Methodology Results

Based on the results of Round 1 of 'Wrecks on the Seabed', Round 2 aimed to further develop and test methodologies for the assessment, evaluation and recording of wreck sites. The project objectives related to three different subheadings:
  • Infrastructure
  • Remotely operated vehicles and
  • Intrasite survey.
The Round 2, Year 1 fieldwork was divided into a 15 day ROV session followed by two 15 day diving sessions.
 
Seven study sites were visited during the ROV session. All of these had been subject to geophysical survey, diver investigation or both in Round 1.
 
Each of these sites were to be recorded to level 2a with the aim to develop methodologies for the use of ROVs in underwater archaeology and to integrate ROV based methods with tracking and recording systems. In addition, the use of ROV survey instead of diver-based methods and the use of ROV survey to complement diver-based methods were to be assessed.
 
The two diving sessions concentrated on three wreck sites that had been very difficult to access during Round 1 of the project. All three sites were recorded to level 3a or 3b in the course of the diving sessions of Round 2.
 
Round 1 diving was conducted from a small workboat that returned to port every night. In Round 2, MV Flatholm, a 26m converted tug, was used as the diving support vessel. Weather permitting, Flatholm was able to stay out on site for the whole 15 day session with crew and dive team living aboard.
 

Infrastructure

When assessing the effectiveness of a larger platform in maintaining station in prevailing conditions it was found that the larger vessel had clear advantages over the small workboat. The position on site could be held in winds up to force 6Bft. When investigating the scope for increasing working time by coping with less favourable conditions, it was found that downtime could be reduced by 10% through the use of a larger vessel. The larger vessel also enabled work on sites that could not be accessed in Round 1.
 
 
By maintaining station on site whenever possible, travel time was reduced and it became possible to work additional slack water periods. By doing this, the total average bottom time per day could be increased by 60 %.
 
The larger work platform also proved to be beneficial for the use of acoustic tracking systems, although it was difficult to quantify this effect.
 
An assessment of the benefits of a larger platform for accommodating and handling diving and ROV systems showed that the larger vessel had several advantages over the small workboat. Equipment could be stored safely, protected from spray and wash. It was also possible to carry out maintenance on board and undertake post-processing tasks, thereby increasing work efficiency.
 

The use of ROVs in underwater archaeology

It has been possible to develop a methodological approach for level 2a surveys with a remotely operated vehicle. In addition, ROV-based methods were successfully integrated with the USBL tracking system and the Wessex Archaeology diver recording system DIVA.
 
The use of ROV survey instead of diver-based methods and as a complement to diver-based methods was also assessed.
 
It was found that when used as a stand-alone tool, the ROV proved to be an efficient mechanism for the rapid survey of large wreck sites with acceptable visibility. The extent and character of wrecks could be determined, and rough site sketches sufficient for a level 2a survey could be made from the tracked ROV video footage in combination with previously obtained geophysical data. However, it was difficult to understand the layout of individual components as the ROV lacked measuring capabilities.
 
Due to the danger of snagging and losing the vehicle, visibility proved to be an important limiting factor for the use of ROVs on wreck sites.
 
A comparison of deployment costs showed that a small survey class ROV such as the Seaeye 600 used was slightly cheaper per day than a dive team, and that overall work rate was significantly greater for level 2 survey.
 
An idealised scenario would be to use the ROV for the level 2a evaluation and then deploy divers for the more detailed level 3, in situ recording. The time consuming tasks of getting an overview of the wreck and defining its extents could be carried out by the ROV and the divers could concentrate on the detailed recording of selected elements identified during the ROV survey.
 

Intrasite survey

A USBL acoustic positioning system was successfully deployed during diving and ROV sessions. The general positioning accuracy achieved with this system using external sensors in single dives varied between 0.5m and 2m and was sufficient for the tolerances/requirements of a level 2a survey. The issue of low repeatability from dive to dive, attributable to GPS, was avoided by conducting designated positioning dives with the aim of positioning all important elements and datum points on sites in a single dive.
 
The Wessex Archaeology recording system, DIVA, was improved and upgraded to run with ArcGIS 9 rather than Arcpad and to include new features. Altogether the recording system with its seamless integration with the acoustic positioning system was found to facilitate decision making in the field and greatly reduce post-processing times.
 
As part of the objective to develop underwater video and still photographic methods, different camera and lighting systems and settings were tested under a variety of conditions to find the best camera and strobe settings.
 
During the post-processing of photography and video, photo mosaics were derived from digital still photos and video captures. Two different software packages were used. Smoky City Designs Panorama factory was found to work best for panoramic underwater still shots taken by swivelling around a single point. The photomerge function in Adobe Photoshop Elements worked well for stitching video captures into plan view photomosaics.
 
 
As part of the development of underwater video methods, video editing facilities were used on site. After each dive the captured video was reviewed and important clips were stored in a site movie project. This allowed site sketches to be corrected and completed and effectively reduced post processing times.
 
As part of the project, the levels of archaeological recording for level 2 and 3 were refined. Recording tasks and related recording foci were added to each of the levels and sublevels.
Altogether the project has improved the knowledge necessary for specifying and conducting rapid and efficient ROV and diver based wreck surveys in areas with conditions similar to the English Channel Coast.
 
PDF files of the project report with a full discussion of project objectives and results can be downloaded in the project download area.