Survey Vessels

Titan ExplorerWessex ExplorerThe geophysical surveys for this project were conducted from dedicated survey vessels.

The sidescan sonar and magnetometer surveys over the 2km x 2km area were conducted aboard the survey vessel Titan Explorer operated by Titan Surveys Ltd. The Titan Explorer is a catamaran 9.7m in length and 2.9m in breadth with a draft of 0.6m.

The sidescan sonar and magnetometer surveys of the three wreck sites in deep water were conducted aboard the survey vessel Wessex Explorer operated by Hayes Marine Ltd. This vessel is 15m in length and has a draft of 2m.

The multibeam echosounder surveys for this project were conducted from the Wessex Explorer.

Sidescan Sonar

Sidescan sonars emit narrow beams of acoustic energy from either side of the towfish. This energy is reflected back from the seafloor which produces an image of the seafloor. Hard objects such as metal reflect more energy than soft objects such as sand and mud.

Objects standing up from the seafloor produce an acoustic shadow. On some shipwrecks this can show individual features and parts of the vessels’ superstructure.

Multibeam Echosounders

Multibeam sonar surveys ensonify the seabed beneath and to either side of the survey vessel. They record continuous and very accurate spot heights for thousands of points on the seabed as the vessel moves forward.

From multibeam data, 3D terrain models are created. Depressions and features that project from the seabed can be displayed and measured accurately.

For this project a Reson SeaBat 8125 multibeam echosounder, provided by NetSurvey Limited, was used and the 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.

Magnetometers

Geometrics G-881 caesium vapour magnetometerMarine magnetometers detect variations in the Earth's magnetic field. These variations may be caused by the presence of ferrous material such as iron on or under the seabed, geological features or the daily variations in the Earth's magnetic field that are caused by the sun. Marine magnetic surveying is now a standard technique for mapping ferrous material on the seabed.

Marine Magnetics ExplorerAs the survey vessels create a magnetic disturbance themselves, the magnetometer is usually towed behind it.

During the 2005 fieldwork, a Geometrics G-881 caesium vapour magnetometer was used for the 2km x 2km area survey while a Marine Magnetics Explorer was used on the three deep wreck sites.

In the post processing the magnetometer data was imported into a Geographical Information Systems (GIS) and overlaid on the results of the sidescan sonar interpretation to compare the results of the two surveys.