Research Of Low-Resolution Pulse-GPR Technology

As a geophysics exploration instrumentation with the highest resolution rising in the last decade, Ground penetrating radar (GPR) has been already widely used in the fields of construction,tunnel engineering,environment,archaeology and military for non-destructive testing.UWB pulse-GPR system is the most successful and has been proved in the practice as a useful geophysics exploration instrumentation.Instrument equipments of GPR system are mainly depended on import now. In order to ensure the safety of tunnel constructing and avoiding geological disaster,it is necessary to predict the geology in front of the tunnel face using scientific instruments.It is called Tunnel geology prediction(TGP).Tunnel seismic method associated with GPR method can accurately predict geology several ten meters in front of the tunnel face.This paper takes this application as background,and attempts to design and produce the low-resolution Pulse-GPR instrument equipments to meet the application requirement of tunnel geological prediction (TGP).The main research work of this paper are as follows:(1) The design of the low-resolution pulse GPR system.The instrument system consists of four parts:ultra-wideband(UWB) transceiver antennas,UWB pulser module, data acquisition module and the host computer.It achieves the assignment as followings:generation of UWB pulse, transmission and receival, data acquisition and imaging.(2) Design of the UWB pulser.UWB pulser is proposed to genetrate nanosecond time domain pulse.The principle of avalanche transistor and the basic circuit of avalanche transistor is analysed.Main methods of generating UWB pulse are compared with each other.Series-wise avalanche circuit and Marx circuit based on avalanche transistor are contrasted.Avalanche transistors that have high avalanche beakdown voltage and high cut-off frequency are elaborately selected and tested.Single stage series-wise avalanche circuit can achieve the proposed nanosecond time domain pulse and requires the least avalanche transistors.Single stage series-wise avalanche circuit is designed and applied in the UWB Pulse-GPR system as the UWB pulser.Testing results show that the pulse outputed by the pulser has several advantages such as high frequency stability,high waveform stability,high repetition frequency. This is necessary that the pulser can be used in the Pulse-GPR system. The amplitude and spectrum of the pulse are suitable in the pulse-GPR system for the application of TGP. The UWB pulser is compared with other pulser used in UWB radar system and they have respective advantages and disadvantages.The pulser designed in the paper has simple structure and stable performance.(3) Design and match methods of the planar bowtie antenna.UWB planar dipoles antennas are familiar in the UWB Pulse-GPR system and they are simulated and analysised by HFSS and CST software. Performance of plannar Bowtie antenna is given under different structure parameter.The paper investigated the split of radiation pattern when Bowtie antenna works on high frequence(compared to its structure).The substaintial reason is analysed.The problem astricted the bandwidth of the radiation pattern and decided the best radiation direction of the antenna.The extent of the split problem is decided by the equivalent degree in the side of the Bowtie antenna.In order to achieve the match between the antenna and the pulser a resistively-inductively loaded (RL-loaded) method for bowtie antenna is theoretically and experimentally investigated.The antenna has been designed for transmission of Gaussian-like pulses with duration of 5ns.The RL-loading method raised in this paper makes the UWB pulse generator and the antenna-feed system get matched fast in the laboratory.The time-domain waveforms and VSWR of the antenna are shown in this paper.RF transformer (JTX-2-10T+) is used as high frequency,wide band balun and it can make that the 75ohm coaxial-cable and the antenna get matched.(4) Design of the SQEA. In order to achieve expected pattern,the edge of the UWB planar dipoles antenna is optimized and a shield quarter-ellipse antenna(SQEA) is raised.SQEA has expected input impedance and its front-to-back ratio (F/B) can reach 35dB which can effectively avoid the unexpected noise from the poor environment.(5) Suppression of jamming pulse in synchronization system.UWB pulse coupled from the pluser and transmitted by the antenna employs itself as jamming in the synchronization system. So the circuit based on optical coupler(OC) and transient voltage suppressor (TVS) is designed to isolate and filter the jamming pulse. Testing results show the the method can effectively avoid error trigger and improve the precision of the synchronization system.(6) Design of the software.The software system is programmed under windows OS.It achieves the performance of real-time data acquisition,transfer,processing, A-scan,B-scan imaging, store and controlling system.The main innovation points of the paper are as follows:(1) Single stage series-wise avalanche circuit is designed and applied in the UWB Pulse-GPR system as the UWB pulser.Testing results show that the pulse outputed by the pulser has several advantages such as high frequency stability,high waveform stability,high repetition frequency. This is necessary that the pulser can be used in the Pulse-GPR system. The amplitude and spectrum of the pulse are suitable in the pulse-GPR system for the application of TGP.(2) The paper investigated the split of radiation pattern when Bowtie antenna works on high frequence(compared to its structure).Two methods(the RL-loading, RF transformer matcher) is raised in this paper to get the match between the pulser and the antenna.(3) SQEA is raised in this paper to achieve expected F/B and shield performance.(4) The circuit based on optical coupler(OC) and transient voltage suppressor (TVS) is designed to suppress jamming pulse.It can improve the precision of the synchronization system.