WHEN you drop a pebble in a pond, you hear a ‘plop’ and see ripples begin to expand across its surface. If you were beneath the surface of the pond, you’d hear a slightly different ‘plop’. All involve waves. And all radiate outwards from the source. So if you can hear it, you can learn a lot about the rock that dropped.
It’s a technique used by the world’s militaries to track and identify submarines and warships over great distances. It’s also a key component in sensing nuclear weapons tests, which is why a worldwide network of hydroacoustic (under water microphones) has been in operation for decades.
Two of these ocean acoustics recorders were in a position to pick up MH370’s impact with the Indian Ocean. But only one, based in Australia, has supplied reliable data.
The other was positioned at the secret US defence facility at Diego Garcia, in the heart of the Indian Ocean. Much of its data from the relevant time frame is distorted. And 25 minutes of it is inexplicably missing.
This is according to mathematicians attempting trying to squeeze every last drop of information out of these recordings in an attempt to narrow down the Boeing 777’s final resting place.
According to Cardif University mathematician Usama Kadri, enough has been gleaned from the available sound recordings to ‘rethink’ the probable point of impact.
Writing in The Conversation , Kadri explains an attempt was made to use recordings of underwater acoustic waves to locate the aircraft in 2017. Noises potentially associated with the crash were detected by hydrophones off Cape Leeuwin, Western Australia, and another at the secret US military facility at Diego Garcia, in the central Indian Ocean.
“Unfortunately this didn’t lead to finding the plane,” he writes.
“However, our research into these waves has moved on since we first proposed the idea … and we have now been able to identify two locations where the aeroplane could have impacted with the ocean, as well as an alternative route that the plane may have taken.”
One puts the crash site further north in the Indian Ocean than previously believed.
The other points to a flight course taking MH370 closer to Madegascar.
The behaviour of sound travelling through water has been the subject of intense study since before World War II.
It’s at the heart of submarine warfare – both for evading and finding hidden hostile vessiles.
It’s long been known that forceful events on and beneath the waves can produce a kind of transmission that can travel thousands of kilometres. Called acoustic-gravity waves, they’re caused by sudden impacts, and the associated change in water pressure forms waves that can reveal considerable detail about the event that caused them.
“In our last study we looked at acoustic-gravity waves picked up by hydrophone (underwater microphone) stations in the Indian Ocean, to narrow down where flight MH370 may have impacted the ocean to two points,” Kadri writes.
But science’s understanding of how these waves perform is not complete.
And small influences can cause large margins of error over great distances.
One such source of error has now been identified.
“We have found another factor that may prove crucial for pinning down the location of the impact: sea floor elasticity (flexibility),” Kadri writes.
“Allowing for the effects of sea floor elasticity, the signal locations that we had previously identified using data from HA01 (Cape Leeuwin) were now different.”
DEVIL IN THE DETAIL
While the Cape Leeuwin recordings (designated HA01) were relatively clear, those from Diego Garcia (HA08s) had been distorted by the noise of a nearby military exercise. This made extracting sound potentially related to MH370 from the recorded data much more difficult.
And at least two separate, clear recordings are needed to provide enough data to produce a reliable ‘intersection point’ identifying where a noise came from.
But one Diego Garcea signal has attracted Kadri’s attention.
And running it through the necessary filters and aligning it with the Cape Leeuwin signal produced a new possible impact point.
“Although the proposed route and point of impact is distant from the 7th arc (produced by satellite signal analysis of MH370’s engine reporting modems), we still recommend further studying a number of signals from HA08 … If the signals are related to MH370, this would suggest a new possible impact location in the northern part of the Indian Ocean.”
But the angle the suspicious sound came from also potentially places the source within the arc of military testing activity.
And there’s another, pressing problem.
“Unfortunately, on top of the noisy recorded signals, 25 minutes of data from HA08s is missing,” Kadri says. “The signals we have analysed indicate that the there was a 25-minute shutdown that has gone unexplained by the Comprehensive Nuclear-Test-Ban Treaty Organisation, which is responsible for the hydrophone stations.”
The mathematician says his analysis, and a request for an independent reanalysis of all hydroacoustic recordings, have been forwarded to the Australian Transport Safety Bureau and the MH370 Safety Investigation Team in Malaysia.