1 Large-scale Geoelectrical Measurements to Investigate a Buried Valley and its Interaction to Deep Salt water Intrusion Andreas Junge 2, Jörn Schünemann 1 and Thomas Günther 1 1 Leibniz Institute for Applied Geosciences, Hannover, Germany 2 Institute for Geosciences, University of Frankfurt, Germany 20th Salt Water Intrusion Meeting June 23-27, 2008 Naples, Florida, USA

2 Outline 1. Introduction Geology Previous Measurements 2. Processing 3. Results 4. Conclusions

3 valley incised into Tertiary not visible at surface Quaternary filling: gravel, sand, clay Lauenburg Clay between m depth important for groundwater supply depth up to 400 m, width 1-2 km, length approx. 40 km Geology

4 Siemon et al., 2001, Identification of salt water Intrusions and Coastal Aquifers Using the BGR Helicopter-borne Geophysical System, SWICA, Morocco Airborne Electromagnetic Measurements (AEM) apparent resistivity map of the frequency 1830 Hz max. depth of 150 m salt water intrusions near the coast at shallow depth glacial valley visible due to clay Geest shows high resistivity Previous Measurements

5 seismics sees boundary of valley AEM (full image) and Skytem (columns) results depth limited due to clay layer => DC measurements complete electric image of subsurface Previous Measurements BurVal Working Group: Kirsch et al., 2006, Groundwater resources in buried valleys - a challenge for geosciences

6 20 receiver stations (red) distances between 500 and 1000 m 10 transmitter stations (black - E1-E10) area of 6 km 2 red lines mark buried valley Measuring area

7 central electrode + 3 directions 75 m dipole length 3 channel MT transient recorder Geolore, sampling rate 8 Hz injected current 1 to 40 A square-wave signal of 9 s period injection time: 20 min Station layout and current injection

8 Signal processing removal of anthropogenic and long-periodic noise

9 Resistance determination Methods stack signals and measure plateaus (time domain) determine FFT amplitudes of current and voltage signal correlation concept (after Kniess, 2002) => with and harmonic functions for :

10 Resistance determination

11 Inversion inversion with FD code DC3dInvRes (Günther, 2004) homogeneous model, 8 layers: 0-20m, …, m individual weighting of data by errors

12 1st layer: medium , 2nd-5th: higher , 6-8th: medium-low  layers 5-6: low resistivity on right hand side

13 clay makes valley visible clear differentiation between salt water and valley

14 3D subsurface model with electrode positions dimensions: 4000 x 3000 x 800 m

15 cut parallel to seismic profile => 2D section

16 able to identify clay layer right hand side: high salt water level sand with freshwater on left hand side transition zone under buried valley salt water clay layer Base of Quaternary sand Tertiary clay and salt water

17 clay layer salt water sand

18 Conclusions 3D resistivity image by large-scale DC dipole-dipole experiment gap between seismics and EM closed clear differentiation between valley and salt water probably limited infiltration of salt water into valley different hydraulic conductivities prevent accumulation of salt water in the valley