The technique works by inducing magnetic eddy currents to flow in close proximity to a transmitter. In a ferrous pipe, the currents migrate with time; these shifts reveal not only variations in wall thickness but also the location of fractures.

However, DiTullio says he tests selectively. "The philosophy of my business is to use the technology to collect enough information to make an assessment without too much detail," he says. By concentrating tests in areas with the highest probability of wall loss, he says valuable conclusions can be drawn.

Two different testing set-ups were used to study the 24- in.-dia, mild-steel force main in Ohio. Both used the same technology but with different equipment configurations.

One used a crown assessment probe, or CAP. The device is a flexible pipe-conforming antenna that is lowered into a relatively small excavation to scan the crown of a buried pipe. Corrosion in the crown is a concern in pressurized sewer mains, especially in high spots along the line, says Cheryl Braun-Thompson, the Ohio utility's project director. "It's almost like a cursory spot-check to see if you want to invest in the more comprehensive full scans," she says.

Testers made full scans in five locations. This effort required complete excavation of the line and manual placement of a scanning block in a series of planned locations.

Braun-Thompson says CAP successfully checked a line running under a major intersection in less than a day. Results showed no unexpected deterioration. "A known risk is better than an unknown risk anytime," she says.