Perth, Australia - Search crews were for the first time sending a sub deep into the Indian Ocean to try and determine whether faint sounds detected by equipment on board an Australian ship are from the missing Malaysia Airlines plane's black boxes, Australia's acting prime minister said today.

Warren Truss, Australia's acting prime minister while Tony Abbott is overseas, said the crew on board the Ocean Shield will launch the underwater vehicle, the Bluefin 21 autonomous sub, today. The unmanned miniature sub can create a sonar map of the area to chart any debris on the sea floor. If it maps out a debris field, the crew will replace the sonar system with a camera unit to photograph any wreckage.

Angus Houston, who is heading the search, said Monday that the Ocean Shield, which is towing sophisticated U.S. Navy listening equipment, detected late Saturday and early Sunday two distinct, long-lasting sounds underwater that are consistent with the pings from an aircraft's "black boxes" - the flight data and cockpit voice recorders. Houston dubbed the find "a most promising lead" in the monthlong hunt for clues to the plane's fate, but warned it could take days to determine whether the sounds were connected to Flight 370.

Crews have been trying to re-locate the sounds since Sunday, but have thus far had no luck, Truss said.

"Today is another critical day as we try and reconnect with the signals that perhaps have been emanating from the black box flight recorder of the MH370," he said. "The connections two days ago were obviously a time of great hope that there had been a significant breakthrough and it was disappointing that we were unable to repeat that experience yesterday."

Truss said the crew would use the sub today to examine the water in the search area in the hopes of another breakthrough.

Finding the black boxes is key to unraveling what happened to Flight 370, because they contain flight data and cockpit voice recordings that could explain why the plane veered so far off-course during its flight from Kuala Lumpur, Malaysia, to Beijing on March 8.

But time was running out to find the devices, whose locator beacons have a battery life of about a month. Today marks exactly one month since the plane vanished.

"Everyone's anxious about the life of the batteries on the black box flight recorders," Truss said. "Sometimes they go on for many, many weeks longer than they're mandated to operate for - we hope that'll be the case in this instance. But clearly there is an aura of urgency about the investigation."

The first sound picked up by the equipment on board the Ocean Shield lasted two hours and 20 minutes before it was lost, Houston said. The ship then turned around and picked up a signal again - this time recording two distinct "pinger returns" that lasted 13 minutes.

"Significantly, this would be consistent with transmissions from both the flight data recorder and the cockpit voice recorder," Houston said.

The black boxes normally emit a frequency of 37.5 kilohertz, and the signals picked up by the Ocean Shield were both 33.3 kilohertz, U.S. Navy Capt. Mark Matthews said. But the manufacturer indicated the frequency of black boxes can drift in older equipment.

The frequency used by aircraft flight recorders was chosen because no other devices use it, and because nothing in the natural world mimics it, said William Waldock, a search-and-rescue expert who teaches accident investigation at Embry-Riddle Aeronautical University in Prescott, Ariz.

"They picked that so there wouldn't be false alarms from other things in the ocean," he said.

But these signals are being detected by computer sweeps, and "not so much a guy with headphones on listening to pings," said U.S. Navy spokesman Chris Johnson. So until the signals are fully analyzed, it's too early to say what they are, he said.

"We'll hear lots of signals at different frequencies," he said. "Marine mammals. Our own ship systems. Scientific equipment, fishing equipment, things like that. And then of course there are lots of ships operating in the area that are all radiating certain signals into the ocean."

Geoff Dell, discipline leader of accident investigation at Central Queensland University in Australia, said it would be "coincidental in the extreme" for the sounds to have come from anything other than an aircraft's flight recorder.

"If they have a got a legitimate signal, and it's not from one of the other vessels or something, you would have to say they are within a bull's roar," he said. "There's still a chance that it's a spurious signal that's coming from somewhere else and they are chasing a ghost, but it certainly is encouraging that they've found something to suggest they are in the right spot."