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Or, at the very least, taking a baseline for when they check again to see if there are any hotspots.

While sitting at work today, we noticed a helicopter zig-zagging back and forth across the downtown area flying very low through the buildings of downtown. Flying low enough to be down between the buildings is unusual. Apparently, we weren't the only ones that thought so.

Here's why low-flying helicopters were zooming around downtown Houston - Houston Chronicle

The U.S. National Nuclear Security Administration, or NNSA, will test for naturally occurring radiation around the city on Tuesday using a low-flying helicopter.

The test is designed to establish a baseline for officials of Houston’s radiation levels in support of Super Bowl LI. The NNSA is a U.S. Department of Energy agency responsible for assisting national security, working to reduce the threat of weapons of mass destruction.

A twin-engine Bell 412 helicopter, operated by the Remote Sensing Laboratory Aerial Measuring System from the Nellis AFB, will be equipped with the radiation sensing technology.

According to a NNSA press release, the helicopter will fly in a grid pattern over the areas at 150 feet or higher above ground surface, at a speed of approximately 80 miles per hour. Officials from NNSA said the radiation assessment will cover approximately 7.5 square miles.

Flyovers will occur only during daylight hours, and it is estimated to take about three hours to complete per area.

Why is that helicopter flying around Houston so low? | khou.com

HOUSTON -- Concerned KHOU 11 viewers have a question: "Why did that helicopter just fly by my office building so low!?"

Several viewers shared videos of a helicopter flying very low around the Galleria area on Monday, including Henri Dubiton.

The chopper zoomed by his office tower window at Sage Road and San Felipe at around lunch time Monday.
"It turned around little bit further south on San Felipe and heads right back toward us," said Dubiton, who works on the 13th floor.

At times the chopper appeared to be only about 10 stories off the ground.

On Wednesday, the chopper was spotted again in downtown and near NRG Stadium.

The Department of Energy was doing a radiation test. KHOU 11 News is still waiting for official word on that but the tail number on the helicopter confirms it belongs to that government agency.

"I was just about able to see the pilot's face and his head gear on and to be honest I wasn't thinking too much other than there's a helicopter 30 feet from me right now," Dubiton said.

Others in the Galleria area did the same thing he did, pulling out their phones to capture the sight.

In April, the Department of Energy did one of these low-altitude flights around Boston before the marathon to measure naturally occurring background radiation, using the same twin-engine Bell helicopter it used Monday.

"There's going to be all sorts of odd things going on in our city that we don't normally see so it's kind of what we chalked it up to," Dubiton added.

The DOE says they do this to establish baseline levels and that it's a normal part of emergency preparedness.

Houston Police tells us the chopper is part of security operations, adding that the public should not be alarmed.