Time-to-depth migration (TDM)
Traveltimes of CDP-stacked or time-migrated reflections tempt seismic in-terpreters to associate them directly with shapes of the reflecting horizons in depth. Such an association is certainly justified in areas of moderate geologic complexity. Moreover, time-migrated reflections often provide a good approx-imation of the true shape of seismic reflectors.
In the last two chapters, however, we showed that time-migrated seismic reflections, like CDP-stacked reflections, rarely fall precisely above their corre-sponding depth points. Lateral position errors in time migration are caused by lateral velocity changes above the reflectors of interest. We will refer, in general, to any of the many processes that transform from apparent positions on seismic time sections (whether stacked or time-migrated) to true positions in depth as time-to-depth migration (TDM).
It has long been recognized (Slotnick, 1936, 1959; Rice, 1949; Krey, 1951; Hagedoorn, 1954; Musgrave, 1964) that TDM is required for unmigrated data such as seismic records and sections of CDP-stacked or normal reflections. That TDM is also required for time-migrated reflections some readers may have realized (perhaps with some disappointment) only upon reading chapter 7. Whenever normal-incidence rays or image rays deviate from the vertical, a time-to-depth conversion that follows a CDP stack or time migration must have some form of TDM as a final step. In section 8.1, we briefly describe a TDM process for normal reflections. In sections 8.2 and 8.3, we show how to perform TDM for time-migrated reflections.
As we shall see in Chapter 9, the subject of computing interval velocities is also very
Figures & Tables
Interval Velocities from Seismic Reflection Time Measurements
“Over the years, ray theory has furnished the exploration geophysicist with most of the working tools for understanding and interpreting events observed on reflection seismic sections. Even today, notwithstanding the pace at which the more powerful acoustic wave theory is introducing its new tools, ray theory, in the hands of the authors, retains its preeminence for providing insights into fundamental problems in reflection seismology. Professor Krey's earlier contributions are part of ray theory's rich heritage. Alongside C. Hewitt Dix and Hans Durbaum, he elucidated relationships between interval velocity and observed reflection moveout.”