The commission that oversees the California's 1,100-mile coastline decided the proposal to use of air guns to send acoustic pulses across the ocean floor near the Diablo Canyon nuclear power plant didn't meet stringent coastal protection rules.
"It's a high bar, and we don't feel like the questions of alternatives and analysis to minimize and perhaps avoid the impacts here have been [answered]," executive director Charles Lester told the Santa Cruz Sentinel.
The testing had been intended to uncover information about known fault lines in the offshore area near the plant to provide scientists with a better idea of the seismic risks.
The proposed testing had been opposed by environmental groups and by commercial fishermen worried it would would impact their industry.
The project could have had considerable environmental impact, wildlife experts said, because marine life relies primarily on sound rather than sight and the acoustic onslaught could silence whales, disrupt foraging and force mammals from the testing area.
"It is sound that marine mammals and many species of fish use to communicate, to mate, to find food, to do many things that they need to do in the wild," Michael Jasny of the Natural Resources Defense Council said. "It is difficult to imagine a worse location environmentally [for the test] than this particular area."