The Migration Velocity Analysis Based On Ocean Bottom Node Geometry

Velocity information is the core content of seismic exploration,and it is no exception for Ocean Bottom Node(OBN)seismic exploration.In Ocean Bottom Node seismic exploration,the geophone is encapsulated in separate node and is deployed on the seabed.Compared with the conventional Marine streamer,it has the advantages of flexible geophone deployment,wide-azimuth acquisition,and four-component geophone can collect more abundant seismic information.The unique observing method leads to special processing before migration velocity analysis.In this paper,starting from the wave equation,the wave equation is extended to the correction of the datum level of the shot and receiver point to turn the special observation system into a conventional observation system where shot point and receiver point located on the same datum plane.At the same time,we study the method of using wave equation to correct the undulating seabed.The combination of the two will lay a good foundation for the migration velocity analysis of OBN seismic exploration.Migration velocity analysis(MVA)uses the velocity-sensitive characteristics of pre-stack depth migration(PSDM)to modify the velocity model,which can obtain a velocity field more suitable for prestack depth migration.PSDM is the basis of migration velocity analysis.In this paper,the wave equation prestack depth migration is first studied.Double square root(DSR)migration is better than single square root(SSR)migration.It does not need to provide source wavelets,and can easily extract common imaging gathers so the DSR migration is more suitable for migration velocity analysis.The angle domain common imaging gathers(ADCIG)does not have multi-path phenomenon and is sensitive to velocity.It is an ideal gather for migration velocity analysis.Starting from the DSR migration,the zero-time imaging condition retains the information related to the offset,and extracts the offset domain common imaging gathers(ODCIG).Then,according to the relationship between the angle and the local offset wavenumber,the ODCIG is successfully converted into ADCIG.There are usually two criteria for the analysis of migration velocity: the depth focus criterion and the gather leveling criterion.The residual curvature analysis(RCA)used in this paper is based on the gather leveling criterion.This criterion considers that if the migration velocity is accurate,the seismic events is concentrated horizontally,and when there is an error in the migration velocity,the seismic events bends.After extracting ADCIG,this paper studies the migration velocity analysis based on ADCIG.Starting from the relationship between ODCIG and migration velocity,we obtain the relationship between ADCIG and migration velocity in horizontal layered media and establish the residual depth equation.At the same time,the residual depth equation in tilted formation is also derived.By comparison,the residual depth equation in tilted formation is closer to the actual result.Afterwards,we analyzed the implementation strategy of migration velocity analysis,pointed out the constraints of the initial velocity model on the velocity analysis,and then we improved the initial velocity model strategy to implement a new analysis strategy—layer-bylayer construct modeling strategy.This strategy is different from the previous method of first giving the initial velocity field and then iteratively correcting the initial velocity field according to the analysis results,but establishing a velocity model from top to bottom layer by layer,avoiding some disadvantages of the initial velocity model strategy.Model testing and actual data processing prove the effectiveness of this method.