Posted on behalf of Declan Butler.

Hopes have faded that hydroacoustic signals picked up on the floor of the Indian Ocean might help to locate the Malaysian Airlines flight MH370 airliner that disappeared in March. Data from an additional sensor suggest that the signal probably resulted from geological activity and not the sound of an aircraft crashing into the ocean’s surface.

In June, Australian scientists had reported that sensitive microphones off the Australian coast had detected a distinctive signal at 01:30 coordinated universal time (UTC) on 8 March, around the time satellites lost contact with the Boeing 777 airliner.

The initially reported signals were discovered by an ocean acoustics group at Curtin University’s Centre for Marine Science and Technology in Perth, Australia. They were studying data from an acoustic station in Perth Canyon, about 40 kilometres west of Rottnest Island off the country’s west coast (see ‘Sound clue in hunt for MH370′).

That station is one of six belonging to Australia’s Integrated Marine Observing System (IMOS). The team then confirmed the signal using data from the Cape Leeuwin acoustic station, operated by the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) southwest of Australia. This gave a rough fix on the location of the origin of the sound as somewhere along a strip in the northwest of the Indian Ocean (see ‘Lost trail’).

On 3 September the researchers recovered data from another IMOS station at Scott Reef, off northwestern Australia. It contains a signal at 01:32:49 UTC that the researchers believe could correspond to the sound event they had detected earlier. Combining the data gave a fix on the location of the sound as the geologically active Carlsberg Ridge, midway between the Horn of Africa and India.

The sound signal also had a low amplitude tail, and taken together these two findings suggest that the event was geological — caused, for example, by an earthquake, underwater landslide or volcanic eruption, says Alec Duncan, a scientist in the Curtin University group.