Research And Implementation Of SFCW Borehole Radar System Based On VNA

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.