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

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 1000 m N Beach Dunes/dike Canal VMEP profile Lagoon

Instrumentation NW SE Sand Silty Finesand Clay E4 E3 E2 E5 E1 8 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]

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) AIR 1, b1, d1 VMEP probe 2, b2, d2 . Model layers . n-1, bn-1, dn-1 n [SELMA – research software, Niels B. Christensen]

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

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

F = 1.6 – 7.5, r = 2 – 6

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.