"Sonic cannons are towed behind boats and emit strong pulses of sound every 10 seconds or so, reverberating beneath the sea floor and bouncing back to the surface. Computers then translate the data into high resolution, three-dimensional images."
"Sonic cannons can be fired consistently for weeks or months depending on the project, and pose real dangers for whales, fish and sea turtles that also use sound to communicate across hundreds of miles. In an environmental impact study of the project,the U.S. government estimates that more than 138,000 sea creatures could be harmed." Jason Dearen of the Associated Press (7/18/14)