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Refraction
An ultrasonic wave transmitted in water does not travel straight but is
more or less refracted. This refraction is caused by the variation of
propagation velocity in water. If the velocity decreases (temperature
decreases) with depth, the top part of the wave front moves faster
than its bottom part, and gradually the front bends downwards. In the
same way, it bends upwards if the sound velocity increases (tempera-
ture rises) with depth.
In other words, the ultrasonic wave refracts toward colder water.
0
50
100
(m)
16°
18°
12°
0.5 1 1.5 2
Water Depth
Distance (Km)
Water Temp.
0
50
100
(m)
20°
18°
18°
0.5 1 1.5 2
Water Depth
Distance (Km)
Water Temp.
Here, a fishing ground off Hokkaido island in Japan is taken as an
example.
Within 100m
Propagation path
A
Big Fish
School
B
small fish
school
Seaface
Large difference in
density
Fishing Ground off Hokkaido (Summer)
In summer, there is a large difference in salinity density below and
above the 100 m deep point. An ultrasonic wave emitted almost in
the horizontal direction propagates within 100 m deep water in the
same way as a radio wave in a waveguide. As a result, even a small
fish school is sometimes detected at an unexpected long range or on
the contrary, detection of a large fish school does not extend to a
relatively long range. These phenomena are encountered when two
fish schools lie in positions “A” and “B” of the illustration.