Because sound travels more slowly in colder water, initially, as depth increases, sound waves move more slowly. However, sound travels faster under high pressure conditions. In deep water the temperature stablizes so the effect of cold temperature is overridden by the effect of pressure, and sound waves move faster. A sound channel is created around the point pressure takes over from temperature as the dominant effect on sound waves. Some sound waves are trapped here, and travel undisturbed for thousands of miles along the channel axis.
Most sound waves travel further from the axis.
They will reach a distant receiver more quickly than sound traveling down the channel axis, but their strength will be greatly diminished by traveling the longer distance. A long-term series of tests that recorded increasingly faster travel times for sound signals over the same path would indicate increasingly warmer ocean temperatures. This could serve as a possible indicator of global warming. Array cable trails across the deck from its attachment beneath the subsurface float. In addition to vertical strings of hydrophones located at remote sites such as Christmas Island, the ATOC system also uses once top-secret arrays of hydrophones originally deployed along the seafloor by the U.S. Navy to detect enemy submarines during the cold war.