| The interval velocity is a very important parameter in the field of seismic exploration,and its accuracy has a great influence on seismic data processing and interpretation.The seismic interval velocity inversion method using VSP first arrival traveltime has a high accuracy and has been widely used.Because VSP has obvious offset in construction,and it is usually assumed that the formation is horizontal when using VSP traveltime to invert interval velocity.In practical application,these factors are mainly eliminated by correction,but the accuracy of seismic interval velocity is still insufficient.Therefore,how to improve the accuracy of interval velocity inversion based on VSP first arrival traveltime,and combine seismic interval velocity with logging velocity to carry out three-dimensional velocity modeling is of great significance to improve the accuracy of velocity model.In this paper,we use the multi-stencils fast marching method to carry out the forward simulation of traveltime by eikonal equation,and analyze the effects of offset and tilted stratum on zero-offset VSP traveltime.In inversion,establish a two-dimensional stratigraphic model based on the actual offset of zero-offset VSP and the estimated dip angle of the seismic profile.Establish the objective function according to the minimum square error between the first arrival traveltime forward result of the eikonal equation and the actual first arrival traveltime.Invert interval velocity layer by layer from shallow to deep using simulated annealing algorithm.The interval velocity inversion test under different offset and different stratum inclination shows that the accuracy of interval velocity inversion method based on eikonal equation is obviously improved compared with the conventional interval velocity inversion method based on corrected traveltime,and the effect of stratum inclination on the accuracy of interval velocity inversion is effectively solved.The test shows that the method can achieve better application effect in actual data.On this basis,the paper further discusses the influence of anisotropy on seismic wave travel.And carry out the forward simulation of anisotropic eikonal equation using factorization,and invert anisotropic parameters based on traveltime constraint.The model and actual data test show that this method can obtain reasonable results.We use VSP traveltime to carry out the joint modeling of logging velocity and ground seismic velocity,and carry out the dispersion correction and upscaling of logging velocity under the constraints of VSP data.Then match and splice with ground seismic depth migration velocity to obtain the seismic interval velocity of the whole well section under the constraints of VSP traveltime.In practical application,the precise calibration results of well and seismic are converted into one-way time-depth control points similar to VSP for constrained inversion,which solves the matching problem between seismic interval velocity and characteristics of traveltime when VSP data is lacking.On this basis,establish the stratigraphic framework with the horizon and fault data of the work area,and establish the three-dimensional velocity model using the seismic scale acoustic logging velocity of the whole well section by Kriging interpolation.The result of Kirchhoff migration shows that the three-dimensional velocity model with VSP traveltime constraint can obtain higher accuracy.It has important practical application value for the comprehensive research of well logging,VSP and ground seismic velocity. |
