1. n. [Formation Evaluation]
In a nuclear magnetic resonance (NMR) measurement, the characteristic time for a loss of coherent energy, or relaxation, by protons in rocks. There are two types of relaxation: longitudinal relaxation, which is the time (T1) needed to align protons in a static magnetic field; and transverse relaxation, which is the time (T2) needed for protons to lose their coherent energy in an NMR measurement. Relaxations are exponential decays, for which T1 and T2 are the time constants. Different mechanisms contribute to T1 and T2. Surface relaxation and bulk relaxation contribute to both T1 and T2. Surface, bulk and diffusion relaxation contribute to T2.