An Investigation of Groundwater Flow on a Coastal Barrier using Multi Electrode Profiling

1
An Investigation of Groundwater Flow on a Coastal
Barrier using Multi Electrode Profiling

• Søren E. Poulsen
• Steen Christensen
• Keld R. Rasmussen

Aarhus Universitets Forskningsfond
2
Outline

• Purpose
• Field site Instrumentation
• Method
• Data acquisition Processing
• Interpretation
• Results Conclusions
• Example 1 March 2008, winter scenario
• Example 2 June 2008, summer scenario

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Purpose

• Obtain detailed information about the resistivity
  and salinity distribution of a shallow coastal
  aquifer by means of vertical multi electrode
  profiling (VMEP)
• Validate modeled formation resistivities by
  calculating formation factors and comparing these
  with expected values

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Fieldsite
1000 m
N
Beach
Dunes/dike
Canal
Lagoon
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Instrumentation
SE
NW
8
E2
E3
E4
E1
E5
Water table
2
0
-2
Meters above m.s.l. m
-4
Expected salt/fresh water interface
-9
155
110
135
60
85
Approximate distance from the coastline m
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Instrumentation

• 32 or 28 electrodes on each probe
• Electrode spacing 0.25 m
• Probes are connected to standard MEP equipment

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Data aquisition Processing

• DC protocols
• GRADIENT, high vertical resolution, suited for
  mapping structures perpendicular to the probe
• DIPOL-DIPOL, large horizontal penetration depth
• Processing
• Identifying and exterminating outliers
• Assessing the amount of reliable data
• Evaluate protocols

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Interpretation

• Layered 1D model SELMA (Simultaneous
  Electromagnetic Layered Model Analysis)

AIR
?1, b1, d1
VMEP probe
?2, b2, d2
.
Model layers
.
?n-1, bn-1, dn-1
?n
SELMA research software, Niels B. Christensen
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Interpretation

• Inversion performed by an L2-norm broad-band
  covariance regularization
• 60 layer model, constant layer thickness
  electrode spacing 0.25 m
• 2 models per probe
• One more tightly bound to the reference model
  than the other

Serban D. Z. and Jacobsen B. H. 2001
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Results

• Example 1 March 2008, winter scenario
• Low potential evapotranspiration
• Following a period of steady freshwater
  infiltration
• Freshwater dominated
• Example 2 June 2008, summer scenario
• High potential evapotranspiration
• Low infiltration
• Brackish/saline dominated

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SE
NW
E5
E3
E4
E1
E2
() water sample point
12
F 1.6 7.5, r 2 6
13
F 2 3.5, r 1 - 2
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Conclusions

• High quality VMEP data can be acquired and
  inverted into reasonable, high resolution models
  of formation resistivity
• Formation factors are within a reasonable range
  especially for the well-determined June 2008
  models

• Acknowledgements Niels B. Christensen, Anders V.
  Christiansen, Jesper S. Mortensen Andrea
  Viezzoli. University of Aarhus.