From an early stage it was recognised that a gyro survey tool had inherent misalignments associated with the modular structure of the Instrument Stack and more particularly the vane type centralization (Weatherford) which was universally adopted for running in both open and cased hole. These centralizers were not a precision piece of manufactured equipment and variants of this centralizer design, all be it with improved centralization capability, continue to be used today for some applications. These misalignments result in small errors in both the inclination and azimuth values calculated and recorded for each survey point.
In 1936 the Sperry-Sun Well Surveying Company developed a ‘True Centre Correction’ methodology which still forms the basis of current applications. The ‘true centre’ and ‘corrected values’ are calculated using a Polar Coordinate Method by representing each Station data set as a radial and angular coordinate. Provided sufficient tool rotation has taken place between the in-run and out-run samples, the common intercept of each sample vector pair denotes the True Centre Correction Value which can then be applied to each individual survey sample. True Centre Correction is particularly important in shallow, low angle, multi-well applications where small errors in Inclination can seriously misrepresent current well position and hence adjacent well displacement.
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