Unmanned sonar sub to be deployed in search

• "It's time to go underwater," Houston says


• The Bluefin-21 will be deployed as soon as possible


• houston cautioned against hopes that underwater vehicle will find wreckage

(CNN) -- With no underwater pulses detected in almost a week, Australian authorities said Monday they will stop listening for pings coming from the floor of the Indian Ocean -- and will now deploy an underwater vehicle.

"We haven't had a single detection in six days. It's time to go underwater," Australian chief search coordinator Angus Houston said.

The Bluefin-21, a probe equipped with side-scan sonar, will be deployed as soon as Monday evening Perth time. Side-scan sonar is an acoustic technology that creates pictures from the reflections of sound rather than light.

Each deployment will last 24 hours.

It will take two hours for the Bluefin-21 to get down to the bottom of the ocean, it will scour the ocean bed for 16 hours, and take another two hours to resurface. It will take take another four hours to download and analyze the data collected, Houston said.

The first mission will cover an area 5 kilometers (3.1 miles) by 8 kilometers (4.9 miles).

Houston cautioned against hopes that the underwater vehicle will find wreckage.

"It may not," he said. "This will be a slow and painstaking process."

The bottom of the search area is not sharply mountainous -- it's more flat and almost rolling, Houston said. But he said the area likely has a lot of silt on the bottom, which can "complicate" the search.

Meanwhile, the Australian ship Ocean Shield detected an oil slick, but it is unclear where the oil came from. Two liters have been collected for examination, Houston said.

The developments come after a weekend of notable developments in the search for MH370, which disappeared March 8 with 239 people on board. Monday marks Day 37 of the search.

Batteries likely dead

The batteries powering the locator beacons inside the so-called black boxes are probably dead, a top official from the company that manufactures the beacons told CNN on Sunday.

That means searchers may not be able to detect any more pings to help lead them to those pieces of the missing plane.

"More than likely they are reaching end of life or already have. We're at Day 37. ... If (a beacon) is still going, it is very, very quiet at this point," Jeff Densmore told CNN's "State of the Union with Candy Crowley."

The time is ripe to move on to other search techniques.

"Every good effort has been expended, but it's now looking like the batteries are failing, and it's time to start mowing the lawn, as we say, time to start scanning the sea floor," said Rob McCollum, a CNN analyst and ocean search specialist.

Search area widens, then shrinks

After days of whittling down the search area in the Indian Ocean, officials expanded the target zone by 40% from Saturday to Sunday.

While that may seem like a setback in the investigation, it might not be, said Mary Schiavo, a former inspector general for the U.S. Department of Transportation.

"What I think they're doing is giving one last final push, a last-ditch effort if you will, to see if by any chance there is any wreckage to be found," she said. "Because even a few pieces would help narrow the search. I think it's one last big push. Maybe just a Hail Mary pass to try and find anything that they can to help them zero in on where to go on the ocean floor."

Monday's search area decreased by 17%. Twelve aircraft and 15 ships were set to participate in the hunt over an 18,400-square-mile (47,600-square-kilomter) area.

CNN's Steve Almasy and Nic Robertson contributed to this report.