Azores, Portugal SWIM21 - 21st Salt Water Intrusion Meeting June 21 - 26, 2010

Coastal aquifers and saltwater intrusions in focus of airborne electromagnetic surveys in northern Germany

Helga Wiederhold1, Bernhard Siemon2, Annika Steuer2, Gerlinde Schaumann1, Uwe Meyer2, Franz Binot1, Klaus Kühne1

1 Leibniz Institute for Applied Geophysics (LIAG), Hannover, Germany 2 Federal Institute for Geosciences and Natural Resources (BGR), Hannover, Germany

ABSTRACT The German North Sea coastal area is part of the Wadden Sea and comprises beside the mainland coast the East Frisian Islands (barrier islands), the North Frisian Islands (moraine ridge islands) as well as the estuaries of the rivers Ems, Jade, Weser and Elbe. Growing awareness of the critical role of freshwater‐saltwater environments on the groundwater supply for coastal residents and tourism as well as on economies and ecosystems in that area led to the conception of an airborne geophysical survey covering the Wadden coastal area. This survey was operated in 2008 and 2009 in cooperation of Leibniz Institute for Applied Geophysics (LIAG) and the Federal Institute for Geosciences and Natural Resources (BGR). Emphasis was placed on the mapping of freshwater‐saltwater interfaces on the North Sea islands (e.g., Borkum, Langeoog and Föhr) and in the coastal aquifers (e.g., Elbe estuary). Furthermore submarine freshwater occurrences were mapped (e.g., Wadden Sea) which are important for the interaction between groundwater and seawater. An additional target of the project was the occurrence of saline groundwater due to salt structures (e.g., salt dome Segeberg). The target areas of the survey (Figure 1) were selected in cooperation with the State Geological Surveys LBEG (Niedersachsen), LLUR (Schleswig‐Holstein) and BSU (Hamburg).

Figure 1. Areas surveyed by airborne electromagnetic methods in Northern Germany. Light black frame: surveyed by BGR before 2008; black frame: surveyed by BGR in 2008 and 2009 (1: Borkum, 2: Langeoog, 3: Esens, 4: Glückstadt, 5: Jever, 6: Nordenham); grey frame: surveyed by SKYTEM ApS in 2008 and 2009 (7: Leck, 8: Föhr, 9: Garding, 10: Bad Segeberg, 11: Niedersachsen).

94 Azores, Portugal SWIM21 - 21st Salt Water Intrusion Meeting June 21 - 26, 2010

METHODS Airborne electromagnetics is widely applied in hydrogeological investigations because the measurements respond to both lithologic and water‐chemistry variations (e.g., Siemon et al. 2009, Steuer et al. 2009a). Two different systems of airborne electromagnetics were used in this survey: the frequency‐domain system RESOLVE (Fugro Airborne surveys) operated by BGR (Siemon 2009); and the time‐domain system SkyTEM, developed at the University of Arhus (Sørensen and Auken 2004, Christiansen et al. 2009) and operated by SKYTEM ApS. Both deliver the distribution of resistivity below the flight lines. By interpolation of the 1D inversion results 3D data volumes are gathered.

RESULTS In the following some results are shown in form of average resistivity maps and vertical resis‐ tivity cross sections (resistivity ρ [Ωm]); see also Steuer et al. (2009b), Wiederhold et al. (2009).

North Sea island Borkum (Figure 2)

Figure 2. Average resistivity maps at 5 and 40 m bsl and vertical resistivity sections. The freshwater/saltwater distribution reflects the contours of the island of Borkum. The freshwater/saltwater boundary was detected down to about 60 m depth.

North Sea island Langeoog and Wadden Sea (Figure 3)

Figure 3. Maps showing the apparent resistivity ρa [Ωm]. The different frequencies mean different penetration depths. 41520 Hz: the arrows point to the freshwater lenses of the islands Langeoog and Spiekeroog; 8390 Hz: the arrows point to tidal creeks in the Wadden Sea; 387 Hz: the arrow points to freshwater discharges to the Wadden Sea.

