Angus Houston, head of a joint agency coordinating the search for the missing plane in the southern Indian Ocean, said that the Australian naval vessel Ocean Shield picked up the two signals on Tuesday, and that an analysis of two sounds detected in the same area on Saturday showed they were consistent with a plane's black boxes.
"I'm now optimistic that we will find the aircraft, or what is left of the aircraft, in the not-too-distant future. But we haven't found it yet, because this is a very challenging business," Houston said at a news conference in Perth, the hub for the search operation.
The signals detected 1,645 kilometers (1,020 miles) northwest of Perth are the strongest indication yet that the plane crashed and is now lying at the bottom of the ocean in the area where the search is now focused. Still, Houston warned he could not yet conclude that searchers had pinpointed Flight 370's crash site.
"I think that we're looking in the right area, but I'm not prepared to say, to confirm, anything until such time as somebody lays eyes on the wreckage," he said.
Finding the black boxes quickly is a matter of urgency because their locator beacons have a battery life of only about a month, and Tuesday marked exactly one month since the plane vanished on March 8 with 239 people on board.
If the beacons blink off before the black boxes' location can be determined, finding them in such deep water - about 4,500 meters, or 15,000 feet - would be an immensely difficult, if not impossible, task.
The Ocean Shield first detected underwater sounds on Saturday before losing them, but managed to pick them up again on Tuesday, Houston said. The ship is equipped with a U.S. Navy towed pinger locator that is designed to detect signals from a plane's two black boxes - the flight data recorder and cockpit voice recorder.
A data analysis of the signals heard Saturday determined they were distinct, man-made and pulsed consistently, Houston said, indicating they were coming from a plane's black box.
"They believe the signals to be consistent with the specification and description of a flight data recorder," he said.
To assist the Ocean Shield, the Australian navy on Wednesday began using parachutes to drop a series of buoys in a pattern near where the signals were last heard.
Royal Australian Navy Commodore Peter Leavy said each buoy will dangle an underwater listening device called a hydrophone about 300 meters (1,000 feet) below the surface. The hope, he said, is the buoys will help better pinpoint the location of the signals.
Houston acknowledged searchers were running out of time, and noted that the signals picked up on Tuesday were weaker and briefer than the ones heard over the weekend, suggesting that the batteries might be dying. The two signals detected on Saturday lasted two hours and 20 minutes and 13 minutes, respectively; the sounds heard Tuesday lasted just 5 and a half minutes and 7 minutes.
"So we need to, as we say in Australia, 'make hay while the sun shines,'" Houston said.
Picking up the sound again is crucial to narrowing the search area so a small, unmanned submarine can be deployed to create a sonar map of a potential debris field on the seafloor. The sub, dubbed Bluefin 21, takes six times longer to cover the same area as the pinger locator, which is pulled behind the boat at a depth of 3,000 meters.
U.S. Navy Capt. Mark Matthews said the detections indicate the device emitting the pings is somewhere within about a 20-kilometer (12-mile) radius. Still, he said, that equates to a 1,300-square-kilometer (500-square-mile) chunk of the ocean floor.
That amounts to trying to find a desktop computer in a city the size of Los Angeles, and would take the sub about six weeks to two months to canvass. So it makes more sense to continue using the towed pinger locator to zero in on a more precise location, Matthews said.
"It's certainly a man-made device emitting that signal," he said. "And I have no explanation for what other component could be there."
The disappearance of Malaysia Airlines Flight 370 on a trip from Kuala Lumpur to Beijing has sparked one of aviation's biggest mysteries. The search has shifted from waters off of Vietnam, to the Strait of Malacca and then to waters in the southern Indian Ocean as data from radar and satellites was further analyzed.
The weakening of the signals could indicate that the batteries were reaching the end of their life, or that the device was farther away, Matthews said. Temperature, water pressure or the saltiness of the sea could also be factors.
Houston said a decision had not yet been made on how long searchers would wait after the final sound was heard before the sub was deployed, saying only that time was "not far away."
"Hopefully in a matter of days, we will be able to find something on the bottom that might confirm that this is the last resting place of MH370," he said.
The Bluefin sub's sonar can scan only about 100 meters and it can "see" with lights and cameras only a few meters. The maximum depth it can dive is 4,500 meters, and some areas of the search zone are deeper.
Search crews are also contending with a thick layer of silt on the seafloor that can both hide any possible wreckage and distort the sounds emanating from the black boxes that may be resting there, said Leavy, who is helping to lead the search.
Meanwhile, the search for debris on the ocean surface picked up intensity on Wednesday, with 15 planes and 14 ships scouring a 75,400 square kilometer area that extends from 2,250 kilometers northwest of Perth.
Despite the challenges still facing search crews, those involved in the hunt were buoyed by the Ocean Shield's findings.
"I'm an engineer so I don't talk emotions too much," Matthews said. "But certainly when I received word that they had another detection, you feel elated. You're hopeful that you can locate the final resting place of the aircraft and bring closure to all the families involved."