New technologies in groundwater exploration. Surface Nuclear Magnetic Resonance

Abstract

As groundwater becomes increasingly important for living and environment, techniques are asked for an improved exploration. The demand is not only to detect new groundwater resources but also to protect them. Geophysical techniques are the key to find groundwater. Combination of geophysical measurements with boreholes and borehole measurements help to describe groundwater systems and their dynamics. There are a number of geophysical techniques based on the principles of geoelectrics, electromagnetics, seismics, gravity and magnetics, which are used in exploration of geological structures in particular for the purpose of discovering georesources. The special geological setting of groundwater systems, i.e. structure and material, makes it necessary to adopt and modify existing geophysical techniques. A new discipline called hydrogeophysics has been formed and is growing fast. Efforts for direct detection of groundwater led recently to a new technique: Surface Nuclear Magnetic Resonance (SNMR). The principle of nuclear magnetic resonance, well known in physics, physical chemistry as well as in medicine, has successfully been adapted to assess the existence of groundwater and the aquifer parameters. This technique allows for the first time detecting and assessing water directly by only surface measurements allowing quantitative information about mobile water content as well as pore structure parameters leading to hydraulic conductivities. Function, results, interpretation, advantages and drawbacks of SNMR are reviewed in this paper showing the current state of art and developments. A comprehensive example of SNMR is presented with measurements conducted at the site of Nauen near Berlin. The site has Quaternary aquifers with differing layering of sand and till. The results are very satisfying as aquifers down to 50 m depth can be identified quite reliably. The water content is estimated with a high degree of accuracy and relaxation times allowed to derive hydraulic conductivities. Supplementary measurements with geoelectrics and radar made possible to complement and confirm the information achieved with SNMR as well as to apply a joint multimethod approach to aquifer assessment.