Jeff Cullison might be excused for aching feet, at least in a figurative sense. Cullison works for Northern Natural Gas Co. of Omaha, Neb., which operates 15,700 miles of natural gas pipeline running from the Permian Basin in Texas to the upper Midwest. The pipeline is part of a national network hundreds of thousands of miles long.Unlike the natural gas distributed to consumers, the colorless gas in these transmission lines has no additives to make it smell, thus making leak detection a challenge. Traditionally, the most sensitive way to spot leaks is for a ground crew to walk the line, using a handheld flame ionization device known as a sniffer.Walking thousands of miles of pipeline is not done easily. Nonetheless, the lines of Northern Natural Gas are checked on a periodic basis, according to Cullison. “On the handheld, they are normally done on a three- to 10-year frequency, depending on the type of line and the population. Those are fairly accurate in picking up smaller leaks, especially as they start,” he said.Another type of leak check is done from the air. Planes fly overhead, and the pilot looks for stunted vegetation and other signs of leaking gas. Unfortunately, such indications appear only after a potentially dangerous leak has been active for a while. Now, however, it is possible to combine the two methods so that an airplane flying overhead can spot leaks with sensitivity equal to that of a handheld device while still covering long distances in a single day.Enabling this efficiency is the use of differential absorption lidar, explained Daniel Brake, director of the airborne natural gas emission lidar (ANGEL) service of ITT Corp.’s Space Systems Div., which is based in Rochester, N.Y.Using lidar for differential absorption measurements allows aerial mapping of natural gas pipe leaks with sensitivity equal to that of a handheld device that is walked along the pipeline. Images courtesy of ITT Corp.’s Space Systems Div.The lidar system uses two lasers — both sitting in the mid-infrared at proprietary wavelengths. One is tuned to an absorption peak of natural gas, while the other is spectrally nearby but not on the peak itself. The wavelengths of the reference and probe beams are selected so that the results will not be thrown off by trace atmospheric gases. As in other lidar systems, the lasers fire thousands of pulses a second toward the ground. The difference between the two in the return signal provides information about the concentration of natural gas present in the air column. The use of a probe-reference pair avoids artifacts and reduces false alarms. The result is an aerial scan that is as sensitive as the 50 to 100 parts per million of a handheld sniffer but that can cover up to 1000 miles in a day — a distance that would take a crew months to walk.A leak from a crack in a buried natural gas transmission line is highlighted by differential absorption lidar data. Such leaks can be dangerous and must be detected as soon as possible along thousands of miles of transmission pipeline across the US.“We have what we believe is equivalent gas sensitivity on a per-sample basis. On top of that, we have this broad-area-scan swath and tremendous collection speed,” Brake said.Besides lidar measurements, the system also collects aerial mapping data, providing visual information about encroaching structures and construction. These visuals can be important in assessing the overall safety of a pipeline and in deciding what actions must be taken if there is a leak.Cullison noted that the airborne approach has proved itself in a pilot run conducted by ITT for Northern Natural Gas. A scan of a pipeline turned up a number of leaks, some resulting from mechanical actuators and similar mechanisms that use gas for fuel. In one case, a detected leak came from another pipeline, which crossed the pipline being scanned. The second pipeline had a leak about 100 yards from the crossing point, producing enough of a signal for the aerial scan to detect.“The [pilot run found] leaks that we hadn’t discovered with handheld detectors,” Cullison said. As a result, he added, plans call for using the service more extensively.Contact: Daniel Brake, ITT Space Systems Div.; e-mail: angel@itt.com; or Jeff Cullison, Northern Natural Gas Co.; e-mail: jeff.cullison@nngco.com.