Multibeam sonar ensonifies a swath of seabed beneath and to either side of the survey vessel, deriving continuous and well- positioned spot heights for many thousands of points on the seabed as the vessel moves forward.
From multibeam data, 3D terrain models can readily be created. Depressions and features projecting from the seabed can be displayed and quantified in terms of their dimensions.
For the "Wrecks on the seabed" project a Reson SeaBat 8125 multibeam echosounder was used.
The survey data was processed by Wessex Archaeology using the IVS Fledermaus visualization system.
The results were 3D terrain models, 3D fly through movies and 2D georeferenced images. The 2D images were used as a base for site plans. Divers used prominent features as datum points and carried out range and bearing measurements or used offset and triangulation to map other objects, as can be seen from this plan of the Portland Stone Wreck.