Could anything be as silent as the deepest depths of the world’s oceans? Actually, yes. It turns out the ocean floor is quite a noisy place because of the din made by whales, ships, storms and even the occasional earthquake.
A group of scientists from the US National Oceanic and Atmospheric Administration (Noaa) and Oregon State University joined with the US Coast Guard to eavesdrop on the sounds reaching the sea floor.
Noaa's Office of Ocean Exploration and Research funded the project in order to establish a baseline for ambient noise in the Pacific Ocean. It also wanted to test designs for the equipment needed to measure underwater noise, which must survive the massive pressures at the seabed.
What better place to test a purpose-built hydrophone than the very deepest place on Earth, the Mariana Trench? It lies in the western Pacific near Micronesia and not far off Guam. It is a huge gash in the seabed that runs for more than 2,500km but remains narrow, with an average width of 69km.
There is a low point along the trench known as Challenger Deep, and it is here that the group decided to sink a hydrophone.
Sea surface
Challenger Deep lies 10,990m down. This is so deep that if you took the highest point on Earth – Everest – and dropped the mountain into it, Everest would vanish out of sight more than 2km below the sea surface.
“Noise in the deep ocean has been on the increase since the 1960s,” says Dr Robert Dziak, a Noaa research oceanographer and chief scientist on the project. Much of this can be attributed to the huge increase in container shipping, he says. “It is not known how man-made noise impacts on marine animals, but it is not such a great thing for these animals,” he says.
For this reason Noaa wanted to take deep-ocean noise measurements to provide a useful baseline to compare with future measurements. “We are trying to develop a noise reference network, a series of hydrophones around the US coast to gauge man-made and natural sounds.”
The Challenger Deep expedition had a slightly different aim: to sample noise at a location as far as possible from the sources at shallower levels. This raised an immediate problem: how do you put together a recording device that can survive the enormous pressures 11km down?
“We had never put a hydrophone deeper than a mile or so below the surface, so putting an instrument down some seven miles into the ocean was daunting,” says Haru Matsumoto, an Oregon State ocean engineer who developed the specialised hydrophone with Noaa engineer Chris Meinig.
“It was a technical challenge to get the instrument package to those great depths, with the intense pressure there,” Dziak says. “It is an underwater mic but it is fluid-filled to protect it from the pressures,” he says. “The electronics are the same as a mic but it is held in an elaborate titanium case. You have to avoid any air spaces or bubbles, as it would be crushed.”
Six-hour journey to the bottom
They deployed the hydrophone in July 2015, and it took more than six hours for it to free-fall to the bottom. They needed a slow, even descent
of about 5m per second. “Structures don’t like rapid change and we were afraid we would crack the ceramic housing outside the hydrophone,” says Matsumoto.
It wasn’t retrieved until last November because of persistent typhoons and scheduling problems. Since then the team has been analysing the sounds and trying to differentiate between natural and man-made sounds. Once they hit playback, there were plenty of surprises. The hydrophone picked up constant noise from natural and man-made sources.
“It is a very seismically active place, so we expected quakes and storms and whales,” he says.
They got all that along with a category- four typhoon and the frequent cacophony of ships’ propellers. They recorded a magnitude 5.0 earthquake that occurred at a depth of about 10km. The hydrophone, at 11km down, recorded the event from underneath.
Retrieving the device involved returning to the site and sending an acoustic signal that caused the instrument to detach from its anchor and float to the surface.
Having proven the system, the researchers are ready to go again next year, led by another Oregon State investigator, Joe Haxel. This hydrophone will record for a longer period and a deep-ocean camera will also be used.
WHALES AND EARTHQUAKES: SOUNDS FROM THE VERY DEEP
First there is silence. Then comes the plaintive call of a baleen whale kilometres above. This haunting sound remains clearly audible despite the great depth of the hydrophone recording it.
The research project to sample noise reaching the deepest place on Earth, the Mariana Trench in the western Pacific Ocean, captured a surprising range of noises. It frequently captured the churn of ships' propellers as they passed, but one of the most frequently heard sounds was the deep rumble of earthquakes.
At one stage a category-four typhoon passed overhead and a succession of smaller storms. These tended to increase overall sound levels, but they were spread out over days.
Perhaps surprisingly, the hydrophone also picked up the sound of winds and waves disturbing the surface, even when there was not a major storm taking place. “Sound doesn’t get as weak as you think it does even that far from the source,” says Haru Matsumoto, who developed the hydrophone package.
His design worked despite the extreme pressures experienced at the bottom of the ocean. Normal sea-level air pressure – caused by the weight of air overhead – is 14.7lb per square inch. The weight of water overhead at 11km deep produces a pressure of more than 16,000lb per square inch.
