SYDNEY—Searchers preparing to resume the underwater hunt for Malaysia Airlines Flight 370 increasingly suspect that some of the electronic signals detected last month didn't come from the jetliner's black-box flight recorders, a senior Australian naval officer said.
The doubts—based on further acoustic analysis of the transmissions by Australian authorities over recent weeks—represent another potential setback in the two-month-old operation. An initial underwater search in the southern Indian Ocean has already failed to find any sign of the missing plane, while a costly air-and-ship search of the ocean's surface turned up only garbage.
Authorities in April clung to hope that electronic transmissions picked up by Australian naval vessel ADV Ocean Shield on four occasions on April 5 and April 8 would provide a breakthrough in the search. But authorities are increasingly considering only the two transmissions on April 5 as relevant to the search, Australian naval Commander James Lybrand, captain of the Ocean Shield search vessel, said in an interview late last week. Further analysis of the streams of signals detected three days later on April 8 has led authorities to doubt that they were from a man-made device, Cmdr. Lybrand said.
Each of the transmissions on April 8 were intermittent and at a frequency of around 27 kHz—much lower than the 37.5 kHz frequency that beacons are designed to emit, and also lower than the 33.3 kHz frequency of other transmissions on April 5. "As far as frequency goes, between 33 kHz and 27 kHz is a pretty large jump," Cmdr. Lybrand said.
The Joint Agency Coordination Center, the Australian agency leading the search, didn't immediately respond to a request for comment on Cmdr. Lybrand's remarks.
Authorities continue to believe that the two April 5 signals—including one held for 2 hours and 20 minutes—are consistent with black-box locator beacons. The signals from that day were detected at a slightly lower frequency than locator beacons are designed to emit, but officials have said this anomaly could have been caused by weakening batteries and the vagaries of deep-sea conditions.
Two other signal streams picked up in the search area had earlier been eliminated as leads, further highlighting the pitfalls of tracking possible noises from locator beacons in the open sea. Signals picked up early in the search by British navy vessel HMS Echo were later shown to have been noises from the ship itself, while a detection from a sonar buoy dropped in the ocean in early April was later determined to have come from a passing commercial freighter.
Initial optimism over the underwater signals led Australian Prime Minister Tony Abbott to express confidence on April 11 that searchers knew the position of Flight 370 "to within some kilometers" based on the ping signals.
However, an underwater drone didn't find any trace of the plane during a two-week search of a narrow area of seabed. That disappointment led Mr. Abbott on April 28 to widen the search area to cover 23,000 square miles (60,000 square kilometers)—about equivalent in size to the U.S. state of West Virginia—and look to hand a bigger role to private companies specializing in lengthy deep-water searches.
The Ocean Shield is due to arrive back in the search area on Monday after replenishing its supplies, and will continue to rely on the Bluefin-21 submersible drone to hunt for Flight 370, which vanished en route to Beijing from Kuala Lumpur with 239 people on board on March 8.
Initially, the focus of the rebooted search will be in an area where the Ocean Shield picked up the first and longest stream of transmissions on April 5 using a towed pinger locator.
However, searchers face significant challenges when using the Bluefin. Cmdr. Lybrand said the undersea hunt for Flight 370 so far has encountered crevasses and extremely deep areas of the ocean floor.
Only around two-thirds of the area of a 6.2-mile radius around the first ping has so far been investigated, with searchers encountering patches of the seabed over 5,000 meters below the ocean surface.
The Bluefin typically operates in depths of up to 4,500 meters, but it was reconfigured to dive 10% below its limit after earlier missions were aborted when its in-built safety device forced it to the surface. Still, the device needs to operate around 40 meters above the ocean floor to get an accurate reading, meaning the Bluefin has been unable to search many areas of seabed in the vicinity of the first ping.
"Some other solution will need to be made to search that area, which is potentially part of the future contract," Cmdr. Lybrand said.
International experts meeting in Canberra this week are discussing what equipment will be used in the expanded search, and which company to hire it from. They want a deep-sea survey ship that can map the area, revealing once and for all the mysteries of its ravines and silt plains. They also want towed sonar equipment that can be operate at depths beyond the Bluefin's reach.
These discussions are due to conclude Wednesday ahead of a tender process to hire civilian contractors that will take another 4-6 weeks.
Cmdr. Lybrand said private contractors will face ravines of up to 70 meters deep that crisscross the ocean floor.
"In parts, it was quite flat at about 4,500 meters, but as you moved to the northwestern area of where we were searching it did become quite up and down—50, 60, 70-meter peaks and crevasses," he said. "It's not flying blind but it's flying with an element of suspicion as to what's down there."