Ronald Harting

The Dutch shallow subsurface is extensively being used for natural aggregate resources, groundwater extraction and construction works. This makes that insights into the composition and heterogeneity of the subsurface and its physical and chemical properties are of vital importance for Dutch society (Vernes et al. 2010, Van de Meulen et al. 2013). Although a large amount of physical and chemical data of the subsurface has been collected in the past, issues like discontinuous sample distributions and/or differences in analysis techniques, have limited their helpfulness for applied research (Harting et al. 2014). We present new results from an analysis of a vast nationwide data set of lithological properties from the shallow subsurface of the Northern Netherlands that were collected, measured and analysed in a systematic and uniform way (Bosch et al. 2015). The dataset, which is managed by the Geological Survey of the Netherlands, contains a large number of parameters that were collected from undisturbed sediments taken from 75 continuous boreholes with a depth up-to 40m. Since the cores have an excellent geographical distribution and the samples are measured on in situ samples with a standard set of techniques, the data overcomes the data density and measurement inconsistencies known from previous studies. In this presentation we will focus on the lithological properties of the Boxtel, Drente, Drachten and Peelo Formations in the northern Netherlands that were deposited under periglacial, glacial, eolian and glacial to glacio-lacustrine conditions respectively. We found that Weichselian (Boxtel Fm.) and early Saalian (Drachten Fm.) eolian sediments are nearly identical, suggesting comparable sedimentary processes during deposition. In several cores we newly identified a Elsterian till belonging to the Peelo Formation, which is very different from the Saalian till belonging to the Gieten Member of the Drente Formation. This difference may be related to the sediment sources available during formation of the till. Finally, we describe a new type of bedded till from the Saalian Gieten Member, strongly contrasting with the well-known massive till. With these examples, we show that using our lithofacies based approach (Harting et al. 2014) is a strong and powerful tool to study and better understand heterogeneity in these sediments (Bosch et al. 2015).

Society has an increasing demand from the subsurface, which in the Dutch shallow subsurface (upper 30 to 40 meters) mainly focuses on natural aggregate resources, groundwater, infrastructure and dike safety. This stimulates the demand for knowledge about the composition and heterogeneity of the subsurface and its physical and chemical properties, including the uncertainties involved. Physical and chemical properties of sediments in the subsurface have been under investigation for decades; however, the usefulness of this data for applied research and the understanding of these properties is limited. This  is due to several factors: studies consist mainly of separately collected datasets, targeted at a limited amount of parameters, focused on a small number of geological units, distributed unevenly with depth and usually collected from clustered drillings with limited spatial extent or are analysed with different techniques and methods, often on disturbed samples. These factors result in a heterogeneous and biased dataset not suitable to function as a reference dataset or to statistically determine regional characteristics of geological units.

To overcome these shortcomings, the Geological Survey of the Netherlands is establishing a nation-wide reference dataset for physical and chemical properties. In 2006, a drilling campaign was started using cone penetration tests, cored drillings and geophysical well logs, choosing the sites for a good geographical distribution. The lithological properties of the undisturbed cores are visually described and interpreted for lithostratigraphy and inferred sedimentary environment based on lithofacies. The location of the samples in the cores are chosen based on this description and interpretation, resulting in an evenly distributed dataset of in situ samples with respect to geological units as well as an adequate number of samples suitable for statistical analysis. Analyses are uniformly performed for grain size distribution, permeability (both high and low permeable lithologies) and geochemical methods (X-Ray Fluorescence, Thermo-Gravimetric Analysis, Total Carbon, Total Sulphur and Total Organic Carbon). These analyses result in a large number of lithological, hydrological and geochemical parameters, i.e. clay content, sand median, vertical and horizontal permeability and CaCO3-content.

We present the results from the analysis of lithological properties for the Northern Netherlands. Besides geology, these properties can be applied directly in studies concerning (amongst others) groundwater, natural aggregates and dike safety. We demonstrate the use of sedimentary environments based on lithofacies as a useful tool for comparison between lithostratigraphic units and lithofacies. These lithofacies match distinct parts of the marine, fluvial, glacial, eolian or organogenic environment, i.e. tidal channel sand, floodbasin clay and subglacial till. This results in lithological properties illustrating the heterogeneity within a geological unit and between equal depositional environments in different lithostratigraphic units.
The acquired data have so far been used in several applied studies, i.e. improving parameterisation of 3D models leading to increased accuracy in groundwater models and dike safety studies concerning dike failure due to undermining. Recently, grain size distributions measured with different methods were recalibrated into a homogeneous dataset using this reference set, which greatly enlarged the dataset to be incorporated in the parameterisation of a 3D voxel model.