Research On Transmission And Acquisition Technology In GPR For Tunnel Geology Forecast

In recent years,with the fast development of economy, many high way projects have been built. Because the heading geologic configuration isn't clear, some accidents such as landslide and roof fall always happen. Therefore it is very important to understand the situation of the heading geologic configuration,surrounding rock, fault, fracture zoo and aquifer case. At present,the practical application methods of engineering are advanced drilling methods,seismic refection and GPR.The characteristics of GPR are low cost,high accuracy,easy to operate,non- destructive nature,very small interfere and it can track the entire processing of construction.GPR is the usage of HF pulse electromagnetic wave through the transmitting antenna coupled to the underground .Because the differences between underground surrounding rock and the goal in electricity, receiving antenna receive the reflection signal and then send to the host computer. When we get the two-way travel time from transmitting antenna to receiving antenna through the underground reflection, we can calculate the goal location and depth on condition that we have known the velocity of electromagnetic wave in the underground medium. According to the intensity of reflected signal and the waveform characteristics after detecting all measuring points continuously,we can get the goal profile image and confirm the working face situation in front.The transmitting and receiving system of the pulse GPR has been designed and produced. GPR transmitting system includes MCU circuit,avalanche pulse generator circuit,balancer,impedance matching circuit and transmitting antenna. MCU circuit module can provided a certain frequency Square-wave by the control words. The MCU synchronous trigger signal is offered to the avalanche pulse generator circuit and Acquisition Module. The two core shielded wire is used for synchronization between transmitting and receiving system. By means of analyzing the principles of avalanche transistor. The avalanche pulse generator circuit module has been designed. The optocoupler can isolate the pathway between the high voltage part of avalanche circuit module and MCU circuit module. According to the spectrum of received waveform,the Data acquisition module was selected for a second development.A UWB bowtie antennas have been designed in the paper. According to the requirement of tunnel geological prediction,the UWB bowtie antennas center frequency has been designed to 100MHz. The structure of the UWB bowtie antennas have been simulated by HFSS. The VSWR and Smith chart have been tested by Vector Network Analyzer. The VSWR of RL-loaded antenna is less than 2.5. The Smith Chart of RL-loaded antenna approach 50?. So the radiation characteristics of the RL-loaded have been improved greatly. Thereby it meets the requirements of the system better. The receiving system includes the receiving antenna,AGC circuit, Data Acquisition Card and PC. The main object of our study is the direct coupled wave. The scope of data acquisition card is fit for the design requirements. However the AGC circuit must be designed in the receiving system. The AGC circuit must take variable Gain Amplifier for different intensity of received signals. The amplified signal can be suitable for the input of AD. The D0 channel of Data acquirement card is external clock input. The external clock can be used for the synchronous signal between transmitting and receiving system. The Data acquisition card can be trigged when there is the transmitting signal otherwise it doesn't work.The data can be transmitted to host computer by the USB bus,processed and displayed on the host computer. A re-development of software is designed,according to the DLL files and program block diagram which manufacturers provided. The re-development software is suitable for pulse GPR operation.The text is divided into six chapters, it describes the system's theories,principles and the process of the realization in detail.Chapter 1 is the introduction ,it introduces the Background , significance , Exploration method and development direction of tunnel geological prediction between home and abroad.Chapter 2 studies the theory and the composition of transmitting and receiving system of GPR, it includes the introduction of the GPR's technical parameters, the typical signal waveforms and the overall program block diagram of GPR's transmitting and receiving system.Chapter 3 introduces various parts design of Pulse GPR's transmitting and receiving system elaborately, including various parts of the circuit working principles,device selection and testing.Chapter 4 describes the detailed design process of the UWB bowtie antennas system. Including A UWB bowtie antennas simulation,production UWB bowtie antennas, through the adoption of impedance matching circuit,let it meet the requirements of the system.Chapter 5 studies the design of the system's software. It includes the MCU's software processing, procedure of Data acquisition module and the PC software.Chapter 6 is the test result. It proves the correctness and feasibility of this system.Chapter 7 is the conclusion. It summarizes the main work of the text, points out the problem in system and the work needed to be studied in the future.