Nuclear magnetic resonance relaxation and diffusion in the presence of internal gradients: The effect of magnetic field strength

It is known that internal magnetic field gradients in porous materials, caused by susceptibility differences at the solid-fluid interfaces, alter the observed effective Nuclear Magnetic Resonance transverse relaxation times T-2,T-eff. The internal gradients scale with the strength of the static background magnetic field B-0. Here, we acquire data at various magnitudes of B-0 to observe the influence of internal gradients on T-2-T-2 exchange measurements; the theory discussed and observations made are applicable to any T-2-T-2 analysis of heterogeneous materials. At high magnetic field strengths, it is possible to observe diffusive exchange between regions of local internal gradient extrema within individual pores. Therefore, the observed exchange pathways are not associated with pore-to-pore exchange. Understanding the significance of internal gradients in transverse relaxation measurements is critical to interpreting these results. We present the example of water in porous sandstone rock and offer a guideline to determine whether an observed T-2,T-eff relaxation time distribution reflects the pore size distribution for a given susceptibility contrast (magnetic field strength) and spin echo separation. More generally, we confirm that for porous materials T-1 provides a better indication of the pore size distribution than T-2,T-eff at high magnetic field strengths (B-0 > 1 T), and demonstrate the data analysis necessary to validate pore size interpretations of T-2,T-eff measurements.