WP1: Repeated hydro acoustic munition and sediment mapping, visual observation and sediment sampling

GEOMAR Helmholtz Center for Ocean Research Kiel

Using historic research and past field measurements, the approximate location of buried munitions in the North and Baltic Sea is quite well documented. However, the exact position on the seafloor and the extent of relocation or burial processes caused by currents and storm events is mostly unknown. Thus, reliable estimates on location and transport of munitions with respect to burial depth and exact position are not possible at the moment.

Aims and responsibilities

The aim of WP1 is to determine the exact location of buried weapons and sediment cover using repeated, ship-based hydro acoustic and visual measurements. By systematically comparing these measurements, a possible transport of the buried weapons can be identified and reconstructed. In addition, local and regional sediment relocation processes, which might lead to burial or exposure of weapons, can also be detected.

UDEMM_WP1_Mines in the study area

Additional hydro acoustic sediment mapping, so-called sub-bottom measurements, is used to determine the general geological condition and sediment dynamics of the study area. Based on these measurements, the small-scale structural geology of the study area and thus the morphodynamic sediment processes (e.g. relocation, ripples) and also buried weapons can be identified. The extensive hydro acoustic mapping with ship-based (multibeam) echosounder systems and sub-bottom profilers is the base of a non-invasive inventory of weapons at submarine disposal sites. This is also part of the partner project RoBEMM and here of specific interest to choose a testing area to conduct two full-scale tests as well as to develope an offshore validation method.

Furthermore, extensive mapping with modern echosounder systems can also be used to identify biological habitats (in accordance with the geological setting) and is therefore closely linked to the tasks of WP4 and also WP2, in combination with visual observations and ground truthing by sediment sampling.

  • Repeated, ship-based echosounder measurements of the study area for bathymetric mapping and exact location of weapons and munition on the seabed
  • Identification of sediment and object relocation processes and their quantification using statistical methods (e.g. cross correlation)
  • Geological characterization of the dumping sites by combining of backscatter, bathymetric and sub-bottom data together with visual observations and sediment sampling
  • Standardized habitat mapping using “supervised” and “unsupervised” classification
  • Achieve the maximum precision for locating the weapons and munition on the seabed by using Real-Time kinetics (RTK) and/or geodetic post-processing positioning services (GPPS-SAPOS)
  • Precise 3D-visualization of the identified weapons using photogrammetric methods (depending on visibility)

Results

As part of the scientific work in UDEMM, we tested two different kinds of state-of-the-art multibeam systems, a NORBIT WBTxxx and a RESON T50. Both systems performed well with respect to their capacity, but most of the mapping finally occurred with the T50 as we could get very good support from a nearby company in setting up the system, properly reference all sensors to each other and provide undisturbed RTK correction for the navigation. The presented results below have been recently submitted by Kampmeier et al. to GGG.

For the compilation of the entire Kolberger Heide area 20 days of surveying were needed  that were acquired between 2016 and 2018. Repeating surveys in Nov. 2015, Feb. 2018 and June 2018 aimed at identifying potential migration of objects. The restricted dumping ground is located in the south of the area in 5–14 m water depth on a shallow platform, which towards the north develops into a more horizontal plain in 19–20 m water depth. Patches of algae, covering the seafloor with varying density could be observed in underwater video profiles and show seasonal variability in their spatial distribution. Small scale ripples of 5 cm height and 20 cm width indicate sediment transport on the seafloor. Their crests are generally N-S oriented and thus perpendicular to the bottom current direction.

In 19 m water depth in the outer area of the dump site, otter trawling marks are clearly visible inside the soft sediments and indicate significant fishing activities. This fishing method uses two doors that are dragged on the seafloor to keep the trawling net open. Since 2004 the trawling is prohibited in the Baltic Sea for areas shallower than 20 m water depth or within 3 miles to the coastline. Despite this, 10-100 hours of otter board trawling were noted within this area in 2006 (Sell et al., 2011). Even though Kolberger Heide is not one of the main fishing grounds in Kiel Bay, bottom trawling seems to occur close to the official dump site and thus can potentially lead to object displacement.

On local-scale the bathymetry is characterized by a shallow platform, which extends from the shore and declines with less than 1 degree towards the north. It builds a plain in 19 m water depth. Patches that show increased rugosity (height differences of 2-4 cm) and ripple- like structures are seen in the feature-scale morphology. Those areas produce high backscatter intensities and contain increased amounts of rocks. As the underwater environment of Kolberger Heide is strongly affected by anthropogenic use artificial objects and remnants of activities are seen on local and feature-scale. There are explosion craters with average diameters of 20 m and depths of 1.5 m, which can be observed as clusters or isolated craters across the entire study area. Craters were formed by blow-in-place detonations that partially acted as test for bubble curtain experiments. Bubble curtains significantly decrease the sound energy of detonations and thus particularly protect marine mammals (Würsig et al. 2000). Remnants of these bubble curtains in form of pipelines and anchor stones were left on the seafloor; they can be reactivated if needed. Other types of feature-scale objects are all kind of UXO and dumped munition in high numbers. Depending on the munition type, they can be more or less accurately identified.

Three hot spot areas have been detected. The first area is composed of ~70 defused moored mines, piled up to a mound-like structure of 30 m length and 15 m width. The height over the surrounding seafloor is about 1.5 m. The second highly contaminated area is located at a cluster of detonation craters. At this location around 90 munition bodies of different types ranging from German and English ground mines, torpedo heads, water bombs and moored mines can be found. Some of these objects have been brought into the dump site after being defused by the EOD. The third area of around 150 m2 in size shows ca. 100 objects of 1 m by 0.6 m.

First validation dives identified these objects as aerial bombs, possibly not defused. A high number of additional suspicious objects can be found all over the research area. Moored mines occur as round shaped objects with ca 1.2 m diameter all over the existing bathymetric data set. Due to their size and elongated shape ground mines and torpedoes can be rather easily identified. Smaller objects such as bombs and torpedo heads are not as easily identified and require validation by divers or underwater video. Joint observation of derivatives with bathymetric data, can greatly enhance the detection and identification of munition. Slope, surface area and curvature highlight distinct objects as rocks and munition on the seafloor very clearly.