VERIFICATION SCIENCE

The science of sound: How ‘soundscapes’ can help us understand the underwater world BY MARK PRIOR

Devices that listen for underwater 100 kilometres (km) long. These cables measured, along with the direction from sounds generated by nuclear tests can provide power and are connected to which they arrive. Time and direction be used to describe the ‘soundscape’ of a satellite link that sends data to the data are used to calculate the times and noises produced by whales, breaking ice, International Data Centre (IDC) at locations of the events that produced earthquakes and volcanoes. the CTBTO's headquarters in Vienna, the signals. Events detected by the IMS Austria. Each cable lies on the seabed network include earthquakes, volcanoes The Preparatory Commission and three hydrophones are floated up and mining blasts, as well as nuclear for the Comprehensive Nuclear-Test- from it. The hydrophones are arranged test explosions. Data from the IMS are Ban-Treaty Organization (CTBTO) in a triangle, two km apart at a depth processed at the IDC 24 hours a day and operates the International Monitoring of about 1,000 metres. This shape a list of events is produced for every day System (IMS): a worldwide network allows the arrival time of signals to be of the year. designed to detect signals caused by nuclear test explosions. The IMS will consist of 337 facilities when complete; almost 90% of these facilities are already operational. The network has seismometers to detect vibrations in the Earth, microphones to listen for very low frequency sounds in the air (infrasound) and radiation sensors that ‘smell the air’ for radioactive gases and particles produced by nuclear explosions. The IMS also uses underwater microphones (hydrophones) to detect sound in the ocean. Since water is an excellent sound conductor – sounds can travel over four times faster through water than air and can also be detected at greater distances - the IMS hydroacoustic network comprises just 11 stations. These stations monitor all of the world’s oceans.

PROCESSING DATA 24/7

IMS hydrophones are attached to shore stations by cables that can be up to Installation of hydroacoustic station HA11, Wake Island, USA.

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CTBTO SPECTRUM 22 | AUGUST 2014 Data from Cape Leeuwin (Aus)

Figure 1: Soundscape for the IMS hydrophone station off Cape Leeuwin, Western Australia. in a 1Hz band) 2

The surface in Figure 1 shows Noise (dB rel µPa Noise (dB rel small ‘hills’ at frequencies around 20 Hz in the early months of each year. These hills are caused by whale calls that are heard at Cape Leeuwin during the Frequency (Hz) yearly migration of fin and blue whales. Recording year The data shown in Figure 1 are useful when working out how sensitive each hydrophone station is. The quietest signal that a station can detect is DESCRIBING HOW THE shows the same data ‘flattened out’ controlled by the noise in the frequency UNDERWATER WORLD SOUNDS so that only the colour shading is left. band containing the signal. The figure This is done because some features in shows how this changes as the source of To understand how well the IMS the data are clearer in the flat image, the noises (ice-breaking, whales, storms hydrophone sensors are working, and while others show up better on the etc.) varies through the year. to help predict how this might change surface. The underwater noise levels in the future, the CTBTO produces in the figure should not be confused Although plots like Figure 1 are ‘soundscapes’. These are descriptions with values quoted for airborne sounds very useful, they do not show important of how the underwater world sounds, made by rock concerts or jet engines. information about the direction from in the same way that a ‘landscape’ is a Different reference levels are used which signals arrive. A second, separate description of how the Earth’s surface for airborne sounds and the numbers plot is used for this and an example is looks. Two different descriptions are cannot be directly compared. shown in Figure 2. used. The first shows how the strength of sound changes with frequency and THE ORIGIN OF ‘OCEAN Figure 2 shows the directions time. The second shows the directions MICROSEISM’ from which signals arrived at the Cape from which signals arrive. Leeuwin station during the year 2010. The ridge in the surface that runs For each day, a grey dot is drawn in Figure 1 shows noise data measured through all years at a frequency of 0.3 the figure for 360 directions moving at the IMS station Cape Leeuwin, off Hz is made by sounds produced by clockwise from north to south then the coast of Western Australia. Data water waves on the sea surface. These back up to north again. Light-grey or are shown for the entire period since waves make noises that travel down white dots show directions and days the station started, with noise levels towards the seabed. The sound waves on which many arrivals were seen. The calculated for frequencies between 0.01 hit the seafloor and make vibrations map to the right of the figure is a ‘Cape Hertz (Hz) and 100 Hz (human hearing in the Earth’s crust. These vibrations – Leeuwin’s eye view’ of the world, where covers an approximate frequency range known as ‘the ocean microseism’ – are continents are placed according to their between 20 Hz and 20,000Hz). The seen even in the middle of continents, direction and distance from the station. height and colours of the surface in the thousands of kilometres away from The “stretching” according to direction figure are set by the noise in decibels, the coast. The bright yellow patches in and distance from Cape Leeuwin is relative to one micropascal, which the ‘carpet’ in the figure show that the responsible for the warped appearance of scientists use as the reference value peaks along the ridge happen in the the map. The blue lines in the map show for underwater sound pressure levels. southern-hemisphere winter, when the mid-ocean spreading ridges. These are The ‘carpet’ on the floor of the figure southern Indian Ocean is roughest. places where earthquakes are common.

