Constraining Groundwater Modeling with Magnetic Resonance Soundings
2012
Boucher, Marie | Favreau, Guillaume | Nazoumou, Yahaya | Cappelaere, Bernard | Massuel, Sylvain | Legchenko, Anatoly
Magnetic resonance sounding (MRS) is a noninvasive geophysical method that allows estimating the free water content and transmissivity of aquifers. In this article, the ability of MRS to improve the reliability of a numerical groundwater model is assessed. Thirty‐five sites were investigated by MRS over a ∼5000 km2 domain of the sedimentary Continental Terminal aquifer in SW Niger. Time domain electromagnetic soundings were jointly carried out to estimate the aquifer thickness. A groundwater model was previously built for this section of the aquifer and forced by the outputs from a distributed surface hydrology model, to simulate the observed long‐term (1992 to 2003) rise in the water table. Uncertainty analysis had shown that independent estimates of the free water content and transmissivity values of the aquifer would facilitate cross‐evaluation of the surface‐water and groundwater models. MRS results indicate ranges for permeability (K = 1 × 10−5 to 3 × 10−4 m/s) and for free water content (w = 5% to 23% m3/m3) narrowed by two orders of magnitude (K) and by ∼50% (w), respectively, compared to the ranges of permeability and specific yield values previously considered. These shorter parameter ranges result in a reduction in the model's equifinality (whereby multiple combinations of model's parameters are able to represent the same observed piezometric levels), allowing a better constrained estimate to be derived for net aquifer recharge (∼22 mm/year).
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