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Crew members are seen aboard a fast response craft (R) from the Australian Defence Vessel Ocean Shield (L) as they continue to search for debris of the missing Malaysian Airlines flight MH370 in the southern Indian Ocean, in this picture released by the Australian Defence Force on April 8, 2014. (HANDOUT/REUTERS)
Crew members are seen aboard a fast response craft (R) from the Australian Defence Vessel Ocean Shield (L) as they continue to search for debris of the missing Malaysian Airlines flight MH370 in the southern Indian Ocean, in this picture released by the Australian Defence Force on April 8, 2014. (HANDOUT/REUTERS)

MH370 search in 'crucial stage' after Australian ship hears two more pings Add to ...

The search for a vanished jetliner has entered a "crucial stage," China said on Wednesday, vowing to boost its search efforts after two new sounds from the ocean depths raised hopes that Malaysia Airlines Flight 370 could soon be found.

Earlier, the Australian Defence Vessel Ocean Shield, which is towing a tool designed to listen for “pings” from an aircraft flight data or voice recorder, heard two new sounds from beneath the ocean. It has now on four separate occasions heard transmissions that appear to emanate from such a device.

Globe and Mail Update Apr. 08 2014, 4:27 PM EDT

Video: This signal could be from the black box of missing flight MH370

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They are located in a roughly triangular shape that is about 25 kilometres top to bottom; the most recent two came Tuesday afternoon and evening. Though it’s impossible to be certain until an underwater vehicle can get visual proof of the plane, Australian search authorities said they believe the long wait to resolve the mystery of Malaysia Airlines flight 370 may be drawing to a close.

“I am now optimistic that we will find the aircraft, or what is left of the aircraft, in the not too distant future,” Australian Air Chief Marshal Angus Houston, who is leading the joint effort to find the plane, said Wednesday morning in Perth. “What we’re picking up is a great lead.”

“The Chinese side will further enhance co-ordination with countries involved in the search operation and build up collaboration among search forces on the water, under the water and in the air so as to push forward the search,” Foreign Ministry spokesman Hong Lei said Wednesday.

Confidence among searchers has been buoyed by an expert analysis of the first two audio signals, which were heard April 5 at 4:45 p.m. local time, and again April 5 at 9:27 p.m. The technical details of the signal – its 33.331 kHz frequency and the fact it pulsed every 1.106 seconds – made clear it came from something electronic, rather than natural.

The Australian Joint Acoustic Analysis Centre therefore believes “the signals to be consistent with the specification and description of a flight data recorder,” Mr. Houston said. He said it does not seem likely the underwater pings have come from anything other than an aircraft.

“We’ve had the analysis done, and it’s nothing natural. It comes from a man-made device, and it’s consistent with the locator on a blackbox. So that’s why we are more confident than we were before,” Mr. Houston said.

“But we have to lay eyes on it.”

Adding to the hope: the signals have been located almost exactly at the site of a partial seventh satellite “handshake” transmission from the aircraft, which disappeared March 8. The partial signal suggests that at that location, the plane ran out of fuel and its “engines might have flamed out,” a point at which it would fall to the surface, Mr. Houston said.

Having four underwater signals allows searchers to more narrowly pin down the location of an underwater search, which will be conducted with a remotely-operated robot capable of operating at the 4.5-kilometre depth of the ocean in the region. Fifteen aircraft on Wednesday also continued to search a 75,000 square kilometre area, the size of New Brunswick, in hopes of finding any floating traces of wreckage. So far, in more than a month of intense aerial flights over vast areas of ocean, not a single sign of Flight 370, which had 239 people on board, has been found at the surface.

Ocean Shield also continues to listen in hopes of hearing more signs, since the ship can cover ground six times more rapidly than an underwater robot, which travels along at a walking pace. Finding more signals will help to further narrow down a location for underwater search.

At the same time, authorities know each passing hour makes it less likely they will hear anything further, given it is now 33 days since the Boeing 777 went missing, and the batteries on the data recorder pinger locators are designed to operate for 30 days. Indeed, the fourth signal was “very weak,” Mr. Houston said.

“The batteries of both devices are past their use-by date and they will very shortly fail,” he said. “So I think we’re very fortunate, in fact, to get some transmissions on day 33.”

To enhance listening efforts, an Australian aircraft will drop sonar buoys Wednesday, which are parachuted to the sea surface and deploy a hydrophone 300 metres deep. The Australian military has specifically modified the acoustic processor on those buoys to listen for the frequency of a flight data recorder’s pinger.

The underwater robot will not be deployed until authorities are convinced they are no longer likely to hear audio signals. That times is now drawing close, Mr. Houston said.

Still, the underwater search phase could be lengthy, both slow and complicated by the ocean’s vast depth, and local conditions.

The bottom in the region is believed to be covered by silt. Silt not only deadens sound – making pings more difficult to hear – but it could also cover parts of the aircraft, making them more difficult to visually locate.

And in a sign of the remoteness of the search location, authorities said the nearest ocean bottom data comes from sampling done years ago at a site some 240 kilometres away. For that same reason, authorities also know little about the behaviour of deep ocean currents in the area, which could prove important in tracking the spread of debris.

“We know more about the surface of the moon than our own seabed,” said Commodore Peter Levy, who heads the military search task force.

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