Marine Geophysics and Hydroacoustics

Multibeam

The AG 'Marine Geophysics and Hydroacoustics' uses modern multibeam echo sounders to efficiently map the seabed. We are specialized in adapting seismic and hydroacoustic systems to various research platforms, from large to small research vessels to very small or towed platforms in shallow water.

In the working group different echo sounders are used. On medium-sized and large German research vessels, the permanently installed systems are usually used (KONGSBERG EM 710, 712, EM 120, 122, ELAC Wärtsilä SB3050). Mobile multibeam echo sounders are installed and used by the working group on various research platforms. In shallow water, we prefer the Chirp-based, integrated iWBMS from NORBIT. In combination with modern RTK-GPS positioning we get up to 2-4 cm resolution in horizontal and vertical direction. As a rule, we use high-precision inertial systems (Applanix Wave Master, Codaoctopus F180) for the precise recording of the position and compensation of the ship's motion.

Pockmarks in der Bathymetrie vor Eckerförde

Abb. 1: High-precision measurement of so-called pockmarks off Eckernförde, they are caused by submarine groundwater discharge..Also clearly visible are traces of fishery activity (trawl marks, approx. 4 cm deep) in WSW-ENE direction. © J. Schneider von Deimling

During the actual echo sounding, the two-way-travel time of sound impulses is measured and the seabed depth is derived (bathymetry). Due to their directional characteristic, modern systems allow recording with an acoustic fan approx. 150° wide at an individual beam resolution between 0.3-2° and a range resolution of up to 1.5 cm. In order to obtain the seabed depth from the angle-time measurements obtained, the sound velocity profile in the water column must also be recorded and the ship's motion compensated. Subsequently, diffraction effects can be considered and an exact morphological image of the seabed surface can be generated (Fig. 1).

  

In addition to mapping the seabed, we adapt acoustic methods for specific measurements on the seabed and in the water column. This enables us to detect and characterize anthropogenic footprints such as old ammunition and mines (UXO), buried cables, or trawl marks. In particular, objects in the water column such as natural and anthropogenic gas leaks can be investigated. Finally, modern multibeam echo sounders offer outstanding capabilities for habitat mapping of the seabed. We have specialized in the acoustic investigation of seagrass meadows in the Baltic Sea. 

 

The accuracy of the method depends on the depth of the water and the methodological challenges increase as the water becomes shallower. In addition to high-precision bathymetric measurements (Fig. 1), we are intensively involved in the backscattering strength of MBES measurements both in the water column and from the seabed itself. The angle dependent backscatter intensity of the seabed is characteristic depending on the composition of the subsoil and the predominant habitat. For example, sand, gravel and silt can be clearly distinguished from each other by their angle-dependent radiation characteristics in modern multibeam echosounder data.

Schwarzweiße Backscatterdaten zeigen einen Munitionskörper.

Abb. 2: Dumped ammunition body (ground mine, UXO) in multibeam Snippet Backscatter-Data imagery, recorded in the Kolberger Heide (ALKOR 447, 2014), (M.Sc.Tina Kunde, 2017).

To process the data we use a combination of open source software under LINUX (MBSystem, GMT) and commercial packages (QIMERA, HDPPost, MATLAB, FLEDERMAUS, HyPACK, KINGDOM), which are also used in teaching. To classify the data, we increasingly rely on artificial intelligence (machine learning methods).
 

 

Gasaustritte (Seiten- und Frontansicht) aus einem unversiegelten Bohrloch.

Fig. 3: (a+b) Gas leaks from an abandoned well. (c) Bathymetric crater formed by accidental drilling of shallow gas in the North Sea in 1990. The diameter is 60 m. To this day, considerable quantities of methane gas are emitted from the crater. (Schneider von Deimling et al., 2015).

 

AG Publikationen zur Methodik

  • Schneider von Deimling, J. S., Weinrebe, W., Tóth, Z., Fossing, H., Endler, R., Rehder, G., & Spieß, V. (2013). A low frequency multibeam assessment: Spatial mapping of shallow gas by enhanced penetration and angular response anomaly. Marine and Petroleum Geology, 44, 217-222.
  • Schneider von Deimling, J., & Weinrebe, W. (2014). Beyond bathymetry: water column imaging with multibeam echo sounder systems. Hydrographische Nachrichten, 31(97), 6-10.
  • Hillman, J. I., Lamarche, G., Pallentin, A., Pecher, I. A., Gorman, A. R., & Schneider von Deimling, J. S. (2017). Validation of automated supervised segmentation of multibeam backscatter data from the Chatham Rise, New Zealand. Marine Geophysical Research, 1-23.
  • Krastel, S., Schmincke, H. U., Jacobs, C. L., Rihm, R., Le Bas, T. P., & Alibes, B. (2001). Submarine landslides around the Canary Islands. Journal of Geophysical Research: Solid Earth, 106(B3), 3977-3997.
  • Held, P., & Schneider von Deimling, J. (2019). New Feature Classes for Acoustic Habitat Mapping—A Multibeam Echosounder Point Cloud Analysis for Mapping Submerged Aquatic Vegetation (SAV). Geosciences9(5), 235.