Comparison of synthetic aperture sonar images and optical images of UXOs from the Skagerrak chemical munitions dumpsite

After World War II chemical munitions were loaded onto large ships and sunk in the Skagerrak Strait in the Norwegian trench, in deep waters between Norway and Denmark. In collaboration with the Norwegian Coastal Administration, Norwegian Defence Research Establishment (FFI) conducted two separate missions in 2015 and 2016, where around 450 square kilometers were mapped using a HUGIN autonomous underwater vehicle (AUV) equipped with a HISAS interferometric synthetic aperture sonar (SAS). 54 wrecks were found, and 36 of these are believed to be part of the chemical munitions dumpsite. Many shipwrecks have suffered severe damage, and their cargo is spread out around the shipwrecks. In January 2019, FFI revisited the Skagerrak dumpsite for additional data collection using the HUGIN AUV with SAS and an optical camera. Multiple wrecks were revisited, and SAS images and optical images of unexploded ordnances (UXO) were gathered. In this paper, we compare SAS images with optical images of UXOs. We assess the retrievable information present in high resolution large area coverage rate SAS images by comparing separate objects and their conditions as seen with optical camera.