Geological Survey of Denmark and Greenland Bulletin 17, 2009, 41-44 Groundwater mapping in Denmark has high priority. It was initiated in the 1990s when the pressure on groundwater resources increased due to urban development and pollution from industrial and agricultural sources. In some areas, the groundwater mapping included survey drillings, modelling based on existing knowledge and geophysical mapping with newly developed methods that made area coverage on a large scale possible. The groundwater mapping that included development of new geophysical methods showed promising results, and led to an ambitious plan to significantly intensify the hydrogeological mapping in order to improve the protec- tion of the Danish groundwater resources. In 1999 the Danish Government initiated the National Groundwater Mapping Programme with the objective to obtain a detailed description of the aquifers with respect to localisation, extension, distribution and interconnection as well as their vulnerability to pollution (Thomsen et al. 2004). This mapping programme covers around 40% of the area of Den - mark designated as particularly valuable water abstraction areas. Water con- sumers fi nance the mapping pro- gramme by paying 0.04 € per cubic metre of consumed water. At the end of the programme in 2015, the total cost is estimated to be about 250 000 000 € with a significant part spent on geo- physical mapping. The mapping programme is admin- istered by seven local offices under the Ministry of Environment, but most of the practical work is carried out by pri- vate consulting companies, and involves the use of geophysical survey methods, survey drillings, well logging, water sampling and hydrological mapping, as well as geological and groundwater modelling. In major parts of the particularly valuable water abstraction areas, it is important to obtain spatially dense geophysical data covering large con- tinuous areas. Geophysical methods used in the hydro- geological mapping The choice of geophysical methods depends on the geologi- cal setting of the aquifers. Those of interest for drinking water are primarily found within the upper 250 m of the subsur- face. The aquifers can be grouped into three main types. In © GEUS, 2009. Geological Survey of Denmark and Greenland Bulletin 17, 41–44. Available at: www.geus.dk/publications/bull 41 Fig. 1. Areal extent of data collected by the end of 2008. A: Areas with TEM and SkyTEM soundings, B: Areas with PACES profiles, C: CVES profiles. D: Seismic profiles. Geophysical methods and data administration in Danish groundwater mapping Ingelise Møller, Verner H. Søndergaard and Flemming Jørgensen C D Jylland Sjælland Fyn A B 50 km Fig. 3 ROSA_2008:ROSA-2008 01/07/09 15:48 Side 41 the western part of Denmark, extensive Quaternary and Pre- Quaternary sand deposits dominate. In the central part, the most important groundwater resources are located in Qua - ternary sand deposits often found in Quaternary valley struc- tures deeply eroded into Palaeogene clay deposits. In the northern and eastern parts of the country, most of the impor- tant aquifers are found in Upper Cretaceous and Danian limestone. The most important geophysical methods are electrical and electromagnetic methods, combined with reflection seis- mic profiling and borehole logging at selected localities. Differences in electrical properties between sandy aquifers and clay sediments favour the use of the electrical and elec- tromagnetic methods (Sørensen et al. 2005), but the ability of seismic methods to reveal detailed internal structures within the aquifers is also important. The most commonly used geophysical method in the groundwater mapping programme is the airborne transient electromagnetic method, SkyTEM (Sørensen & Auken 2004), which is one of the new methods that has been developed to improve and optimise groundwater mapping. The first Sky- TEM groundwater mapping project was carried out in 2003. Since then the SkyTEM method has been developed further and has proved faster and more powerful than the ground- based, single-site transient electromagnetic method, TEM, which was previously widely used. The SkyTEM method is used for mapping to a maximum depth of 250–300 m. Nu - me rous buried valleys have been mapped in Denmark by the TEM method, in particular in the central parts of the coun- try, where highly impermeable and low-resistive Palaeogene clay layers form the lower boundaries of the aquifers and the valleys are easily detected. At the end of 2008, TEM and SkyTEM data cover an area of more than 11 000 km2 (Fig. 1A), which is about one quarter of the area of Denmark. Electrical methods are used for near-surface mapping pur- poses. The pulled array continuous electrical sounding meth - od (PACES; Sørensen 1996) has been extensively used to map layers within the upper 20–30 m. This method works well in combination with TEM measurements, and the com- bined methods provide data from the surface down to 200–300 m. This combination of methods has mainly been used in eastern Jylland and on Fyn. A total of around 9000 effective line kilometres of PACES data have been collected, corresponding to a coverage of more than 3000 km2 (Fig. 1B). The continuous vertical electrical sounding method (CVES; e.g. Dahlin 1996) is used in areas where it is unne - cessary to map deeper layers, and where the subsurface resis- tivity values are too high for the SkyTEM method. About 4000 line kilometres have been collected, mainly in the cen- tral part of Jylland and on the eastern part of Sjælland (Fig. 1C). The reflection