Accuracy Verification And Application Of Fluid-solid Coupling Wave Simulation Method

With the advancement of China 's " Marine Power Strategy," the construction of islands and reefs in the South China Sea and major projects in coastal areas have developed rapidly.At the same time,the regional division of China ' s sea area is not perfect,and the seismic safety problem of marine engineering is prominent,so it is urgent to carry out the research of sea area ground motion.Due to the late start of China's ocean observation network and the scarcity of sea area ground motion data,numerical simulation is an important means of studying sea area ground motion.Compared with the land area,the key to the construction of marine seismic simulation method is to accurately simulate the influence of seawater-seabed fluid-solid interaction effect on wave propagation.In this paper,the fluid-solid coupling wave simulation method based on Legendre spectral element is studied.Firstly,the process of using the coefficients of reflection and transmission of the fluid-solid coupling to verify the accuracy of the simulation method is established.On this basis,the method is applied to the simulation of the ground motion field of the reservoir gas gun with a large number of measured records,and the feasibility and limitations of the numerical simulation of the fluid-solid coupling effect on the wave propagation are clarified.The research contents and results are as follows:1.The establishment of accuracy verification process of fluid-solid coupling numerical simulation method : Firstly,the reflection and transmission coefficient matrix of plane wave at solid-solid interface is derived.According to the different boundary conditions,the reflection and transmission coefficient matrix of fluid-solid interface is obtained by deleting the corresponding elements and the energy distribution is discussed.Then,the fluid-solid coupling numerical simulation scheme is constructed to simulate the incident plane wave,and the coefficient of reflection and transmission is calculated according to the obtained records.In this process,the Hilbert envelope amplitude ratio is proposed to replace the peak ratio to eliminate the error caused by the waveform change due to the phase shift when the incidence angle exceeding critical angle.Finally,the simulation results and theoretical derivation results of Hilbert envelope amplitude ratio under the conditions of water-soft bedrock and water-hard bedrock are compared.The results show that the relative error of the two is less than 1 % in most cases,which proves the reliability of the proposed scheme.2.Application of two-dimensional fluid-solid coupling wave simulation method in the study of the excitation law of reservoir airgun seismic field : The rationality of using trapezoid to approximate the reservoir base is verified based on the measured two-dimensional reservoir profile.On this basis,combined with the layered wave velocity structure in Fujian,the influence of reservoir width,depth,oblique angle and focal depth on the reservoir airgun seismic field is analyzed.The results show that the variation of near-field P wave amplitude of reservoir is most sensitive to the variation of focal depth,and the P wave amplitude increases significantly with the increase of focal depth.The secondary influencing factors are reservoir depth and reservoir width.With the increase of depth and width,the P wave amplitude increases first and then decreases.The amplitude of P wave only slightly changes with the reservoir oblique angle.On this basis,combined with the ray theory,the physical mechanism behind the above laws is preliminarily explained.3.Application of three-dimensional fluid-solid coupling wave simulation method in the study of excitation law of reservoir gas gun seismic field : In view of the fact that two-dimensional wave numerical simulation cannot realize the dominant phenomenon of S wave on some observation lines when thegas gun is excitated in irregular reservoirs and,three-dimensional wave numerical simulation is carried out combined with the measured three-dimensional base of reservoir.The results show that the S-wave dominant phenomenon can be effectively reproduced by considering the three-dimensional measured irregular reservoir basement.On the contrary,the two-dimensional numerical simulation results on these observation lines are always P-wave dominant Therefore,the three-dimensional model must be used to accurately simulate the seismic field of irregular reservoirs.