"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 urgent because their locator beacons have a battery life of about a month, and Tuesday marked one month since the plane vanished with 239 people on board. If the batteries fail before the black boxes are located, finding them in such deep water — about 4,500 meters, or 15,000 feet — would be immensely difficult, if not impossible.
The Ocean Shield is towing a pinger locator from a U.S. Navy 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 the ship heard Saturday determined they were distinct, man-made and pulsed consistently, Houston said.
"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 was using parachutes to drop buoys in a pattern near where the signals were last heard.
Royal Australian Navy Commodore Peter Leavy said each buoy will dangle a hydrophone listening device 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, noting the last two signals were weaker and briefer than the first pair, suggesting the batteries are failing. One Saturday lasted two hours and 20 minutes and the second lasted 13 minutes; those 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.
The weakening of the signals also could indicate the device was farther away, U.S. Navy Capt. Mark Matthews said. Temperature, water pressure or the saltiness of the sea could also be factors.
Leavy said thick silt on the ocean floor also could distort the sounds, and may hide wreckage from the eventual visual search.
Houston said a decision had not yet been made on how long to use the towed pinger locator while knowing the beacons' batteries will likely fail soon, saying only that a decision to deploy an unmanned submarine 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.
When the pinger locator's use is exhausted, the unmanned sub will be sent to create a sonar map of a potential debris field on the seafloor. The Bluefin 21 sub takes six times longer to cover the same area as the pinger locator.