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ACOUSTICS IN THE OCEAN

VIEWS: 18 PAGES: 3

									ACOUSTICS IN THE OCEAN

Timothy K. Stanton
Department of Applied Ocean Physics and Engineering
Woods Hole Oceanographic Institution
Woods Hole, MA 02543-1053
e-mail: tstanton@whoi.edu
Ph: (508)-289-2757

         Acoustic signals can travel great distances in the ocean and for that reason, they are used in a
wide range of scientific applications. The applications include two broad categories-- 1) a remote sensing
tool in investigating oceanographic processes and 2) wireless underwater communications. Although the
focus of this talk is scientific, there are also important Navy applications of sound involving detection of
targets (in which the oceanographic processes may be sources of interference) and acoustic
communication as well. Given the broad applicability of acoustics in the ocean, there exist a
correspondingly broad array of sensors and deployment methods.

REMOTE SENSING OF OCEANOGRAPHIC PROCESSES

        Acoustics can be used in two distinctly different ways in remotely sensing oceanographic
processes-- in the active and passive modes. In the active mode, sound is emitted by a transmitter and the
echoes are received by a receiver. In the passive mode, only a receiver is used to receive sounds naturally
occurring in the ocean.

        Active mode. There is a diverse range of scatterers of sound in the ocean that correspond to
important oceanographic entities, ranging from the sea surface to the seafloor. When sound is transmitted
into the water, and depending upon the direction of the incident signal, it can scatter off of some
combination of the sea surface, near-surface bubbles, marine biota such as fish and zooplankton, physical
microstructure, suspended sediment, surficial roughness of the seafloor (including rocks and shells), and
sub-bottom heterogeneities. The scattering is commonly displayed in an image referred to as an
echogram (Fig. 1).

         Quantitative interpretation of the echoes is a challenge. The strength of the echoes from the
objects or structures in the ocean depends upon the size (or spatial scale), shape, orientation, and material
properties as well as the wavelength of the sound. Because of all of these dependencies, there are
inherent ambiguities in the interpretation and ground truth must be used. Frequently, multiple acoustic
frequencies are used to reduce the number of ambiguities. A typical quantity to extract from the strength
of the echo is the size or characteristic scale of the scatterer.

        In addition to the strength of the echo, the Doppler shift is used in some systems to study the
velocity of the scatterer. In many applications, the Doppler shift is used to study currents.

         Passive mode. There are many natural and anthropogenic sources of sound. Among the natural
sources are rainfall, breaking waves, cetaceans (whales, dolphins), and snapping shrimp. A major source
of sound caused by humans is from ships. Specifically, the cavitation from the ship propellor is a
significant source of sound.
WIRELESS UNDERWATER COMMUNICATIONS

         Acoustic signals that are transmitted through the water can also be used to carry information. The
information can be carried either by modulating the amplitude or phase (much like in an AM/FM radio).
Since underwater sound travels so much slower than radio waves and has much lower frequencies, the
data rate is subsequently much lower. Typically, the data rate is at the highest when there is a direct path
between the transmitter and receiver and there is not significant interference due to reverberation from the
sea surface and seafloor.

ACOUSTIC SENSORS

         The most commonly used sensors are piezoelectric ceramics. They are sometimes used one at a
time to form single beams of sound or in arrays to form one or more beams (such as in a multibeam
system). The ceramics normally have efficiencies (input electrical energy to output acoustical energy) of
better than 50%. However, any given ceramic sensor may be heavy (especially at the lower frequencies)
and operate over a narrow range of frequencies (within 10% of the resonance frequency). Thus, other
technologies have been developed, although with more specialized use. For example, there are many
types of broadband sensors, such as piezoelectric films, although many of them can only be used to
receive the signals.


DEPLOYMENT METHODS

         There is a diverse set of ways to deploy acoustic sensors. For active acoustic sensing of an
oceanographic process, a sensor can be mounted onto the hull of a ship, towed from a platform, or
mounted onto an underwater vehicle, mooring, or tripod on the seafloor. Passive systems may be
mounted in a similar way, although many passive systems come in the form of long line arrays. In this
latter case, the line array may be towed behind a ship or strung in the vertical dimension via use of a
mooring. There are usually at least two if not many more communication sensors used at a time. For
example, one may be on a mooring while the other on the sensor platform, which is making
measurements of an oceanographic process.
Figure 1. Acoustic echogram from downlooking single-beam echosounder towed near the sea surface
over Georges Bank. The variability in echo levels generally correspond to patches or layers of
zooplankton.

								
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