A fast inversion recovery NMR imaging technique for mapping two-dimensional tracer diffusion and dispersion in heterogeneous media

Abstract

A nuclear magnetic resonance imaging method is demonstrated for measuring diffusion of a paramagnetic tracer in an aqueous gel, and the hydrodynamic dispersion of the same tracer in a porous medium. The fast inversion recovery method for determining the longitudinal relaxation times T₁ is used in conjunction with a standard Fourier imaging scheme to obtain images with null contours, which track particular tracer concentrations. The fast inversion recovery method offers considerable saving of experimental time relative to the standard inversion recovery protocol, but without introducing dependences on transverse relaxation time T₂, which would be hard to quantify. With correct parameter choice, null contours can be localized in the images within two pixels and the corresponding tracer concentrations identified from longitudinal relaxation data only. In a cylindrical gel sample, concentration fronts are tracked and yield diffusivities in agreement with results previously reported for a similar one-dimensional method; in a core of natural sedimentary rock, we image clearly the markedly irregular advance of a concentration front caused by natural heterogeneity in the medium. The method is applicable to a variety of fields involving diffusion or dispersion in microscopically or macroscopically heterogeneous media.