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Seventh International Williston Basin Symposium, July 23,
Seismic reflection data can be imaged in depth to form an accurate picture of the true geology of the earth in complex areas where the velocity changes abruptly, both vertically and laterally, such as some areas of the Williston Basin. A subsurface velocity model necessary to form an accurate image can be obtained from the data through self-consistent computer modeling. Where available, the modeling is controlled with nearby well data, and the imaging is done in 3-D.
A dome 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 migration process versus time migration plus conversion to depth in complex geological areas. A case study from the Williston Basin is examined to help delineate a possible anticlinal prospect. Initially, stacking velocities were used to derive interval velocities for modeling, but this led to incorrect, contradictory geology. Subsequently, a well was drilled, and the sonic data helped produce a much improved depth model for the depth imaging process. A 2-D depth imaged section leaves unexplained, crossing events due to seismic energy coming from out-of-the plane. Consequently, a two-pass, 3-D depth image was produced, which provides the "best" depth image, and the final interpretation is much closer to reality. Three-D depth imaging is essential for clarifying structure beneath complex overburden to help delineate and assess subsurface petroleum traps.
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