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Underwater Sound Waves Locate Ocean Impacts

24 October 2017

Towing tank at the Faculty of Applied Science and Engineering Towing Tank at Memorial University of Newfoundland (Cardiff University)Towing tank at the Faculty of Applied Science and Engineering Towing Tank at Memorial University of Newfoundland (Cardiff University)

Scientists have developed a new method to locate the precise time and location that objects fall into Earth’s oceans.

This method, developed by Cardiff University, uses underwater microphones, called hydrophones, to listen to underwater sound waves that are emitted when an object hits the sea surface.

The researchers believe the new method could be used to locate meteorites, satellites or parts of an aircraft that may have entered the ocean. It could also be used to locate underwater explosions, landslides or the epicenter of earthquakes far out at sea.

The new method relies on the measurement of acoustic-gravity waves (AGWs), naturally occurring sound waves that move through the deep ocean at the speed of sound and can travel thousands of meters below the surface.

AGWs can measure tens or hundreds of kilometers in length and it is thought that certain lifeforms like plankton, that are unable to swim against a current, rely on waves to aid their movement and enhancing their ability to find food.

When objects hit the sea surface, they cause a sudden change in water pressure that leads to the generation of AGWs.

In the first part of the study, the team dropped 18 spheres onto the surface of a water tank at varying distances and heights and measured the subsequent AGWs that were emitted from the hydrophone.

The team analyzed hours of data from hydrophones off the coast of Western Austrailia. The hydrophones are operated by the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) for detecting underwater nuclear tests and they can pick up signals form AGWs.

Using this data, the research team was able to validate their method by successfully calculating the time and location of recent earthquakes in the Indian Ocean.

"By using existing detectors dotted all around our oceans and listening out for signatures from these deep ocean sound waves, we've uncovered a completely novel way of locating objects impacting on the sea surface," said lead author of the study Dr. Usama Kadri, from Cardiff University's School of Mathematics. “Tracking these acoustic gravity waves opens up a huge range of possibilities, from locating falling meteorites to detecting landslides, snow slides, storm surges, tsunamis and rogue waves."

The team went one step further and analyzed data from the same hydrophones from March 18, 2014, when the Malaysian Airlines flight MH370 disappeared over the South Indian Ocean.

"Our study was initially motivated by a desire to gain more knowledge about the incident involving flight MH370, using data analysis techniques that can pick up and locate much weaker signals," said co-author of the study Dr. Davide Crivelli, from Cardiff University's School of Engineering.

"Though we've located two points around the time of MH370's disappearance from an unknown source, we cannot say with any real certainty that these have any association with the aircraft. What we do know is that the hydrophones picked up remarkably weak signals at these locations and that the signals, according to our calculations, accounted for some sort of impact in the Indian Ocean.

"All of this information has been passed on to the Australian Transport Safety Bureau and we anticipate that both now, and in the future, this new source of information could be used in conjunction with a whole of host of other data that is at the disposal of the authorities."

A paper on this research was published in Scientific Reports.

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