Study On First Break Velocity Inversion And Near Surface Structure Modeling In Piedmont Zone

With the deepening of exploration tasks,seismic data processing technology is also developing,and the processing focus has changed from post-stack to pre-stack and from time domain to depth domain.The imaging of prestack depth migration is highly dependent on the near-surface velocity model,and its accuracy directly affects the imaging accuracy.First-arrival traveltime tomographic inversion is based on the first-arrival traveltime information in seismic data,which is used to reconstruct the underground near-surface velocity structure,thus reflecting the characteristics of underground geological structure and stratum distribution.At present,firstarrival traveltime tomographic inversion is one of the important methods to obtain near-surface velocity model and has been widely used.However,in the complex piedmont area,the velocity model obtained by the conventional first break travel time tomography inversion technology is not accurate enough to meet the increasing demand for high-precision imaging.To solve this problem,based on the theory of tomographic inversion,this paper studies the near surface structure investigation,high-precision first break pickup method,ray forward modeling theory and tomographic inversion theory.Three kinds of first break travel time tomographic inversion techniques are adopted:(1)the initial model is established with the constraint of micro logging,and the inversion result of small offset restricts the inversion of near surface model with large offset;(2)First break constrained inversion near surface model in CMP domain;(3)Feasible variable tomographic static correction and inversion near surface model.The accuracy of several sets of velocity models is verified by six verification methods,including:(1)the coincidence between the thickness map of low deceleration zone and the trend of surface elevation and geomorphic characteristics;(2)Geological outcrop and microlog information;(3)Apparent velocity of first break of single shot;(4)Common offset first arrival;(5)VSP well information;(6)Prestack depth migration imaging.Finally,it also analyzes whether the inversion results are accurate through forward modeling.Form a set of tomographic inversion technical process and verification ideas suitable for complex mountain data.Through the research of this paper,the following conclusions and understandings are obtained:(1)The surface conditions of complex mountain data are poor and change violently.Due to the high cost and certain layout conditions,micro logging cannot be implemented on a large scale.This paper proposes a method of constrained tomography inversion in CMP domain.By uniformly setting control points,collecting the included CMP points,and linearly fitting the time distance curve of CMP,a priori information can be obtained,which can solve the problem of insufficient constraint information.This method can effectively control the horizontal and vertical velocity changes of the model,so as to improve the accuracy of the velocity model.(2)Conventional tomographic methods use the same fixed grid for inversion.Local special geological anomaly bodies are often not accurately described,and they are equivalent to the surrounding velocity bodies in velocity performance,and this part of velocity information is the focus of subsequent depth migration processing.Deformable tomography is divided by irregular grid,and the geometric shape of the inversion velocity interface is used to replace the inversion velocity.It can be used to invert the layer velocity and interface shape at the same time.It can more accurately depict the boundary of different formation velocity changes,and can effectively improve the accuracy of near surface velocity model,with the best effect.(3)When the near surface velocity is verified to be accurate,the static correction amount is generally calculated after the subsequent processing,and the application effect of single gun and superimposed section is analyzed.However,the accuracy of the near surface velocity model cannot be judged because the velocity in different low deceleration zones may get little difference in static correction.Through the analysis and comparison of six verification means,forward and inverse,this paper greatly improves the reliability of verification,enriches and summarizes a complete set of verification means,and provides detailed ideas for the verification of the model.