Sonar and seismic impacts
Sonar is an acronym for Sound Navigation and Ranging. There are two broad types of sonar in use:
- Passive Sonar. Passive sonar is a listening device that can determine the presence, characteristics and direction of marine noise sources. These sources may include biological noise (animal communication) and human sounds (eg ship or submarine noise). Passive sonar equipment is essentially an acoustic receiver which emits no sound and therefore has no potential to disturb marine life.
- Active sonar. Active sonar is a technique that uses sound to determine relative positions of submerged objects (including submarines, fish, mines and wrecks of ships and aircraft) and the sea floor, by emitting a sound signal and listening for the echoes from the objects. Many different types of active sonar are used throughout the world's oceans by private, commercial and military vessels. These systems mirror the purpose of sonars used by some marine animals. Active sonar devices locate objects by the reflection of sound-waves and remain an important means of underwater detection and navigation.
Potential impacts on the marine environment
The potential for sonar to impact on the marine environment is a function of the output power, the transmission frequency, and the sound transmission characteristics of the marine environment. The potential for impact on an individual species depends on its hearing response characteristics and behavioural sensitivity, and its distance from the sound source.
The strength of the active sonar transmission is an important factor in considering potential impact on marine species. The output power of active sonar is dependent on the designed purpose of the system. If the system is required to detect objects or the sea floor at great distances then high power transmissions will be required. This is because only a small amount of the output power may be expected to be reflected back to the detector from an object distant from the source. High power sonars include military antisubmarine sonars, commercial sidescan sonars, deep water echo sounders and fish finders. Lower output powers are used for sonars designed to detect objects in shallow water or close to the source.
Active sonars use a range of frequencies depending upon the role of the equipment. Lower frequencies (below 20 kHz) have potentially greater ranges in seawater due to lower rates of sound attenuation with distance. Detection of distant objects therefore relies on the use of lower frequencies, the lower the better, however, lower frequencies are not suitable for detection of small objects.
High to very high frequencies (above 100 kHz) provide excellent resolution of objects such as fish, small objects, and the sea floor but suffer from severe attenuation with distance from the source. These systems are only practical in shallow water or for short range detection of objects near the source.
Output power levels versus received levels
The sound source level will appear to an observer to reduce with increasing distance from the source. This is because the sound is reduced in intensity as it travels as a result of loss of energy from the sound wave and spreading the energy more thinly as the wave spreads out. The perceived output at distance is called the Received Level. Determination of potential impact on a species must use the received level, not the source level which is an abstract measure of the source characteristics (received levels are always lower than source levels). It should also be noted that sound levels quoted for various sources in air bear no direct relation to in-water sound sources. For example, comparing some sonar output power levels with jet aircraft noise is inaccurate and misleading.
Receptor species auditory characteristics
The potential for active sonar to impact on a species is dependent on the ability of the species to hear the sound. Species hear sounds over different frequencies ranges, and the efficiency of sound detection varies markedly with frequency. Additionally, species behavioural responses to a detected sound may vary according to the sensitivity of the species to disturbance and what activities the animals are engaged in at the time.
Determination of potential impact on a species must therefore include estimation of the ability of the species to detect the sound, and the likelihood of disturbance to critical activities such as feeding or parental protection of juveniles.
The main types of active sonar used by military forces are:
- Low-frequency (LF). Low frequency sonars have been defined as those that emit sound below 1000 Hz. These sonars are designed to provide theatre level protection, such as for an Aircraft Carrier Task Group out to many miles (up to 200 miles) from the ships. This is possible because of the extended propagation possible at low frequencies. Outputs are similar to medium frequency sonars (described below) but the sound travels further because of the significantly enhanced seawater propagation. The low frequencies employed require large transmission arrays and are fitted to large ships. Two examples are the US Surveillance Towed Array Sensor System Low Frequency Active sonar (SURTASS LFA) and the Type 2087 Sonar being deployed by the UK Navy. Only a small number of these LFA sonars exist. They are not used by the Royal Australian Navy, and have not been operated in Australian waters by other Navies.
- Medium frequency (MF). Medium frequency active sonars emit sounds at frequencies between (1000 and 10,000 Hz). These sonars represent a sliding scale of compromise between possible detection range and size of the transmission array. At the lower end of the frequency range (1000- 3000 Hz) the systems are capable of extended detection ranges using high output power, but the size of the transducer limits applications to large warships. These systems are designed to provide area protection for a small Task Group out to a few tens of miles. An example is the US Navy SQS-53 series of sonars fitted to larger US warships. The Royal Australian Navy does not operate these systems. At the upper end of this frequency range (3,000 – 10,000 Hz), sonar arrays are smaller and output powers are less, but the systems can be fitted to smaller vessels. These systems are designed for ship self protection out to a few miles. Examples include the SQS-56 and Spherion systems fitted to Australian warships.
- High frequency (HF). High frequency active sonars operate between approximately 30,000 and 500,000 Hz (30 kHz and 500 kHz). These systems allow increasingly greater resolution as the frequency increases but at the expense of range. The highest frequencies are only effective over short distances because of the rapid attenuation of high frequency sounds in seawater. These systems are used by the Royal Australian Navy for minehunting and sea floor searches, and for hydrographic survey. These sonars are similar to commercial fish finders and sidescan sonars designed to search for fish, map the sea floor and search for sunken objects.
Typically high power military active sonars are operated infrequently during voyages and the sounds are not emitted continuously but as short bursts ('pings') during operation.
Commercial and civilian sonars are generally designed to detect the sea floor (echo sounders), map the sea floor and search for sunken objects (sidescan sonars) and to locate fish (fish finders). Sonars of at least one of these types are fitted to nearly all vessels. Even some small boats have fish finding and echo sounders. The characteristics of these sonars are broadly similar to the high frequency military sonars described above.
The Royal Australian Navy in consultation with the Department of the Environment, Water, Heritage and the Arts has adopted mitigation procedures for the use of military sonar to avoid potential impacts on marine mammals. Activities involving the use of military sonar are subject to thorough defence environmental assessment procedures. This includes use of active sonar during joint exercises involving foreign military forces, including consultation with the department.
There have been a few overseas instances in recent years where marine mammal strandings have occurred in the same area and at the same time as naval forces were operating certain types of sonar equipment. It is important to note that there is no evidence to suggest that human sound sources have been the cause of any cetacean stranding events in Australia.
Seismic is different from sonar. The petroleum industry uses a technique which involves venting high-energy air pressure into the water which can generate seismic waves in the earth's crust beneath the sea. The resulting waves can then be studied to show geological structures of types often associated with petroleum deposits. Pneumatic air-guns are the most common energy source for marine geophysical surveys. These seismic surveys are usually conducted by towing an array of air-guns just below the surface behind a small ship. Sound pulses from these surveys are often detectable in the water at some level tens or even hundreds of kilometres from the source. Just because they are detectable, does not necessarily mean that they are harmful to marine life for any or all of this distance.
During seismic surveys, a predominantly low frequency (10 - 300 Hz), high intensity (215-250 dB) sound pulse is emitted every few seconds by the array of guns with the sound pressure depending on the size of the array.
- Seismic guidelines - Interaction between offshore seismic exploration and whales - EPBC Act Policy Statement 2.1