Raymarine DSM250 Owner's Handbook Manual page 92

6-4
A
Hard bottom (sand)
B Soft bottom (mud)
C Rocky bottom
D White line feature on:
Figure 6-2: Sounder Bottom Indications
An uneven bottom which is covered with rocks or coral produces
a complex display; the bottom image is irregular, with long tails
pointing upward. A wreck produces a similar image.
A softer bottom or a bottom covered with seaweed produces a
wide line, as the sound waves reflect from different layers of the
bottom. In Figure 6-2 (section B), the sounder is getting a good
echo from the bottom, so the upper layer of the bottom is shown
as dark gray.
Notice that the lower layer of the bottom is shown as a lighter
gray, this indicates a weaker echo. This could mean that the upper
layer is soft; some sound waves may get through this layer and be
reflected by a more solid layer below.
It is also possible that the sound waves are making two complete
trips – hitting the bottom, bouncing off the boat, then reflecting
off the bottom again; this can happen if the water is shallow, the
bottom is hard, or gain is set high.
DSM250 Digital Sounder Module
D5016-2