An Investigation of Groundwater Flow on a

Coastal Barrier using Multi Electrode Profiling

Søren E. PoulsenSteen ChristensenKeld R. Rasmussen

0102030405060708090100

1.11.21.31.41.51.61.71.81.92.0

VMEP profile [m]

Hyd

raul

ic h

ead

DN

N [m

]

Hydraulic head during autumn 2007/winter 2008

04-10-200715-11-200707-12-200701-02-200802-02-2008

East

01020304050607080

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

VMEP profile [m]

Hyd

raul

ic h

ead

DN

N [m

]

Vertical variation hydraulic head during a winterstorm primo Februrary 2008

01-02-2008: -1.8 m01-02-2008: -4.8 m02-02-2008: -1.8 m01-02-2008: -4.8 m

Aarhus Universitets Forskningsfond

Outline

Purpose

Field site & Instrumentation

Method– Data acquisition & Processing– Interpretation

Results & Conclusions– Example 1: March 2008, winter scenario– Example 2: June 2008, summer scenario

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

Fieldsite

Lagoon

Dunes/dikeBeach 1000 m

N

VMEP profile

Canal

Instrumentation

155 60 85 110 135

SE

0 2

8

-4-2

E1

-9

E2E3E4E5

SandSilty FinesandClay

Expected salt/fresh water interface

Water table

NW

Met

ers

abov

e m

.s.l.

[m

]

Approximate distance from the coastline [m]

Instrumentation• 32 or 28 electrodes

on each probe

• Electrode spacing 0.25 m

• Probes are connected to standard MEP equipment

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

Interpretation

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

[SELMA – research software, Niels B. Christensen]

VMEP probe 1, b1, d1

2, b2, d2

n

.

.

AIR

n-1, bn-1, dn-1

Model layers

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]

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

Water sample salinity March 2008 [ppm]

Elev

atio

n ab

ove

m.s

.l. [m

]

0102030405060708090100-5

-4

-3

-2

-1

0

Water sample salinity June 2008 [ppm]

Elev

atio

n ab

ove

m.s

.l. [m

]

Profile [m]

0102030405060708090100-5

-4

-3

-2

-1

0

0

5000

10000

15000

20000

25000

30000

35000SENW

(+) = water sample point

E5 E3E4 E1E2

10-1 100 101 102 103-6

-5

-4

-3

-2

-1

0

1

2

3

4

5E1 March 2008

LooseTightLoose 10*STDTight 10*STDWater tableWater sample

10-1 100 101 102 103-6

-5

-4

-3

-2

-1

0

1

2

3

4

5E2 March 2008

10-1 100 101 102 103-6

-5

-4

-3

-2

-1

0

1

2

3

4

5E3 March 2008

10-1 100 101 102 103-6

-5

-4

-3

-2

-1

0

1

2

3

4

5E4 March 2008

10-1 100 101 102 103-6

-5

-4

-3

-2

-1

0

1

2

3

4

5

f [

m]

Elev

atio

n m

.s.l.

[m]

E5 March 2008

F = 1.6 – 7.5, r = 2 – 6

10-1 100 101 102 103-6

-5

-4

-3

-2

-1

0

1

2

3

4

5E1 June 2008

LooseTightLoose 10*STDTight 10*STDWater tableWater sample

10-1 100 101 102 103-6

-5

-4

-3

-2

-1

0

1

2

3

4

5E2 June 2008

10-1 100 101 102 103-6

-5

-4

-3

-2

-1

0

1

2

3

4

5E3 June 2008

10-1 100 101 102 103-6

-5

-4

-3

-2

-1

0

1

2

3

4

5E4 June 2008

10-1 100 101 102 103-6

-5

-4

-3

-2

-1

0

1

2

3

4

5

f [

m]

Elev

atio

n m

.s.l.

[m]

E5 June 2008

F = 2 – 3.5, r = 1 - 2

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.