| Tunnel seismic advanced prediction is the use of seismic methods to predict the geological conditions of the section about 150 m in front of the tunnel face during the tunnel excavation to guide construction to reduce risks.This method is currently the most common geophysical method because of its long prediction distance and sensitivity to geological interfaces,but there are still some shortcomings: the seismic wave field is difficult to identify under the condition of a complex boundary tunnel in full space;the design principle of the geometry is not clear;the speed sensor is difficult to meet the wide-band high-sensitivity signal acquisition requirements of the tunnel seismic;It is difficult to couple the geophone with the surrounding rock of the tunnel;the imaging diffraction and the resolution is low.Aiming at the above-mentioned problems,this paper has conducted a systematic study on tunnel seismic advanced prediction.The main research results are as follows:(1)Using the finite difference method,the forward modeling of the tunnel full-space full-wave field is carried out,and the forward modeling method based on the tunnel air boundary conditions is studied.By setting the tunnel wall and the tunnel face to free boundaries and setting the stress to 0,the full wave field of the tunnel can be obtained,which mainly includes direct waves,Rayleigh waves,high-energy RS waves and P-wave S-wave from different geological interfaces.Analysis of the amplitude characteristics and events of these wave field can provide a theoretical basis for sensor design,geometry design,and signal processing.(2)The design principle of the geometry is studied and a new two-dimensional irregular geometry suitable for the actual tunnel is proposed.Studies have shown that increasing the offset in the horizontal direction(tunnel excavation direction)can reduce the arc of imaging,and increasing the offset in the vertical direction can suppress the mirror image.The vertical offset of the tunnel space is limited,and the tunnel height can be used the richer vertical offset seismic data further suppresses the mirror image.Based on this conclusion,a new two-dimensional irregular geometry is proposed and applied to the actual tunnel engineering.(3)Based on the shaking table test and tunnel explosive blasting experiment,the acceleration sensor for tunnel seismic signal acquisition was studied.The sensor sensitivity is 2.8V / g,which is higher than the current foreign top instruments 1V / g,which is the highest level in the current literature and investigation.The sensor bandwidth range is 10Hz-5k Hz,which is much wider than conventional velocity sensors.The blasting experiment also found for the first time that the frequency spectrum of the tunnel seismic signal can be as high as 1500 Hz,indicating that the wide-band acceleration sensor can greatly broaden the acquisition frequency of the traditional velocity sensor,and ultimately improve the imaging resolution.(4)Based on the acceleration sensor,a direct coupling geophone was developed.The mechanical structure design of the pulley and hard spring is used to make the geophone and the wall of the tunnel borehole tightly fit together.For the first time,the direct coupling between the geophone and the surrounding rock medium of the tunnel borehole is realized.The sdirect coupling geophone has higher fidelity and richer high-frequency information than the conventional geophone.It reduces the installation time of the conventional geophone from about half an hour to 1 minute,shortens the working time in the harsh environment of the tunnel under construction,and saves installation cost.(5)A small aperture imaging method based on wave equation-based reverse time migration is studied.The forward modeling shows that the arc drawn by the reverse time migration is significantly smaller than the conventional equal travel time migration,indicating that the wave equation based migration method is closer to the true propagation law of seismic waves and the imaging resolution is higher.The two methods are used in engineering practice at the same time.The area of the geological anomaly body in the reverse-time migration profile is more convergent than the conventional equal-travel time migration profile,and the resolution is higher,indicating that reverse time migration imaging can be used for small-aperture tunnel seismic imaging.(6)Developed TETSP tunnel seismic advance prediction system.Based on practical engineering application,a simple and efficient tunnel seismic signal acquisition software was designed.According to the law of tunnel seismic wave full-space propagation,a process of tunnel seismic data processing was designed and data processing software was compiled.Combined with the above research,a new TETSP tunnel seismic advance prediction system has been formed.This system breaks the monopoly of foreign systems and has been widely used in domestic railway,water conservancy,subway and other tunnels.The detection results are stable and reliable,which can provide a safe reference for tunnel construction. |
PDF Full Text Request