Three-dimensional Numerical Simulation Of Karst Cave In Tunnel Seismic Advanced Prediction

Underground projects such as tunnels and metros belong to concealed projects.Because of the complex geological structure,tunnel construction often faces various security risks,such as karst caves,fractured zones and gas,which seriously threaten the safety of tunnel constructors.Therefore,it is necessary to forecast the geological condition in front of tunnel face before construction.Seismic reflection wave method is widely used in tunnel geological prediction because of its simple operation,solid theoretical foundation,scientific and reasonable seismic record processing process and reliability of prediction.In this paper,the seismic reflection method is used to analyze the three-dimensional Tunnel Cave model.The X,Z and Y components of different scale caves are studied from three aspects: seismic track data,three-dimensional wave field snapshot and P-S wave field separation.The main contents of the study are as follows:(1)Because the transverse resolution of shear wave is the smallest resolution,this paper makes relevant research based on Fresnel zone radius R of shear wave,establishes three-dimensional Tunnel Cave model with cave diameter ∞R,5R,2R,1R,0.667 R,0R,and studies the seismic wave propagation law of caves of different scales.Seismic trace data are extracted by numerical simulation,snapshots of wave field are drawn by surfer software,wave field is separated by τ-p transform,and the law of the amplitude and energy of reflected wave changing with the scale of karst cave is studied and analyzed.(2)The amplitude of three-component seismic trace records extracted from numerical simulation is studied.The amplitude and energy of X,Z and Y components are compared based on the relationship between the energy groups of each component reflected wave in the three-dimensional wave field snapshot graph,and the applicability of each component of P and S waves in the advance prediction of tunnel earthquakes is obtained.(3)Because the damage degree of caves with different fillers is different,this paper makes a detailed study of the caves filled with air,water and soft soil,and analyses the magnitude relationship of reflected longitudinal and shear wave amplitude of each component of different fillers.(4)Seismic data are processed by multi-channel tunnel seismic reflection method in practical engineering.The process of data processing is elaborated in detail.The relationship between the strength of effective signals in X,Z and Y component seismic trace records is compared.The wave field separation characteristics of seismic waves are studied,and the rationality of interpretation and prediction is evaluated.From the above research,the following conclusions are drawn:(1)In advance prediction of tunnel caves,the smaller the S-wave velocity,the larger the main frequency of seismic wave and the closer the distance between the caves and the palm face,the easier it is to identify the smaller caves,and the more conducive to accurate prediction of small caves.In addition,when the diameter of karst cave is equal to D=5R,the amplitude of reflected wave can reach 60% of that of fault reflection wave at the same location.At this time,the amplitude of reflected wave is strong,and the karst cave can be accurately identified and predicted.When the diameter of karst cave is equal to D=0.667 R,the amplitude of reflected wave can only reach 15% of the fault at the same location,which is already on the edge of unrecognizable.It can be seen that when the diameter of karst cave is less than 0.667 R,it can hardly be predicted.(2)The amplitude of reflected P-wave of X component is the largest,which is larger than that of Y and Z component.The amplitude of reflected S-wave of Z component is the largest,and that of reflected P-wave and S-wave of Y component is the smallest.So we usually use X-direction to study reflected P-wave,and Z-direction to study reflected S-wave.By analyzing seismic trace data,we can see that compared with P-wave,S-wave has stronger energy and signal-to-noise ratio,so the prediction effect of S-wave is better.(3)The amplitudes of reflected waves are different when the fillings are soft soil,water and air respectively.According to the research,it can be concluded that the amplitudes of reflected waves of fillings with different materials are the largest in air,the smallest in soft soil,and the water is between air and soft soil.In practical engineering,because the amplitude of reflected shear wave of air and water is much larger than that of soft soil,the reflection shear wave can be used to quickly determine whether the filling of karst cave is soft soil or not.The amplitude of reflected P-wave of air is much larger than that of water.Reflected P-wave can be used to further determine whether the cave filler is water or air.(4)In the actual project,the effective signal of the Z component is stronger than the X component and the Y component through data comparison analysis.Therefore,the Z component data is mainly used for wave field separation,inversion analysis and interpretation.At the same time,the comparison between the forecasting excavation result and the theoretical calculation is carried out to verify the correctness of the theoretical calculation.