Parameter identification in fresh-saltwater flow based on borehole resistivities and freshwater head data

By means of a series of borehole resistivity measurements and a resistivity -salinity relation, a particular salt-freshwater inversion was found under the shore with semi-diurnal tides at the French-Belgian border. These resis tivity data provide valuable information about the vertical variation of th e saltwater percentage in different boreholes. At different places and dept hs fluctuations of freshwater heads are observed. A regression modelling pr ocedure is proposed in which the hydraulic parameters for density dependent flow and solute transport can be simultaneously considered with the parame ters of the resistivity-salinity relation. The object function comprises re sistivity residuals and freshwater head residuals along with saltwater perc entage residuals and parameter residuals. First, a synthetic problem is ela borated with this regression modelling procedure. It is followed by the app lication of the procedure on the observed fresh-saltwater flow problem unde r the shore. In the synthetic problem the identification of the hydraulic p arameters was demonstrated without the inclusion of prior information about these parameters. The resistivity-salinity relation was slightly adjusted in this regression. During the regression modelling of the observations mad e in one of the shore-normal cross sections, the horizontal and vertical co nductivity are identified along with the effective porosity and the longitu dinal and transverse dispersivity. The optimal values of the dispersivities are very small. Finally, it is shown that the high waters on the back shor e forms the main threat of saltwater enchroachment from the sea side of the dunes and that the isolated fresh-brackish lens under the lower part of th e shore before the build up area of De Panne can be explained by overexploi tation. (C) 1999 Elsevier Science Ltd. All rights reserved.