First International Conference on Frontiers in Shallow Subsurface Technology

Nanoseismic monitoring techniques are able to detect extremely low-energy (ML > -4.0) signals generated by active subsurface instabilities within cavities, cliffs, landslides and sinkholes. The data is acquired by portable sparse seismic arrays, which are deployed, within minutes, in varying geometries as close as possible to a presumed zone of instability. Events detection is carried out by semi-automated pattern recognition-supported schemes4, which scan for broad-band energy spikes within continuous data records sampled at 200 to 500 Hz. The authenticity of source signals is verified either by true-scale simulation in the field -when possible- or through a multi-parameter validation process that uses a custom library of reference patterns. This comprehensive waveform characterization process includes full-spectral signal analysis, 3-D source location, waveform cross correlation and source magnitude evaluation. Study cases will be reviewed to show how failure generated within cavities5,6, cliffs7 and sinkholes1 can be located and characterized. Additional analyses include magnitude-time series, source time migration and event coincidence with external parameters as well as a custom calibration of source energy dissipation in unconsolidated material.

The properties of sand are important for its suitability for the use in construction works. For dredgers, this information needs to be available in advance of dredging activities. Shear wave velocities are linked to the strength of the sediment. In underwater environments, shear wave velocities can be extracted from seismic data using the dispersive properties of Scholte waves. We used forward modelling to investigate whether we can regard Scholte waves as their equivalent on land, Rayleigh waves. The difference between Rayleigh and Scholte waves is related to the relative water depth and the Poisson ratio distribution. For a halfspace of unconsolidated muddy sediments (high Poisson ratio) on deep water layer, the Scholte wave velocity is nearly 15% lower than the Rayleigh wave velocity. For sandstone (low Poisson ratio), there is no difference. When the water is shallow, the Scholte wave velocity deviates from Rayleigh wave velocity by less than 5%. Shear wave velocities were extracted from Scholte waves observed in data sets from the intertidal Wadden Sea (The Netherlands) and the river Danube near the village of Kulcs (Hungary). Based on the results from the forward modelling, we treated the Scholte waves as Rayleigh waves. The shear wave velocities in the top layer of the Wadden Sea sediments were approximately 150 m/s, which fall in the expected range for poorly consolidated material. The shear wave velocities of the Kulcs sediment were higher (400 to 600 m/s). The transition from clay to sand as apparent from a borehole log was successfully recovered in the subsurface shear wave velocity model.

During the last decades, the pressure for space in dense urbanized areas has led to an increasing use of the underground for a variety of functions such as infrastructure, utilities and buildings. In the last decade, an exponential growth in systems using the underground for thermal energy storage fueled by the need for renewable energy sources is added to this. These relative novel underground functions can compete with existing users of the underground. This includes both active (e.g. drinking water) and passive (groundwater dependent ecosystems) users of these so called ecosystems services. Dutch policy makers of various governmental levels (municipality, province, national) are currently exploring methods to deal with these conflicting interests. A few examples of issues to deal with are: 1) how do we deal with existing rights versus new claims for the underground? 2) how do we manage the exponential growth of subsurface functions of which the long-term environmental effects are unknown? KWR and University Utrecht have recently started a project funded by the Dutch Ministry of Housing, Spatial Planning and the Environment investigating these questions. In this project we will look at both the technical and legal aspects of subsurface planning and specifically focus on underground thermal energy storage in spatial planning. This extended abstract details the preliminary results of this project.

The climate change may pose threats to the way the Dutch live in deltas below sea level. Peat districts below sea level are at more risk, when peat gets oxidized to Green House Gas (GHG) emissions, followed by land subsidence due to low groundwater levels. However, new sustainable concepts can handle the climate change. Local materials like manure, sediment, water treatment sludge, debris from plants, and construction materials can reverse the soil subsidence. Inspiration comes from 500 BC, when farmers erected mounds for seasonal refuge. They mixed sods with manure and wooden debris to a fertile soil with high bearing capacity and resistance to failure. Today, the Cradle-to-Cradle (C2C) approach aims at a new life for waste materials in a transition of the rural farming to a sustainable one (the “Peat in balance” concept.

The infiltration rate in soils is important for determining the propagation of saturation, and hence for the stability of slopes subjected to overrunning water. The paper suggests a simple practical formula for interpretation of infiltration tests, validated by various laboratory and field test results.

There is an increasing demand for detailed information on the spatial distribution of geological units and their physical and chemical properties. However, existing studies on sediment properties are scarce. The project TOPINTEGRAAL aims to build a geological model of the upper 30 to 50 meters of the subsurface and to characterize the lithological, hydraulic, geochemical and geotechnical properties of the sediments. A drilling campaign was launched in 2006 in order to obtain additional soil samples for the analyses and measurement of physical and geochemical properties. So far 157 cored wells were drilled. The results of the analysis and measurements of the samples are interpreted and published per GeoTOP-main area. In 2009 a first interpretation of the grain size and geochemical data of main area “Noord-Nederland” in the northern part of the Netherlands has been performed. The data and their interpretations will become a unique national database which will help researchers and planners to obtain better and more accurate results.

A recent development in the Netherlands is the implementation of a Key Register Subsurface (“Basisregistratie Ondergrond”). Part of this Key Register Subsurface is a new generation of 3D subsurface models that provide estimates of stratigraphy, lithology (clay, sand, peat) and sand-grain size class data at voxel-resolutions of 100*100*0.5m (referred to as GeoTOP models). These regional-scale models can be used as a starting point for detailed investigations of risks and opportunities that may evolve from intensive and multiple uses of the subsurface. An example of this is the planning of a new subway in the city of Rotterdam.

The Geological Survey of the Netherlands aims at building a 3D geological property model of the upper 30 meters of the Dutch subsurface. The model schematises the subsurface in millions of grid cells each measuring 100 by 100 meters in the horizontal directions and 0.5 meters in the vertical direction. Each grid cell of the model includes estimates of stratigraphy, lithofacies and lithology (clay, sand, peat) and if applicable, sand-grain size class data. Stochastic interpolation techniques are used to compute probabilities for these parameters, providing a quantification of model uncertainty. The model provides a sound framework for subsurface related questions on, amongst others, groundwater management, land subsidence, natural resources and infrastructural issues.

The main objective of this research is to enhance our understanding of the relation between pore-scale adsorption and Darcy-scale parameters using a 3D pore-network model. This helps to scale up from a simplified but reasonable representation of microscopic physics to the scale of interest in practical applications. This upscaling will be carried out in two stages: i) from pore scale to the scale of a tube, which is the main element of a pore-network model, and ii) from tube scale to the scale of a core represented by the pore-network model. The first stage of this upscaling from pore to tube scale has been reported in an earlier manuscript. There we found relationships between micro-scale parameters (such as equilibrium adsorption coefficient, kd, and Peclet number, Pe) and tube-scale parameters (such as attachment coefficient, katt, and detachment coefficient, kdet). Here, we perform upscaling by means of a 3D multi-directional network model, which is composed of a large number of interconnected pores bodies and pore throats (represented by tubes). We use the expressions which were developed in our earlier work to estimate tube-scale adsorption parameters for each and every pore in the network. Transport parameters are then upscaled over the whole network by fitting average concentration breakthrough curves at the outlet to the solution of classical advection-dispersion equation with adsorption. This procedure has resulted in relationships for upscaled adsorption parameters in terms of micro-scale coefficients.