Nuclear magnetic resonance (NMR) has been used for decades in the petroleum industry to characterize fluid flow through reservoir materials. The potential for NMR tools to improve remote estimates of permeability led to the development of several NMR well-logging tools. Along with surface NMR systems, these same tools and techniques can be used to measure how water flows through near-surface materials to characterize hydraulic conductivity (K). Several methods to estimate hydraulic conductivity from NMR measurements have been developed, but all require extensive empirical calibration to accurately determine K. Controlled laboratory studies of how NMR parameters correlate with K could improve how K is estimated using NMR for aquifer characterization.
We use controlled laboratory studies to investigate the dependence of K on NMR measured material properties for a range of artificial and field samples. In our lab, we have a 2 MHz (~0.005 T) tabletop Rock Core Analyzer (RCA) made by Magritek for NMR investigations of rock and soil samples. This machine is capable of applying strong magnetic field gradients along the z-axis of our samples which enables us to study additional properties such as diffusion through the pore space. NMR tools are typically used in geophysics to measure the relaxation time distribution of hydrogen spins in porous media which can provide information about the pore size distribution. By measuring the relaxation times, diffusion coefficients, and other parameters, NMR provides a tremendous amount of information about how water interacts with near-surface materials. In the lab, we can use this information to potentially establish better relationships between NMR measurements and material parameters. These laboratory studies are an excellent testing ground of new ideas before they are tried on NMR tools used at the field scale.