Slang for fluid mud or very soft sediment lying above the consolidated bottom in ports and rivers. A high-frequency sounder reflects off the top of the fluff while a low-frequency pulse passes through to firmer material, so the two can differ by a metre or more. Some ports define a nautical bottom at a density near 1.2 tonnes per cubic metre and use density probes to find it, since ships can pass safely through the lighter layer.
200 and 33 kilohertz are a metre apart in the turning basin, it's all fluff after the floods.
A channel depth means little until you know its frequency: over fluff, 200 kHz and 33 kHz depths can differ by more than the whole dredging tolerance.
Also heard as
- fluid mud
- soft bottom
Related terms
Singlebeam echo sounder
An echo sounder that measures one depth directly beneath the vessel on each ping, using a single vertical beam.
Bottom pick
The depth chosen from an echo trace as the true seabed, either by the sounder's tracker or by an operator.
Sub-bottom profiler
A low-frequency sonar that penetrates the seabed to image buried layers and objects beneath the bottom.
Backscatter
The strength of sound returned from the seabed, used to map bottom type alongside depth.