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mh-370-searchOver lunch in Shanghai on April 11, Australian Prime Minister Tony Abbott signaled that the answer to one of the world’s biggest aviation mysteries was close.

His audience in China’s commercial capital–a mix of local business leaders and Australian executives–heard Mr. Abbott express confidence that Australia knew the position of missing Flight 370 “to within some kilometers.” Days earlier, an Australian naval vessel had detected streams of signals consistent with an aircraft’s “black-box” locator beacon. Back home in Australia, officials leading the search were also expressing cautious optimism that the plane may soon be found.

On Monday, Mr. Abbott had a more sobering message. An initial underwater search of 400 square kilometers (154 square miles) around where the Australian naval ship Ocean Shield detected the electronic signals hadn’t found any trace of the plane—the latest disappointment in a 52-day search littered with false leads.

As a result, the operation was refocusing on an area equivalent to the size of the U.S. state of West Virginia, spanning some 60,000 square kilometers (23,000 square miles). Searchers who had hoped to find plane wreckage within days were now told it would take roughly eight months to scan the new area, possibly longer.

The new search area is not only vastly larger but also in most places believed to be far deeper than the area just explored, with ancient abyssal plains and remote plateaus about which scientists know relatively little.

“It has been said we know more about the surface of the moon than our own seabed,” Australian Navy Commodore Peter Leavy, one of the officials involved in the search, told reporters this month.

mh-370-search1Mr. Abbott signaled other big shifts in the search. Military air crews and ships deployed by nations including the U.S. to scan the southern Indian Ocean surface were being stood down immediately. Stepping up in their place are private contractors and organizations with specialist equipment capable of searching at depths greater than those already scanned.

One of the only surveys of the ocean floor in the search zone was taken in 1960 by the Soviet research vessel Vityaz, which used a sextant and dead reckoning to plot its course. The ship has since been turned into a floating museum in Kaliningrad.

Crude bathymetric data has also been taken of the area in focus for the wider search, using satellites bouncing radar off the ocean to predict changes in undersea terrain based on peaks and troughs in the water surface.

These sources were recently brought together by government-run Geoscience Australia and James Cook University, based in the northeastern Australian city of Townsville, to build a rough map of the patch of ocean floor some 1,675 kilometers, or around 1,000 miles, northwest of Perth.

“It’s not great but it’s the best we have,” said Robin Beaman, an oceanographer from James Cook who helped to compile the map.

A Bluefin-21 underwater drone was only able to operate at depths of around 4,500 meters, or about 15,000 feet, with an in-built system forcing it to the surface when it went too low. According to the map, it would fall far short for much of the expanded search area.

Taking the search even a few kilometers north of its current location would immediately encounter the deep waters of the Wharton Basin. This frigid, barren plain stretches toward Indonesia at depths consistently over 5,000 meters. If the search heads in the other direction further south it would enter areas even deeper, in places dropping below 7,000 meters, said Mr. Beaman.

Australia plans to switch from autonomous drones such as the Bluefin to remotely operated submersibles connected by cables to ships on the sea surface. These submersibles can cover more ground as they don’t need to return to the surface, and sonar data is directly fed to computers on board the ships. Mr. Abbott said relying more heavily on private contractors for the expanded search could cost around 60 million Australian dollars (US$56 million) if it goes for the full eight months. He said Australia would make contributions alongside countries including Malaysia and China.

Officials haven’t said why they waited until now to bring in more deepwater search equipment. The Bluefin was readily available and contracted to the U.S. Navy by Phoenix International Holdings Inc. Mr. Abbott said acquiring other equipment needs to follow a tender process, which can take weeks.

Assumptions that have guided the search so far are now being re-evaluated.

Mr. Abbott said Australia wanted investigators in Kuala Lumpur to evaluate satellite and radar data again to pinpoint the area where it is most likely that Flight 370 went down.

The fruitless initial underwater search has dented confidence that the signals detected three weeks ago are from the black boxes.

The four signals were detected up to 25 kilometers, or around 16 miles, apart, well beyond the around 1 mile distance sound is supposed to travel from a locator beacon before dissipating to levels undetectable by pinger locators.

“I’ve got no theory to explain that,” said U.S. Navy Capt. Mark Matthews, who led the undersea search using the Bluefin, earlier this month. “I’d like to see a much tighter grouping.”

At extreme depths, sound can sometimes defy expectations and travel extraordinary distances along conductive bands of cold, saline water. “When temperature changes, the speed of sound changes,” said Charitha Pattiaratchi, head oceanographer at the University of Western Australia. “Temperature layers can trap sound and transmit it.”

An experiment conducted in 1960 near the current search zone, where three bombs were detonated off Perth by the Australian navy ship HMAS Diamantina at a depth of 3,400 meters, revealed that sound could easily travel half way around the world to an antipodal American listening station in Bermuda.

The nature of the ocean floor is also relatively unknown.

Some decades after the Vityaz venture through the area, another Soviet ship, the RV Dmitriy Mendeleev, took a core sample of the nearby seabed. The 3-meter yellow plug brought back to the surface revealed an ancient, compact mud made from dead plankton. Carbon dating showed that each thousand years only around 4 millimeters of new waste settled on the seabed.

While the top layer of this rotting sea matter may be soft and silty, what lies beneath is thought to be relatively firm, like potters clay, Mr. Pattiaratchi said.

Air Chief Marshal Angus Houston, the Australian head of the search, said Monday that experts hoped larger pieces of debris would rest on top of this substance without sinking through and disappearing from the view of sonar devices.

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