Research On Joint Inversion Method Of Seismic Compressional And Shear Waves From Ocean-Bottom Nodes (OBN)

To explore offshore oil and gas,the OBN(Ocean Bottom Nodes)seismic exploration method can not only obtain full azimuth fidelity data,but also improve the repeatability of 4D exploration and reservoir monitoring results.One of the most important purposes of seismic exploration is to obtain the subsurface structure information in depth domain.In order to obtain the depth information,it is necessary to have accurate velocity value.The elastic parameters obtained from the P-wave velocity information are not closely related to the underground structure and pore fluid.However,the multi-component seismic data acquired by OBN,include P-wave and S-wave information of subsurface media.Since the S-wave velocity is only related to the rock skeleton,combination of P-and S-wave data can reduce the uncertainty of reservoir detection and improve the accuracy of fluid identification and lithology interpretation.Therefore,it is of great significance to invert the P-and S-wave velocity by OBN data.After performing some preprocessing on OBN data,such as relocation,reorientation and wave field separation,the velocity model can be obtained by inversion.Based on the inverted P-wave and S-wave velocity model,the Vp/Vs ratio,Poisson's ratio and Lame constant can be calculated.These parameters are related to the characteristics of rock skeleton and reservoir fluid,which can provide abundant and valuable information for reservoir prediction and oil&gas identification.Therefore,in consideration of the advantages of multi-component seismic data in the exploration and development of tight gas reservoirs,we studied the joint inversion method of P-wave and S-wave velocity of the OBN seismic data in this paper.Firstly,according to the migration section and geological conditions,the layered media model is established with reasonable interpretation,and the time-depth transformation was carried out to obtain the velocity-depth model.Then the linear traveltime interpolation ray tracing forward algorithm and particle swarm optimization nonlinear inversion algorithm are comprehensively adopted to perform joint inversion of prestack seismic Pand S-waves to obtain the velocity distribution of underground media,which provides a basis for migration velocity modeling and oil and gas identification.In the process of joint inversion,the matching of P-and S-waves from the same horizon is a difficult problem.We introduce the coherent inversion algorithm to effectively solve the matching problem of P-and S-waves based on the obtained P-wave velocity.After giving the range of S-wave velocity and updating interval of S-wave velocity according to the empirical formula,the coherent values corresponding to different shear wave velocities may be calculated afterwards.The seismic event corresponding to maximum coherent value is the converted shear wave to the corresponding horizon.The experimental results of theoretical models and actual OBN data show that the method is feasible,and a high accuracy joint inversion of P-and S-wave velocities can be achieved with this algorithm.