Resistivity borehole image and dipmeter tools acquire data from sides of a well bore. Electrical current needs to pass through button electrodes and penetrate into the formation. As these devices “see” into the formation, boundaries do not lie on the borehole wall, but are in the formation. This effect needs to be taken into account when calculating orientations. The depth of penetration, or depth of investigation, needs to be added to the hole size (derived from the calipers) to give the electrical radius or diameter, allowing the correct calculation of dips. In many cases these values are small (less than 1-2 cm), but can be as large as 11.5 cm or even deeper.
THEYS, P. 1999. Log Data Acquisition and Quality Control, Second Edition. Éditions Technip, Paris, 453.