Deep Scattering Layers

The Oxford Companion to Ships and the Sea | 2006 | 700+ words | Copyright

Deep Scattering Layers (DSLs) appear on echo sounders and other sonar devices, and often resemble a false bottom. Immediately after the Second World War (1939–45) shoals shallow enough to be a danger to shipping were shown on some charts of the open ocean, because very dense DSLs were mistaken by chartmakers for shallows. DSLs are the result of the sound transmissions of the echo sounder being reflected by dense concentrations of fish or plankton. Fishes with swim-bladders containing gas bubbles reflect the sounds efficiently, especially if the diameter of the gas bubble is more than half the wavelength of frequency of the transmitted sound. The higher the frequency of sound used, the smaller the organisms that reflect the sound. Some DSLs ‘behave’ in that they migrate up towards the surface at dusk and sink down again at dawn, indicating that the animals causing them are undertaking vertical migrations.

M. V. Angel

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