Gaining insight into the T2*-T2 relationship through complex inversion of surface NMR free-induction decay data

Abstract

One of the primary shortcomings of the standard surface nuclear magnetic resonance (NMR) measurement, called the free-induction decay (FID), is the uncertainty about the link between the signal’s time dependence and the geometry of the pore space. Ideally, the FID signal’s time dependence, described by the parameter T2*, carries an intimate link to the geometry of the pore space allowing robust estimation of pore-size and permeability. However, T2* can also be strongly influenced by background magnetic field (B0) inhomogeneity, which can mask the link to pore geometry. To improve the utility of surface NMR FID measurements, we investigate whether complex inversion of surface NMR data can be used to provide insight into the link between T2* and T2 (the parameter carrying the link to pore geometry). Synthetic and field measurements are presented to demonstrate that an alternative forward modelling approach that involves direct modelling of relaxation during pulse (RDP) effects can help constrain the relationship between T2* and T2. Complex inversions are performed using forward models that include RDP for varying magnitudes of B0 inhomogeneity (consistent with observed T2* values) and it is observed that satisfactory data fits can only be obtained given reliable T2 estimates. Thus providing insight into the T2*-T2 relationship. We aim to demonstrate that an alternative forward modelling approach may help improve the utility of FID measurements for estimation of pore-scale properties.