Characterization and mitigation of off-resonance effects in surface NMR [electronic resource]

Surface nuclear magnetic resonance (NMR) is a young geophysical technique providing non-invasive characterization of groundwater resources. To build images or interpretations using surface NMR that are reliable representations of the subsurface we require accurate modeling of the physics during the experiment. In this thesis we examine the impact of background magnetic field (B0) inhomogeneity on the surface NMR experiment. This is done in two parts. The first part aims to advance the fundamental understanding of how magnetic field inhomogeneity impacts the surface NMR measurement; we aim to improve our understanding of how these mechanisms impact the measured signal and how the predicted results are obscured from being representative of the true subsurface as a result. The second portion of the thesis aims to build on our improved understanding of these mechanisms by developing novel methodologies to mitigate the impact of magnetic field inhomogeneity. Background magnetic field inhomogeneity arises due to magnetic susceptibility contrasts in the subsurface that lead to a spatially varying B0 field, which impacts both the ability to excite an NMR signal and the time-dependence of the signal. Both paramagnetic and ferromagnetic grains lead to B0 inhomogeneity. During excitation, B0 inhomogeneity leads to a condition called off-resonance excitation, which violates an implicit assumption in the standard surface NMR forward model. Through synthetic and field experiments the influence and characteristics of artifacts in the estimated aquifer properties were characterized. Neglected off-resonance effects were observed to both lead to underestimated water contents, and degrade the accuracy of the estimated water content profile. Unfortunately B0 inhomogeneity remains extremely difficult to characterize and detect; we therefore require methods to mitigate its influence to ensure reliable aquifer characterization despite its presence. To mitigate the influence of B0 inhomogeneity during excitation we develop a method called frequency-cycling to ensure an accurate forward model despite a spatially varying B0. This method was demonstrated to significantly improve survey performance in the presence of B0 inhomogeneity. Given that frequency-cycling requires off-resonance excitation, but in a controlled manner, we perform a resolution analysis contrasting off-resonance and on-resonance excitations' abilities to produce high-resolution images of the subsurface. We demonstrate that off-resonance excitation effectively allows a second independent spatial sampling to be performed after each excitation. As a result, off-resonance excitation is observed to provide better resolution at shallow depths. A novel methodology to characterize the B0 distribution is also developed to address signal loss due to off-resonance effects and to characterize the influence of B0 inhomogeneity on the time-dependence of the signal in a proof of concept study conducted under controlled laboratory conditions on fluid samples. Specially designed pulses are used to characterize the B0 distribution, allowing its influence on the signal's time-dependence to be removed; thus ensuring reliable characterizations of the pore geometry. The goal of this thesis is to better understand the influence of B0 inhomogeneity in surface NMR, and to develop methods to ensure accurate aquifer characterizations. The improved understanding and novel methods to mitigate B0 inhomogeneity allow more accurate interpretation of surface NMR results, and ensure an accurate forward model. This will enhance the reliability of surface NMR, allowing broader implementation of the technique in more challenging environments.

Keywords: Geophysics.
Advisor: Knight Rosemary (Rosemary Jane)
Mavko Gary
Mukerji Tapan