Nuclear Magnetic Resonance (NMR) methods offer the determination of pore size distributions of loose sediments, which is the basis of investigations concerning ground water dynamics, i.e. water content and hydraulic conductivity of the subsurface. The NMR resolution range of pore radii depends on many parameters such as the technology of the applied NMR system, the Larmor frequency and the properties of the pore fluid and the pore surface, respectively. Different NMR methods do not necessarily show the same results at the same samples. At core samples from a shallow aquifer we compare Earth’s field NMR measurements with conventional lab NMR measurements and show the influence of the dead time and the Larmor frequency on the resolution limits of the pore size distribution.