1. n. [Formation Evaluation]
During a nuclear magnetic resonance measurement, the loss of energy by hydrogen atoms in a rock as they align themselves with the static magnetic field. The atoms behave like spinning bar magnets so that when a static magnetic field is applied, they initially precess about the field. Then, through interactions with nuclei and electrons, they lose energy, or relax, and align themselves with the magnetic field. The relaxation of the hydrogen atoms does not occur immediately but grows exponentially with a time constant T1. There are two mechanisms for longitudinal relaxation, surface relaxation and bulk relaxation.