Investigators hunting for the missing Malaysian Air jet will deploy an underwater vessel to scan the ocean floor as the absence of new signals suggests the black boxes have run out of power.

Australian ship Ocean Shield will send the Bluefin-21 to search in the area where the pings were heard, Angus Houston, who heads Australia’s Joint Agency Coordination Centre, said in Perth today. The boat will stop using the towed pinger locator and instead rely on the submersible to find debris as the search stretches into a 38th day.

Scouring the ocean surface for debris with planes and ships will be called off in two to three days as the chance of any floating material being recovered has “greatly diminished,” Houston said. An oil slick has also been found in the same area and will be analyzed.

The towed pinger locator pulled by the Ocean Shield heard no further signals after picking up four sounds between April 5 and April 8. Australian officials said today they are deploying planes to scour about 47,644 square kilometers (18,380 square miles) of water, down from 57,506 yesterday. Pings heard by Chinese vessel Haixun 01 were analyzed and have been discounted, Houston told reporters.

“At the moment this is really all we’ve got,” Houston said. “We’ve got no visual objects, the only thing we have left at this stage are four transmissions and an oil slick in the same vicinity. So we will investigate those to their conclusion.”

The Bluefin-21, equipped with side-scan sonar, will be deployed for 24 hours at a time. It will spend two hours descending, 16 hours on the ocean bottom, two hours returning to the surface, and four hours having its data downloaded, Houston said. It will be launched tonight Australian time and search an area of about 40 square kilometers to produce a high-resolution, three-dimensional sonar map of the seabed, he said.

The towed pinger locator can’t be operated at the same time as Bluefin-21, which doesn’t relay information while diving, Houston said. Data returned by today’s search will be used to determine search zones for subsequent sweeps.

At the search area, the depth of the ocean is “on the right side” of 4,500 meters (14,765 feet), the maximum diving depth of the submersible, Houston said. The sea floor is probably covered in silt with a topography that’s “flat and almost rolling,” he said.

“This is an area that is new to man,” he said of the ocean bottom in the area. “We’re actually gathering information about the search environment all the time.”

The center of today’s search areas is 2,200 kilometers (1,367 miles) northwest of Perth, with as many as 12 aircraft and 15 ships taking part, Australia’s Joint Agency Coordination Center said. No confirmed acoustic detections have been made over the past 24 hours.

The black boxes are key to determining why the Malaysian Airline System Bhd. plane disappeared March 8 en route to Beijing from Kuala Lumpur, reversing course and flying into some of the world’s most remote ocean waters. The pingers’ batteries are now a week beyond their 30-day projected life at full power.

The Ocean Shield detected two signals on April 5 and two more on April 8, which authorities have linked to the beacons on the Boeing Co. 777-200ER’s black-box recorders. That raised optimism in the search. Hopes faded on April 11 when the JACC said an initial assessment of a fifth potential transmission wasn’t related to an aircraft black box.

Malaysia’s Attorney General is in the U.K. discussing with officials of the International Civil Aviation Organization and other legal experts on who gets the custody of the black boxes, when they are found, Malaysia’s acting Transport Minister Hishammuddin Hussein said in Kuala Lumpur yesterday, according to comments confirmed by his office.

“I would caution you against raising hopes that the deployment of the autonomous underwater vehicle will result in the detection of the aircraft wreckage,” Houston said. “It may not. However, this is the best lead we have and it must be pursued vigorously.”