Image polarity

End member colour assignment of borehole image or dipmeter data based on the range of resistivity or sonic response.  Usually (convention) for resistivity tools is for conductive features to map to dark (black) colours and resistive ones to map to brighter (white) colours.  Image polarity can be checked by comparison with a resistivity log or against a known reference lithology such as a resistive carbonate body.  With acoustic imaging tools, high values of the reflected amplitude signal and smaller travel time are mapped to light (white) colours with lower values mapped to dark (black) colours.  With televiewers image polarity can again be calibrated against a lithology with a known or expected response.


During interpretation of borehole images, the eye is drawn to darker colours, leading to a visual bias in the analysis of a given section.  To offset this bias it is often sufficient to reverse the polarity of the images interactively, remembering to change back so as to keep a standard scheme.




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