Furuno TS-331A - Manual (Page 25)

2. SONAR OPERATION
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4. The ship has accelerated to clear a shoal. The trawl is rising off the bottom
and the bottom is displayed as if it has a sharp undulation though it actually
does not.
5. The trawl is passing over the shoal. The ground rope seen off the bottom
indicates that the shoal has been cleared.
6. The trawl has settled on the bottom again.
7. The trawl is being towed along the bottom.
8. Hauling of the trawl has begun.
9, 10.As the hauling proceeds, the bottom echoes are becoming distant and the
sea surface echoes are getting nearer to the transmission line.
11. Hauling is completed.