Underwater Exploration with Sonar of the Flooded Basement of a WW-II Submarine Bunker in the Context of the Digitization of Cultural Heritage

Underwater mapping based on the registration of data from a mechanical scanning sonar (MSS) is presented. The specific use-case is the exploration of the flooded basement of the U-Boot Bunker Valentin in the context of the digitization of this World-War-II memorial. The visibility in the basement, which is inaccessible for divers due to safety reasons, is very poor. Hence, maps are very helpful if not essential for the exploration of this environment, for which - despite its rather recent history - open questions exist. The mapping is conducted with a very robust method in form of Fourier-SOFT 2D (FS2D), which is well suited for real-time registration of the very noisy scans generated with simple motion-compensation from the MSS data. More precisely, the exploration of the bunker basement is conducted with low-cost hardware in form of a BlueROV2 vehicle, simple navigation sensors, and a Ping360 MSS. We show that reasonable maps can be generated in real-time with this equipment when using FS2D.