Protons, or hydrogen nuclei, have both an electrical charge and a spin. A rotating electrical charge generates a magnetic field. The hydrogen nuclei therefore behave as small magnets. NMR measurements take advantage of this property.
The data acquisition sequence at a given depth is made up of the following steps:
The formation is exposed to the magnetic field, B0, of the permanent magnet in the logging tool. The hydrogen nuclei in the formation align themselves in this magnetic field, like the needle of a compass aligns itself in the earth’s magnetic field. This first phase is called polarization. The amount of polarization increases at an exponential rate with a time constant T1, called longitudinal relaxation time. The magnetic field is applied for a period of time called Wait Time before the actual measurement sequence begins. The wait time should be long enough for most of the nuclei to be polarized. In other words, it should be about three times the value of the highest T1 component at that spot.
A radio-frequency electromagnetic pulse is applied with the antenna to tilt the spins of the nuclei 90 degrees into the transverse plane. This is referred to as the 90-degree pulse.
The nuclei (also called spins) start recessing while reorienting themselves into the longitudinal axis. Due to heterogeneity in the local magnetic field, individual spins recess at different speeds and get out of phase very rapidly. The net signal drops to zero.
A so-called 180-degree electromagnetic pulse is applied to reverse the apparent direction of rotation of the spins, so that they can re-phase. When spins are back in phase, the detected signal in the transverse plane reaches a maximum value, called an echo. As spins continue recessing, they de-phase again and the signal drops to zero.
Another 180-degree pulse is applied to re-phase the spins. This procedure is repeated a large number of times. The time between 180-pulses is referred to as echo spacing, and the number of times the procedure is repeated is called number of echoes.
The peaks of the echoes define the decay of the signal in the transverse plane. For a single hydrogen proton the signal decays exponentially with a time constant T2, called transverse relaxation time. At a given depth in the formation, hydrogen protons relax at different speeds, depending mostly on the size of the pores in which they are located. The decay cannot be represented by a single T2 value but is shown as a continuous spectrum called T2 distribution.