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Self-consistent depth imaging of seismic data in three dimensions is used to map the complex subsurface geology in northern Utah and southwestern Wyoming. Concepts are illustrated with a prototype thrust-fault model of the region. Data from the model illustrate the complications produced by rapid lateral velocity variations in the rocks and the importance of using depth imaging versus conventional time imaging of seismic data when mapping complex geology.
Case studies from northern Utah and southwestern Wyoming are examined to demonstrate new methods that provide improved images of seismic data. For both locations, stacking velocities initially were used to derive interval velocities for modeling, but this leads 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 should typically be done in three dimensions.
In the first case study from southwestern Wyoming, many diffraction events are produced by the sharp velocity contrasts across the Absaroka Fault. A depth-imaged section cleans up these diffractions and greatly aids the interpretation of the faulting. Two good producing wells are located through the Absaroka fault plate.
In the second case study from northern Utah, a two-dimensional depth-imaged section does not explain crossing events due to seismic energy coming from out-of-the-plane. Subsequently, a two-pass, three-dimensional depth image was produced, which provides the “best” depth image; and the final interpretation, as confirmed by drilling, is much closer to reality. Depth imaging, and when the data are available, three-dimensional depth imaging, are essential for clarifying structure beneath complex overburden to help map and assess subsurface hydrocarbon traps.
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