The Sonar Image - Raymarine e7 User Reference

11.2 The sonar image

Interpreting the seabed using sonar
It is important to understand how to correctly interpret the seabed
structure represented in the fishfinder display.
The seabed usually produces a strong echo.
The following images show how different seabed conditions are
represented in the sonar display:
1
2
3
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Item
1
2
3
The dark layers indicate a good echo; the lighter areas indicate
weaker echoes. This could mean that the upper layer is soft and
therefore allowing sound waves to pass to the more solid layer
below.
It is also possible that the sound waves are making two complete
trips – hitting the seabed, bouncing off the vessel, then reflecting
off the seabed again. This can happen if the water is shallow, the
seabed is hard, or the gain is set to high.
Factors influencing the sonar display
The quality and accuracy of the display can be influenced by a
number of factors including vessel speed, depth, object size,
background noise and transducer frequency.
Vessel speed
The shape of the target changes along with your speed. Slower
speeds return flatter, more horizontal marks. Higher speeds cause
the target to thicken and arch slightly, until at fast speeds the mark
resembles a double vertical line.
Target depth
The closer the target to the surface, the larger the mark on screen.
Description
A hard bottom (sand) produces a thin
line.
A soft bottom (mud or seaweed cover)
produces a wide line.
A rocky or uneven bottom or a wreck
produces an irregular image with
peaks and troughs.
e7 / e7D — User reference