Nuclear magnetic resonance (NMR) logging is a type of well logging that uses the NMR response of a formation to directly determine its porosity and permeability, providing a continuous record along the length of the borehole.
[1][2] NMR logging exploits the large magnetic moment of hydrogen, which is abundant in rocks in the form of water.
The NMR signal amplitude is proportional to the quantity of hydrogen nuclei present in a formation and can be calibrated to give a value for porosity that is free from lithology effects.
Uniquely, a petrophysicist can also analyse the rate of decay of the NMR signal amplitude to obtain information on the permeability of the formation - a crucial quantity in hydrocarbon exploration.
[2] Molecules in fluids are in constant Brownian motion, diffusing about the pore space and bouncing off the grain surfaces.