Magnetic resonance free induction decay in geological porous materials

Magnetic resonance is an important noninvasive technology across life sciences and industry. Free induction decay is the simplest H-1 magnetic resonance measurement method and an important means of probing fast-decaying signals in porous materials such as rocks, lung, and bone. It is commonly assumed that the free induction decay in geological porous materials is single-exponential. We experimentally observed two regimes of free induction decay behavior in geological porous materials: single-exponential and non-exponential decay. Numerical simulations that match experimental data highlight the effect of mass diffusion, especially in the single-exponential behavior. These two regimes of free induction decay in porous materials are associated with a bifurcation point in the solutions of the Bloch-Torrey equation for diffusion of fluids in confined domains in the presence of internal magnetic field gradients. This finding facilitates the extraction of absolute internal magnetic field gradient intensities from simple free induction decay measurements in the laboratory and field. This work also warns against common single-exponential assumptions in surface magnetic resonance methods employed in surveying underground water aquifers.

Automated summary

In geological porous materials, free induction decay can exhibit both single-exponential and non-exponential decay behaviors. Numerical simulations show different effects of mass diffusion in these two regimes. This finding warns against the common single-exponential assumptions in surface magnetic resonance methods for surveying underground water aquifers.