The magnetic thickness tool is based on an electromagnetic phenomenon that can be observed by placing coils in a conductive pipe. The measurement is made with a coil arrangement that produces a magnetic field that opposes the primary field casing attenuation and phase shift.
The magnitude of the phase shift is a function of the casing electrical conductivity, magnetic permeability, and thickness of the metal present in the field. Defects in the pipe disrupt the eddy currents and are detected by the far field detector as increases in amplitude and shorter transit times. Multiple coil spacing and frequencies are used to control the depth of investigation and measure electromagnetic properties. The magnetic thickness tool measures casing wall thickness using a remote eddy current field measurement.
Phase shift of the signal received by the far field detector is proportional to wall thickness. A differential thickness measurement is made by a second receiver coil closely spaced to the far field detector. This provides an enhanced resolution of small defects in the casing wall.
An additional coil pair array provides an electromagnetic caliper which measures casing inner diameter. This measurement can be used in conjunction with a mechanical caliper to determine magnetic properties of an indicated defect.