Bay City News Service—A helicopter may be seen flying at low-level altitudes over parts of San Francisco, Pacifica and Oakland this week, but authorities say the public shouldn't be alarmed. The helicopter isn't searching for suspects on the loose, but rather is taking measurements of naturally occurring radiation, Oakland police said.

The helicopter appeared to be flying over parts of Berkeley and the North Oakland hills this afternoon (Wednesday). The flyovers, which are taking place Monday through Saturday, are a part of a joint research project by the Department of Homeland Security's Domestic Nuclear Detection Office and the National Nuclear Security Administration to assess natural background radiation levels.

The survey will cover about 69 square miles, and will be performed by the NNSA's twin-engine Bell 412 helicopter, which is operated by the Las Vegas-based Remote Sensing Laboratory Aerial Measuring System in Las Vegas. The helicopter will fly in a grid pattern about 300 feet above ground. The flights will be limited to daylight hours. Federal officials say the data will be used as background to help local, state and federal entities measure radiation levels in the future.