Earthquake faults would have been mapped by firing loud air cannons periodically over a four-year period near the Diablo Canyon Power Plant. Opponents feared that would harm thousands of sea mammals.
"It will affect the larvae meters down into the ground. When you start taking from the bottom of the food chain, the top of the food chain is going to suffer," said Robert Black with Greenpeace.
Pacific Gas and Electric Company said the study was necessary to understand seismic hazards near the plant, and that nearby sea life would not be affected.
"If we come within 0.6 miles of any marine mammal, the ship has to shut down," said Mark Krausse with PG&E.
The California Coastal Commission held a public hearing in Santa Monica Wednesday to decide the issue. It voted unanimously to deny a permit for the ocean blasting.