Diving Methodology

The Wessex Archaeology dive team consisted of five team members: archaeological supervisor; diving supervisor; diver; standby diver and tender. All divers held commercial diving qualifications.
 
The diving support vessel EMU Surveyor was hired for all diving fieldwork in the ALSF project "Wrecks on the Seabed". Access to the water was provided by a transom gate and diving ladder at the stern.
 
Surface Supplied Diving Equipment was used throughout the project. The air supply on board consisted of four 300bar 50l J-cylinders.
 
The air supply was controlled with a DIVEX two diver panel with integrated pneumofathometers. An AMRON communication box was used for diver communication.
 
Divers were linked to the surface by umbilicals which included air supply, pneumo tube, communication, light and camera cables. Standard KMB 28 band masks with neoprene hoods were used. Faceplate mountings held video camera and light.
 
Underwater recording was carried out using the hat mounted camera, handheld digital stills camera and by making measured sketches on drafting film. Watch an extract of the footage (opens your media player, MPEG1, 1.3Mb).
 
For each dive the wreck was marked with a buoy, before anchoring the vessel on a two point mooring for the duration of the dive. If longer fieldwork was planned, a shot was secured to a part of the wreck.
 
Due to strong tidal currents in the area, diving was only carried out in slack water periods.

Acoustic Diver Tracking

Throughout the project, diver orientation was enhanced by an acoustic USBL diver tracking system, manufactured by Sonardyne.
 
In this system, range and time difference measurements of sound signals emitted by a beacon attached to the diver on the seabed allow to calculate the divers position.
 
Further details on acoustic theory can be found at: http://www.sonardyne.co.uk/theory.htm#USBL
 
Equipment setup on EMU SurveyorEquipment setup on EMU SurveyorA transceiver assembly mounted over the side of the vessel received the sound signals. A navigation processor calculated and displayed the relative diver position. Further software packages were used to transform the relative coordinates into real world geographical coordinates and to display the diver position overlayed on a georeferenced multibeam or sidescan image of the wreck.
 
As the transceiver and thus the coordinate reference frame was fixed to the vessel and influenced by yaw, pitch and roll, additional equipment such as a motion reference unit and a gyrocompass had to be used to compensate for the boat’s movement and enhance tracking accuracy.
 
The average positioning accuracy achieved with the USBL system ranged from 2m to 6m. This made the system suitable for basic orientation on large wreck sites, but not for surveying or detailed orientation in low visibility conditions. The main factors responsible for inaccuracies were related to boat positioning and boat movement compensation rather than the acoustic tracking system itself.