Diving Operations on Archaeological Sites

Archaeology often entails teams diving on the same site for days, weeks, months or even years at a time. The range of tasks carried out run from visual inspection, through various forms of survey and recording, including photography, to excavation and recovery requiring heavy tools and lifting equipment. In some phases projects can seem nearer civil engineering than anything else. For this reason many projects have adopted the techniques and proceedures used in the inshore and offshore commercial diving industries. One of the first to do so on a large scale was the Mary Rose Project. In the final season alone, the Trust's 'Salvage and Recovery Diving Team' (consisting of archaeologists, engineers and staff divers) chalked up 3,500 hours under pressure leading up to the ship's salvage. This doesn't include the time spent by the Trust's other archaeological team (other staff archaeologists together longstanding avocational volunteers) and the Royal Engineers.

The success of this eclectic approach led to similar systems being deployed elsewhere. However, most sites that used this sort of equipment were relatively shallow. Increased depth brings other factors into play. In 1994 the opportunity to record a wreck on the limits of the air-diving range.

The Kravel

This site is the wreck of ship believed to have sunk in 1525. It lies between 30 - 56m depth, so technically, SCUBA with air as the breathing gas could have been used on the shallower parts of the site. However, limited 'productive' bottom time, narcosis and the safety implications of repeat air diving in cold water were all good reasons for another solution. A system was needed that would be both safe and archaeologically productive.

The first requirement was a diving support vessel and the project was fortunate to secure the loan of Nigel Boston's Terschelling. This seaworthy, 40m vessel berthed 19, has a capacious hold and is well equiped (Figure 1).

Figure 1

The best of both worlds

The system subsequently installed aimed to combine the security and other advantages of surface-supplied diving with the mobility and convenience of SCUBA. We did this by deploying two or three divers using Terschelling's wet-bell. One or two divers would use Kirby Superlite helmets with head-mounted video and light. The other diver(s) would use SCUBA but be on the same schedule (time/depth/breathing gas). The helmet diver would effectively be the 'minder' for the scuba diver, talking to the surface, etc.

Figure 2	Figure 3

The more mobile scuba diver would do any necessary swimming backwards and forwards (though with correctly trimmed gear and floating umbilicals, helmet divers can do a pretty good 'self-contained' impersonation if they want to). The bell was fitted with a video camera, lights, safety strobe, on-board gas, communications to the surface, spare kit, tools, and even a spare hot water hose for the scuba divers to squirt inside their gloves on deco stops. The bell was therefore the 'lift' to and from work, a refuge and a decompression stage (Figures 2 and 3). Used this way scuba dives were effectively run by the dive supervisor (Figure 4).

Figure 4

Nitrox (in fact enriched air) was used on the shallower part of the site between 30 - 36m. This was mixed on board to give 1.4 ppO2 at the working depth and stored in 130m2 quads supplying the panel. Nitrox was not used to extend bottom times but to reduce errors in surveying (which it did dramatically) and to build in a large safety margin for a team doing repeat dives two out of every three days for three weeks. Nitrox also virtually eliminated the post-dive fatigue often experienced after deep air dives.

Below 36m no SCUBA was used. All dives were carried using surface-supply, premixed heliox (80/20) and O2 decompression. All heliox diving was also carried out using hot water suits. A two compartment chamber completed the system.

The wet bell was part of Terschelling's equipment. The rest was borrowed from Stolt Comex Seaway, one of the world's longest established diving companies and associated with archaeology for a long time. Many of the archaeological team on the kravel project had worked on the Mary Rose and the Amsterdam Projects. On both projects a similar system had also been lent by Comex. Subsequently many of the team had also worked for them in the North Sea (moonlighting between archaeology jobs!) For these reasons the kravel project used their diving tables too.

Terschelling steamed from Plymouth to Dundee to load the system along with several tons of gas, then headed across the North Sea to the Baltic. By early September she was on site and ready to go. Figure 5 shows the Terschelling moored over the site with diving operations in progress.

Figure 5

Obviously everyone had to have appropriate qualifications to use this stuff. The team of 17 divers all had professional qualifications. The Brits ranging from HSE 3 (surface-supplied air-diving) to HSE 1 & 2 offshore air & mixed-gas bell diving, etc. The Swedes had their equivalent 'A' & 'B' certificates. Ten members of the team were archaeologists and the others were NAS instructors and/or had extensive archaeological experience as project staff divers. Needless to say it involved a lot of other crucial people, too numerous to name here, but without whom it wouldn't have happened.