A helicopter will be flying low over San Francisco, Oakland, Berkeley and Pacifica through Saturday to record natural radiation levels, federal authorities said Monday.

The flyovers are part of research by two federal security agencies to compare aerial and ground based mapping of radioactivity.

“In some cases, ground-based monitoring may be superior. In others, aerial detection may be superior,” said Alan Remick, emergency response program manager for the National Nuclear Security Administration. The goal, he said, “is detection of nuclear and radioactive materials both external and internal, such as terrorist threats.”

The twin-engine Bell 412 helicopter will fly a grid pattern about 300 feet above the ground, covering about 70 square miles, including areas near the Oakland-Berkeley border, along the coast in Pacifica and several in San Francisco.

The project will help local, state and federal authorities’ measure radiation, according to an announcement by the National Nuclear Security agency. The agency said the mapping will help set baseline radiation levels. Radioactive sources — such as uranium, radon gas and carbon-14 — have been present in the Earth’s crust since it was formed.

The Department of Homeland Security is charged with keeping terrorists from smuggling radioactive material into the country. The Nuclear Security agency is responsible for securing the U.S. military’s nuclear stockpile and monitoring international nuclear threats at border crossings and foreign airports and seaports.

President Obama in a 2009 speech warned that nuclear terrorism is the most immediate and extreme threat to global security.

One watchdog group saw the survey as important but also called for better planning for accidental or terrorist release of radioactive materials.

“Protection of the public from radiation exposure after a nuclear accident or radiological attack depends critically on the ability to track the path of plumes of radioactive materials,” said Edwin Lyman of the Union of Concerned Scientists, a Washington D.C.-based nonprofit which advocates for a smaller global nuclear stockpile.

Aerial mapping “will help to distinguish the signals of such events from (background) noise and improve the speed and accuracy of such tracking,” he said.

He also used the survey to call for more training to enable the public to act in emergencies and for larger safety zones around known hazards.

“For instance, the U.S. government requires only that people living within 10 miles of nuclear power plants be provided with information on evacuation in the event of an accident or terrorist attack,” Lyman said.

He noted that the Fukushima, Japan nuclear reactor accident resulted in high radioactive contamination more than 20 miles from the site.

The Philadelphia Inquirer contributed to this article. Contact Sean Maher at 925-943-8013 or smaher@bayareanewsgroup.com. Contact Lisa M. Krieger at 650-492-4098 or lkrieger@mercurynews.com Follow them at Twitter.com/OneSeanMaher and Twitter.com/lisamkrieger.