A method and apparatus for measuring casing wall thickness using an axial main coil for generating a flux field enveloping the casing wall, and sensing actual field distribution by means of plural radial sensing coils each disposed at a radial position within the casing thereby to derive data sufficient to model a reference calibration for good casing wall structure; and, sensing flux leakage from the casing wall to detect anomalies of wall structure relative to the established reference calibration for output indication versus position along the casing.
