Bathymetry and soil composition

The seabed within the Belgian part of the North Sea (BNS) forms a trapezoidal area of barely 3500 km², which slopes steadily towards the northwest where it reaches a maximum water depth of 40 to 45 metres. The bathymetry is characterised by the presence of a complex system of channels and some 30 sandbanks (Mathys, 2009).

Based on their location and orientation, the sandbanks are divided into four groups. The Coastal Banks and the Zeeland Banks are more or less parallel to the coastline, while the Flemish Banks and the Hinder Banks form an angle with the coast. Some sandbanks are dozens of kilometres long and rise almost 30 metres above the neighbouring bottom (Mathys, 2009; 2010).

The substrate of the soil usually consists of non-consolidated Quaternary sediments, with a thickness that varies from a few metres in the channels to 50 metres at the level of the sandbanks. The grain size increases as the distance from the coast increases, varying from silt-rich sediment close to the coast to fine and rough sand, and even layers of gravel deeper into the sea (MAREBASSE project). The Quaternary sediments lie on top of much older Tertiary clay deposits, which can only be seen locally in the surface trenches (TILES project).

Since aggregate and sand extraction are important economic activities in the BNS, it is very important to find out how much sediment is present on the soil. Several projects have already been set up for this purpose, such as the MAREBASSE project and the TILES project. These have largely contributed to the mapping of bathymetry and the soil composition of the BNS.

This mapping often combines two large groups of techniques, namely reflection seismics and sampling. In the case of reflection seismics, an acoustic source produces a sound wave that is reflected back to a receiver in the subsurface by the boundary between two layers. The result is a vertical cross-section of the subsurface. Sampling of the soil is often done by means of drill cores, but can also be collected by a variety of other equipment (Van Veen grab, Hamon grab, Box corer, etc.).

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Soil composition