Imaging Velocity of Pre-Stack Depth Migration in Steep and Complicated Structures

Abstract During the processing of pre-stack depth migration (PSDM) of seismic data in complex structures in western China, an anomaly of interval velocity inconsistent with geological features appears, even when the CRP gather is flattened and the residual velocity (or the residual moveout) returns to zero(Reshef, 2001). The model data analysis in this paper suggests that velocity errors in the shallow layer lead to these anomalies. In order to improve PSDM results of near-surface complicated zones, the velocity variation trend of the deep layer is restrained by geological constraints and residual spectra and afterwards an updated velocity modeling method using the corrected shallow layer velocities is applied. The crucial point in this paper is to understand the reasoning for velocity anomalies and to build a relatively correct velocity model using the PSDM velocity analysis. Introduction The pre-stack depth migration technique has recently been widely used to obtain better structural images of overthrust nappes in the piedmont zone of western China. Since the surface conditions are complex, poor shooting and receiving conditions result in a low signal-to-noise ratio (SNR) of seismic data and as a result, it is hard to identify effective seismic reflection information of shallow layers data. The very low SNR makes PSDM velocity modelling difficult. The PSDM velocity model is derived from the initial interval velocity by velocity iteration and the interval velocity is converted from pre-stack time migration (PSTM) RMS velocity according to the DIX formula. Whether the iteration velocity is correct or not depends on if CRP gathers are flattened and reflection stack energy reaches a maximum. Using this rule, the accuracy of picking velocity is not guaranteed when CRP events can't be identified. There are more picking velocity errors of shallow layers in continental piedmont zone. The main reason is that piedmont lithology and the velocity of shallow layers is various in transverse and longitude. At the same time, the precision of the velocity spectrum is affected by poor shooting and receiving conditions, less valid multiple stacks and a low SNR of shallow seismic data. This paper focuses on the deep velocity error of complex structures affected by incorrect shallow velocity models that may cause structural deformation of PSDM results.