Depth Conversion and Depth Imaging
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. The way this has occurred for depth conversion is shown in Figure 10-1. Depth imaging is used when the velocity distribution and structural complexity are such that the time image of the subsurface does not permit the interpreter to understand the geology (Figure 10-2).
Depth imaging is difficult, expensive and never completely accurate. The most accurate depth imaging uses pre-stack depth migration of the 3-D seismic data volume, a computationally intensive task which is critically dependent on an accurate velocity field. The velocity field cannot be known until the geological structure is known, and the geological structure cannot be known until the seismic volume has been migrated. Consequently, the depth imaging process usually involves iteration.