[OpendTect_Users] 3D survey coordinates

[robert baker]

When interpreting, it is easy to forget that 3D coordinates are inexact.  To make a grid, points on the Earth are projected to a plane.  Spacing between points are equal on the plane but not equal on the Earth.  The difference is usually not great, but, as a rule, we should look at the original CDP location, not the software location, when spotting a well.  The larger the survey, the larger the error, and the worst error can occur when the processor uses a standard grid.  A minimum error grid would have the meridian pass through the center of the survey, but standard grids could have the meridian far from the center, and distortion increases with distance from the meridian.  Geographic distortion has little effect on interpretation because features and geologic sequence are not affected, but distortion could put a well on the wrong side of a fault.  Merged surveys can have substantial error, similar to what happens when maps are pasted together. 
 A proper merging of surveys would require reprocessing, including restacking, to a non-standard meridian.
Bob Baker
 
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