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Near Surface 2005 — Palermo, Italy, 4 - 7 September 2005

Introduction

Buried valleys are deep glacial erosion structures with depth down to more than 500 m and length of up to 100 km. They occur in southern Scandinavia, Poland and the Baltic States, Northern Germany, the Netherlands and the eastern part of the UK. The increasing impor-tance of the buried valleys for the water supply makes systematical hydrogeological and geo-physical investigations necessary to enable a water management in a sustainable and balanced way. Basically buried valleys influence – depending on their sedimentary filling – the groundwater conditions in two ways: as a groundwater reservoir when their filling is of sandy material (Fig. 1, case 1) or as a barrier for water flow when they are filled with low permeable clayey material (Fig. 1, case 2). In practice, both fills can occur in the same valley. Moreover, as the valleys are in hydraulic contact with their surrounding, they can provide hydraulic con-nection between different groundwater layers.

The BurVal project

Comparison and further development of geophysical methods for the investigation of buried valleys is the aim of the project BurVal (http://www.burval.org). Within this project geologists and geophysicists from water branches of public authorities and research institutes of three countries get together to systematically investigate buried valley groundwater reserves for quantity, quality and vulnerability. For this six pilot project areas are defined in the Netherlands, Germany and Denmark. The activities include geophysical exploration e.g. with airborne electromagnetic and magnetic methods, seismic reflection surveying, VSP (vertical seismic profiling), VES (vertical electrical sounding), TEM (ground based and airborne), gravity, georadar, PACES (pulled array continuous electrical sounding) and well logging as well as sedimentological exploration. The data will be given into the national geophysical databases as GERDA (http://gerda.geus.dk) or FIS Geophysik (https://www.gga-hannover.de/app/fis_gp/startseite/start.htm). The detailed results, findings and recommen-dations on geophysical and other investigation methods, on geological and hydrogeological properties of buried valleys will be compiled in a technical handbook, as well as recommen-dations on how to carry out mapping and protection of groundwater resources in connection with buried valleys are compiled in a planning handbook. BurVal started in January 2004 and ends December 2006.

Examples from pilot areas

The base of Quaternary deposits (Fig. 2) after Stackebrandt et al. (2001) delineates well the system of buried in Northern Germany and Denmark. The six pilot project areas where the survey activities take place are outlined in Fig. 2. Three pilot areas are located in Denmark, two in Germany, and one in the Netherlands. In the following, new results from the german pilot areas Cuxhavener Rinne and Ellerbeker Rinne are presented.

P035 Buried valleys in the North Sea region: New investigations in the project BurVal

HELGA WIEDERHOLD1, HANNA-MARIA RUMPEL1 AND THE BURVAL WORKING GROUP2

1Leibniz Institute for Applied Geosciences (GGA-Institut), Stilleweg 2,D- 30655 Hannoverh.wiederhold@gga-hannover.de

2Members of the group come from: TNO-NITG (Utrecht, NL), BGR (Hannover, D), BSU (Hamburg, D), LANU (Flintbek, D), Vejle Amt (Vejle, DK), Ringkjøbing Amt (Ringkøbing, DK), Sønderjyllands Amt (Tønder, DK), GEUS

(Kopenhagen, DK), University of Aarhus (Aarhus, DK)

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NEOGENE QUATERNARY

m m.s.l. m m.s.l.

Quaternary

boulder clay

clay

coarse, medium,

fine sand

Tertiary

Upper Glimmerton

Hamburg Clay (HT)

coarse, medium,

fine sand

clay, gypsum, salt

salt pillow

Lower Glimmerton (UGT)

lignite

Base Pleistocene

Pleistocene valleys

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1

Salt structure

Cuxhavener Rinne area

In the area between the cities of Bremerhaven and Cuxhaven the conditions are good to study buried valley aquifers in coastal aquifer environment. The tidal flat coast and the estuaries of the Elbe and Weser rivers define a coastal aquifer test field (CAT-Field) established by the GGA-Institut and the Geological Survey of Lower Saxony (NLfB). The resistivity map from an airborne electromagnetic survey (Siemon et al. 2001a, Siemon et al. 2001b, Gabriel et al. 2003) gives an impressing view of two valleys (Fig. 2): The Cuxhavener Rinne runs NNE to SSW and is evident by a good conductive clayey layer (Lauenburger Ton) for about 15 km. In the NNE part a near-surface clay layer not restricted to the valley is superposed, in the SSW part the clay layer seems to vanish but boreholes and new seismic results show that the valley continues. From seismic measurements the depth and the bedding of the valley in the envi-ronment is identified. Gravity and resistivity measurements complement the surveys very well (Fig. 3). Results from a SkyTEM survey (Sørensen & Auken 2004) conducted February 2005 are in process.

