Bottom reflectivity varies from low (blue, relatively soft, fine-grained sediments) to high (green, relatively coarse and/or hard sediments), and are depicted as a semi-transparent layer above a shaded relief map.
The map of hard and soft bottoms is based on data acquired using a multi-beam echo sounder. This instrument is used for detailed mapping of water depths (bathymetry), but also supplies information on the composition of the seabed through the strength of the audio signals reflected back from the seabed. When the bottom reflectivity data are processed, it is possible to acquire detailed information on variations in the types of seabed.
The bottom reflectivity depends upon several factors. A hard or coarse grained, stony type of seabed usually gives a strong signal whereas fine-grained sediments on the bed give weaker signals because the audio signals are absorbed. Other factors influencing the bottom reflectivity are how well the sediments are sorted and how even the seabed is.