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SPECIAL SECTION: GROUND PENETRATING RADAR IN HYDROGEOPHYSICS

Time-Lapse GPR Tomography of Unsaturated Water Flow in an Ice-Contact Delta

^a Dep. of Geosciences, Univ. of Oslo, 1022 Blindern, 0315 Oslo, Norway
^b Schlumberger Cambridge Research, Madingley Rd., Cambridge CB3 0EL, UK
^c Bioforsk, Frederik A. Dahls vei 20, 1432 Ås, Norway
^* Corresponding author (m.b.farmani{at}geo.uio.no).

Received 6 September 2006.

Cross-well ground penetrating radar (GPR) data sets were collected in the vadose zone of an ice-contact delta near Oslo's Gardermoen Airport (Norway) before, during, and after snowmelt in 2005. The observed travel times were inverted using curved-ray travel time tomography. The tomograms are in good agreement with the local geologic structure of the delta. The tomographic results were confirmed independently by surface GPR reflection data and x-ray images of core samples. In addition to structure, the GPR tomograms also show a strong time dependency due to the snowmelt. The time-lapse tomograms were used to estimate volumetric soil water content using Topp's equation. The volumetric soil water content was also observed independently by using a neutron meter. Comparison of these two methods revealed a strong irregular wetting process during the snowmelt. This was interpreted to be due to soil heterogeneity as well as a heterogeneous infiltration rate. The geologic structure and water content estimates obtained from the GPR tomography can be used in forward and inverse flow modeling. Finally, the water balance in the vadose zone was calculated using snow accumulation data, precipitation data, porosity estimates, and observed changes in the groundwater table. The amount of water stored in the vadose zone obtained from the water balance is consistent with the amount estimated using GPR tomography. Alternatively, the change in water storage in the vadose zone can be estimated using GPR tomography. This may then permit estimates of evapotranspiration to be made, provided other components of the water balance are known.

Abbreviations: EM, electromagnetic • GPR, ground penetrating radar

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