Ellerbeker Rinne area

The Ellerbeker Rinne in southern Schleswig-Holstein stretches from the small town Barmstedt in south-east direction at a length of about 25 km to the downtown area of Ham-burg. The water reservoir of this valley is used by the Hamburger Wasserwerke and the Pinneberger Wasserwerke. So a number of observation and production wells have been drilled in this area. As we know from the drillings, the composition of the aquifer in the valley shows distinct lateral variations from medium to coarse sands in the southern part to fine sand to silt in the northern part. That means that the yield of the aquifer is high in the south and low in the north. The aquifer is covered with mica clay (Lauenburger Ton). In the area of Tangstedt the Ellerbeker Rinne is crossed by another, probably younger, valley. In this area the covering mica clay is replaced by boulder till. Seismic reflection and gravity surveys combined with borehole logging display the valley well. An airborne electromagnetic survey is projected.

Conclusions

Airborne electromagnetic methods have turned out to be a good tool for mapping a buried valley that is covered by a good conductive clayey layer (Lauenburger Ton) and embedded in less conductive sandy surrounding (Cuxhavener Rinne). A detailed picture of the internal structure can be given by reflection seismic in combination with boreholes and logging results incl. vertical seismic profiling (VSP). With the BurVal project a systematic investigation of different buried valley test sites is in process.

Fig. 1: Schematic geological cross section with characteristic structures from Northern Ger-many. For 1 and 2 see text.

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Near Surface 2005 — Palermo, Italy, 4 - 7 September 2005

B C

D

E

A

F

B

Fig. 2: Base of Quaternary deposits after Stackebrandt et al. (2001). Pilot study areas of the BurVal project are outlined: A Groningen valley, B Cuxhavener Rinne, C Ellerbeker Rinne, D Rødekro-Aabenraa valley, E Tyrsting valley, F Bording valley. The detail map of the Cuxhavener Rinne B shows the resistivity map from an airborne electromagnetic survey (from Siemon et al. 2001a). The Cuxhavener valley is identified by the light color running NNE to SSW in the eastern part belonging to resistivity values less than 20 Ohmm.

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Fig. 3: Geophysics across the Cuxhavener Rinne: a) resistivity cross section based on airborne electro-magnetic survey, b) seismic section c) gravity anomaly and model.

Acknowledgment

The BurVal project is part financed by the European Union in the framework of the INTERREG IIIB North Sea Region programme.

References

[1] Gabriel, G., Kirsch, R., Siemon, B. & Wiederhold, H. (2003): Geophysical investigation of buried Pleistocene subglacial valleys in Northern Germany. Journal of Applied Geo-physics, 53:159-180.

[2] Siemon, B., Voß, W., Röttger, B., Rehli, H.-J. & Pielawa, J. (2001a): Forschungs-vorhaben "Detaillierte aerogeophysikalische Landesaufnahme" (DAGLA). Messgebiet Cuxhaven, Mai 2000. - Technischer Bericht, BGR, Archiv-Nr. 121 236; Hannover.

[3] Siemon, B., Röttger, B. & Eberle, D. (2001b): Airborne geophysical investigation of saltwater intrusions and coastal aquifers in NW Germany. 7th Meeting Environmental and Engineering Geophysics, Proceedings: 228-229; Birmingham, UK.

[4] Sørensen, K.I. & Auken, E. (2004): SkyTEM – a new high-resolution helicopter tran-sient electromagnetic system. Exploration Geophysics, 35: 191-199.

[5] Stackebrandt, W., Ludwig, A.O. & Ostaficzuk, S. (2001): Base of Quaternary deposits of the Baltic Sea depression and adjacent areas. Brandenburgische Geowiss. Beitr., 1/2001:13-19.