Abstract：It's difficult to directly use poststack seismic data for studying time difference of time-lapse because the poststack seismic data corresponds to zero-offset data and the time difference of time-lapse is very small.The authors presented adopting prestack seismic data on far-offset to calculate the time difference of time-lapse.The order of time difference by finely calculation for mufti-layered theoretical model can be reached to 10 ms and above.But for a layered medium,it's difficult to compute the vertical time difference directly by using prestack data on far-offset because the travelling paths of seismic wave before and after the reservoir variation are very complex and shot-points and receiver-points are changed.i aking a double-layered medium as an example,the paper deduced and studied the time uifference of layered medium,considered that using time difference of time-lapse produced by large incident angle can accurately calculate the velocity difference produced by replaced reservoir fluid,which attained the goal by using 4-D seismology to monitor the reservoir.
