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

3. 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

4. Fieldsite
1000 m N
Beach
Dunes/dike
Canal
VMEP profile
Lagoon

5. Instrumentation NW SE Sand Silty Finesand Clay E4 E3 E2 E5 E18 E4 E3 E2 E5 E1
Water table 2
Meters above m.s.l. [m] -2 -4
Expected salt/fresh water interface -9 60 85
110
135
155
Approximate distance from the coastline [m]

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

7. 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

8. 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]

9. 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]

10. 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

11. NW SE E5 E4 E3 E2 E1
(+) = water sample point

12. F = 1.6 – 7.5, r = 2 – 6

13. F = 2 – 3.5, r = 1 - 2

14. 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.