text: PERTH, Australia -- Search crews hunting for the missing Malaysia Airlines jet have failed to relocate faint sounds heard deep below the southern Indian Ocean that officials said were consistent with a plane's black boxes, the head of the search operation said Tuesday. 
   
   Angus Houston, the retired Australian air chief marshal who is heading the search far off Australia's west coast, said sound locating equipment on board the Ocean Shield has picked up no trace of the signals since they were first heard late Saturday and early Sunday. 
   
   Time may have already run out to find the devices, whose locator beacons have a battery life of about a month. Tuesday marks one month since the plane vanished. Once the beacons blink off, locating the black boxes in such deep water would be an immensely difficult, if not impossible, task. 
   
   Up to 14 planes and 14 ships are scouring the 30,000-square-mile search zone. 
   
   The wait has been devastating for weary families, CBS News correspondent Seth Doane reports. Many insist the search continues at all costs, despite the frustrations. 
   
   It is unclear whether a sub on board the Ocean Shield will be deployed to try to chart out any debris on the sea floor when searchers are certain the black boxes' locator beacons have gone silent. If the sub maps out a debris field, the crew will replace the sonar system with a camera unit to photograph any wreckage. 
   
   
   
   Houston's comments about deploying the sub contradicted an earlier statement from Australia's acting prime minister, Warren Truss, who said search crews would launch the Bluefin 21 autonomous sub on Tuesday. 
   
   The towed pinger locator 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 said, dubbing the find a promising lead in the month-long hunt for clues to the plane's fate. 
   
   Still, officials warned it could take days to determine whether the sounds were connected to Flight 370, which vanished March 8 on a flight from Kuala Lumpur, Malaysia, to Beijing with 239 on board. 
   
   "This is an herculean task - it's over a very, very wide area, the water is extremely deep," Defense Minister David Johnston said. "We have at least several days of intense action ahead of us." 
   
   Houtson said finding the sound again was critical to narrowing down the search area before the sub can be used. If the vehicle went down now with the sparse data collected so far, it would take "many, many days" for it to cover all the places the pings might have come from. 
   
   "It's literally crawling at the bottom of the ocean, so it's going to take a long, long time," Houston said. 
   
   Despite the excitement surrounding the Ocean Shield's sound detections, Houston warned that the search had previously been marred by false leads - such as ships detecting their own signals. Because of that, other ships cannot be sent in to help with the underwater search, as they may add unwanted noise. 
   
   "We're very hopeful we will find further evidence that will confirm the aircraft is in that location," Houston said. "There's still a little bit of doubt there, but I'm a lot more optimistic than I was one week ago." 
   
   Finding the black boxes is key to unraveling what happened to the Boeing 777, because they contain flight data and cockpit voice recordings that could explain why the plane veered so far off-course. 
   
   "Everyone's anxious about the life of the batteries on the black box flight recorders," said Truss, who is acting prime minister while Tony Abbott is overseas. "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. That would be consistent with transmissions from both the flight data recorder and the cockpit voice recorder. 
   
   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. 
   
   Houston said the frequency of the sounds heard was considered "quite credible" by the manufacturer, and noted that the frequency from the Air France jet that crashed several years ago was 34 kilohertz. Pressure from being so deep below the surface and the age of the batteries can also affect the transmission level, he said. 
   
   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. 
   
   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." 
   
   The Ocean Shield is dragging a ping locator at a depth of 1.9 miles. It is designed to detect signals at a range of 1.12 miles, meaning it would need to be almost on top of the recorders to detect them if they were on the ocean floor, which is about 2.8 miles deep. 
   
   Meanwhile, the search for any trace of the plane on the ocean's surface continued Tuesday. It is taking place 1,400 miles northwest of the Australian west coast city of Perth, with good weather predicted, said the Joint Agency Coordination Center, which is overseeing the operation. 
   
   One-hundred-thirty-three missions have been completed so far in the hunt from the air for debris that could be from Flight 370, but no debris from the plane has been found. ||||| 1 of 7. A fast response craft from Australian Defence Vessel Ocean Shield tows Able Seaman Clearance Diver Michael Arnold as he searches the ocean for debris in the search zone in the southern Indian Ocean for the missing Malaysian Airlines flight MH370 in this picture released by the Australian Defence Force April 8, 2014. 
   
