By KEITH BRADSHER
New York Times
JUNE 17, 2014

CANBERRA, Australia — Australia plans to resume searching for Malaysia Airlines’ missing Flight 370 to the southwest of the area in the Indian Ocean where the seafloor was scanned in detail last month, Australian officials say.

The shift to the southwest reflects analyses of a series of electronic “handshakes” between the Boeing 777-200 and a satellite operated by the London-based company Inmarsat in the hours after the plane vanished before dawn on March 8 during a flight from Kuala Lumpur, Malaysia, to Beijing with 239 people aboard.

The satellite data, suggesting that the aircraft turned south across the Indian Ocean after skirting the northern tip of the Indonesian island of Sumatra, remains the best lead that investigators have in trying to find the plane, said Angus Houston, the retired chief of the Australian military who is overseeing the search.

“We’re going to have to go deep and do a comprehensive look at the ocean floor,” he said, later adding, “The handshakes are the most robust information we have at the moment.”

The Australian Transport Safety Bureau hired a private company, Fugro Survey, on June 10 to conduct a three-month survey of the ocean floor along the arc of possible final locations derived from the satellite data. The bureau is also seeking tenders by the end of the month for a commercial contractor to deploy a towed deep-sea submersible to scour the seafloor; Mr. Houston said that the towing operation would not need to wait for the completion of the undersea mapping but could start this summer.

Australia’s own long-range, over-the-horizon radar, known as the Jindalee Operational Radar Network, was not focused on the area where the plane apparently flew and did not detect it, he said. Investigators have also not gleaned any useful clues, he said, from an undersea thud detected by acoustic researchers at Curtin University, near Perth, Australia, that roughly coincided with the time when the plane stopped its electronic handshakes and appears to have run out of fuel.

An Australian vessel, the Ocean Shield, did an exhaustive search last month of a small section of seafloor in the northeast half of the arc of possible final locations for the plane. The search was conducted after the ship detected sounds there that were initially interpreted as locator pings from the aircraft’s so-called black boxes, sounds that were later reanalyzed and found not to be the pings.

Chris McLaughlin, an Inmarsat vice president in London, said Tuesday in a telephone interview that the satellite company did not fault searchers for pursuing those signals, which were detected in April to the northeast of the zone identified by Inmarsat’s calculations.

“The Inmarsat model indicated a more southerly reach for MH370 than the earlier pickup of pings appeared to suggest,” Mr. McLaughlin said. “Four other independent analyses of the data,” conducted by experts at Boeing, the French electronics group Thales and investigators in Australia and Malaysia, “have also indicated a more southerly position, closer to the seventh arc” calculated from the final signal Inmarsat’s satellite received from the plane, he said.

“So we believe the next stage of the search will be concentrated around the outcome of this data,” Mr. McLaughlin said.

On Tuesday, the BBC quoted officials at Inmarsat who said that while Australia had understandably paid considerable attention to the detected sounds, Inmarsat’s modeling of the satellite handshakes had long showed the highest-probability zone for the aircraft’s final location as lying farther to the southwest.

But Inmarsat said in a statement on Tuesday, “Because there are many uncertainties due to the lack of aircraft performance and tracking data, a specific final location in the Southern Ocean cannot be identified.”

Australian officials said that their next priority was to map the ocean floor in enough detail that the deep-sea submersible could be safely towed for long distances at a fairly brisk speed in the coming months with little risk that it will slam into previously undetected seamounts.

Australia has no plans for any further searches from the air for floating debris, having concluded that any debris would have sunk by now or would have spent so much time in the water that it would no longer be recognizable as having come from the plane. “After a period of time, nearly everything sinks, including seat cushions and so on, because they become so waterlogged,” Mr. Houston said.

When an Air France flight crashed off the coast of Brazil in 2009, considerable debris was visible on the ocean’s surface for the first few days, but nothing was left on the surface by the 16th day, and by the 26th day the visual search was halted.

In the early days of the search for the Malaysia Airlines plane, one of the worries was that the plane might have landed on a smooth patch of sea somewhere and then sunk, in which case it might have left no floating debris for airborne searchers to find.

But now the possibility, however small, that the plane might have sunk intact is actually the best-case scenario for searchers. An intact aircraft would be less difficult to find on the vast expanses of the ocean floor than one that had broken into many pieces upon hitting the water.

Such pieces would have descended at different rates and in different directions based on their size, shape and water resistance as they glided or tumbled through water as deep as six kilometers, or nearly four miles.

“We could be confronted with a very dispersed debris field,” Rear Adm. Trevor Jones of the Royal Australian Navy said in an interview on Monday.

A further worry has to do with the topography of the seafloor. When the Air France jet crashed, it came to rest on a fairly level sandy plain at the bottom of the Atlantic Ocean, which facilitated recovery of both black boxes: the cockpit voice recorder and the flight data recorder.

By contrast, sizable areas of the Indian Ocean seafloor where the plane might be located are crisscrossed with hills and ravines and with silt dozens of meters deep in some places. One of the concerns, Australian officials said, is that the black boxes may have plunged deep into the silt, even as lighter articles like clothing may have drifted down slowly and settled on the silt’s surface.

Deep-sea submersibles like the United States Navy’s Bluefin-21, which the Ocean Shield towed last month, have sonar that can penetrate light silt suspended in the water at the ocean floor but may not be able to penetrate thick layers of denser silt.

Australian officials and oceanographers say that silt drifting down on the debris is very unlikely to cover it up in the months and years to come. The rate of deposition is extremely slow, as little as one or two micrometers a year in some areas, a tiny fraction of the thickness of a human hair.

Deposition may be a little faster in the more southerly areas now slated to be searched, as upwelling cold water may sustain more life in the ocean waters above. But the rate of deposition still should not be fast enough to bury debris, said Robin Robertson, an oceanographer at the Australian Defense Force Academy here in Canberra.

Group Captain Craig Heap, the Royal Australian Air Force officer who oversaw the airborne search of large swaths of the Indian Ocean in late March and through April, said that aircraft from Australia, China, Malaysia, New Zealand, South Korea and the United States had logged about 3,000 hours of flying time during the search for debris, including nearly 900 hours of flying over remote areas of ocean where the plane might have come down.

“We’re still very disappointed, and always will be, that we didn’t find anything,” he said, while adding that the search had nonetheless been unusual in producing cooperation among military aviators from China, Japan and South Korea — countries that have had considerable frictions lately over air and maritime sovereignty issues in the East China Sea.

Mr. Houston expressed optimism that the missing plane would eventually be found. But other Australian officials voiced worries that the combination of silt on the ocean floor and the dispersion of debris and the uncertainties about the final flight path may not make it feasible for years, if ever, to find the aircraft and recover its black boxes.

Nicola Clark contributed reporting from Paris.