1. n. [Formation Evaluation]
In nuclear magnetic resonance (NMR) logging, the characteristic time for transverse relaxation. In rocks, there are three components of the transverse relaxation: surface, Ts; bulk, Tb; and diffusion relaxation, Td. T2 is the inverse sum of each component for each fluid, as follows: 1/T2 = 1/Ts + 1/Tb + 1/Td. Because of the reciprocal sum, the smallest of the three types of relaxations is the most important in determining the final T2 for each fluid. There is not one single value of T2 for a rock but a wide distribution of values lying anywhere between fractions of a millisecond and several seconds. The distribution of T2 values is the principal output of an NMR log.