Gradient-based surface NMR for groundwater investigation

Abstract

In medical MRI, spatial localization (imaging) is based upon the application of controlled magnetic field gradients on top of the main magnetic field, to spatially modulate the frequency and/or phase of the NMR across the volume of investigation. In this work, we have applied similar physical principles to produce controlled magnetic field gradients during surface NMR-based groundwater investigations. In this approach a gradient pulse of variable amplitude or duration is applied immediately after the excitation pulse, to cause predictable phase encoding of the NMR signal as a function of depth. This approach is also applicable to emerging surface NMR detection methods that use a pre-polarization field with fast non-adiabatic turn off to generate detectable NMR signals from the shallow subsurface. In this case, the gradient pulse is applied after terminating the pre-polarization field and provides a heretofore unavailable means of localizing the NMR response as a function of depth. The application of gradients can also be combined with tip-angle based modulation to yield higher imaging resolution than can be achieved through either gradient- or tip-angle based imaging alone. We implemented this new gradient-based capability into a surface NMR gradient generation accessory that is compatible with the GMR-Flex instrument and developed surface NMR-specific forward modeling and linear inverse models. We validated the accuracy of this novel gradient-based sNMR technology using computer simulations, experiments using a small pool filled with a discrete layer of bulk water, and field experiments at well-characterized groundwater test sites along Ebey Island, WA, and Larned, KS. The gradient-based sNMR imaging observations were compared with high resolution direct push NMR results observed at these sites. The results of computer simulations and field experiments indicate improvements in both detection (signal-to-noise ratio) and spatial resolution of shallow surface water content using surface NMR, compared to traditional surface NMR imaging methods.