Although doing it right the first time is most desirable, there is never a situation where this is possible when estimating an earth model in depth. The velocity-depth ambiguity that is inherent to inversion makes it very difficult getting the right answer — the true geological model, let alone the first time. Limitations in the resolving power of the methods to estimate layer velocities that arise from the band-limited nature of the recorded data and finite cable length used in recording further compound the problem. Finally, traveltime picking that is needed for most velocity estimation techniques and time-to-depth conversion as well as picking depth horizons from depth-migrated data to delineate reflector geometries are all adversely affected by noise present in the data.
All things considered, we can only expect to do our best in estimating what may be called an initial model, and update this model to get an acceptable final model. In this section, we shall discuss ways to estimate an initial model, and in the next section, we shall discuss application of residual moveout analysis and reflection traveltime tomography to update the initial model.
We shall discuss two strategies applicable to both 2-D and 3-D seismic data for initial model building:
- A time-to-depth conversion strategy based on interpretation in the time domain, and
- A layer-by-layer inversion strategy based on interpretation in the depth domain.
Practical methods to estimate layer velocities and delineate reflector geometries used in implementing the two strategies are listed in Table 9-1. A widely used combination for time-to-depth conversion is Dix conversion to estimate layer velocities and image-ray depth conversion to delineate reflector geometries. Whereas for layer-by-layer inversion, a widely used combination is coherency inversion to estimate layer velocities and poststack depth migration to delineate reflector geometries.
- Time-to-depth conversion
- Time structure maps
- Interval velocity maps
- Depth structure maps
- Calibration to well tops
- Layer-by-layer inversion
- Structure-independent inversion