Migration Velocity Analysis Based On Common Image Point Gathers In Angle Domain

As it is well known, velocity is one of the key techniques in the flow of seismic data processing. And the pre-stack depth migration (PSDM) method needs a exact layered velocity model. The uncertainty of the velocity and depth makes it difficult of PSDM method to have a best imaging section because of the sensitivity of PSDM method to velocity. On one hand, the sensitivity is not good for the migration; on the other hand, the PSDM method is the effective tools for the velocity analysis. Therefore, we can construct velocity model by means of PSDM method and the optimization of imaging result should be the criterion to judge the validity of migration velocity.Generally speaking, PSDM method require migration result (usually named as common imaging gathers (CIGs)) to update and construct the migration velocity model. If the migration velocity is not correct, the residual curvature would be shown in the CIG(velocity lower,CIGs bend;velocity larger,CIGs under bend)s. Angle domain common imaging gathers(ADCIGs) (or ray paramete Ph common imaging gathers (PhCIGs)) characterize the relationship between the local migration depth at imaging points and the incident angles (or ray parameter Ph). In comparison with conventional Kirchhoff type surface offset-domain CIGs (Kirchhoff-ODCIGs), the local offset could not influence the generation of CIGs, so we can say, ADCIGs and PhCIGs gained from wave equation migration reduce the generation of kinematics artifacts as much as possible. This is the best property suited for velocity analysis and later AVA (Amplitude Versus Angle) anal??.The characteristics of PSDM based on double square root (DSR)are little alias, simply boundary treatment, not required seismic source wavelet and great computational efficiency.In this paper,we obtain local subsurface ODCIGs based on DSR-PSDM,then transform to ADCIGs using the equation about the relationship of local subsurface offset direction wave number ,depth direction wave number and incident angle.We also extract true amplitude ADCIDs based on DSR- wave field extrapolation ,then overlay imaging.The residual curvature analysis(RCA) migration velocity analysis method makes use of these residual curvatures to updata the velocity value. The key technique of RCA is how to relate the residual curvatures with the velocity perturbation. For ADCIGs, based on the assumption of level layered media, the residual depth equation is obtained from wave equation ODCIGs. Under the circumstance of constant velocity at local imaging point, through the slant stack technique, wave equation ODCIGs can be transformed to ADCIGs.Based on this equation,we design velocity analysis process layer by layer. The tests prove the velocity analysis methods derived in this paper are exact,robust and valid.