Subsurface Modelling

The need for geological models

The Geological Survey of the Netherlands (GDN) is part of TNO and it makes models of the earth beneath our feet, sometimes kilometres deep. These models are a summary of all the basic geological information. They combine geological data and knowledge, and provide the most faithful representation possible of the subsurface. They predict strata structure and characteristics, including the gases and fluids present. We can use these models to answer the following important questions:

• where you can safely dig tunnels
• where you can erect large buildings
• where you can extract gravel, sand, clay, limestone, and groundwater
• where there will be subsidence and how much
• using Seasonal Thermal Energy Storage
• producing thermal energy

Four key models

The GDN has developed 4 models for the shallow subsurface, down to a depth of 500 metres. It created these on the basis of hundreds of thousands of drillings and cone penetration tests recorded in the DINO database (Data and Information of the Dutch Subsurface). The GDN also develops models for the deep subsurface, down to depths of 4000 to 5000 metres, using seismic data and data from deep drillings. The models are freely available to everyone on GDNdata. Below is an overview of the 4 most important models for the shallow subsurface.

1. The Digital Geological Model

In the Digital Geological Model (DGM), the Dutch subsurface is built by digitally stacking the lithostratigraphic layers in the subsurface on top of each other from a depth of 500 metres to the surface. The model provides insight into the stratification through thickness and depth planes. The spatial relationships of the layers are made clear on a regional scale as map images and profiles.

2. REgional Geohydrological Information System

The REgional Geohydrological Information System (REGIS-II) is the hydrogeological refinement of the DGM in which each geological unit is subdivided into layers with good permeability (sandy) and poor permeability (clayey). These layers have been assigned mean geohydrological parameters for use in groundwater studies.

3. GeoTOP

GeoTOP provides a detailed three-dimensional image of the subsurface to a depth of 50 metres below the Amsterdam Ordnance Datum. This is the part of the subsurface that’s used most intensively by humans. This model is a refinement of the upper side of the DGM and REGIS-II, in which the subsurface is divided into millions of voxels (little blocks) of 100 x 100 x 0.5 metres. Each voxel contains information on the type of soil and its physical and chemical properties.

4. DGM-Deep

The fourth model is DGM-Deep, which closely resembles DGM but covers a different depth range, down to 6 kilometres. This makes it suitable for calculating the potential of geothermal energy.

Predicting soil subsidence

Soil subsidence in the Netherlands is a cause of societal problems and costs. We measure, monitor, and model subsidence. The main consequences are subsiding roads, bridges, dykes, or houses, and an increased risk of flooding of low-lying areas. Therefore, we want to know whether and how we can influence subsidence.

Research into soil subsidence in the Netherlands

Effects that occur in both the shallow and the deep subsurface may cause soil subsidence. Subsidence in the shallow subsurface can be caused by the shrinkage of clay and the oxidation and compression of peat. We use models to investigate the causes, with the aim of better understanding and predicting subsidence. We do this with data about the structure of the subsurface, the speed of the subsidence, and the amount of CO2 emissions that this causes. We investigate the following important questions:

• What processes cause soil subsidence?
• What’s the extent of the subsidence?
• How much greenhouse gas is released?
• Can we predict how much more subsidence will occur?

Soil subsidence due to human activity

In many places in the Netherlands, soil subsidence is caused by human activity. Major causes are:

• lowering the groundwater level
• building residential areas and roads on soft soil
• extracting gas, oil, and salt
• clay and peat in the soil

Places where the soil consists of clay and peat run a particularly high risk of subsidence, because that material is very compressible. Keeping the groundwater level low makes the layers more compacted. The soil subsides and large amounts of CO2 are emitted.

Taking account of soil subsidence in policy

For many subsurface activities, it’s good to know whether there’s soil subsidence. Our research enables us to predict subsidence increasingly accurately. In this way, risks and opportunities can be analysed and evaluated during policy development. Those responsible can then adjust policy accordingly, to minimise subsidence. With this work, we ensure a safe and sustainable living environment.