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
1
Outline
1. Introduction
• Geology
• Previous Measurements
2. Processing
3. Results
4. Conclusions
2
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
3
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
4
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
5
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
6
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
7
Signal processing
• removal of anthropogenic and long-periodic noise
8
Inversion
• inversion with FD code DC3dInvRes (Günther, 2004)
• homogeneous model, 8 layers: 0-20m, …, 600-800 m
• individual weighting of data by errors
11
• 1st layer: medium r, 2nd-5th: higher r, 6-8th: medium-low r
• layers 5-6: low resistivity on right hand side
12
• clay makes valley visible
• clear differentiation between salt water and valley
13
• 3D subsurface model with electrode positions
• dimensions: 4000 x 3000 x 800 m
14
• cut parallel to seismic profile => 2D section
15
clay layer
sand
Tertiary clay and salt water
Base of Quaternary
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
16
clay layer
sand
salt water
17
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
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