Displaying Seismic Inversion Traces
Closely spaced Seislogs are presented in cross section format, similar to a seismic section, but on a depth scale instead of a time scale. Stratigraphic relationships are mapped on the continuous log section, under the assumption that a given horizontal lithologic unit maintains a constant velocity until the lithology changes.
 

 

 


Inverted seismic section

This assumption must be tempered by knowledge of gas zones and lithology variations defined by log analysis. Depth plots of gamma ray, density, sonic, and neutron logs, or computed lithology logs, will be a great help in understanding a Seislog section.

Below, a number of velocity breaks have been contoured. In normal use, the entire section is contoured. This procedure lends itself readily to automation. The contours were machine drawn, illustrating the potential for automated stratigraphic mapping. Individual lithological units can be outlined whenever there is a small velocity contrast between adjacent units. While the Seislog velocities may not match the borehole sonic velocities exactly, the relative change in velocity from trace to trace is normally quite reliable and very sensitive to changing lithology, porosity, or fluid content.


Seismic inversion section with interpreted lithology based on velocity contours

Unfortunately, several closely spaced contour lines can be confusing, making it difficult to distinguish individual units. This can be remedied by the addition of color coding, which relates velocity to estimated rock type.


Contoured velocity mapping on a seismic inversion

In general, the color scheme is grouped into three major division: blue tones for the high velocities most commonly associated with carbonates; yellow and orange tones for the intermediate velocities most commonly associated with sandstones; and green tones for the lower velocities commonly associated with shales. By custom, as the velocity of sandstones and carbonates increases, the colors become darker suggesting denser material and less porosity. At the other end of the scale the darkest green colors correspond to the lowest velocity, generally shales. In practice the interpreter is given the option of varying the color scheme to fit known lithology from logs and samples.

The color code is strictly a function of velocity (or acoustic travel time) and only indirectly indicates lithology. For instance, the green colors above are somewhat ambiguous. Although they represent shale in the Paleozoic section, they correspond to sandstones having the same velocity in the Cretaceous. Such ambiguities must be recognized and considered in the interpretation.

Good log analysis results, plotted to the same vertical scale as the inverted seismic section, will help calibrate lithology, porosity, and fluid changes. Raw sonic, density, and gamma ray logs overlaid on the Seislog would also be a tremendous help. It is surprising how few presentations of this type are actually made, considering that seismic inversion that is not calibrated to ground truth is merely colorful, expensive wall paper.
 

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