NMR logging in the earth's magnetic field

摘要

An apparatus and a method for measurement of NMR relaxation parameters of earth formations uses a coil on a logging tool to substantially polarize nuclear spins within a volume of earth. The polarizing pulse then switched off adiabatically. The magnetization of the spins then precesses around earth's magnetic field at a Larmor frequency corresponding to the earth's field. A second coil on the logging tool is pulsed with a carrier signal having this frequency and modulated by a CPMG sequence to obtain spin echo signals. The spin echo signals may be received by the same coil or by a second coil. Alternatively, beam steering of two coils on the tool may be carried out to ensure a resultant field that has a significant component transverse to the earth's field for carrying out the spin echo measurements.