The Sonar Image - Raymarine e7D Installation And Operation Handbook

14.2 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
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.
The depth of individual targets can be displayed by switching on the
Target Depth ID in the fishfinder menu Menu > Presentation. The
number of target depths displayed is influenced by the fish alarm
sensitivity level.
Using the fishfinder
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.
Water depth
As sea depth increases signal strength decreases, resulting in a
lighter on-screen image of the bottom.
Size of the target
The larger the target, the larger the return on the fishfinder display.
The size of a fish target is also dependent upon the size of the fish's
swim bladder rather than its overall size. The swim bladder varies in
size between different breeds of fish.
Transducer frequency
The same target will appear differently when the transducer
frequency is changed. The lower the frequency the broader the
mark.
Clutter / Background noise
The fishfinder picture may be impaired by echoes received from
floating or submerged debris, air bubbles or even the vessel's
movement. This is known as 'background noise' or 'clutter' and is
controlled by the gain modes. The system will automatically control
the gain settings according to the depth and water conditions. You
can however adjust the gain settings manually if you prefer.
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