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Depth conversion is an important step of the seismic reflection method, which converts the acoustic wave travel time to actual depth, based on the acoustic velocity of subsurface medium (sediments, rocks, water).
Depth conversion integrates several sources of information about the subsurface velocity to derive a three- dimensional velocity model:
- "Well tops", i.e., depth of geological layers encountered in oil and gas wells.
- Velocity measurements made in oil and gas wells.
- Empirical knowledge about the velocities of the rocks in the area investigated.
- Root Mean Square (RMS) stacking velocities which are derived from the processing of the seismic reflection data.
The conversion permits the production of depth and thickness maps that depict subsurface layers that are based on reflection data. These maps are crucial in hydrocarbon exploration because they permit the volumetric evaluation of gas or oil in place. Depth conversion concerns the seismic interpreter because seismic measurements are made in time, but the wells based on a seismic interpretation are drilled in depth. The depth conversion can now be carried out as part of the data processing, but this depth imaging is only done in special circumstances. Historically, geophysical interpreters have relied more and more on automatic data processing to prepare the data for interpretation
Time-domain seismic imaging is a robust and efficient process routinely applied to seismic data (Yilmaz, 2001; Robein, 2003). Rapid scanning and determination of time-migration velocity can be accomplished either by repeated migrations (Yilmaz et al., 2001) or by velocity continuation (Fomel, 2003). Time migration is considered adequate for seismic imaging in areas with mild lateral velocity variations. However, even mild variations can cause structural distortions of time-migrated images and render them inadequate for accurate geological interpretation of subsurface structures. To remove structural errors inherent in time migration, it is necessary to convert time-migrated images into the depth domain either by migrating the original data with a prestack depth migration algorithm or by depth migrating post-stack data after time demigration (Kim et al., 1997). Each of these options requires converting the time migration velocity to a velocity model in depth. The connection between the time and depth domain coordinates is provided by the concept of image ray, introduced by Hubral (1977). Image rays are seismic rays that arrive normal to the Earth's surface. Hubral's theory explains how a depth velocity model can be converted to the time coordinates. However, it does not explain how a depth velocity model can be converted to the time-migration velocity. Moreover, image-ray tracing is a numerically inconvenient procedure for achieving uniform coverage of the subsurface. This may explain why simplified image-ray-tracing algorithms (Larner et al., 1981; Hatton et al., 1981) did not find widespread application in practice. Other limitations of image rays are related to the inability of time migration to handle large lateral variations in velocity (Bevc et al., 1995; Robein, 2003).