Hydrogeological experience in the use of MRS
Résumé
Nowadays, MRS contribution to characterize aquifers in common conditions down to about 100 meters deep is highly valuable in rocks that exhibit hydraulic behaviour of non-consolidated aquifer at the sounding scale (e.g. sediments, weathered and fissured hard-rocks, densely fissured or highly interstitial porous carbonates). In rocks that exhibit behaviour of fractured aquifer (e.g. low density fractured crystalline basements and limestone, karsts) MRS is a useful complementary method but is not always effective for common engineering studies. In magnetic rocks MRS measurements are often impossible. On the one hand, field experiences reveal that MRS is useful to characterize aquifers. (1) The geometry of saturated aquifer can be estimated in 1D case. Interpolation in-between 1D soundings also reveals 2D geometry if the size of MRS loops is smaller than about half the heterogeneity size. (2) Links between MRS water content and aquifer total porosity and storativity have been found. (3) The transmissivity is accurately estimated from MRS parameters in several geological contexts, using the appropriate conversion equation. (4) Thanks to the integrative property of the sounding, the spatial scale of measurement is considered by hydrogeologists as appropriate for aquifer characterization and modelling. (5) The aquifer characterization is improved when MRS is used in the framework of a hydrogeological methodology and jointly with complementary geophysical methods. On the other hand, field experiences reveal the main limitations encountered in the use of MRS with the actual instrumentation. (1) MRS is not yet self-sufficient to characterize aquifers as the MRS output parameters still need to be compared to hydrogeological properties to achieve quantitative estimation of transmissivity. Storage related parameters are not yet quantitatively accessible from MRS. (2) The electromagnetic noise lowers the signal to noise ratio and makes urban areas but also low interstitial porosity and poorly fractured rocks difficult to survey. (3) Heterogeneity of the magnetic properties of rocks makes measurements impossible or interpretations wrong. (4) Electrically conductive layers reduce the investigation depth of MRS in salty water and clayey environments.