seismic method is also of great value as a geophysical groundwater mapping tool, particularly follow- ing the development of a land-streamer and a new vibroseis- mic system (e.g. Vangkilde-Pedersen et al. 2006). Although the reflection seismic method is expensive, it can be success- fully combined with SkyTEM measurements, and the deci- sion about where to acquire seismic data can be based on the SkyTEM results (Jørgensen et al. 2003). Successful mapping of the outline of buried valleys and their internal structures has been based on the interpretations of seismic profiles; SkyTEM data do not allow such interpretations. The reflec- tion seismic method has also been used successfully to map Palaeogene and Neogene sediments in the western part of Denmark (Rasmussen et al. 2007), where thick and extensive layers of sandy deposits constituting important aquifers are bounded by thinner layers of clayey deposits, and to map faults in Danian and Cretaceous limestone in the eastern part of Denmark. Around 1400 km of seismic lines have been col- lected, particularly in the western and central parts of Jylland (Fig. 1D). Borehole logs are crucial for the geological and hydrologi- cal interpretation of boreholes. It is now common practise to log boreholes following survey drilling, and older water sup- ply wells have also been logged. Particularly in areas with chalk and limestone or Neogene groundwater reservoirs log stratigraphy has provided valuable information. About 1500 boreholes have been logged. Administration of the geophysical data The Groundwater Mapping Programme is split up into many smaller areas to ease the administrative handling and to be able to meet priority criteria. Careful and standardised treat- ment of data is required to ensure that the resulting ‘patch- work’ is of high and uniform quality and has no visible seams. Therefore, standards and guidelines are worked out for geo- physical data acquisition, calibration of instruments, data pro- cessing, interpretation (e.g. HydroGeophysics Group 2007a) and geological modelling (Jørgensen et al. 2008). Without a predefined system of archiving the geophysical data and modelling results, the data logistics of the ground- water mapping programme would be overwhelming. The national GEophysical Relation DAtabase (GERDA; http:// gerda.geus.dk) hosted at the Geological Survey of Denmark and Greenland (GEUS), is used for archiving these geophys- ical data. The development of the database began more than ten years ago. The database contains geophysical data of var- ious types such as Wenner profiles, Schlumberger soundings, pulled array continuous electrical soundings, continuous ver- tical electrical soundings and induced polarisation, transient electromagnetic data including the airborne SkyTEM data, 42 ROSA_2008:ROSA-2008 01/07/09 15:48 Side 42 frequency domain electromagnetic data, reflection seismic profiles and borehole logs. Various kinds of 1-D models and 2-D models resulting from inversion of electrical and elec- tromagnetic data are also saved, securing an immediate use of the results. All information about data acquisition, data pro- cessing and inversion can be stored, which facilitates repro- cessing of data and makes the inversion and interpretation of data transparent. GEUS also hosts another database (Jupiter; http://jupiter. geus.dk) for borehole data. Jupiter contains information on, for example, geological and lithological descriptions, ground- water level and water quality observations. Both the Jupiter and GERDA databases have web-based graphical user inter- faces, where any user can search for and download data free of charge. Geophysical data are handled from data processing to ge - ological interpretation in an integrated system formed by the GERDA and the Jupiter databases and two software pack- ages, the Aarhus Workbench and the Geoscene3D in combi- nation with a geological model database hosted at GEUS (Fig. 2; Møller et al. in press). The Aarhus Workbench (Hydro Geophysics Group 2007b) has modules for handling, processing, inverting, interpreting and visualising electrical and electromagnetic data, all combined on a common GIS platform and a common database. The Aarhus Workbench enables anybody to work with the geophysical data in the GERDA database without having to know the complicated data model of GERDA or to be able to carry out a database query. By using the GIS platform at the Aarhus Workbench it is easy to produce various types of maps compiled from the geophysical data. The different maps are entered into the 3-D visualisation and modelling tool Geoscene3D (I-GIS, http://www.i-gis.dk) together with all the geophysical data stored in GERDA and the borehole information stored in Jupiter, and the geophys- ical data are ready to be used in the geological modelling process carried out in Geoscene3D. An example of the strength of the integrated data handling system is illustrated for a 50 × 60 km2 area in eastern Jylland (Fig. 3). Large parts of this area are covered by TEM sound- ings (c. 83 000 soundings), collected during more than 90 mapping campaigns (Fig. 3B) and with five different TEM methods (Fig. 3C) over a time span of more than ten years. Figure 3A shows a map of the surface of the deepest low-resis- tive model layer based on interpretation of all the TEM soundings in the area. The deepest low-resistive model layer represents Palaeogene clay deposits except in the north-east- ern corner, where it represents salty pore water in Danian limestone. The most prominent features found in the area are