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CTBTO SPECTRUM 22 | AUGUST 2014 Cape Leeuwin, Australia Brightness set by number of signals per day from direction North

level of microseism sound is related to sea surface roughness and this can be useful to scientists interested in West climate modelling. Ice-breaking noises received from Antarctica are of interest to polar scientists studying changes in ice-cover; and whale noises recorded on CTBTO hydrophones can be used by scientists studying marine mammal South behaviour. These and many other scientific fields can all benefit from the

Received direction Received pictures of the ocean soundscape that can be built from the data provided by the hydrophone stations of the East International Monitoring System.

This is a popular version of a Paper that was presented on 5 December 2013 at the 166th Meeting of the Acoustical Society of America, San Francisco, USA.

Jan. April July Oct. Dec. The views expressed in this paper are Time those of the author and do not necessarily represent the views of the CTBTO Preparatory Figure 2: Commission. The directions from which sound arrives at the IMS hydrophone station off Cape Leeuwin, Western Australia.

IDENTIFYING ICE NOISE This type of feature is made by large AND T PHASE SIGNALS earthquakes and the aftershocks that follow them. In this case, the line in BIOGRAPHICAL NOTES The broad, grey stripe in Figure 2 shows Figure 2 was made by a magnitude 7.7 that signals arrive at Cape Leeuwin from earthquake on 25 October 2010 off the the south all year round. The map region west coast of Sumatra in Indonesia. matching this stripe covers Antarctica This earthquake caused a 3-metre- and this helps to identify ice noise as the high tsunami and killed over 400 source of many of the arrivals. Inside people. Data from the IMS, including the broad stripe, thinner, brighter lines hydrophone stations, which are can be seen. These are caused by single, currently sent to 11 tsunami warning drifting icebergs that leave Antarctica centres around the world, helped to and move with ocean currents, breaking produce a tsunami warning that was up as they melt. sent on the day of the earthquake that caused the line in Figure 2. The thin, grey line near the top of the figure shows a direction to the SOUNDSCAPES OFFER A northwest of the station from which RANGE OF BENEFITS signals arrive throughout the year. The MARK PRIOR map shows that this direction points Soundscapes help the CTBTO to is a seismic/acoustic officer at the to the coast of Indonesia where there understand the current performance International Data Centre of the are many earthquakes. These signals of their hydrophone stations. CTBTO where he has worked since are called T-phases and are noises in Trends in noise that show up in the 2007. He has worked in the field of underwater acoustics since the ocean made when earthquakes soundscapes also help to predict graduating from university and has make the coast or underwater how station performance might previously studied oceanography mountains shake. Towards the end of change in the future. Furthermore, and sonar design. Before joining the year, Figure 2 shows a bright line the information contained in the the CTBTO Dr Prior worked at the just above the thin stripe. This line soundscapes is useful to scientists NATO Undersea Research Centre in La Spezia, Italy, and prior to that in begins suddenly then gradually fades involved in areas outside nuclear Dorset, England. from white, through grey to black. test-ban monitoring. For example, the

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CTBTO SPECTRUM 22 | AUGUST 2014