| Nowadays China has already developed into a country which has the greatest number and largest scale of tunnels, as well as, the fastest growing pace, in the meanwhileThe booming tunnel construction not only brings many opportunities, but also brings many challenges. During the tunnel construction, with a variety of complex geological condition and poor geological frequently being encountered, disasters and serious accidents are easily evoked such as water inrush, mud outburst, landslides, TBM (Tunnel Boring Machine) cards and so on. These disasters often result in schedule delays, mechanical damage, serious economic losses and even heavy casualties. Therefore, it is vital for the tunnel construction safety and disaster prevention to early detect the occurrence of faults, karst caves, underground rivers and other adverse geology ahead of the tunnel face.The ahead prospecting method based on seismic waves has become a leading geological method in tunnel ahead prospecting for its farther detection distance and better recognition effects of the interfaces. However, due to the complex tunnel environment, many problems still exist with the existing tunnel seismic ahead prospecting method:①the cognition of the propagation characteristics of seismic waves in tunnel environment is not clear enough, and lack of systematic research on the 3D seismic wave filed response characteristics for typical adverse geology;② the common observation modes used by the existing seismic ahead prospecting are always the straight-line classes, which cannot get accurate seismic wave velocity distribution information of the rock ahead of the tunnel face, resulting in lower location accuracy and identification reliability of anomalies;③the 3D space-class observation modes have the advantage in achieving more accurate estimate of the seismic wave velocity, but the research on the arrangement of space-class observation modes in tunnel environment is not deep enough, and lack of reasonable data processing and imaging methods that applied for the 3D space-class observation modes. For these issues above, taking the 3D space-class observation modes, effective imaging methods and 3D seismic wave filed response characteristics as the starting point, systematic studies were carried out in this paper. First, the 3D space-class observation modes in tunnel environment and its data processing and imaging method were proposed. Through systematic forward modeling and imaging test, the seismic wave filed response pattern and imaging characteristics of typical adverse geologies were revealed. Second, in order to suppress the multiply solutions problem derived from a single ahead prospecting method, a comprehensive ahead prospecting method ahead of tunnel face for adverse geologies based on 3D seismic waves with prior constraint information was proposed. Eventually, a complete set of methods with integrated prediction and joint interpretation method of tunnel 3D seismic wave ahead prospecting based on 3D space-class observation modes was formed. Serious of aspects were significantly improved in terms of the separation of 3D seismic wave field, the 3D velocity analysis and imaging, the suppression of multiply solutions and so on. Successful application has been carried out in site detection with the mentioned achievements.The main research work and achievements in this paper are as follows(1) For the problem that accurate seismic wave velocity of the rock in front of tunnel face cannot be obtained with the common straight-line observation modes, a kind of tunnel seismic wave ahead prospecting 3D space-class observation mode with three-directional offsets was proposed. Taking into account the separation requirements of the seismic wave field, the parameters of the 3D seismic space-class observation modes such as the track pitch and the offset were designed. The offset of this observation modes has three directions that are along the tunnel axial, vertical and horizontal, respectively. Through theoretical analysis and numerical forward modeling, it was found that this observation modes maximized the use of the limited observation space of the tunnel cavity, which was beneficial to achieve reliable results of the velocity distribution, improve the location accuracy of the anomalies and improve the detection results of the anomalies with small intersection angles to the tunnel axial to a certain extent.(2) In terms of the new 3D space-class observation mode with three-directional offset, the tunnel 3D seismic ahead prospecting data processing and imaging method was proposed and the processing and imaging program with independent intellectual property rights was developed. For the time-varying frequency distribution pattern of the seismic record, the time-varying filtering method was adopted to effectively filter out clutter and to improve the detection resolution. Considering the problem that the spectral components of partial interference waves are close to the effective waves, F-K and τ-p joint filtering method was adopted which realized the extraction of the reflected waves from the tunnel complex wave field and the filtration of interference waves. The 3D velocity analysis of tunnel seismic wave ahead prospecting under the new 3D space-class observation modes with three-directional offset was realized through adopting the velocity-scan analysis method based on diffraction stack. On this basis, the tunnel seismic wave migration imaging method based on equal travel time was realized by utilizing the principle of geometric seismology, the principle of wave kinematics and the Huygens’ principle, which further improved the detection results of the anomalies with small intersection angles to the tunnel axial.(3) Through systematic forward modeling and imaging tests, the 3D seismic wave field response pattern and imaging characteristics of the typical adverse geologies such as faults, lithologic interface and karst caves were revealed. The 3D seismic wave field propagation and response patterns produced from the lithologic interfaces with different angles, faults with different thickness and karst caves with different diameters were studied and processed by the tunnel 3D seismic wave ahead prospecting imaging method and program proposed in this paper. The imaging characteristics of the typical adverse geological bodies were achieved which would provide the theoretical basis for the site data processing and interpretation of the tunnel 3D seismic wave ahead prospecting.(4) Taking into account the multiple solutions and non-reliability of each single geophysical method in the traditional integrated advanced prediction, using the constraint inversion and joint interpretation as the core, the tunnel comprehensive advanced prediction method of adverse geologies was proposed based on 3D seismic prior constraint information. First, take the geological structure boundary and morphology information obtained by the seismic wave advanced prediction method as space structural constraints and apply the constraints into the 3D resistivity inversion equation. Combined with the inequality constraints theory, the 3D resistivity constraint inversion method was proposed based on the inequality constraints and space structural constraints. Second, the different response patterns of adverse geological bodies derived from different detection methods were summarized from different aspects in terms of differences in elasticity, conductivity and dielectricity, and the joint interpretation rules of the aqueous body was proposed. On the basis of constraint inversion and joint interpretation, the tunnel adverse geology comprehensive prediction method and system based on 3D seismic prior constraint information were then built.On the basis of the above studies, a complete set of methods with integrated prediction and joint interpretation method of tunnel 3D seismic wave advanced prediction based on 3D space-class observation modes was formed eventually. The research achievements have been applied to serious of tunnel projects like Jilin Songhuajiang Water Supply Project (Tenders TBM2, TBM3), a diversion project in Liaoning Province, Wudongde Hydropower Traffic Tunnel Project and so on. The prediction results were consistent with the actual excavation results, which verified its effectiveness and reliability from the view point of site and practical application. |
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