Sediment

The Seabed Page allows the properties of the seabed layers to be changed. The seabed may consist of any number of layers.

The currently active layer is selected in the top dropdown menu. Layers may be added with the add and remove buttons. 

If multiple positions have been entered in Setup project → Properties map there will be a dropdown menu at the top right, for selecting the relevant location.

Every location must have the same number of layers!

Everything below the checkbox Layer depths referenced from local seafloor refers to the currently selected seabed layer. The layer may be named, and the depth where the layer starts is entered in the Depth text box.

A sediment type from the database may be selected via the dropdown menu. The selected properties can be modified by entering values in the text boxes.

Attenuation is given in units of dB/wavelength. The 20 log and 10 log solvers ignore the seafloor altogether.

If the checkbox Layer depths referenced from local seafloor checkbox is checked, then the solvers will take the layer depths as being referenced from the local seafloor at each point in the solution. If it is not checked, the layers are taken as referenced from the sea surface. So with this option it is possible to choose between seafloor layers that are horizontal throughout the entire project area, or that follow the local variations in bathymetry.

The calculation area for the dBSeaModes and dBSeaPE solvers extends lower than the water column, to a total depth of three times the depth of the water column. Any layers that fall within this are will be included in the calculation. The dBSeaRay solver does not allow rays to enter the seabed, however an effective complex reflection coefficient is calculated based on frequency, incidence angle and sediment layers. This coefficient is applied to the ray upon reflection at the seafloor.

It should be noted that the speed of sound and density of typical sediment layers are generally similar to those of the overlying water. Therefore the seafloor allows a large portion of incident energy to enter, in contrast to the air-water interface.