Whale signals detected by International Monitoring System (IMS) facility
The International Monitoring System utilises hydroacoustic stations to monitor the earth's seas and oceans for indications of a possible nuclear test explosion. The highly sensitive station hydrophones also pick up signals from other sources, which must be identified so that they can be distinguished from an underwater explosion.
Hydrophones are deployed at the depth of the SOFAR axis, the centre of the layer in the ocean through which sound travels to great distances. The hydrophones are connected to the International Monitoring System stations at BIOT/Chagos Archipelago and Cape Leeuwin to detect the signals from nuclear explosions which might be generated underwater, in the atmosphere just above the water, and on land near the coastline. Two triplets of hydrophones are deployed from BIOT/Chagos Archipelago, one about 220 km northwest of the island of Diego Garcia at a depth of 1200 m, the other about 70 km south of the island at a depth of 1400 m. The triplet at Cape Leeuwin is about 135 km southwest of the Cape at a depth of 1000 m.
In addition to being able to detect nuclear explosions, the hydrophone triplets also record low frequency (1 to 100 Hz) acoustic signals associated with man-made and natural phenomena in the ocean. Efforts are underway to catalogue the signals associated with these events, as the International Data Centre analysts must be able to quickly distinguish these signals from the important signals of an underwater explosion. Signals associated with man-made and natural activity include undersea earthquakes, undersea volcanic activity, ocean seismic exploration (air-gun signals), ice generated noise from Antarctica, storm generated noise and biological activity.
Of particular general interest are the signals which appear to originate from whales. These signals are generally of short duration and typically have a defined frequency band. Thus they appear as short duration tones or often as series of such tones.
The nature of the tones, for example their duration, pattern, and frequency content, can be used to identify the whale species. The species observed to date in the Indian Ocean include blue whales, pigmy blue whales, minke whales, and finback whales. Some characteristic signals have not yet been associated with a particular whale species.
The figure shows a signal from the Cape Leeuwin hydrophone triplet which appears to originate from a nearby whale. The time series (upper panel) shows a pattern of several short pulses of energy at intervals of 10 to 20 seconds. The lower panel (a spectrogram showing frequency content versus time) shows that the frequency content of the signal is confined to a fairly narrow band primarily near 20 Hz but occasionally extending to 40 Hz. Based on these characteristics, it is likely that these signals are from a fin whale.