After 4 days of no pings, what's next?

(CNN) -- Once investigators looking for Malaysia Airlines Flight 370 decide to shift from listening for pings emanating from the floor of the Indian Ocean to poring over its terrain, they will begin drawing from a whole new set of tools.
Among them will be the Bluefin-21, a probe equipped with side-scan sonar -- an acoustic technology that creates pictures from the reflections of sound rather than light.
"That is a piece of equipment that does assist in locating where the wreckage may be," said Sylvia Earle, an oceanographer from National Geographic who was chief scientist for the National Oceanic and Atmospheric Administration.