1. n. [Formation Evaluation]
In a nuclear magnetic resonance measurement, the characteristic time for longitudinal relaxation. In rocks, longitudinal relaxation is the inverse sum of the surface relaxation and bulk relaxation. T1 is not normally measured in NMR logging, but is an important parameter in deciding the polarization time and hence the logging speed. T1 is closely related to the transverse relaxation time, T2. The ratio T1/T2 in water-filled rocks is typically between 1.5 and 2.5. In light hydrocarbons and gas, the ratio increases up to 10 and more as the viscosity decreases.