Forward And Inversion Of Geological Prediction In Tunnel With Negative Visual Velocity

With the development of society and the progress of science and technology, the quality of the project is more and more high, the railway and highway need to be built in the tunnel,then the excavation of the tunnel has become an important link. But due to the geological conditions of work face is not clear, the geological risk factors potentially more, if we dig, it will appear a variety of geological situations, such as collapse, mud and water bursting, the construction process of the tunnel will be greatly affected, even cause casualties and economic losses. Therefore, research on tunnel advance geological forecast is imperative,various experts and scholars study hard, and hope as much as possible understand geological conditions in front of tunnel face,it can provides the basis for more secure tunnel construction.There are many kinds of geological prediction methods in tunnel, which can be divided into three kinds of geological, drilling and geophysical prospecting methods, because of the accuracy and the actual needs, most of them choose to use the method of geophysical prospecting. Commonly used geophysical prospecting methods are TSP, geological radar and transient electromagnetic, seismic methods is one of the best and gain widespread use.Domestic negative apparent velocity method, the International said, tunnel VSP". In simple terms, the tunnel negative apparent velocity method is what layout shot points and receiver points through the tunnel wall, shot points and receiver points on a straight line and vertical to the tunnel face, so as to get the reflected wave time distance curve and the direct wave time distance curve of the intersection is the position of the tunnel face.In this paper, the finite difference forward modeling of wave equation is performed.on the front of the tunnel face exist various interface when the seismic wave field numerical simulation, And in the vertical and horizontal direction, the seismic record is obtained.Through the study of the wave field snapshots and seismic records, the characteristics of the interface reflection wave under the excitation condition of the tunnel wall are studied, which provides a reasonable basis for the interpretation of the negative visual velocity method. In forward calculation, partitions with attenuation method to deal with the free boundary in the tunnel, tunnel internal wave field attenuation value is zero, effectively reduce the influence ofthe tunnel; perfectly matched layer(PML) to deal with artificial boundary, eliminating the interference caused by the boundary.For small angle interface(< 60o) negative apparent velocity method of conventional extension of homeopathy. There exists the problem of big error, the paper through theoretical model analysis establish polynomial fitting, even fractional interpolation, three point interpolation distance curve, can effectively reduce the errors to 1%-5.5%, improves the accuracy of the interpretation. The five models to study in front of tunnel face seismic wave field characteristics and seismic direct wave and reflected wave time distance curve characteristics. For forward modeling of seismic record, the traditional way is through the extension of homeopathy distance curve to determine the location of the interface, so as to obtain the error with the interface of smaller inclination is more and more big, this paper puts forward the interpolation fitting way to deal with distance curve, comparison of the two methods, interpolation intends to the error is very small, and summarizes the negative apparent velocity method to time distance curve. For multiple interface, but also pair distance curve correction, the energy conservation demand areas adjacent to the velocity values, then pair distance curve correction, the comparative study can be obtained in processing of multiple interface, you can reduce the error. Forward modeling provides reference for the study of the kinematics and dynamics of tunnel seismic wave propagation, and provides a theoretical basis for the processing and interpretation of the seismic wave data.