Synthetic aperture sonar images and bathymetries from the 2015 survey of the Skagerrak World War II Chemical Munitions Dump Site

This report is a summary of the synthetic aperture sonar (SAS) processing results from the HUGIN HUS survey of the Skagerrak dump site in April 2015. The cruise was conducted from H. U. Sverdrup II for the Norwegian Coastal Administration. This report is not intended to be a complete cruise report, but rather to document the collected data. Interpretations of the data is only included to a limited extent. The exact positions of the wrecks are omitted from this report since the Norwegian Coastal Administration does not want them to be public knowledge. Tables with positions of all the wrecks presented in this report can be found in a separate report which is exempt from public disclosure [26]. After World War II, about 168.000 tons of chemical weapons were dumped at the deepest place in the Skagerrak, at about 600 meters water depth. Estimates indicate that the chemical weapons were disposed of in 38 ships. The Norwegian Defence Research Establishment (FFI) has previously investigated the dump site in 1989, 2002 and 2009. In 2009 the HUGIN HUS autonomous underwater vehicle with a sidescan sonar as its primary sensor was used. HUGIN returned with the 2015 cruise, but now upgraded with a synthetic aperture sidescan sonar with considerably improved performance compared to the sensor used in 2009. This cruise is part of the Norwegian Coastal Administration’s surveys of the state of the dump site in the Skagerrak. The surveys are to be concluded with another cruise in 2016, with further mapping using the HUGIN HUS and collection of core samples from the seabed. The primary sensor for this cruise is the HISAS 1030 high resolution interferometric synthetic aperture sonar. HISAS can produce sonar images out to about 200 meters to each side of the vehicle, with up to 3x3 cm resolution. It also produces bathymetric depth maps, typically of a chosen resolution of 18x18 cm. This report is divided into two main sections. The first section is based on a conference article published for the OCEANS’15 in Washington D.C., October 2015 [23]. In this section we present the challenges of using interferometric SAS to map large structures on the seabed, such as wrecks. We compare the SAS data from this cruise with the sidescan sonar data from 2009, and demonstrate how the multibeam echosounder that is also equipped on HUGIN can be combined with the SAS data for increased mapping efficiency. The second section starts with a brief review of all the dives that were conducted in the 2015 cruise. We then present SAS images of all the wrecks discovered. 34 wrecks were found, of which 20 was known from the 2009 cruise. We also show bathymetric maps of all the wrecks. A lot of the results were produced onboard H. U. Sverdrup II during the cruise, and got wide national media coverage from NRK and Aftenposten, among others. This section of the report is intended as a documentation of the data collection, to make it easier to revisit the data and do further work if required.