Still, underwater microphones have picked up blasts from these sonic cannons over distances of thousands of miles, and the constant banging — amplified in water by orders of magnitude — poses unavoidable dangers for marine life, scientists say.
Whales and dolphins depend on being able to hear their own much less powerful echolocation to feed, communicate and keep in touch with their family groups across hundreds of miles. Even fish and crabs navigate and communicate by sound, said Grant Gilmore, an expert on fish ecology in Vero Beach, Fla.
"We don't know what the physiological effects are. It could be permanent hearing damage in many of these creatures just by one encounter with a high-energy signal," Gilmore said.
More than 120,000 comments were sent to the government, which held hearings and spent years developing these rules. The bureau's environmental impact study estimates that more than 138,000 sea creatures could be harmed, including nine of the world's remaining 500 north Atlantic right whales.
These whales give birth and breed off the coast of Florida, Georgia and the Carolinas before migrating north each year. Many other species vital to East Coast fisheries also travel up and down the Gulf Stream.
"Once they can't hear -- and that's the risk that comes with seismic testing -- they are pretty much done for," said Katie Zimmerman, a spokeswoman for the South Carolina Coastal Conservation League based in Charleston, S.C.
"Even if there were oil out there, do we really want that? Do we really want to see these offshore rigs set up? Do we really want our tourism industry to suffer? Do we really want our environment to suffer?" she asked.
Some of these animals are so scarce that intense noise pollution could have long-term effects, agreed Scott Kraus, a right whale expert at the John H. Prescott Marine Laboratory in Boston. Scientists can't even approach them without extensive permits from federal marine mammal regulators.
"No one has been allowed to test anything like this on right whales," Kraus said of the seismic cannons. "(The Obama administration) has authorized a giant experiment on right whales that this country would never allow researchers to do."
Before the U.S. Atlantic seabed was closed to oil exploration in the 1980s, some exploratory wells were drilled, but the region has never had significant offshore production.
"One thing we find is, the more you get out and drill and explore to confirm what you see in the seismic -- you end up finding more oil and gas than what you think is out there when you started," Radford said.
Opposition to oil development has been abundant along the coast, where people worry that oil will displace fisheries and tourism. More than 16 communities from Florida to New Jersey passed resolutions opposing or raising concerns about the seismic testing and offshore drilling. Some states have passed
The local economy is fueled by beach tourism and fishing in St. Augustine in north Florida, where rare turtles come ashore to lay their eggs.
"Florida has already felt the devastating effects of an uncontrolled oil release with the Deepwater Horizon event of which cleanup efforts are still on-going," said John Morris, a county commissioner whose constituency includes St. Augustine Beach. "Any oil spill, large or small, off the coast of St. Johns County, would greatly affect the county's economy."
Associated Press Writers Bruce Smith in Charleston, S.C.; and Alex Sanz in Vero Beach, Fla., contributed to this story. Jason Dearen can be reached at http://www.twitter.com/JHDearen