The time-to-depth conversion strategy involves the following steps:
- Interpret a set of time horizons from an image volume derived from time migration; these time horizons are usually associated with layer boundaries with velocity contrast or geological formations of interest.
- Intersect rms velocity functions picked at specified analysis locations over the survey area with the time horizons from step (a) to derive horizon-consistent rms velocity maps. The rms velocity functions are preferably picked from gathers derived from prestack time migration.
- Perform Dix conversion of the rms velocity maps from step (b) to derive interval velocity maps.
- Perform vertical-ray or image-ray depth conversion of the time horizons from step (a) using the interval velocity maps from step (c).
The combination of the interval velocity maps from step (c) with the depth horizons from step (d) constitutes the initial model derived from time-to-depth conversion. This initial model may then be calibrated to well data if the depth horizon maps are to be used as depth structure maps for well positioning. Alternatively, the estimated initial model may be updated and used to perform 3-D post- or prestack depth migration to derive an image volume in depth.
In reference to step (c) of the procedure outlined above, interval velocities may be estimated by methods other than Dix conversion (Table 9-1). Nevertheless, as part of the common strategy for time-to-depth conversion, interval velocities are derived from Dix conversion of rms velocities. In reference to step (d) of the procedure outlined above, depth conversion of time horizons may be performed by one of the following three strategies:
- Most commonly applied strategy is based on a combination of Dix conversion of rms velocities to interval velocities and image-ray depth conversion of time horizons interpreted from the time-migrated volume of data. This is the usual implementation of map migration. Stacking velocity inversion sometimes may be substituted for Dix conversion to estimate interval velocities.
- Alternatively, depth conversion may be performed using vertical rays. This is acceptable only if lateral mispositioning because of lateral velocity variations is negligible (introduction to earth imaging in depth). Again, the interval velocities are estimated by Dix conversion.
- Albeit rarely, a third option is to use normal-incidence rays for depth conversion. Time horizons interpreted from the time-migrated volume of data may first be forward-modeled to derive 3-D zero-offset traveltimes, which are then depth-converted using normal-incidence rays. Dix conversion still is the robust method for interval velocity estimation.
- Model building
- Time structure maps
- Interval velocity maps
- Depth structure maps
- Calibration to well tops
- Layer-by-layer inversion
- Structure-independent inversion