Underwater noise: Causes
What are the sources of ocean noise pollution?
Levels of anthropogenic (human-generated) noise have doubled every decade for the past 60 years in some regions. This destructive underwater noise is mostly caused by the military, the oil and gas industry and shipping traffic:
Seismic airguns are primarily used for oil and gas exploration below the seabed and for geophysical surveys of the seafloor. Air is driven into the water at high pressure, sending intense and explosive sound pulses towards the seabed. These sounds can permeate thousands of metres of ocean before penetrating hundreds of kilometres into the ocean floor. Up to 40 airguns are fired in a tight sequence, each of them emitting sound every ten to fifteen seconds, often for 24 hours a day and for several weeks in the same area. Hydrophones are used to record and analyse the sound that reflects back to the sea surface. As easily extractable resources are already depleted, seismic surveys keep spreading into ever more sensitive marine habitats and into ever greater depths.
Active sonar is used by military vessels during exercises and routine activities to search for objects such as enemy submarines. These mid- and low-frequency sonar systems emit sound pulses of more than 100 seconds at a time for hours on end. These pulses are emitted with as much energy and in as narrow a range as possible. Low-frequency sonar serves as a way of putting large areas under surveillance and pervades thousands of cubic kilometres of water with sound. Mid-frequency sonar uses 0.1 to 10 kHz and can reach up to 230 decibels.
90% of globally traded goods are transported by ships which are generating an ever-present and constantly rising acoustic “fog” that masks natural sounds. Ships tend to produce low-frequency sound between 10 Hz and 1 kHz that can spread over huge distances and is the most common source of ocean noise.
Explosives are detonated in the ocean by the military, for demolition purposes, or for testing equipment – e.g., ship-shock trials, whereby ships are deliberately struck with explosives to test their durability. These explosions cause extremely high noise levels across a wide frequency range and are characterised by rapid rise times.
Construction works in harbours as well as pile-driving for offshore wind farms and oilrigs are generating intensive noise emissions, too.