Nuclear magnetism well logging - by establishing free precession of chosen nuclei

摘要

A nuclear magnetism well logging tool simultaneously establishes a strong DC magnetic field, to polarise chosen nuclei, and an AC magnetic field in a direction at right-angles to the earth's field and at a frequency corresponding to the Larmor precession frequency of the chosen nuclei. By terminating the DC field adiabatically over a period less than the thermal relaxation time of the chosen nuclei, the latter are swept into driven resonance in the AC field and when that field is then abruptly terminated the nuclei precess freely about the earth's magnetic field and a characteristic of the free precession is monitored.