seasonal and geographic variations, currents, tides, swells, internal waves, and
even time of the day. The average velocity of a sound-wave travelling through
seawater is approximately 1,500 m/s, but changes in water temperature, pressure
and salinity cause important deviations (Wilson 1960; Medwin and Clay 1998).
The sound speed plotted as function of depth for a particular location is called a
sound speed profile and provides a visual indication of location-specific sound
speed variations (Fig. 8.3). It is observed that close to the surface, the controlling
variable for the speed of sound in water is temperature, while at larger depths the
controlling factor is depth. Salinity has only a small influence within the range
from 34 to 35 parts per thousand (34–35 Practical Salinity Units, PSU). As a
practical approximation, per every degree centigrade of temperature increase,
sound speed close to the surface increases by 3 m/s; per every increase of one PSU
increase in salinity, sound speed increases 1.4 m/s; and per every kilometer of
depth increase, sound speed increases by 17 m/s.
Localized sound speed can be measured using an instrument such as a Time of
Flight Probe, which uses an acoustic transducer transmitting a pulse that is
reflected by a plate located at a fixed distance. It measures the two way travel time,
divides it by two, and thereby computes the sound speed. For acoustic remote
sensing, however, it is necessary to measure the sound speed profile for the entire
water column. The sound speed profile can be obtained using instruments such as a
CTD, which is lowered from the surface while periodically measuring the conductivity ‘‘C’’ (salinity), temperature ‘‘T’’ and depth ‘‘D’’ (pressure) through the
water column. The United Nations Educational and Scientific Organization
(UNESCO) algorithm to compute the sound speed profile based on CTD data,
based on the Chen and Millero (1977) equation, is considered a universal standard
in oceanography. Alternatively, an eXpendable Bathy Thermograph (XBT), which
is less accurate but cheaper and easier to use, can similarly be used to compute the
sound speed profile. While free falling at a known rate through the water column,
the XBT sends temperature information to the surface using two very thin wires,
before breaking loose. For a vessel that is underway, the Moving Vessel Profiler
(MVP) utilizes a computer controlled winch and a custom ‘fish’ (a general term for
a hydrodynamically shaped instrument) to measure sound velocity without the
need to stop the vessel.
An important factor for acoustic remote sensing is attenuation in the water
column, where active SONAR suffers two-way attenuation and passive SONAR
only one-way attenuation. Since sound spreads geometrically from its source and
then again upon reflection, it is attenuated by this spreading. The classic example
to visualize a wave spreading is to throw a rock into a calm pond. A circular wave
is produced when the rock hits the pond, where circles with increasing circumferences, but decreasing amplitudes, are seen. The total amount of energy in a
wave remains constant as it spreads out from a source, but as the circle gets bigger,
the energy spreads to fill it. So the energy per unit length of the surface wave gets
smaller, decreasing the height of the wave. If a disturbance is created in the water
column and the waves spread out in all directions, they get smaller even more
rapidly than surface waves; this is called spherical spreading. Sooner or later, the
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B. Riegl and H. Guarin
even time of the day. The average velocity of a sound-wave travelling through
seawater is approximately 1,500 m/s, but changes in water temperature, pressure
and salinity cause important deviations (Wilson 1960; Medwin and Clay 1998).
The sound speed plotted as function of depth for a particular location is called a
sound speed profile and provides a visual indication of location-specific sound
speed variations (Fig. 8.3). It is observed that close to the surface, the controlling
variable for the speed of sound in water is temperature, while at larger depths the
controlling factor is depth. Salinity has only a small influence within the range
from 34 to 35 parts per thousand (34–35 Practical Salinity Units, PSU). As a
practical approximation, per every degree centigrade of temperature increase,
sound speed close to the surface increases by 3 m/s; per every increase of one PSU
increase in salinity, sound speed increases 1.4 m/s; and per every kilometer of
depth increase, sound speed increases by 17 m/s.
Localized sound speed can be measured using an instrument such as a Time of
Flight Probe, which uses an acoustic transducer transmitting a pulse that is
reflected by a plate located at a fixed distance. It measures the two way travel time,
divides it by two, and thereby computes the sound speed. For acoustic remote
sensing, however, it is necessary to measure the sound speed profile for the entire
water column. The sound speed profile can be obtained using instruments such as a
CTD, which is lowered from the surface while periodically measuring the conductivity ‘‘C’’ (salinity), temperature ‘‘T’’ and depth ‘‘D’’ (pressure) through the
water column. The United Nations Educational and Scientific Organization
(UNESCO) algorithm to compute the sound speed profile based on CTD data,
based on the Chen and Millero (1977) equation, is considered a universal standard
in oceanography. Alternatively, an eXpendable Bathy Thermograph (XBT), which
is less accurate but cheaper and easier to use, can similarly be used to compute the
sound speed profile. While free falling at a known rate through the water column,
the XBT sends temperature information to the surface using two very thin wires,
before breaking loose. For a vessel that is underway, the Moving Vessel Profiler
(MVP) utilizes a computer controlled winch and a custom ‘fish’ (a general term for
a hydrodynamically shaped instrument) to measure sound velocity without the
need to stop the vessel.
An important factor for acoustic remote sensing is attenuation in the water
column, where active SONAR suffers two-way attenuation and passive SONAR
only one-way attenuation. Since sound spreads geometrically from its source and
then again upon reflection, it is attenuated by this spreading. The classic example
to visualize a wave spreading is to throw a rock into a calm pond. A circular wave
is produced when the rock hits the pond, where circles with increasing circumferences, but decreasing amplitudes, are seen. The total amount of energy in a
wave remains constant as it spreads out from a source, but as the circle gets bigger,
the energy spreads to fill it. So the energy per unit length of the surface wave gets
smaller, decreasing the height of the wave. If a disturbance is created in the water
column and the waves spread out in all directions, they get smaller even more
rapidly than surface waves; this is called spherical spreading. Sooner or later, the
200
B. Riegl and H. Guarin