Steel, cast-iron and ductile-iron water and waste water pipelines which span ravines or are suspended under bridges pose some unique inspection challenges. Accessing the pipeline is the first, and usually the most difficult challenge; however, manipulating a NDT technique in confined spaces and at height can be just as daunting.

Where internal Tools are used, the pipeline can easily be inspected; however, sometimes it is not possible to insert internal Tools, and an external inspection is necessary. This paper presents a new, low-frequency AC technique that
has been successfully used to inspect such pipelines from the outside

A case study is presented on a large potable water pipe crossing the Mississippi River, slung under a highway bridge. The technique is described and some of the results of the inspection.

Difficulties such as access and pipe supports, as well as safety constraints are also discussed. The advantages and disadvantages of electromagnetic techniques compared to other NDT techniques are presented.