The oldest First World War German U-boat and the earliest German submarine to be sunk in English territorial waters – the U-8 – has been given protection by the Secretary of State for Culture as a Protected Historic Wreck site, on the advice of Historic England.
Pioneering underwater survey techniques were used in 2015 to survey the site and assist the case for its protection.
Exploring New Technologies for Underwater Research
Historic England recently commissioned an innovative survey of a First World War submarine wreck in order provide data to support its protection and to test the application of new equipment for archaeological research.
Autonomous Underwater Vehicles (AUVs) have been used offshore for some time and the development of smaller systems has opened up a range of inshore opportunities for archaeological investigation. With recent advances in technology, these small AUV systems boast a suite of remote sensors that can include impressive underwater survey tools: side-scan sonar, multibeam echosounder, sub-bottom profiler, magnetometer and an underwater camera.
In order to test such a system for use in underwater archaeology, we commissioned Wessex Archaeology to carry out an AUV survey of the German U-Boat U-8 located some 10 nautical miles off Dover. Our First World War wreck diary provides more details of the loss of the U-8.
For the investigation, we deployed an Ocean Server Iver3 AUV which carried Edgetech 2205 sonar transducers and towed a Marine Magnetics Explorer magnetometer (Fig. 1). The AUV was about 2m in length and weighed approximately 40kg; light enough to be deployed by two people. The stated endurance of 8 hours was enough to ensure sufficient coverage of the U-8 target area. However, it was not known how the system would cope with a moderate sea state and tidal streams of up to 2.6 knots, so it was therefore decided to deploy the system to coincide with slack water during neap tides to give the best operational window possible.
Before the AUV could be used to acquire data over the U-8, its buoyancy needed to be adjusted for the salinity and density of the seawater and the underwater survey lines were planned on a laptop with software calculating where the AUV needed to dive down and where it was to come back up. Survey positioning was provided by a GPS receiver within the AUV when at the surface and below the surface positioning was provided by a RDI Doppler velocity log, depth sensor and corrected compass. The AUV can only be communicated with via Wi-Fi when it is at the surface.
Deployment of the AUV at the wreck site was relatively straightforward, even in the slight to moderate sea state encountered, and it was programmed to fly around 10m off the seabed. Unlike a conventional towed system though, the geophysicist was unable to see live images of data as the sensor passed over the seabed: there is no way of knowing that the data is of sufficient quality or that the survey lines have been positioned correctly to ensonify (image) the target site, until the AUV is recovered to the vessel. This can make for a nervous time whilst the geophysicist is waiting to see the data!
The sea state did have an effect on the performance of the AUV whilst in the water in two ways. Firstly, the waves tended to swamp the ‘conning tower’ containing the GPS tracking system, which meant that the AUV sometimes had difficulty acquiring a GPS signal, causing it to refuse to start surveying. Secondly, when slack water was lost, the AUV struggled to get into its start of line position as it laboured against the tide, unable to dive. However, its endurance seemed good despite this, with the AUV deployed for about 6 hours with no requirement for a battery change.
Following recovery of the AUV and data download we could see that the side-scan sonar imagery showed a clearly defined submarine with detail of the conning tower visible. Sharp detail was observed in the acoustic shadow that shows the presence of three distinct upstanding narrow features, two on the conning tower (possibly periscopes) and one just behind (interpreted as the radio mast) (Fig. 2).The magnetometer data was also of good quality with a large magnetic anomaly observed over the location of the wreck, as would be expected.
Some challenges were identified with the bathymetry data, particularly where some of the smaller features observed in the side-scan sonar imagery were not visible owing to the relatively low resolution of the bathymetry (Fig. 3). In addition, the on-board camera did not pick up any footage of the U-8 despite the visibility being around 8m.
During this, our first, archaeological trial of an AUV, the system performed reasonably well, giving sharp imagery that has aided our interpretation of the U-8’s condition. We’ve learnt some important lessons for future operations, particularly in understanding the effects of tidal currents on the AUV during data collection. Use of the AUV proved a cost-effective method of survey in a busy shipping channel with the same methodology being applicable to other sites that are similarly difficult to reach, such as those in proximity to shore, those in deep water, or otherwise restricted in some way. The system is also particularly well suited to more benign waters such as ports, or natural and manmade harbours, and if the circumstances allow the system to be launched from shore, then the cost savings could be considerable when compared to established survey methods.
Toby Gane and Dr Stephanie Arnott, with Mark Dunkley
Toby Gane is a Senior Project Manager and Stephanie Arnott is a Senior Marine Geophysicist at Wessex Archaeology.
Mark Dunkley is Historic England’s marine designation adviser.