Sabina Haber-Pohlmeier

Small-scale instrumentation for nuclear magnetic resonance of porous media

The investigation of fluids confined to porous media is the oldest topic of investigation with small-scale nuclear magnetic resonance (NMR) instruments, as such instruments are mobile and can be moved to the site of the object, such as the borehole of an oil well. While the analysis was originally restricted by the inferior homogeneity of the employed magnets to relaxation measurements, today, portable magnets are available for all types of NMR measurements concerning relaxometry, imaging and spectroscopy in two types of geometries. These geometries refer to closed magnets that surround the sample and open magnets, which are brought close to the object for measurement. The current state of the art of portable, small-scale NMR instruments is reviewed and recent applications of such instruments are featured. These include the porosity analysis and description of diesel particulate filters, the determination of the moisture content in walls from gray concrete, new approaches to analyze the pore space and moisture migration in soil, and the constitutional analysis of the mortar base of ancient wall paintings.

Relaxation in a Natural Soil: Comparison of Relaxometric Imaging, T1 – T2 Correlation and Fast-Field Cycling NMR

Longitudinal and transverse relaxation times are used to characterise the pore system of a natural Kalden- kirchen sandy loam. Here we present new results obtained by relaxometric imaging (MEMS) and two-dimensional T1-T2 correlation relaxometry, and compare these with available T1- relaxation time distributions of water obtained by the analy- sis of fast field cycling relaxometry (FFC) data. The soil shows relatively broad bimodal distribution functions P(T1) and P(T2) with a T1/T2 ratio of about 2:1. The average T1 as well as the spatial distribution, which are obtained from the re- laxometric imaging corresponds well to the relaxometric results. From the analysis of the field dependent FFC data at low field including T1 data obtained at high field the basic locally averaged relaxation mechanism is derived from the disper- sion curve, i.e. the dependence of the relaxation rate from the magnetic field strength over five orders of magnitudes. From this we conclude that two-dimensional diffusion at locally flat surfaces controls the relaxation, i.e. the shapes of the distribution functions are controlled by surface relaxation.