PERTH, Australia (AFP) - The hunt for missing Malaysian Flight MH370 was set to switch to the ocean floor on Tuesday using a robot submersible after suspected black box signals were detected, one month to the day since the jet vanished.
After describing sonic "pings" picked up by the naval vessel Ocean Shield as the "most promising lead" so far, the head of the Australian-led search operation warned that hopes of finding surface debris were fading.
Retired Air Chief Marshal Angus Houston said that the moment was nearing to deploy the US-made autonomous underwater vessel Bluefin-21 to scour for wreckage on the seabed.
"I haven't had the discussion this morning, we'll be having that discussion a little later on," he told national radio. "I imagine we'd be getting very close to that point." The search is now focusing on a 600km arc of the southern Indian Ocean, far off the coast of western Australia.
Deputy Prime Minister Warren Truss sounded a note of optimism.
"Today is another crucial day as we try and reconnect with the signals that perhaps have been emanating from the black box flight recorders of MH370," he said.
"The connections two days ago were obviously a time of great hope that there had been a significant breakthrough." But he added: "It was disappointing that we were unable to repeat that experience yesterday." Ocean Shield is criss-crossing the area to try to home in on the signals again before launching the five-metre long submersible sonar device, and Truss indicated the deployment was imminent.
"I understand that we are using the autonomous vehicle to examine the waters in the areas of interests of today," he said.
The signals are being investigated as the clock ticks past the 30-day lifespan of the emergency beacons on the two data recorders from the Malaysia Airlines jet, which vanished on March 8 with 239 people on board en route from Kuala Lumpur to Beijing.
"As a consequence there is a chance the locator beacon is about to cease transmission, or has ceased transmission," Houston said.
"I think it's absolutely imperative to find something else and hopefully when we put the autonomous vehicle down, its capability is such that it'll be able to find wreckage.
"Unfortunately with the passage of time, oceanic drift and all the rest of it - particularly as a cyclone went through that area in the last few days - the chances of finding anything on the surface are diminishing with time." Houston explained that the 4.5 kilometre (nearly three mile) depth of the ocean floor was the absolute operating limit for a Bluefin-21, which is designed for deep sea surveying and can carry video cameras.
"It can't go deeper than that, so it's quite incredible how finely balanced all of this is," he said.
"It's a long, painstaking process, particularly when you start searching the depths of the ocean floor." When the sonar detects unusual formations on the ocean floor, it is brought back to the surface to fit cameras before returning to the seabed.
"You can't have the side sonar and the camera down there together, it's one or the other," Houston said.
"We will continue sortie after sortie until such time as we pick up evidence that there's something unusual on the ocean floor. We would then send down the camera.
"What we're after is wreckage, a debris field as people would say." Up to 11 military planes, three civilian planes and 14 ships were Tuesday set to take part in the unprecedented search 2,268 kilometres northwest of Perth, the Joint Agency Coordination Centre (JACC) said.
Ocean Shield, which picked up two series of pulses lasting two hours and 20 minutes and then 13 minutes, are operating at the northern end of the defined search area. China's Haixun 01 and Britain's HMS Echo will work the southern end.
Commander William Marks of the US Seventh Fleet has said one of the signals picked up strengthened for a time, then weakened, indicating crews were near its source.
"That is encouraging because that is what you would expect if you are indeed moving toward the black box - that it should get stronger and as you move away from it, it should get weaker," he told AFP.