Longitudinal to transverse relaxation time ratio (T₁/T₂) in unconsolidated geological materials: a perspective from 2-D borehole and laboratory NMR measurements

This study investigates the longitudinal (T₁) to transverse (T₂) relaxation time ratios in unconsolidated geological materials to determine how they vary across different geological units. Assessing the T₁/T₂ ratio can inform about the validity of the presumed relationship between T₁ and T₂ relaxation times in steady-state surface nuclear magnetic resonance (NMR) modelling (i.e.T₁/T₂ ratio is assumed to be constant and equal to one). The T₁/T₂ ratio investigation is conducted by 2-D T₁–T₂ correlation data using laboratory and borehole NMR measurements at a Larmor frequency of 2 MHz and 430 kHz, respectively. Laboratory NMR measurements were performed on 73 sediment samples from nine sites in Denmark and Germany, and borehole NMR measurements were conducted at 59 selected depth intervals in unconsolidated geological units across eight sites in the same countries. Volumetric magnetic susceptibility of the laboratory samples was measured to evaluate the effects of magnetic susceptibility on the T₁/T₂ ratio. Our results indicate that the T₁/T₂ ratios in mineral soils and sediments are pretty similar for borehole NMR and lab NMR data sets, regardless of the geological unit. In these geological materials, the mean value of the T₁/T₂ ratios is 1.64 in lab-NMR and 1.82 in borehole NMR data sets. In contrast, in our in-situ borehole NMR measurements in organic peat soils, the mean value of the T₁/T₂ ratios was higher (i.e. 2.77), exhibiting a broader distribution ranging from 1 to 4.8. Moreover, we observed that magnetic susceptibility did not have a significant effect on the T₁/T₂ ratio in the investigated samples. More importantly, the findings in this study can be adopted in the modelling of steady-state surface NMR modelling routines where a constant ratio of 1 for T₁/T₂ is assumed when solving the Bloch equations. It is expected that updating the T₁/T₂ ratio can improve the accuracy of water content and relaxation time estimations derived from steady-state surface NMR measurements.