Image Processing Reference
the target, while the strengths of the return pulses depends on the density of
the target (higher density equals stronger return pulse) and the distance to the
target. The strength, shape, and timing information about the return pulses is
processed by a computer and turned into a visual image on a computer screen.
One commonly-used imaging sonar system works in the following manner: An
ultrasonic transducer and electronics for transmitting and receiving ultrasound
pulses are housed in a torpedo-shaped float known as a towfish. A fan-shaped
beam of sound emanates from the sonar transducer in a direction perpendicular
to the towfish direction of motion. Figure 5.2 shows an advanced towfish design
with its underwater cable attached. Figures 5.3(a) and 5.3(b) show the acoustic
beam pattern emitted by a typical towfish.
The transducer detects reflected portions of the sonar pulse and measures their
arrival time, shape and strength. As the towfish moves along, the data it receives
are used to generate a sonar picture of the bottom, building up an image line by
line. The smallest feature that can be resolved by imaging sonar depends on the
wavelength of the sonar pulse. Sonar that uses sound with wavelengths of a few
millimeters can detect features as small as 20 cm at an imaging range of about 100
m. The price one pays for this resolution is its effective range.
Sonar imaging has come a long way since its inception in the 1950s, and modern
sonar images are often very striking, as though they were actual photographs.
Figures 5.4 and 5.5 are images of theEmpireKnight, a British freighter that sunk in
90 feet of water off the coast of Maine during World War II. The viewer is looking
straight down at the sea bottom. The color saturation corresponds to the strength of
the return signal—white pseudocolor indicates the strongest return and black the
weakest return. Note the black streak that runs from top to bottom along the middle
and the sonar “shadows” thrown by the ship. These are artifacts of the towfish
sonar pattern, which scans to the side and therefore cannot “see” objects directly
below the swath or behind objects with height above the bottom. The length of the
shadow is directly related to the height of the object and can be used to give such
information; the same shadowing effect is seen in radar images of mountainous
terrain, since the imaging technique is very similar.
Figure5.2 Sonar towfish with undersea cable. (CourtesyofGarryKozak,L-3Klein)