General concepts in Hydrogeology and Geophysics related to MRS

Abstract

A tight collaboration between hydrogeologists and geophysicists is necessary to achieve the most appropriate use of Geophysics in Hydrogeology, mainly for planning the geophysical activities in accordance to the hydrogeological target to be investigated and for the interpretation or translation of the geophysical documents into hydrogeological documents. For this collaboration to be fruitful it is of great importance to share a common language allowing communication feasible: geophysicists have to know the fundamentals of the hydrogeological process, and hydrogeologists have to know the fundamentals of geophysical methods. This is the main objective of this paper, where definitions of basic concepts of both disciplines related to Magnetic Resonance Sounding (MRS) are reminded. All surface geophysical methods are actually used in groundwater studies. MRS deserves special attention because of its singularity and novelty: it is the only method able to detect directly the presence of water in the underground. As research is going ahead, MRS reveals also its ability for the evaluation of hydraulic parameters, being nowadays a real alternative to the use of boreholes tests in some circumstances. The other standard surface geophysical methods are valid to determine the geometrical parameters of aquifers, and just in a few cases they allow the evaluation of hydraulic properties. The MRS method is at present limited to the investigation of the first 150 m of depth. Most groundwater catchments areas, at any geological environment, fall within the category of shallow confined or unconfined aquifers; otherwise hydrogeological research in the first 150 m is basic not only from the standpoint of groundwater supply but also for geotechnical and environmental groundwater related studies. The use of MRS can be adapted to any scale of the hydrogeological study. In regional research the main goal is the evaluation of water resources and the acquisition of parameters to make a mathematical model to the aquifer control. To achieve such a model the area in which the aquifer is situated, as well as its recharge and discharge zones, are divided into cells where the flux variables or parameters that characterise the aquifer have to be evaluated. MRS can be used to get part of these parameters adapting the sampling or distance between MRS measurements to the desired scale: cells size of kilometres or hundreds of meters. Local hydrogeological surveys are mainly focused at water extraction for human supply or for agricultural and/or industrial use, requiring a higher resolution in the methodology applied, for which MRS can play again an important role for the best location of sites for well drilling. Evaluation of hydrodynamic parameters from MRS data needs an initial calibration with known reference data, being necessary to fully understand the relation between hydrogeological and MRS parameters, for what a description of the former ones as well as a summary of the hydrological tests for its evaluation, in special of pumping tests, is presented.