1. n. [Formation Evaluation]
A measurement of the nuclear magnetic resonance (NMR) properties of hydrogen in the formation.
There are two phases to the measurement: polarization and acquisition. First, the hydrogen atoms are aligned in the direction of a static magnetic field (B0). This polarization takes a characteristic time T1. Second, the hydrogen atoms are tipped by a short burst from an oscillating magnetic field that is designed so that they precess in resonance in a plane perpendicular to B0. The frequency of oscillation is the Larmor frequency. The precession of the hydrogen atoms induces a signal in the antenna. The decay of this signal with time is caused by transverse relaxation and is measured by the CPMG pulse sequence. The decay is the sum of different decay times, called T2.
The T2 distribution is the basic output of a NMR measurement. The NMR measurement made by both a laboratory instrument and a logging tool follow the same principles very closely. An important feature of the NMR measurement is the time needed to acquire it. In the laboratory, time presents no difficulty. In a log, there is a tradeoff between the time needed for polarization and acquisition, logging speed and frequency of sampling. The longer the polarization and acquisition, the more complete the measurement. However, the longer times require either lower logging speed or less-frequent samples.
See: dephasing, diffusion relaxation, echo spacing, logging tool, longitudinal relaxation, nuclear magnetic resonance, polarization time, relaxation time
