Whale and Dolphin Conservation
Sonar is an acronym for Sound Navigation and Ranging. There are two broad types of sonar in use:
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.
Output power
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.
Output frequency
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:
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 environmental assessment and include consultation with the Department. This includes use of active sonar during joint exercises involving foreign military forces.
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. In the case of the Bahamas stranding event in 2000, the U.S. Navy acknowledged that use of mid-frequency, antisubmarine warfare sonar in the area appeared to be a factor in the mass strandings of beaked whales. 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.
A recent event in Southern Tasmania has shown that the cause of a mass stranding of long-finned pilot whales at Marion Bay was not related to the presence of Royal Australian Navy mine-hunting vessels which were operating high-frequency sonar in the area for a period after the strandings had begun. To investigate this event the Australian Government convened a Panel of experts to examine the circumstances and determine what effect, if any, the two ships had on the event. The panel membership included whale and acoustic experts, non-government conservation groups, the Tasmanian Environment Department and the Australian Government Departments of Defence and Environment and Heritage.
The panel found the first stranding event on October 25 could not have been caused by sonar activities because it happened at least six hours before the Navy ships arrived in the region.
Defence and DEH continue to support marine mammal research in Australia to further develop the efficacy of mitigation measures and to ensure the ongoing protection of marine mammals. Read the Panel’s complete findings.
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 tens or even hundreds of kilometres from the source.
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.