IMPROVING THE SPATIAL RESOLUTION OF SURFACE NMR FOR IMAGING THE UNSATURATED ZONE

Laboratory studies have shown that there are many potential ways in which proton nuclear magnetic resonance (NMR) measurements could be used to obtain information about properties and processes in the unsaturated zone. The NMR measurement directly detects the nuclear magnetization associated with the hydrogen nuclei in the pore water and provides a distribution of NMR relaxation times. The initial signal amplitude is a measure of water content in the sampled material. The distribution of relaxation times can be related to the pore-size distribution of the water-filled pores in the sampled material. The average relaxation time is related to the average surface-area-to-volume ratio of the material which can be used to estimate permeability or hydraulic conductivity. In addition to the link to physical properties, NMR is also a very sensitive indicator of changes in the mineralogic form of any iron in the sampled material.

Surface NMR employs a wire loop on the ground surface to make a non-invasive NMR measurement of the subsurface. The primary focus of our research is the challenge of acquiring sufficient vertical resolution in studies of the top few meters or tens of meters of the unsaturated zone. We are exploring the use of novel data acquisition techniques designed to provide improved resolution in the water content and relaxation time depth profiles estimated in surface NMR. Using excitation schemes that generate a complex NMR signal, we are able to employ the real and quadrature components to spatially sample the subsurface in different ways; effectively allowing us to perform two soundings in a single measurement. This strategy is able to better reproduce water content and relaxation time contrasts, and identify fine-scale structures in the shallow subsurface (<12 m). This represents an important advancement in the surface NMR method, producing acquired images that more accurately reflect the subsurface properties and structure.