providing its own pulse energy and thus differing from passive sensors that
measure energy emanating from the environment (e.g., listening arrays). SONAR
technologies are largely based on measurements of the speed of sound in water
(i.e., the travel time between a transmitted and received sound pulse), as well as
the characteristics of the scattered sound. Dependent on the strength, width and
orientation of the pulse, the acoustic signal can be used to investigate features of
the water body, the sediment surface, or the sediment (or bedrock) interior.
SONAR methods are therefore among the most versatile and frequently used
oceanographic remote sensing tools in the open ocean, as well as in coastal and
reef environments. Listening arrays are also increasingly used in fisheries science.
Fish and/or marine mammals are either tracked by using their own vocalizations
(in particular whales lend themselves very well to such applications) or acoustic
emitters are attached to animals and then tracked by passive listening arrays.
Absorption of sound in water is dependent mostly on the frequency of the acoustic
pulse. At the high-frequency end, above 1 Hz (Hz = cycles per second), absorption
by seawater is important and applications are largely limited to acoustic imaging and
side-looking SONAR. At the low end, below 1 Hz, the generation of sound is
technically challenging, needing small earthquakes or large explosions to create
measurable events. Thus, ocean acoustics generally concerns itself with a band of
frequencies between 1 Hz and several hundred KHz (Tolstoy and Clay 1966).
SONAR technology is not new and has served many purposes. Early in its
development, it was mainly dedicated to shipping safety and military use. Soon
after the Titanic struck an iceberg and sunk under great loss of life in 1912, patents
were filed to use the echo of sound waves to detect large objects underwater
(Medwin and Clay 1998). The technology was based on the idea that once speed of
sound in water is precisely known, measurement of its travel time from a transmitter source and back to a receiver would allow an equally precise measurement
of the scattering object’s distance from the sensor. The earliest measurements of
sound velocity in water were performed by Colladon and Sturm (1827) in Switzerland’s Lake Geneva. In 1916 Chilowsky and Langevin (1916) obtained echoes
from the shallow seafloor and from an iron plate at 200 m depth. They used an
electrostatic sound source as transmitter and a carbon button microphone as
receiver. During WWI, the Canadian physicist Boyle and the British scientist
Wood used quartz piezoelectric crystals to build the first SONAR (Medwin and
Clay 1998), which became known as Anti-Submarine Detection Investigations
Committee (ASDIC). Langevin used quartz piezoelectric transducers both as
projector and receiver to obtain sound transmission up to 8 km. In 1919, Marti
patented an ‘echosounder’ that was capable of producing a continuous visual
record of the seafloor, and in 1925, the German ship Meteor ran echosounding
lines across the South Atlantic. With this, a scientific revolution began in the
understanding of the deep seafloor following the detection of the mid-oceanic
ridges and abyssal plains. By 1935, acoustic bathymetry was developed enough to
allow soundings of ocean depth, and backscatter began to be used for the detection
of fish schools (Medwin and Clay 1998). The onset of WWII gave great impetus to
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B. Riegl and H. Guarin
measure energy emanating from the environment (e.g., listening arrays). SONAR
technologies are largely based on measurements of the speed of sound in water
(i.e., the travel time between a transmitted and received sound pulse), as well as
the characteristics of the scattered sound. Dependent on the strength, width and
orientation of the pulse, the acoustic signal can be used to investigate features of
the water body, the sediment surface, or the sediment (or bedrock) interior.
SONAR methods are therefore among the most versatile and frequently used
oceanographic remote sensing tools in the open ocean, as well as in coastal and
reef environments. Listening arrays are also increasingly used in fisheries science.
Fish and/or marine mammals are either tracked by using their own vocalizations
(in particular whales lend themselves very well to such applications) or acoustic
emitters are attached to animals and then tracked by passive listening arrays.
Absorption of sound in water is dependent mostly on the frequency of the acoustic
pulse. At the high-frequency end, above 1 Hz (Hz = cycles per second), absorption
by seawater is important and applications are largely limited to acoustic imaging and
side-looking SONAR. At the low end, below 1 Hz, the generation of sound is
technically challenging, needing small earthquakes or large explosions to create
measurable events. Thus, ocean acoustics generally concerns itself with a band of
frequencies between 1 Hz and several hundred KHz (Tolstoy and Clay 1966).
SONAR technology is not new and has served many purposes. Early in its
development, it was mainly dedicated to shipping safety and military use. Soon
after the Titanic struck an iceberg and sunk under great loss of life in 1912, patents
were filed to use the echo of sound waves to detect large objects underwater
(Medwin and Clay 1998). The technology was based on the idea that once speed of
sound in water is precisely known, measurement of its travel time from a transmitter source and back to a receiver would allow an equally precise measurement
of the scattering object’s distance from the sensor. The earliest measurements of
sound velocity in water were performed by Colladon and Sturm (1827) in Switzerland’s Lake Geneva. In 1916 Chilowsky and Langevin (1916) obtained echoes
from the shallow seafloor and from an iron plate at 200 m depth. They used an
electrostatic sound source as transmitter and a carbon button microphone as
receiver. During WWI, the Canadian physicist Boyle and the British scientist
Wood used quartz piezoelectric crystals to build the first SONAR (Medwin and
Clay 1998), which became known as Anti-Submarine Detection Investigations
Committee (ASDIC). Langevin used quartz piezoelectric transducers both as
projector and receiver to obtain sound transmission up to 8 km. In 1919, Marti
patented an ‘echosounder’ that was capable of producing a continuous visual
record of the seafloor, and in 1925, the German ship Meteor ran echosounding
lines across the South Atlantic. With this, a scientific revolution began in the
understanding of the deep seafloor following the detection of the mid-oceanic
ridges and abyssal plains. By 1935, acoustic bathymetry was developed enough to
allow soundings of ocean depth, and backscatter began to be used for the detection
of fish schools (Medwin and Clay 1998). The onset of WWII gave great impetus to
196
B. Riegl and H. Guarin