnetic wave detecting system, and the transmitting and receiving antennas play an important role in the effective detection of the complete system characterized by its energy transformation effect and efficiency associated with the detected and received signals. The completed SFCW radar antenna based on VNA is designed to work within wide bandwidth. According to the experiences of radar detection and the borehole-radar detecting features, with the matching characteristics for the antenna being guaranteed, realized gain of radar antenna as a parameter that denotes the real efficiency of an antenna is proposed to be taken as the main design object parameter for the antenna bandwidth. Following the analysis of the principal parameter characteristics of cylindrical dipole antenna, improved design and simulation research has been carried out to expand the bandwidth for radar antenna. Core components have been designed and manufactured and two types of general borehole radar antennas that are easy to assemble have been produced. To satisfy the stable gain within wide bandwidth for borehole radar antenna, simulations of the directional parameters of antenna are performed and matching performance tests of antenna has been carried out. Some constructive conclusions have been made to guide the practical detection, and the performance of the antenna designed and produced has been discussed.In contrast with ground GPR, the advantage of borehole radar in practice is that deep subsurface structure detection can be carried out by electrical means. Thus longer signal transmission wires are required, and the relatively shorter coaxial transmission wires in the prototype system cannot be used any more. Due to high signal attenuation for unit extent coaxial cable, with the amount of attenuation varying with different frequency, analysis of the features of coaxial transmission wires shows that long distance coaxial cable are no more suitable for the demand of borehole radar detection using analog signal in practical borehole radar field test. In this thesis, with optical fiber being the transmission cable, optical fiber analog transmitting system of borehole radar can be constructed employing photoelectric converting device and single-mode optical fiber. The principle of optical fiber transmitting system, key devices and main parameters are considered, and the construction and function of optical fiber transmitting system are also discussed. The relevant device are designed and developed. Equations of the linear dynamic range of some photoelectric converting device are given, which are helpful in system specification design and device selection. Primary field detection is carried out using this practical borehole radar system, and the detection results and system performance are discussed.During the research and development of borehole radar, the restrictions and requirement for the subsurface device scale is severe, for example, the scales of all the devices such as amplifier, photoelectric converter, and transceiving antenna are limited by the borehole cross section. Given the guaranteed design specifications, one of the basic requirements of borehole radar development is to accomplish the overall design and components development within the scale limitations. The auxiliary system of borehole radar includes probe design and subsurface power design and manufacture. The detected target of borehole radar in this project is at least 500 meters in depth, thus the encapsulation and compression design of the probe and transmission line is another important topic of concern. The present complete antenna system, optical fiber transmission system, subsurface probe and power system meet the design specifications, and the design and manufacturing of the novel fiberglass probe have been done.Through the primary research and development of borehole radar system, a platform for the borehole radar application research and further system development has been provided. Relieved from the restrictions of virtual system simulation, the research level of borehole radar can be furthered and deeper and detailed research results are promising.更多还原