Acoustic gravity waves pinpoint where and when objects hit ocean surface

Scientists at the University of Cardiff in Wales have developed a method for tracing acoustic gravity waves to their point of origin.

Researchers can use the method to identify the precise time and location that an object splashed into the ocean. The technique could be used to locate the entry point of meteorites, spacecraft, satellites, aircraft and more.

The method -- detailed in a new paper published this week in the journal Scientific Reports -- relies on hydrophones, microphones that can pick up underwater sound waves emitted when an object hits the surface of the ocean.

The underwater sound waves are called acoustic gravity waves, or AGWs. The waves are triggered by sudden shifts in water pressure and can travel for several miles beneath the surface of the ocean. Scientists at Cardiff have previously considered redirecting AGWs to break up onrushing tsunami waves. Researchers have also tracked similar waves in the atmosphere to predict the path of large storms.

In the lab, researchers used hydrophones to measure AGWs produced when various spheres were dropped into a water tank. Scientists analyzed the different wave patterns to develop a predictive formula. They tested the accuracy of their method using recordings from hydrophones operated by the Comprehensive Nuclear-Test-Ban Treaty Organization. The hydrophones are designed to listen for possible underwater nuclear weapons tests, but they can also record AGWs. Researchers used their method to accurately predict the time and location of an earthquake 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," Usama Kadri, from Cardiff's School of Mathematics, said in a news release. "Tracking these acoustic gravity waves opens up a huge range of possibilities, from locating falling meteorites to detecting landslides, snowslides, storm surges, tsunamis and rogue waves."

Researchers used their method to analyze hydrophone data recorded on the day the Malaysian Airlines Flight MH370 disappeared over the South Indian Ocean. Their analysis revealed a pair of faint signals near the projected path of the airplane.

"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," said Davide Crivelli, from Cardiff's School of Engineering. "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."

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