Surface Nuclear Magnetic Resonance for direct assessment of groundwater

The principle of nuclear magnetic resonance, well known in physics, physical chemistry as well as in medicine, has now successfully adapted to measurements at the Earth\'s surface to assess the existence of groundwater and aquifer parameters. This new geophysical technique of Surface Nuclear Magnetic Resonance (SNMR) in the last 10 years or so has already passed the experimental stage to become a valuable tool in hydrogeophysical investigations. Function, results, interpretation, advantages and drawbacks of the method are reviewed in this paper, showing the current state of art and developments. Basically, in SNMR the response of excited hydrogen protons is measured in terms of relaxation strength and time yielding the water content and pore sizes of the subsurface. The hydrogen protons of water, being magnetic dipoles of spinning charges, are excited with magnetic fields at Larmor frequency using large loops and different pulse moments. Subsequently, the protons relax their precession around the Earth\'s field and their magnetic field is measured with the same loop. 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 us to derive hydraulic conductivities. Supplementary measurements with geoelectrics and radar are made possible to confirm the information achieved with SNMR as well as a joint multimethod approach to aquifer assessment.