Research On Tunnel Advanced Detection Imaging Technology Based On Elastic Wave Reverse Time Migration

Tunnel advanced detection is also called tunnel advanced geological prediction.It is a technology to predict the geological information of the area behind the tunnel face by geophysical prospecting or drilling.The construction party can use this information to formulate corresponding construction plans and preventive measures,so as to reduce the occurrence of accidents and ensure the safety of human life and property.As a common means of geophysical prospecting method,seismic exploration method has developed maturely.However,there are still some shortcomings,such as low prediction accuracy,slow operation speed,and difficult to predict complex geological structure.Aiming at these problems,this paper proposes a tunnel advanced detection imaging method based on elastic wave reverse time migration,and studies the following aspects:(1)Research and improvement of finite difference method for solving wave equation.In this paper,the acoustic wave equation and the elastic wave equation are solved by the conventional grid finite difference method and the staggered grid finite difference method respectively,and four key problems in the finite difference method are discussed in detail,including analyzing the stability conditions of the finite difference equation,using staggered grid and FCT method to suppress the numerical dispersion,using PML method to deal with the artificial boundary problem,and discussing the space complexity and time complexity of the algorithm.(2)Research on reverse time migration technology and selection of imaging conditions.This paper first expounds the working principle of reverse time migration technology,gives the specific imaging process,and completes the design of the program.Then,the commonly used excitation time imaging conditions,amplitude ratio imaging conditions and cross-correlation imaging conditions are simulated respectively.The experimental results show that the imaging effect of cross-correlation imaging conditions on the vertical interface is better than that of the other two.Finally,the artifacts and noise in the imaging results are suppressed by filtering the direct wave,using the normalized imaging condition and solving the time-space domain finite difference coefficient.(3)Design of control software for GEI-TSP tunnel advanced detection system.This paper uses C++ language to write the computer software used with GEI-TSP tunnel advanced prediction system,and designs the graphical operation interface based on QT platform.The software realizes the command sending and receiving function based on UDP protocol and the seismic data transmission function based on FTP passive mode protocol.(4)Tunnel model establishment and simulation experiment.In this paper,the inclined layered tunnel model and the hollow tunnel model are established,and based on these two models,the forward simulation is carried out using the finite difference theoretical method.Then based on the reverse time migration,the imaging results of three different observation systems are compared,and the optimal observation system is determined.Then,the white noise from small to large is added to the collected signal for imaging.The results show that when the maximum amplitude of 50 %white noise is added to the seismic data,and the signal-to-noise ratio is-9.5d B,the image signal-to-noise ratio of the imaging results is only about 0.12 d B lower than that of the imaging results without noise,which proves that the imaging method designed in this paper is anti-noise and can meet the specific needs of practical engineering.