Ocean and Seabed Acoustics: A Theory of Wave Propagation

Acoustic-gravity waves in the atmosphere generated by infragravity waves in the ocean

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The conversion process is greater at high source levels than small ones. Because of the non-linearity there is a dependence of sound speed on the pressure amplitude so that large changes travel faster than small ones. Thus a sinusoidal waveform gradually becomes a sawtooth one with a steep rise and a gradual tail.

Use is made of this phenomenon in parametric sonar and theories have been developed to account for this, e. Sound in water is measured using a hydrophone , which is the underwater equivalent of a microphone.

Acoustic Fluctuations and Their Harmonic Structure

A hydrophone measures pressure fluctuations, and these are usually converted to sound pressure level SPL , which is a logarithmic measure of the mean square acoustic pressure. The scale for acoustic pressure in water differs from that used for sound in air. Similarly, the intensity is about the same if the SPL is Many measurements have been made of sound absorption in lakes and the ocean [6] [7] see Technical Guides — Calculation of absorption of sound in seawater for an on-line calculator.

Measurement of acoustic signals are possible if their amplitude exceeds a minimum threshold, determined partly by the signal processing used and partly by the level of background noise. Ambient noise is that part of the received noise that is independent of the source, receiver and platform characteristics.

This it excludes reverberation and towing noise for example. The background noise present in the ocean, or ambient noise, has many different sources and varies with location and frequency. Transient sound sources also contribute to ambient noise. These can include intermittent geological activity, such as earthquakes and underwater volcanoes, [26] rainfall on the surface, and biological activity. Biological sources include cetaceans especially blue , fin and sperm whales , [27] [28] certain types of fish, and snapping shrimp.

Rain can produce high levels of ambient noise. However the numerical relationship between rain rate and ambient noise level is difficult to determine because measurement of rain rate is problematic at sea. Many measurements have been made of sea surface, bottom and volume reverberation. Empirical models have sometimes been derived from these. A commonly used expression for the band 0. For bottom reverberation a Lambert's Law is found often to apply approximately, for example see Mackenzie.

Bottom loss has been measured as a function of grazing angle for many frequencies in various locations, for example those by the US Marine Geophysical Survey. Graphs have been produced for the loss to be expected in particular circumstances.

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In shallow water bottom loss often has the dominant impact on long range propagation. At low frequencies sound can propagate through the sediment then back into the water. As with airborne sound , sound pressure level underwater is usually reported in units of decibels , but there are some important differences that make it difficult and often inappropriate to compare SPL in water with SPL in air. These differences include: [34]. High levels of underwater sound create a potential hazard to human divers.

Diver aversion to low frequency sound is dependent upon sound pressure level and center frequency. For example, the hearing threshold of the killer whale occurs at an RMS acoustic pressure of 0. High levels of underwater sound create a potential hazard to marine and amphibious animals. The hearing sensitivity of fish is reviewed by Ladich and Fay. Sonar is the name given to the acoustic equivalent of radar.

Pulses of sound are used to probe the sea, and the echoes are then processed to extract information about the sea, its boundaries and submerged objects. An alternative use, known as passive sonar , attempts to do the same by listening to the sounds radiated by underwater objects. The need for underwater acoustic telemetry exists in applications such as data harvesting for environmental monitoring, communication with and between manned and unmanned underwater vehicles , transmission of diver speech, etc.

A related application is underwater remote control , in which acoustic telemetry is used to remotely actuate a switch or trigger an event. A prominent example of underwater remote control are acoustic releases , devices that are used to return sea floor deployed instrument packages or other payloads to the surface per remote command at the end of a deployment.

Acoustic communications form an active field of research [45] [46] with significant challenges to overcome, especially in horizontal, shallow-water channels. Compared with radio telecommunications , the available bandwidth is reduced by several orders of magnitude. Moreover, the low speed of sound causes multipath propagation to stretch over time delay intervals of tens or hundreds of milliseconds, as well as significant Doppler shifts and spreading. Often acoustic communication systems are not limited by noise, but by reverberation and time variability beyond the capability of receiver algorithms.

