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Large-scale Geoelectrical Measurements to Investigate a Buried Valley and its Interaction to Deep Salt water Intrusion

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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

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Presentation Transcript
slide1

Large-scale Geoelectrical Measurements

to Investigate a Buried Valley and its

Interaction to Deep Salt water Intrusion

Andreas Junge2, Jörn Schünemann1 and Thomas Günther1

1Leibniz Institute for Applied Geosciences, Hannover, Germany

2Institute for Geosciences, University of Frankfurt, Germany

20th Salt Water Intrusion Meeting

June 23-27, 2008

Naples, Florida, USA

slide2

Outline

  • 1. Introduction
  • Geology
  • Previous Measurements
  • 2. Processing
  • 3. Results
  • 4. Conclusions
slide3

Geology

  • valley incised into Tertiary
  • not visible at surface
  • Quaternary filling: gravel, sand, clay
  • Lauenburg Clay between 50-70 m depth
  • important for groundwater supply
  • depth up to 400 m, width 1-2 km, length approx. 40 km
slide4

Previous Measurements

  • 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

Siemon et al., 2001, Identification of salt water Intrusions and Coastal Aquifers Using the BGR Helicopter-borne Geophysical System, SWICA, Morocco

slide5

Previous Measurements

BurVal Working Group: Kirsch et al., 2006, Groundwater resources in buried valleys - a challenge for geosciences

  • 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
slide6

Measuring area

  • 20 receiver stations (red) distances between 500 and 1000 m
  • 10 transmitter stations (black - E1-E10)
  • area of 6 km2
  • red lines mark buried valley
slide7

Station layout and current injection

  • 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
slide8

Signalprocessing

  • removal of anthropogenic and long-periodic noise
slide9

Inversion

  • inversion with FD code DC3dInvRes (Günther, 2004)
  • homogeneous model, 8 layers: 0-20m, …, 600-800 m
  • individual weighting of data by errors
slide10

1st layer: medium r, 2nd-5th: higher r, 6-8th: medium-low r

  • layers 5-6: low resistivity on right hand side
slide11

clay makes valley visible

  • clear differentiation between salt water and valley
slide12

3D subsurface model with electrode positions

  • dimensions: 4000 x 3000 x 800 m
slide14

clay layer

sand

Base of Quaternary

Tertiary clay and salt water

salt water

  • able to identify clay layer
  • right hand side: high salt water level
  • sand with freshwater on left hand side
  • transition zone under buried valley
slide15

clay layer

sand

salt water

slide16

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