The effect of pore size and magnetic susceptibility on the surface NMR relaxation parameter T 2

Surface nuclear magnetic resonance (NMR) is a non-invasive geophysical method that can provide valuable information about aquifer properties related to groundwater flow and storage. Our ability to extract such information from surface NMR data, however, is limited by an insufficient understanding of the relaxation parameter T 2 * governing the decay rate of the surface NMR signal in Earth’s magnetic field. In this study, we use a combination of numerical and laboratory experiments to systematically explore the effect of two key geologic properties, pore size and magnetic susceptibility, on the T 2 * relaxation process. A one-dimensional numerical model is developed and parametrized to simulate the surface NMR response for a wide range of geologic materials. These simulations illuminate the processes controlling T 2 * relaxation and identify conditions under which T 2 * exhibits varied sensitivity to pore size. For materials with low magnetic susceptibility, T 2 * is highly sensitive to pore size; however, as susceptibility increases, this sensitivity diminishes and T 2 * becomes dominated by complex dephasing effects, particularly when pores are large. Laboratory Earth’s field NMR experiments complement the numerical simulations. Measurements on water-saturated quartz sands show that for weakly magnetic materials, T 2 * can be sensitive to pore size and thus could provide useful information about aquifer properties.