A Deterministic Evaluation of eismic Fidelity using Velocity Modeling ...
A Deterministic Evaluation of eismic Fidelity using Velocity Modeling ...
A Deterministic Evaluation of eismic Fidelity using Velocity Modeling ...
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procedure making the application <strong>of</strong> PSDM more common. Improved computing<br />
provides motivation for a reevaluation <strong>of</strong> the most critical phase <strong>of</strong> the process, the<br />
velocity modeling. Accurately estimating an interval velocity model from a 3-D land<br />
survey was essential to improving the fidelity <strong>of</strong> the subsurface image at Vinton Dome.<br />
In the next section I will begin by reviewing the two most common velocity modeling<br />
approaches, then I will discuss the well-based velocity modeling approach that I applied<br />
to the Vinton Dome 3-D surface s<strong>eismic</strong> survey. I designed the method to make use <strong>of</strong><br />
the sonic logs and extensive well log database.<br />
4.4 <strong>Velocity</strong> Model Building<br />
4.4.1 Grid-Based and Layer-Based <strong>Velocity</strong> <strong>Modeling</strong><br />
Grid-based velocity modeling begins by converting RMS velocities to time interval<br />
velocities. Converting RMS velocities to time interval velocities requires an x, y<br />
location, <strong>of</strong>ten referred to as a velocity location. The velocity location is typically<br />
associated with a reference horizon that serves to identify the interval. Using a flat<br />
horizon that is spaced evenly and increases with depth is one approach to designating the<br />
reference horizon. Interpreting a surface such as a water bottom in a marine survey that<br />
is spaced evenly at increasing intervals with depth is another common approach to<br />
identify the reference horizon. Interpreting s<strong>eismic</strong> horizons is a third method to select<br />
the intervals used to determine the time interval velocity. Once the time interval velocity<br />
has been determined, application <strong>of</strong> the Dix formula (Dix, 1955) converts it to a depth<br />
interval velocity for use in the migration.<br />
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