Listening in order to see
Marine acoustics uses sounds to study the submarine environment. There are active acoustics and passive acoustics.
Active acoustics allows to “see” under water down to depths of several hundred metres, where not even light penetrates. Echo sounders emit sounds which travel and strike obstacles, including living organisms. Based on the echo retransmitted by these obstacles, their position and properties can be deducted and an image of their distribution in the water or submarine relief can be generated. At different frequencies, echo sounders can distinguish echoes originating from fish and those from zooplankton. Krill for example have a stronger echo at 120 kHz than at 38 kHz, frequency at which capelin have a stronger echo. Zooplankton smaller than krill has a stronger echo at frequencies exceeding 120 kHz.
Opposite is an echogram (38 kHz) recorded on board Parks Canada’s L’Alliance at the edge of Tadoussac Bay on August 4, 2009. Three humpback whales photographed when the boat passed by were in the area of the yellow and red mass (small fish).
Sound propagation in water can also be studied to determine the latter’s characteristics such as its average temperature. More powerful low-frequency sounds can be used to detect modern submarines at great distances or to explore underground structures, notably for oil prospecting.
Telemetry project on rorquals of the Estuary and acoustic prey census
- Video credit: © Ocean Mysteries – Georgia Aquarium
- On October 3, 2013, while the GREMM team is in the St. Lawrence Estuary, the Parks Canada crew is simultaneously performing an acoustic prey census aboard L’Alliance in order to determine at what depths and on what types of prey the seven-tonne giant is feeding. The acoustic images, obtained using the echo sounder submerged in the water column, reveal schools of fish, probably sand lances, also being pursued by several other minke whales as well as hundreds of gray seals and seabirds.
This rorqual marking project is being conducted conjointly by the GREMM and Fisheries and Oceans Canada, with the participation of Parks Canada for the prey census.
Did you know that sound travels approximately four times faster in water than in the air? And that it travels much farther? Passive acoustics is the study of submarine sounds. To capture these sounds, researchers place hydrophones (waterproof microphones) under the sea. But whales can emit sounds that are inaudible to the human ear, e.g. infrasounds and ultrasounds. Hydrophones can detect them, but in order for us to be able to hear sounds below 60 Hz and above 16,000 Hz, they must be transformed. They are either accelerated or slowed down.
Here’s what a low-frequency sound emitted by a blue whale sounds like when it’s accelerated times four.
- Sound of a blue whale
- Credit: © Mériscope
Using a network of hydrophones placed under water, we can even pinpoint the source of the sound, i.e. the location of the whale. Over time, it is thereby possible to gain an overview of the use of the territory and understand correlations with certain climatic and oceanographic factors. Lastly, this technique can be used to study the ambient noise of the sea, including natural sources (such as earthquakes) and anthropogenic sources (such as boats), and thus to assess sound pollution levels and their effects on different marine species.
- Sound of a freighter
- Ian McQuinn © Fisheries and Oceans Canada
- Sound of a zodiac
- Credit: Ian McQuinn © Fisheries and Oceans Canada