Raymarine 465 - Manual (Page 50)

39 Operating Instructions
Fig. 3-6
Bottom
Conditions
reach through this layer, and may be reflected by a more solid layer
below. It is also possible that the sound waves are making
two complete
trips – hitting the seabed, bouncing off of the bottom of the boat, then
reflecting off the seabed again. This can happen if the water is shallow,
the seabed is hard, or the sensitivity is set to a high value.
Hard
(Sand)
Soft
(Mud)
Rocks