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Seismic time must be related to a common datum in
each shot point, in order that the measured travel time to the reflecting horizon may be compared with all other shot points. The reflection seismograph method was first used in the Gulf Coast province, where the present method of correcting seismic time to a horizontal datum was developed. The special geological conditions of the Gulf Coast allow the correcting of seismic time to a horizontal datum to be usable and accurate. However, when this method of correcting seismic time is applied under other geological conditions, serious errors are introduced.
Three types of errors are inherent in correcting seismic time to a horizontal datum. These are: (1) if velocity is inaccurate, each change in elevation of the shot causes a change in the amount of error introduced; (2) constant velocity is used to correct to the horizontal datum, through different formations in each shot point; (3) formations cross the horizontal datum. When beneath the horizontal datum, these are calculated as having the average velocity to the reflecting horizon. When above the datum, the same travel time is calculated at a much lower velocity. The effect is that all near-surface structure cut by the horizontal datum is exaggerated two to three times.
Seismic time can be corrected to a geological datum if each shot hole is logged. By plotting uphole times on the log, seismic time can be corrected to a geological marker without actually placing a shot opposite the bed. By noting seismic travel time between different beds, seismic time in a whole shooting program may be corrected to a particular geological datum, by using the same methods as have been applied in surface and coredrill mapping.
Seismic time corrected to a geological datum has several advantages over time corrected to a horizontal datum. The seismograph is directly applied to local geology and seismic time is measured through the same near-surface geological section in each shot point. Variations in the seismic time between shot points are related to the deeper geological conditions and therefore more directly related to the purpose of seismic exploration.
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