   SYDNEY/PERTH, Australia (Reuters) - An Australian ship which picked up possible "pings" from the black box recorders of a missing Malaysia Airlines jetliner has been unable to detect any further signals and time is running out to narrow the massive search, officials said on Tuesday. 
   
   Angus Houston, head of the Australian agency coordinating the search for Malaysia Airlines Flight MH370, said the month-long hunt in the Indian Ocean was at a critical stage given the batteries in the black box beacons had already reached the end of their 30-day expected life. 
   
   A U.S. Navy "towed pinger locator" onboard Australia's Ocean Shield picked up two signals consistent with black box locator beacons over the weekend - the first for more than two hours and the second for about 13 minutes. 
   
   Houston said the signals represented the best lead in the search yet, but efforts to pick up the pings again had so far been unsuccessful. 
   
   "If we don't get any further transmissions, we have a reasonably large search area of the bottom of the ocean to prosecute and that will take a long, long time. It's very slow, painstaking work," said Houston. 
   
   The black boxes record cockpit data and may provide answers about what happened to the plane, which was carrying 227 passengers and 12 crew when it vanished on March 8 and flew thousands of kilometers off its Kuala Lumpur-to-Beijing route. 
   
   Authorities say evidence, including the loss of communications, suggests the plane was deliberately diverted by someone familiar with the aircraft, but have not ruled out mechanical problems. 
   
   Analysis of satellite data led investigators to conclude the Boeing 777 came down in an area some 1,680 km (1,040 miles) northwest of Perth, near where possible pings were picked up and the search is now focused. 
   
   BLUEFIN ON HOLD 
   
   An autonomous underwater vehicle (AUV) named Bluefin-21 is onboard the Ocean Shield and could be sent to look for wreckage on the sea floor, but narrowing the search zone first was critical, Houston said. 
   
   "It is a large area for a small submersible that has a very narrow field of search, and of course, it is literally crawling along the bottom of the ocean," he said. 
   
   "That's why it's so important to get another transmission and we need to continue until there's absolutely no chance the (black box) device is still transmitting." 
   
   The Bluefin will scour the ocean floor in 20-hour missions using sonar in an attempt to find the aircraft, before its findings are downloaded and analyzed on board the Ocean Shield. 
   
   If anything unusual is spotted, the sonar on board the robotic vehicle will be replaced with a camera to take a closer look. The potential search area is about 4.5 km (2.8 miles) deep, the outer reach of the Bluefin's range. 
   
   Some 133 missions have been completed so far in the multinational aerial hunt for debris in the southern Indian Ocean and would continue, officials said. 
   
   Up to 11 military planes, three civilian planes and 14 ships planned to take part in the search on Tuesday, with good weather in the search area. But so far, searchers have only turned up fishing gear and other detritus. 
   
   "Even after the black box is found, the surface search will continue because they're looking for evidence to investigate why the aircraft went down," Lieutenant Commander Adam Schantz, the officer in charge of U.S. Navy P-8 Poseidon search unit, told Reuters. 
   
   "So, we'll continue for a while trying to find anything on the surface that might give a reasoning behind the accident." 
   
   (Additional reporting by Jane Wardell in Sydney and Anuradha Raghu in Kuala Lumpur; Editing by Dean Yates and Michael Perry)
target: – When an Australian ship heard pings possibly from Malaysia Airlines Flight 370, officials saw a big lead open up in the search—but since then, investigators haven't been able to recapture the signal, Reuters reports. That's particularly worrying since the batteries on the jet's black boxes have likely just about run out. But "we need to continue (searching) for several days right up to the point at which there's absolutely no doubt that the batteries will have expired," says Angus Houston, the Australian official leading the search, per CBS News. "If we don't get any further transmissions, we have a reasonably large search area of the bottom of the ocean to prosecute and that will take a long, long time," Houston says. The ship is carrying an autonomous underwater vehicle that can search the depths for the plane, "literally crawling along the bottom of the ocean," Houston notes. Trouble is, findings so far point to "a large area for a small submersible that has a very narrow field of search." Once the vehicle, called Bluefin-21, does head downward, it will conduct 20-hour sonar missions to try to find the plane.