Abstract

Seismic data from the Overthrust Belt of southwestern Wyoming should typically be depth imaged in 3-D. A simplified thrust model is studied to illustrate (a) the complications produced by rapid lateral velocity changes, velocity pull-up, etc., and (b) the necessity of using a depth imaging process versus time imaging plus conversion to depth in complex geological areas. Two case studies from southwestern Wyoming are examined todemonstrate new methods that provide improved images of seismic data. For both cases, stacking velocities were initially used to derive interval velocities for modeling, but this led to incorrect, contradictory geology. Subsequently, velocity information from available well logs helped produce much-improved depth models for the depth imaging process. When a grid of data is available, the imaging is typically done in 3-D.

In the second case study, a 2-D depth-imaged section leaves unexplained, crossing events due to seismic energy coming from out-of-the-plane. Subsequently, a two-pass, 3-D depth image was produced, which provides the "best" depth image, and the final interpretation, as confirmed by drilling, is much closer to reality. The objective of depth imaging, and when the data are available, 3-D depth imaging, is to produce as accurate a picture of the actual geology of the earth as possible in areas of complicated geological structures.