Study On Velocity Inversion For Walkaway-VSP First Arrival Travel Time

Compared with seismic exploration techniques,Vertical Seismic Profile(VSP)technology has the advantages of low acquisition cost,high signal-to-noise ratio,and high resolution.VSP has been one of the key technologies of interest in the oil and gas exploration industry and academia since the 1940 s.Walkaway-VSP(WVSP)technology has a higher coverage and a larger coverage range.At the same time,WVSP technology can obtain accurate and reliable time-depth relationships around the wellbore through first arrival wave picking.How to effectively use first arrival information for velocity inversion has become the key to WVSP velocity modeling.This article focuses on the problem of first arrival inversion in WVSP.The main technical route is to use first arrival travel time and first arrival time difference as the main optimization objectives,reduce the ill-posedness and multi-solution nature of the problem through regularization methods,and realize first arrival inversion of WVSP.The main innovations are as follows:(1)Thesis proposes an initial arrival inversion method based on the joint constraint of low-frequency velocity model and layered structure for the underdetermined problem of Walkaway-VSP velocity inversion.First,the ground seismic velocity is used as the initial velocity model,and the ray tracing forward calculation of the grid velocity model is performed to obtain the time difference between the picked first arrival and the forwardcalculated first arrival travel time.Then,low-frequency velocity field and structural information are extracted from both ground seismic velocity model and structural interpretation data to form low-frequency background constraint regularization term and layered structure constraint term,and the mathematical model of the inversion method is established based on this.Finally,the inversion problem is solved through the interface provided by Eigen.This article verifies the effectiveness of the method through theoretical model calculation and demonstrates its adaptability to complex media on the Marmousi model.(2)In first arrival velocity inversion,the rays excited by near-offset distances pass through fewer velocity grids compared to those by far-offset distances,thus the former has higher reliability in velocity adjustment.Based on the work in(1),this article proposes a Walkaway-VSP first arrival inversion method based on offset distance weighting.The method divides the observation system into near,medium,and far offset distances,and iteratively modifies the velocity model to reduce first arrival differences and improve the accuracy of the velocity model.Finally,it is shown through calculation of different models that this method has better convergence effect on the objective function than the conventional first arrival inversion method,and the velocity field is further corrected.