The complex conductivity of soils remains poorly known despite the growing importance of this method in hydrogeophysics. In order to fill this gap of knowledge, we investigate the complex conductivity of 71 soils samples (including four peat samples) and one clean sand in the frequency range 0.1 Hz to 45 kHz. The soil samples are saturated with six different NaCl brines with conductivities (0.031, 0.53, 1.15, 5.7, 14.7, and 22 S m−1, NaCl, 25°C) in order to determine their intrinsic formation factor and surface conductivity. This data set is used to test the predictions of the dynamic Stern polarization model of porous media in terms of relationship between the quadrature conductivity and the surface conductivity. We also investigate the relationship between the normalized chargeability (the difference of in‐phase conductivity between two frequencies) and the quadrature conductivity at the geometric mean frequency. This data set confirms the relationships between the surface conductivity, the quadrature conductivity, and the normalized chargeability. The normalized chargeability depends linearly on the cation exchange capacity and specific surface area while the chargeability shows no dependence on these parameters. These new data and the dynamic Stern layer polarization model are observed to be mutually consistent. Traditionally, in hydrogeophysics, surface conductivity is neglected in the analysis of resistivity data. The relationships we have developed can be used in field conditions to avoid neglecting surface conductivity in the interpretation of DC resistivity tomograms. We also investigate the effects of temperature and saturation and, here again, the dynamic Stern layer predictions and the experimental observations are mutually consistent.

The Geological Survey of the Netherlands aims at building a 3D geological 'GeoTOP' model of the upper 30 m of the subsurface of the Netherlands in order to provide a sound basis for answering subsurface related questions on, amongst others, groundwater extraction and management, land subsidence studies, aggregate resources and infrastructural issues. The model provides a uniformly constructed framework for subsurface information and serves as an easily accessible data source for any subsurface activities in the Dutch delta. Earlier large-scale 3D voxel models of the Netherlands were tailor made to specific applications, i.e. aggregate resources (Van der Meulen et al., 2005) and clay resources (Van der Meulen et al., 2007).The GeoTOP model is a fully three-dimensional, multi-purpose subsurface model of the onshore part of the Netherlands. In contrast to other nation-wide subsurface models, the Digital Geological Model (DGM; www.dinoloket.nl) and the REgional Geohydrological Information System (REGIS-II; Vernes and Van Doorn, 2005;Vernes et al., 2010; www.dinoloket.nl)

Seawater intrusion has often resulted in scarce fresh groundwater resources in coastal lowlands. Careful management is essential to avoid the overexploitation of these vulnerable fresh groundwater resources, requiring detailed information on their spatial occurrence. Airborne electromagnetics (EM) has proved a valuable tool for efficient mapping of ground conductivity, as a proxy for fresh groundwater resources. Stakeholders are, however, interested in groundwater salinity, necessitating a translation of ground conductivity to groundwater salinity. This paper presents a methodology to construct a high-resolution (50 × 50 × 0.5 m 3 ) 3D voxel model of groundwater chloride concentration probability, based on a large-scale (1800 km 2 , 9640 flight line kilometres) airborne EM survey in the province of Zeeland, the Netherlands. Groundwater chloride concentration was obtained by combining pedotransfer functions with detailed lithological information. The methodology includes a Monte Carlo based forward uncertainty propagation approach to quantify the inherent uncertainty in the different steps. Validation showed good correspondence both with available groundwater chloride analyses, and with ground-based hydrogeophysical measurements. Our results show the limited occurrence of fresh groundwater in Zeeland, as 75% of the area lacks fresh groundwater within 15 m below ground surface. Fresh groundwater is mainly limited to the dune area and sandy creek ridges. In addition, significant fresh groundwater resources were shown to exist below saline groundwater, where infiltration of seawater during marine transgressions was hindered by the presence of clayey aquitards. The considerable uncertainty in our results highlights the importance of applying uncertainty analysis in airborne EM surveys. Uncertainty in our results mainly originated from the inversion and the 3D interpolation, and was largest at transition zones between fresh and saline groundwater. Reporting groundwater salinity instead of ground conductivity facilitated the rapid uptake of our results by relevant stakeholders, thereby supporting the necessary management of fresh groundwater resources in the region.

In the Netherlands, the bulk of the Miocene to lowest Pliocene sedimentary succession is currently assigned to a single lithostratigraphical unit, the Breda Formation. Although the formation was introduced over 40 years ago, the definition of its lower and upper boundaries is still problematic. Well-log correlations show that the improved lecto-stratotype for the Breda Formation in well Groote Heide partly overlaps with the additional reference section of the older Veldhoven Formation in the nearby well Broekhuizenvorst. The distinction between the Breda and the overlying Oosterhout Formation, which was mainly based on quantitative differences in glauconite and molluscs, gives rise to ongoing discussion, in particular due to the varying concentrations of glauconitic content that occur within both formations. In addition, the Breda Formation lacks a regional-scale stratigraphic framework which relates its various regionally to locally defined shallow marine to continental members. In order to resolve these issues, we performed renewed analyses of material from several archived cores. The results of archived and new dinocyst analyses were combined with lithological descriptions and wire-line log correlations of multiple wells, including the wells Groote Heide and Broekhuizenvorst. In this process, the updated dinocyst zonation of Munsterman & Brinkhuis (2004), recalibrated to the Geological Time Scale of Ogg et al. (2016), was used. To establish regionally consistent lithostratigraphic boundaries, additional data was used along a transect across the Roer Valley Graben running from its central part (well St-Michielsgestel-1) towards the southern rift shoulders (well Goirle-1). Along this transect, chronostratigraphic and lithostratigraphic analyses were integrated with well-log correlation and the analyses of seismic reflection data to constrain geometrical/structural relationships as well. The results led to the differentiation of two distinct seismic sequences distinguished by three recognisable unconformities: the Early Miocene Unconformity (EMU), the Mid-Miocene Unconformity (MMU) and the Late Miocene Unconformity (LMU). The major regional hiatus, referred to as the Mid-Miocene Unconformity, occurs intercalated within the present Breda Formation and compels subdivision of this unit into two formations, viz. the here newly established Groote Heide and the younger Diessen formations. Pending further studies, the former Breda Formation will be temporally raised in rank to the newly established Hilvarenbeek subgroup, which comprises both the Groote Heide and Diessen formations. Whereas these two sequences were already locally defined, a third sequence overlying the LMU represents two newly defined lithostratigraphical units, named the Goirle and the Tilburg members, positioned in this study at the base of the Oosterhout Formation. Besides their unique lithological characteristics, in seismic reflection profiles the Goirle and the Tilburg members stand out because of their distinct seismic facies. Use of an integrated, multidisciplinary and regional approach, an improved southern North Sea framework and more comprehensive lithostratigraphic subdivision of Neogene successions is proposed for the Netherlands, to make (cross-border) correlations more straightforward in the future.

A 3D geological raster model has been constructed of the onshore of the Netherlands. The model displays geological units for the upper 500 m in 3D in an internally consistent way. The units are based on the lithostratigraphical classification of the Netherlands. This classification is used to interpret a selection of boreholes from the national subsurface database. Additional geological information regarding faults, the areal extent of each unit and conceptual genetic models have been combined in an automated workflow to interpolate the basal surfaces of each unit on 100 × 100 metre (x,y dimensions) raster cells. The combination of all interpolated basal surfaces results in a 3D Digital Geological Model (DGM) of the subsurface. A measure of uncertainty of each of these surfaces is also given. The automated workflow ensures an easily updatable subsurface model. The outputs are available for end users through www.dinoloket.nl.