The remote field non-destructive inspection technique is an eddy current technique. Its primary application is in the full wall inspection of steel pipes with an internal probe.

Eddy currents are induced in conductors by fluctuating magnetic fields from exciter (exploring) coils. These eddy currents get their name from their continuous loops which resemble eddies in liquids. Eddy currents, in turn, produce their own magnetic fields that always oppose the exciter field and attenuate and delay the exciter field as it penetrates into the conductor. They act as a shield.

These induced eddy currents are the key to defect detection. Defects block and distort their preferred flow patterns. This disruption can be detected. Defects reduce the eddy current shield. The ways in which these eddy currents are detected and analyzed give rise to distinctly different forms of eddy current testing.

The remote field inspection technique is a through transmission technique. Only the interrogating field that has completely penetrated the pipe walls is detected. This indirect coupling path is “non-intuitive” in that both exciter and detectors are in the pipe interior. Interpretation usually uses a voltage plane polar plot of the penetrating field vector to examine defect indications.

In the more common and conventional form of eddy current testing, the fields from induced eddy currents retracing the path of the penetrating exploring field affect exciter coil impedance in a reflection like process. An impedance plane presentation is used to interpret defect indications.

While both are eddy current techniques, these two approaches differ widely in their characteristics, capabilities and applications.

The conventional exploring coil technique has had lengthy development and is, by far, the most common. It is severely limited by depth of penetration, however, and is seldom used to inspect ferrous materials with the exception of detecting near surface breaking cracks.

The remote field technique was originally developed for the in-situ detection of external corrosion in oil well casings, but its application is expanding for inspecting the wide range and large quantities of steel tubulars from heat exchanger tubes to pipe lines. It is not limited to ferrous pipes and works equally well in non-ferrous tubes.

Remote field, being a through-transmission technique, is now significantly easier to understand and interpret than the conventional reflected impedance technique.

This paper will show how simple skin effect equations can be used to explain and analyze the remote field, through-transmission inspection. Results of simulations based on these skin effect equations will then be used to demonstrate effects of relative size of defects and detectors, and response to standard defect geometries. Both a single, absolute detector and dual, differential detectors patterns are illustrated. Once these basic patterns can be recognized and understood, the more subtle patterns due to specific defect perturbations such as pits with metal loss or cracks can be singled out for interpretation. This paper will introduce the basic patterns called "signatures".

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Updated August 2002