The NMR tool measures the spin (gyromagnetic ratio) of hydrogen nuclei and thus the properties of pore fluids. The response is essentially only that of the protons in fluids as the nuclei in solids have little direct effect on the measurements. Hydrogen nuclei are aligned with a static magnetic field and then disturbed by a radio-frequency pulse that involves transverse magnetization. Changing their alignment involves a time delay (relaxation time) that depends on their immediate surroundings in the molecule and thus on the molecule’s environment. Bound water results in a shorter relaxation time and relaxation-time analysis can give the distribution of the pore-sized in which the water molecules reside as well as the kind of molecule (water of oil). The nuclear magnetic resonance imaging log (NMRIL) permits determining the fluid-filled porosity independently of the lithology, water saturation without knowing the salinity, and estimates of the permeability, fluid type, percentage of moveable fluid (free-fluid index, q.v.), and other factors under certain circumstances. It measures the decay rate of the nuclear spin of hydrogen in water and oil within a thin annulus several inches into the formation (Chandler et al., 1994). The output is water-filled porosity units. The rate of alignment gives T1 relaxation time, decay gives T2 relaxation time. The data can be transformed to the proton population in various relaxation time gates (Prammer, 1994). Bound-water volume and free-fluid index calculated from these data correspond to the small and large pores to distinguish between immobile and mobile fluids. Permeability can be calculated from porosity and relaxation-time calculations (Coates et al., 1994) based on locally-established relationships. Also called nuclear magnetism log, free-fluid log, and NMRIL log. (a Halliburton trade name). See Coates et al. (1999) and Western Atlas (undated). See also proton-resonance magnetometer.