Scientists Defend Ocean Noise Tests : Environment: Transmission of sounds will be halted if there are signs of harm to marine mammals, Scripps Institution officials say.

Scientists seeking to transmit loud sounds through the Pacific Ocean to measure global warming defended their experiment Wednesday, insisting that the noise would not deafen whales and pledging to halt the project if it appeared to be causing harm.

“We are not out to harm a single whale with these underwater sounds,” said David Hyde, director of the research project at Scripps Institution of Oceanography in San Diego. “We will cease sound transmissions to avoid harm to marine mammals.”

The controversial experiment, which could begin as early as next month, would broadcast a high-volume, low-frequency rumbling sound for 20 minutes, six times a day, from the ocean floor off Big Sur and Kauai, Hawaii. The sound would be detected at U.S. Navy listening posts in the ocean as far as 10,000 miles away.

Some whale experts have charged that the unusual project could deafen or damage the hearing of acoustically sensitive marine mammals and have sought to block a federal permit needed for the study.

But Scripps scientists said the experiment is essential to learn if the ocean’s temperature is rising--a phenomenon they say would pose a far greater risk to marine life than the loud sounds broadcast during what they hope will be a 10-year experiment.

The researchers also said they have included in the project a comprehensive study of marine mammals near the broadcast sites to determine if the noise is disturbing the whales or driving them from the area.

“We are an environmental research project and we are sensitive to environmental concerns, particularly with respect to marine mammals,” Hyde said. “We share that concern with our critics.”

Project leaders acknowledged that Scripps had done little to prepare the public for the experiment and said they were caught off guard by a sudden outpouring of opposition to the federally funded project. Scripps scientists did not respond to The Times’ requests earlier this week for interviews.

Responding to calls from constituents, both California senators, Democrats Dianne Feinstein and Barbara Boxer, urged the National Marine Fisheries Service on Tuesday to move cautiously in approving the experiment.

But Andrew Forbes, the project’s deputy director, blamed poorly informed marine mammal experts for irresponsibly alarming the public with misconceptions.

The sound would be broadcast at 75 hertz, which is at the low end of the range of human hearing but may not even be audible to many marine mammal species, Hyde said.

Transmitted from loudspeakers about 900 meters deep, the initial volume of 195 decibels would quickly drop as the sound travels away.

Only an animal within 200 meters of the sound source would suffer ear damage, Hyde said. To warn animals away, the sound would be turned up gradually for the first five minutes of each 20-minute broadcast.

Also of concern to whale researchers is the possibility that the frequent transmission could alter the behavior of endangered marine mammals, reduce feeding or reproduction, or drive them from the area.

Whales are known to change their behavior in response to noises of 120 decibels. Hyde said the experiment’s transmission would be heard at that volume or higher within a 30-mile radius of the sound source, potentially affecting 677,000 marine mammals near California that live or travel through the area off Big Sur.

But the scientists said their noise would not be as loud as other sounds that pollute the ocean, including the engine noise of large ships, explosions detonated during oil exploration and naval activities.