Spectral Inversion and Interpretation of Surface Nuclear Magnetic Resonance Data

The Surface Nuclear Magnetic Resonance (SNMR; Magnetic Resonance Sounding, MRS) method is a non-invasive geophysical method that allows direct determination of the water distribution in the subsurface. In SNMR the hydrogen protons in the pore water are excited with an artificial magnetic field, generated by an antenna loop on the surface. The loop is energized by an alternating electrical current oscillating with the local Larmor frequency of the protons. After termination of the exciting pulse, the magnetic field due to the relaxation of hydrogen protons is measured. The signal amplitude is directly proportional to the amount of free water in the subsurface. The decay time constant (spin-spin-relaxation time) of the relaxation signal is linked to the effective pore size. Increasing the intensity of excitation by raising the duration or strength of the exciting pulse increases the depth of investigation. So far SNMR is only used in sounding mode and the inversion of SNMR data mainly concentrates on the interpretation of the water content distribution. Thereby, the inversion of the decay time is performed by fitting only a single relaxation constant (mono-exponential fit), i.e. assuming a mean pore size for each inversion layer. This approximation, however, often does not comply with the hydrogeological properties of the subsurface. In analogy to borehole and laboratory NMR we introduce a new approach to the inversion of SNMR data in terms of decay time spectra analysis, i.e. a multi-exponential fitting of the SNMR relaxation signals. This allows a more quantitative characterization of an aquifer with respect to the distribution of pore radii. Multi-exponential interpretation of SNMR data has been carried out for datasets obtained from sites in northern Germany and the Netherlands. Along with an improved data fit compared to the conventional mono-exponential fitting the inversion also yields a depth dependent decay time distribution corresponding to the pore radii distribution in the subsurface. The inversion results correlate well with borehole measurements carried out at these sites.