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rSOUNDER MODEq HOW TO READ IMAGES
(16)-6 Influence of air bubbles
If the ultrasonic beam passes through air bubbles, reflections from a shoal of
fish or the sea bottom may deteriorate or be interrupted, resulting in voids
occurring on the screen image.
This symptom is apt to occur when a low frequency is used, the ship turns and
passes through its own track, or the sea is rough. Voids on the screen mean that
the ultrasonic energy is dissipated or absorbed by air bubbles.
Thermocline
H
315°T 10.2KT 58°F
FT
43.4
GPS 48°21.417N 125°26.643W
20
40
60
80
0
100
The side lobe from the transducer may cause phantom images.
These images tend to appear above the contour of the sea bottom in a hilly terrain,
or under the contour in a flat terrain.
(16)-7 Phantom images
Phantom images
H
Main lobe
Side lobe
315°T 10.2KT 58°F
FT
43.4
GPS 48°21.417N 125°26.643W
20
40
60
80
0
100
H
4
Voids due to air bubbles
315°T 10.2KT 58°F
FT
43.4
GPS 48°21.417N 125°26.643W
20
40
60
80
0
100
Thermocline is a lay of a body of water at which the rate of temperature
decreases with depth is at a maximum, and is displayed as a screen image.
As a thermocline reflects ultrasonic waves, it appears as a very thin line between
the sea level and bottom on the screen.
(16)-5 Thermoclines