Detailed seismic imaging of a chemical munition dumpsite in the Bornholm Basin, south-western Baltic

Very high resolution seismic investigations were carried out over a munition dumpsite in the Bornholm Basin, south-western Baltic Sea. After WW2 over 32,000 tons of chemical weapons were dumped in this area. The aim of the investigations was to image the internal structure of the dumpsite and to identify possible natural hazards with regard to the dumped war material. Two geophysical surveys were carried out in the summers of 2006 and 2007. During the surveys, acoustic sources with a distinct frequency spectrum (parametric echosounder, sparker, boomer) were deployed simultaneously. This approach proved very fruitful as it allowed to study the sediments in a wide depth range and in the highest possible detail. In total, seven seismic-stratigraphic units have been identified, related to different stages in the Holocene and late glacial history. Water depths range roughly between 96 m in the north and 70 m in the south where the basin shallows and older glacial sediments are outcropping. The seafloor topography at the dumpsite is generally quite flat, locally marked by small pit-like features that are probably related to the dumping impact. Small-scale fluid escape features are present throughout the dumpsite area. Large-scale doming occurs locally and is possibly linked to the migration of deep thermogenic gas. A large number of buried objects were identified on the acoustic data. Their burial depth is generally less than 1 m. The data confirm the wide variety of dumped war material, ranging from bombs and shells to encasements and containers. The distribution of the objects seems to be very heterogeneous, with locally high concentrations and areas of low object density. Four different shipwrecks were identified but their relation to the dumped warfare is not clear.