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Surface NMR- Addressing Field Inhomogeneity

The Problem

Surface NMR is a relatively young geophysical technique that can be used to non-invasively characterize aquifer properties, such as aquifer depth and thickness, water content, and also be used to investigate pore-scale properties. However, there is a challenge in surface NMR regarding the connection between the measured NMR parameters and the aquifer properties of interest due to the presence of background magnetic field (B0) inhomogeneity. This field inhomogeneity, which occurs naturally due to the presence of magnetic susceptibility contrasts in the subsurface, can impact the measured NMR signal potentially biasing estimates of aquifer properties. We are interested in further exploring and accounting for the impacts of B0 inhomogeneity on the surface NMR measurements to ensure accurate estimates of aquifer properties. To further improve the utility of surface NMR for aquifer characterization, we are also investigating the use of novel strategies for data acquisition in order to improve surface NMR images of aquifer properties.

Our Approach

To improve the understanding of how the impacts of B0 inhomogeneity manifest in the surface NMR signal and the subsequent images of aquifer properties we are conducting laboratory experiments, numerical modeling, and field studies to quantify and characterize the effects. We are developing a new form of data acquisition - measuring the NMR signal following a suite of preparatory pulse sequences designed to encode information about B0 inhomogeneity in the signal. An inversion is then used to predict the B0 distribution to allow its impacts on the NMR signal to be accounted for.

We are also exploring the potential for novel excitation schemes to improve the ability of surface NMR to generate images more reflective of the true subsurface properties. Building on previous studies that demonstrate resolution improvements by inverting a complex NMR signal, we propose to generate a complex signal in a controlled manner in order to encode additional spatial information in the quadrature component of the measured signal. This has the potential to provide improved spatial resolution in the surface NMR images.

Results to Date

We have also completed a synthetic proof of concept study demonstrating potential resolution improvements using novel excitation schemes. A field study demonstrating the feasibility of these methods in a real world setting is planned for Spring 2014.

Project Publications and Presentations

Quantifying background magnetic field inhomogeneity for improved interpretation of free induction decay measurements, (In review for Geophysics). (2013)