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Jelte Stam,
GeologistCall
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DIS North Sea
Sand is the second most widely used natural resource in the world after water. Every year, around 35 million cubic metres of sand is extracted from the Dutch part of the North Sea. This sand is used for coastal protection and for construction. Together with Deltares, the Geological Survey of the Netherlands (GDN) has been working on the Minerals Information System (DIS) for years. On behalf of the Netherlands’ Directorate-General for Public Works and Water Management (Rijkswaterstaat), sand reserves in the North Sea subsurface are being mapped out using ever-better 3D subsurface models.
Quality and quantity
Several kilometres off the Dutch coast, a strip along the coastline has been reserved for sand extraction. The subsurface of the North Sea is not only made up of sand. Layers of clay, peat, gravel, and shells are also present. In addition, the grain size of sand can vary from very fine to extremely coarse. For our national sand extraction strategy, it is important to know the exact locations of different types of sand. In this way, the most optimal location for each application can be found.
Geological archive
GDN has an archive of borehole and seismic data that provides information about the structure of the subsurface beneath the sea floor. Using these point and line data, we can clearly see how the subsurface varies in spatial terms. We use geostatistical methods to predict the composition of the subsurface between these points and lines. This gives us an area-wide model.
Layer and voxel model
For DIS, we first create a geological layer model using geostatistics. This means that we divide the subsurface into layers with well-defined lithological characteristics. Based on this layer model, we then create a voxel model, dividing the subsurface into blocks of 200 x 200 x 0.5 metres. For each 20,000-cubic-metre block, we calculate the type of sediment it is likely to contain. When making this calculation, the layer model is leading.
Sand quality
All kinds of questions about the subsurface can be answered using this voxel model, such as, ‘Where is the greatest chance of finding gravel?’, ‘Where are we likely to find clay?’, or ‘Is the sand here suitable for construction or is it more suitable to use for beach elevation?’
How much sand can we extract?
By combining the voxel model with data on spatial planning and large-scale sea floor infrastructure (such as power lines and gas pipelines), we can see where sand can and cannot be extracted. Using the same techniques, we can also calculate how much sand would be available in different scenarios for the near or more distant future. This information forms the foundation of a sustainable sand extraction policy.
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