The fidelity of underwater communication links can be greatly improved by the use of hydrophone arrays, which allow processing techniques such as adaptive beamforming and diversity combining. Underwater navigation and tracking is a common requirement for exploration and work by divers, ROV , autonomous underwater vehicles AUV , manned submersibles and submarines alike. Unlike most radio signals which are quickly absorbed, sound propagates far underwater and at a rate that can be precisely measured or estimated. Starting in the s, this has given rise to underwater acoustic positioning systems which are now widely used.

Despite the relatively poor resolution due to their long wavelength, low frequency sounds are preferred because high frequencies are heavily attenuated when they travel through the seabed. Sound sources used include airguns , vibroseis and explosives. Acoustic sensors can be used to monitor the sound made by wind and precipitation.

For example, an acoustic rain gauge is described by Nystuen. Large scale ocean features can be detected by acoustic tomography. Bottom characteristics can be measured by side-scan sonar and sub-bottom profiling. Due to its excellent propagation properties, underwater sound is used as a tool to aid the study of marine life, from microplankton to the blue whale. Echo sounders are often used to provide data on marine life abundance, distribution, and behavior information. Echo sounders, also referred to as hydroacoustics is also used for fish location, quantity, size, and biomass.

Acoustic telemetry is also used for monitoring fish and marine wildlife. An acoustic transmitter is attached to the fish sometimes internally while an array of receivers listen to the information conveyed by the sound wave. This enables the researchers to track the movements of individuals in a small-medium scale. A neutrino is a fundamental particle that interacts very weakly with other matter. For this reason, it requires detection apparatus on a very large scale, and the ocean is sometimes used for this purpose. In particular, it is thought that ultra-high energy neutrinos in seawater can be detected acoustically.

From Wikipedia, the free encyclopedia. The study of the propagation of sound in water and the interaction of sound waves with the water and its boundaries. Main article: Sonar. Main article: Underwater acoustic communication. Main article: Underwater acoustic positioning system. Main article: Reflection seismology.

This section needs expansion. You can help by adding to it. May Main article: Acoustical oceanography. Main article: Bioacoustics.

Principles of Underwater Sound, 3rd Edition. Ray picture for downslope seabed case 2.

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Figure 9. Ray picture for wavy terrain when setting sound source at 50 m below sea surface case 1. Figure 1 0. Ray picture for wavy terrain when setting sound source at m below sea surface case 1. Figure 1 1. Figure 1 2. Figure 1 3. Ray picture for wavy terrain case 2. Because the phenomenon is iterative, it forms the accumulation area in wavy terrain and sound ray refracts iteratively in upslope ; 3 when sound source is set as m under water in upslope, sound ray propagates between m and m in vertical, and propagates with submarine topography; 4 when sound source is m under water, sound ray always bends to the general trends of sea surface, and refracts to the sea bottom, then refracts to the surface again.

This phenomenon is iterative.

Reflection and refraction of acoustic waves at poroelastic ocean bed

Sound ray propagates with submarine topography when sound ray come under the topography just as upslope. When sound source is 50 m under water, sound ray of certain angles propagate as surface channel within sea surface. Case 1: when upslope and wavy terrain, angle of sound ray is. When the angle is , sound ray bends to sea surface as biggish angle, then refracts rapidly because of the biggish angle.

The Science of Sound

Biggish angle makes sound ray in this angle range not refract in the speed boundary of constant speed structure, and can not propagate as surface channel; when the angle is , sound ray refracts in the speed boundary of constant speed structure instead of refraction, bends to the bottom instead of surface channel.

Case 2: when downslope, sound ray propagates as surface channel in all angle, because sound rays of biggish angle bend to surface and bottom, form multiple refraction, original propagation angle are changed. This phenomenon makes sound ray propagate as sound channel. The distance of sound ray propagating minish erratically as angle increases; when the angle of sound ray refracting around sea surface is in agreement with topographical grade, sound ray refracts at sea bottom and propagates with topography, then separates from the bottom at 47 km in horizon as wave propagation instead of accumulation area Figure 1 7.

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