95 Azores, Portugal SWIM21 - 21st Salt Water Intrusion Meeting June 21 - 26, 2010

North Sea island Föhr (Figure 4)

section section

5 km 110100Om

Figure 4. Average resistivity map for the depth interval 30‐40 m bsl and two vertical resistivity sections for the Island Föhr. The arrows point to the freshwater lenses. In the northern part of the island saltwater bearing sediments are dominant. In the western and southern coastal region low resistive spots point to salt water intrusions from the North Sea.

Saltdome Bad Segeberg (Figure 5)

110100Om1000

saltdome Segeberg section 4

saltdome Segeberg saltdome Segeberg se c s tio ec n section 3 tion 4 3

Figure 5. Average resistivity for the depth interval 100‐80 m bsl and two vertical resistivity sections. The arrows point to potential saltwater intrusions (<10Ωm) in the surrounding of the saltdome Segeberg. The dashed arrows points to the axis of the structural lineament. The salt rock body is clearly indicated by high resistivities (>300Ωm). Low resistivities on top and southwestern flank are interpreted as Tertiary clays.

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DISCUSSION AND CONCLUSIONS In 2008 and 2009, 13 400 line kilometer using the RESOLVE system and 4 500 line kilometer using the SkyTEM system were flown at the German North Sea coast. Some of these data are used in other actual projects, e.g., CLIWAT (EU Interreg IVB North Sea Region; Auken et al. 2010) or KLIMZUG‐NORD (German research fund of BMBF) where the 3D resistivity distribution supports groundwater modelling. A powerful web‐interface (www.geophysics‐database.de) will provide the data for the scientific and engineering community. In addition to geological and geophysical information, changes of surface and subsurface conditions in time and space may be documented by repeated surveys in future.

REFERENCES Auken, E. et al. (2010): CLIWAT: a transnational project about climate change and coastal groundwater in the North Sea Region. 21st Salt Water Intrusion Meeting 2010, Azores, Portugal.

Christiansen, A.V., Auken, E. and Sørensen, K. 2009. The transient electromagnetic method. In Groundwater Geophysics ‐ A tool for Hydrogeology, 2nd Edition, ed R. Kirsch, 179‐225. Heidelberg: Springer.

Siemon, B. 2009. Electromagnetic methods – frequency domain: Airborne techniques. In Groundwater Geophysics ‐ A tool for Hydrogeology, 2nd Edition, ed R. Kirsch, 155‐170. Heidelberg: Springer.

Siemon, B., Christiansen, A.V. and Auken, E. 2009. A review of helicopter‐borne electromagnetic methods for groundwater exploration. Near Surface Geophysics, 7: 629‐646.

Steuer, A., Siemon, B and Auken, E. 2009a. A comparison of helicopter‐borne electromagnetics in frequency‐ and time‐domain at the Cuxhaven valley in Northern Germany. Journal of Applied Geophysics, 67: 194‐205. doi:10.1016/j.jappgeo.2007.07.001.

Steuer, A., Siemon, B. and Grinat, M. 2009b. Helicopter‐borne Electromagnetics and Geoelectrics to Investigate the Fresh‐water Lenses of the North Sea Island Borkum. ‐ Ext. abstract, EAGE Near Surface 2009, 07.‐09.09.2009; Dublin, Ireland.

Sørensen, K. I. and Auken, E. 2004. SkyTEM ‐ A new high‐resolution helicopter transient electromagnetic system. Exploration Geophysics 35: 191‐199.

Wiederhold, H., Schaumann, G. and Steuer, 2009. An Airborne geophysical investigations for hydrogeological purposes in Northern Germany. ‐ Ext. abstract, EAGE Near Surface 2009, 07.‐09.09.2009; Dublin, Ireland.

Contact Information: Helga Wiederhold, Leibniz Institute for Applied Geophysics (LIAG), Stilleweg 2, 30655 Hannover, Germany, Phone: +49‐(0)511‐6433520, Fax: +49‐(0)511‐6433665, Email: helga.wiederhold@liag‐hannover.de

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