Hard and soft bottom

Geology for society

Published: October 23rd 2015 | Last change: March 15th 2016

Multibeam echosounders are used to collect backscatter data that is processed to provide detailed information on variations in seabed types. This vital data set is used to produce geological maps of the seabed.

Bottom reflectivity varies from low (blue, relatively soft, fine-grained sediments) to high (green, relatively coarse and/or hard sediments), and are shown as a semi-transparent layer above a shaded relief map that displays the contours of the seabed.

The basis of NGU's "Hard and Soft Bottom" maps is information collected from multibeam echosounders. This instrument is used for detailed mapping of water depths (bathymetry), but researchers can also interpret the morphology and composition of the seabed through the strength of the signals reflected back. When the bottom reflectivity data (back scatter) are processed, it is possible to acquire detailed information on variations in the types of seabed.

Bottom reflectivity depends upon several factors. A hard or coarse-grained, cobble type of seabed usually gives a strong signal; whereas, weaker signals are reflected by fine-grained sediments which tend to absorb signals. Other factors influencing the bottom reflectivity are evenness of the surface, and how well the sediments are sorted.

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