By Dugald McConnell and Brian Todd

Residents of Washington may be surprised by a helicopter flying low overhead this week, endlessly prowling the city to map its radiation signature.

The helicopter is crisscrossing the city, like a lawn mower covering a lawn, flying as low as just 150 feet off the ground. CNN spotted it northwest of downtown on Monday, flying low over the buildings, back and forth, east to west.

The purpose: to produce a baseline scan of the natural radiological readings in the capital. Once the map is done, any new anomalies - or suspicious radioactive activity - could be more easily detected.

If authorities ever need to find stolen radioactive material in the capital or a dirty bomb, they would want to be able to separate out any new spikes in the radioactivity readings.

"If sometime in the future you have a reason to be looking for something radiological, it's very necessary to have the original background so you don't chase a high-radiation area that's part of the background," said Joseph Krol of the National Nuclear Safety Administration, which is conducting the work.

"If there was some type of threat," he said, "we would do the aerial survey first." Then, officers with sensors would go on foot to the location where an anomaly was found. A team could be deployed anywhere in the country within two hours, he said.

"The way we operate," he said, "we assume every time we get a call, that this is the big one."

The scan of the District of Columbia will cover 70 square miles and take about two weeks to complete. But if authorities were urgently trying to find nuclear material, Krol said, "we would hope that scanning the entire city would not be necessary. You always have other indicators, like intelligence, that narrows down the search area."

A dirty bomb - a bomb with no atomic explosion, but spewing enough radioactive material to require evacuations and quarantines - is a method of attack that terrorists would be highly interested in, according to Randy Larsen with the Institute for Homeland Security.

"The good news is, it wouldn't kill a lot of people," he said. "But if you did it here in downtown Washington D.C., or Wall Street in New York City, and the people can't go back in that area for a long time, that's a pretty significant event."

But could a hidden dirty bomb be detected from a helicopter?

"Absolutely," said Larsen, but only after a detailed scan has been done. "This would tend to make something stick out like a sore thumb. Without it, it would be looking for a needle in a haystack."

Two pilots, a scientist, a technician and equipment for detecting gamma radiation are aboard the flights.

Everyday radiation can come from several common urban sources, including industrial sites, construction sites, hospitals that use radiological materials, and even the stone used in buildings and monuments.

The agency has already scanned the capital once before, as well as New York, Baltimore, and San Francisco.

Larsen said the New York scan turned up unexpected sources of radiation - industrial accidents that were never been reported. "They were surprised how many hot spots they found," he said.

The helicopter in Washington is making an exception for the National Zoo, giving it a wider berth to avoid scaring the animals. But Krol said the public should not be worried about the survey's potential impact on people.

"These scanners are passive," he said. "All they're doing is measuring the emanations coming from the ground."

Watch: Surviving a "dirty bomb"