Surface nuclear magnetic resonance (NMR) is a geophysical technique that takes advantage of the NMR-response of hydrogen protons to image the distribution of groundwater in the subsurface. The macroscopic spin magnetization that results from water molecules in the pore spaces of rocks and sediments is usually in equilibrium, aligned along the Earth's magnetic field. Tipping the spins using a secondary magnetic field generated by a current pulse in a large surface loop forces them into precessional motion. After the pulse is cutoff, the continuously precessing spins generate a measurable electromagnetic signal that is received by the same or a second surface loop. The recorded signals are directly related to the subsurface water distribution that can be detected.
Traditionally, surface-NMR is carried out in a 1-D sounding mode (Magnetic Resonance Sounding, MRS), in which the distribution of water with depth is determined from measurements using a single loop that operates as both a transmitter and receiver. The induced pulse intensity is varied in order to investigate different depth ranges. Here, we present the latest developments that now allow the surface-NMR method to be employed in a 2-D tomographic mode (Magnetic Resonance Tomography, MRT). These developments include:
application of a sophisticated 2-D inversion scheme;
implementation of multi-offset loop measurements to maximise subsurface resolution;
identification of the most sensitive loop configurations to provide the best coverage from a minimum number of measurements;
application of advanced finite-element modeling of the loop electromagnetic fields to allow for arbitrary topography and 3-D electrical resistivity distributions.
In petrophysical applications of NMR (e.g. borehole logging and laboratory studies), the relaxation constants of the NMR signals are used to provide various hydrological parameters. In surface-NMR, the relaxation information extracted from the exponential decaying signal is not an appropriate proxy for petrophysical properties, because it is biased by temporal and spatial variations of the Earth's magnetic field. Consequently, more sophisticated pulse sequences have to be applied in order to recover unbiased relaxation parameters. Adaptation of well-established pulse-sequences used in sample-scale applications to surface-NMR conditions is not straightforward. We present the first steps for reliable determination of relaxation constants by means of surface-NMR.
In many geological situations where conventional geophysical techniques (e.g. geoelectrics, georadar or seismics) provide only limited or ambiguous information, surface-NMR can provide unique details about subsurface physical properties and structure. Some relevant case studies are shown to demonstrate the potential of surface-NMR in hydrogeological investigations.