a large number of buried valleys incised into the Palaeogene clay deposits. The buried valleys show no direct correlation to the overall topography. Even though the data have been acquired by different companies, with different instruments and methods, and at different times, the data can be com- bined without showing any discrepancies at survey borders. Concluding remarks Geophysical measurements play an important role in the National Groundwater Mapping Programme and have con- tributed significantly to the mapping of aquifers in Den - mark. In heterogeneous regions the data density needs to be high in order to provide acceptable mapping results. Geo - physical methods like TEM/SkyTEM and electrical methods can provide sufficient data density and reflection seismic pro- files can resolve internal structures in specific areas in combi- nation with detailed borehole information such as litho- logical descriptions, geophysical logs, data on water chem- 43 GERDA JUPITERAarhus Workbench Data processing Raw electrical and electromagnetic data Inversion Preparation of data to GERDA 3-D visualisation Data quality control Geological modelling Geological models Hydrostratigraphical models Groundwater models Borehole information Location Lithology Waterlevel Water chemistry Wenner Seismic data, borehole logs CVES Schlumberger IP TEM PACES SkyTEM EM3X HEM Seismics Borehole logs 1-D models 2-D models Reprocessing Re-interpretation Visualisation on maps, profiles Advanced data analysis Advanced data interpretation Geoscene 3D Modeldb Hydrogeological data Fig. 2. Sketch of the integrated system of databases and program pack- ages handling geophysical data and geological modelling. The arrows show the flow of data between the geophysical database GERDA, the borehole database Jupiter, the Aarhus Workbench program package, the Geoscene3D visualisation and modelling tool and the geological model database Modeldb. ROSA_2008:ROSA-2008 01/07/09 15:48 Side 43 44 istry and hydraulic parameters. These data form the basis for detailed hydrogeological models. An integrated data hand - ling system makes it possible to merge geophysical data acquired over long periods by different companies with dif- ferent instruments. This is of great value for future mapping and administrative purposes. References Dahlin, T. 1996: 2D resistivity surveying for environmental and engineer- ing applications. First Break 14, 275–283. HydroGeophysics Group 2007a: Guide to processing and inversion of SkyTEM data. Version 1.2. Århus: Department of Earth Sciences, Uni ver - sity of Aarhus. http://www.hgg.geo.au.dk/HGGSoftware/work-bench/ Workbench_SkyTEM.pdf. HydroGeophysics Group 2007b: Aarhus Workbench A-Z Reference, Ver - sion 2.2. Århus: Department of Earth Sciences, University of Aarhus. http://www.hgg.geo.au.dk/HGGSoftware/workbench/Workbench_A- Z_reference.pdf. Jørgensen, F., Lykke-Andersen, H., Sandersen, P.B.E., Auken, E. & Nør - mark, E. 2003: Geophysical investigations of buried Quaternary valleys in Denmark: an integrated application of transient electromagnetic soundings, reflection seismic surveys and exploratory drillings. Journal of Applied Geophysics 53, 215–228. Jørgensen, F., Kristensen, M., Højberg A.L., Klint, K.E.S, Hansen, C., Jordt, B.E., Richardt, N & Sandersen, P. 2008: Opstilling af geologiske modeller til grundvandsmodellering. Geo-Vejledning 3, 176 pp. Copen hagen: Geological Survey of Denmark and Greenland. Møller, I., Søndergaard, V.H., Jørgensen, F., Auken, E. & Christiansen, A.V. in press: Integrated management and utilisation of hydrogeophysical data on a national scale. Near Surface Geophysics. Rasmussen, E.S., Vangkilde-Pedersen, T. & Scharling, P. 2007: Prediction of reservoir sand in Miocene deltaic deposits in Denmark based on high- resolution seismic data. Geological Survey of Denmark and Greenland Bulletin 13, 17–20. Sørensen, K. 1996: Pulled array continuous electrical profiling. First Break 14, 85–90. Sørensen, K.I. & Auken E. 2004: SkyTEM – A new high-resolution heli- copter transient electromagnetic system. Exploration Geophysics 35, 191–199. Sørensen, K.I., Auken, E., Christensen, N.B. & Pellerin L. 2005: An integrated approach for hydrogeophysical investigations. New tech- nologies and a case history. In: Butler, D. (ed.): Near-surface Geo - physics Part II, SEG Investigations in Geophysics Series 13, 585–603. Tulsa: Society of Exploration Geophysicists. Thomsen, R., Søndergaard, V.H & Sørensen, K.I. 2004: Hydrogeologi - cal mapping as a basis for establishing site-specific groundwater pro- tection zones in Denmark. Hydrogeology Journal 12, 550–562. Vangkilde-Pedersen, T., Dahl, J. F. & Ringgaard, J. 2006: Five years of ex - perience with landstreamer vibroseis and comparison with conven- tional seismic data acquisition. Proceedings of the 19th Annual SAGEEP Sym posium on the Application of Geophysics to Engineering and En - vironmental Problems, Seattle, USA, 2–6 April, 2006. (Published on CD- ROM, 1086–1093). Authors’ address Geological Survey of Denmark and Greenland, Lyseng Allé 1, DK-8270 Højbjerg, Denmark. E-mail: ilm@geus.dk Fig. 3. Data coverage and results from an area in eastern Jylland. For location see Fig. 1. A: Map showing the elevation of the surface of the deepest low-resistive layer in the area relative to sea level. B: The data come from 94 different mapping projects (shown by different colours). C: Five diffe rent TEM methods were used to produce map A. Århus 10 km –200 –100 500 TEM 40 HMTEM 1 HMTEM 2 PATEM SKYTEM CA B Elevation (m) ROSA_2008:ROSA-2008 01/07/09 15:48 Side 44