Patent ID: 6559640
Filing Date: 2003-05-06
Classification: G01V

Abstract:
A method for nuclear magnetic resonance (NMR) sensing of earth formations comprising:(a) using an electromagnet on a logging tool in a borehole within the earth formations for inducing a time varying magnetic field for polarization of nuclei within a region of the earth formations, said electromagnet being used between an activation time and a deactivation time; (b) generating during a first time interval a first radio frequency (RF) magnetic field having a first frequency for causing precession of said nuclei, said first frequency being related to a first intensity of the static field induced during said first time interval; (c) generating during a second time interval later than the first time interval a second RF magnetic field having a second frequency for refocusing said precessing nuclei, said second frequency being related to a second intensity of the static field induced during the second time interval; and (d) receiving at a third time interval after the second time interval an NMR signal from said nuclei at a third frequency, wherein said third frequency is related to a third intensity of the static field induced during said third time interval, wherein said first, second and third frequencies comprise at least two different frequencies.