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Fine whale songs can reveal hidden features of the ocean floor

A new study finds that the call of the fin whale is one of the strongest in the ocean: it can even penetrate the earth's crust. Echoes of whale songs recorded by seismic instruments at the bottom of the ocean reveal that sound waves pass through layers of sediment and underlying rocks. These songs can help probe the structure of the crust when more conventional survey methods are not available, the researchers reported on Feb. 12.

Six songs, all from a single whale singing while swimming, were analyzed by seismologists Václav Kuna of the Czech Academy of Sciences in Prague and John Nábělek of Oregon State University in Corvallis. They recorded the songs, from 2.5 to 4.9 hours, in 2012 and 2013 with a network of 54 ocean-bottom seismometers in the northern Pacific Ocean.

The songs of fin whales (Balaenoptera physalus) can reach up to 189 decibels, as loud as a large boat. Seismic instruments detect the sound waves of the song, just as they pick up pulses from earthquakes or air pistols used for ship lifts. Underwater sounds can also produce seismic echoes: when sound waves traveling through water meet the ground, some of the energy from the waves becomes a seismic wave (SN: 17/09/20). Such seismic waves can help scientists “see” underground: as penetrating waves bounce off different layers of rock, researchers can estimate the thickness of the layers. Changes in wave speed can also reveal what types of rocks the waves traveled through.

Echoes recorded in the Pacific Ocean revealed a classical structure of oceanic crust beneath three sites along the whale’s swimming path: layers of sediment between 400 and 650 meters thick on a 1.8-kilometer layer of basaltic rock. Beneath that basalt lies a dense oceanic rock known as gabbro. The findings suggest that fin whale songs may be effective seismic tools for studying the seabed.

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