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The Preparation of Seismic Depth Maps in Oil Exploration
Sufficient subsurface geologic and seismic velocity control are now available in Western Canada to warrant the preparation of depth maps from seismic reflection results. The need for this refinement is realized from the knowledge that misleading results sometimes are portrayed by seismic maps measured in reflection time due to lateral velocity changes within a prospect.
The specific factors contributing to these velocity variations are examined by referring to an area of concentrated geologic and velocity control. Seismic reflection times to several key horizons were calculated at each well in the Innisfail Field and environs, using known geologic intervals together with their known velocities. The resulting reflection times were contoured as seismic reflection-time maps, to produce maps similar to those resulting from an actual seismic survey. The seismic time maps thus prepared showed the combined influence of three components: 1, the velocity gradient across the area; 2, differential erosion on the Paleozoic surface; and 3, actual structure of the reflecting horizon mapped.
In order to prepare useful maps from seismic surveys the first two of these influences must be removed. From the knowledge gained from this simulation of seismic time maps from known velocities and known geology, a method of depth conversion of seismic reflection data is suggested. This method utilizes a regional geologic structure map of a shallow formation on the basis that the major scene of velocity gradient is in the interval between this formation and the surface. Deeper maps can be prepared by adding thickness intervals to this regional map, with these intervals being calculated in a series of steps determined by velocity interfaces and recorded reflection times.
As an illustration of the effectiveness of this depth conversion method the Innisfail Field is once again considered, but this time on the basis of a minimum amount of well control. Using the seismic reflection times calculated at each well, together with a regional Blairmore geologic map, seismic depth maps were prepared. A comparison of these seismic depth maps with seismic time maps illustrates that the application of some elementary geologic structural knowledge, together with a restricted amount of velocity data, can result in the preparation of seismic depth maps of value to oil exploration.
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