Surface Nuclear Magnetic Resonance (SNMR)
Water belongs to the most important basic needs of mankind. Worldwide, about 1,2 billion people do not have access to clean drinking water, another 2,5 billion suffer from problems due to inadequate water quality. In order to emphasize the importance of the raw material water, the UNO declared the year 2003 "International Year of (Fresh) Water". International agreements aim at cutting down to half the number of people with no access to clean water. If they are not successful, soon there could be warlike operations for the access to water. Therefore, the shelter and the sustainable use of the world’s freshwater resources is one of the most important tasks of our time.
Geophysical methods are well-established and indispensable in the prospection for drinking water. A new, non-destructive measuring method, the only geophysical surface method allowing the direct determination of the underground’s water content, is the Surface Nuclear Magnetic Resonance (SNMR or MRS). Unlike SNMR, conventional methods only permit the indirect detection of water and the determination of water content and hydraulic parameters (porosity, permeability). With the help of several petrophysical relations (ARCHIE formula, miscibility law, WYLLIE equation etc.) these parameters are deducted from electrical conductivities (geoelectrics, electromagnetics), dielectricity number (georadar) or seismic velocities. These estimation methods, however, are often only applicable under ideal conditions and, owing to cross-sensitivities of the measured values to the clay content of the sediments, the salinity of the pore water etc., yield only insecure information on the water content.
The SNMR method uses the magnetic properties of the water on the atomic level. Hydrogen nuclei (protons) have a magnetic momentum, so they can be stimulated into nuclear magnetic resonance by an external alternating magnetic field. The amplitude of the answer signal returning from the underground is directly proportional to the number of hydrogen nuclei available there. Thus, it is possible to determine the water content of loose and solid rock and of soils. Furthermore, from the fading behaviour of the answer signal, information on material properties like porosity, permeability and hydraulic perviousness can be obtained.
The first precise measurements of the Nuclear-Magnetic-Resonance-(NMR) signals generated by hydrogen nuclei were done by BLOCH and PURCELL in the year 1946. For this scientific achievement the two researchers were awarded the Nobel Prize for Physics in 1952. Meanwhile, NMR is being used worldwide in chemistry, physics and medicine and also in borehole geophysics for hydrocarbon exploration. The basic idea for the SNMR method dates from the 1960s, when VARIAN and BARRINGER suggested to transform the principle of the proton magnetometer into a measuring instrument for groundwater exploration. But only in the 1980s, Russian scientists in Novosibirsk developed a serviceable instrument and used it for the exploration of pore aquifers. Since 1990, extensive SNMR tests have been carried out worldwide under various hydrogeological conditions. The first successful SNMR measurements in Germany were done in April 1997 at the Haldensleben test site. BGR carried out measurements with low geomagnetic field intensities in 1999 in Namibia.
The NMR methods in the application areas mentioned above use two technically generated strong magnetic fields perpendicular to one another. The SNMR method, on the other hand, uses the relatively low geomagnetic field as a static field and an external magnetic excitation field, generated by a big conductor loop lying on the earth’s surface as an emitting inductor. This inductor also records the answer signal returning from the underground after switching off the excitation field.
Selected publications on the SNMR-method:
- BGR und Technische Universität Berlin
- Surface-NMR in an area with low geomagnetic field and low water content – a case history from Namibia