Multibeam Sonar Survey of HMS Colossus

In April 2017 a multibeam sonar survey of the designated historic wrecks in Scilly was undertaken on behalf of Historic England. The wrecks surveyed were Association, Colossus, Tearing Ledge, Bartholomew Ledges and The Wheel Wreck.

Multibeam sonar is a method for mapping the seabed. An array of sound pulses sent from the survey vessel measure the depth of the seabed. This can produce a quick and accurate 3D map of the seabed and any wreckage lying on it. However, the survey will not usually detect material which is buried within the seabed sediments.

Guns 8 and 10 shown on the multibeam survey
Guns 8 and 10 shown on the multibeam survey

A small section from the multibeam sonar survey of HMS Colossus showing two of the 32 pound cannons lying on the seabed to the south of the main section of wreckage (shown arrowed as Gun 8 and Gun 10). By way of comparison a photograph of gun 10 on the seabed is shown below.

An archaeologist diver surveying Gun 10
Survey of Gun 10 by a CISMAS diver
detail from the HMS Colossus multibeam sonar survey shows a small anomaly
Detail from the HMS Colossus multibeam sonar survey showing a small anomaly

This detail from the HMS Colossus multibeam sonar survey shows a small anomaly (outlined with a blue rectangle) on the seabed. When divers investigated it they found it was an iron anchor.

Diver visiting an iron anchor on the seabed
Iron anchor discovered by the multibeam survey