Method and apparatus for evaluating an earth formation using nuclear

Abstract

This invention makes an NMR measurement at extended depths into the formation using the earth's magnet field to generate an NMR measurement and using a magnet field from a permanent magnetic to suppress the NMR signal in the borehole and shallow formation. The magnetic field from a permanent magnet increases the Larmor frequency in the borehole and the shallow earth formation. An oscillator applies an RF magnetic field to the formation for a period of time in order to align nuclei spins approximately perpendicular to the earth's magnetic field. At the end of this period, the field is turned off rapidly. Since the spins are unable to follow this sudden change, they are left aligned perpendicular to the earth's field. These nuclei precess about the earth's field at the approximate Larmor frequency of the earth's field. The RF receiver detects the RF signals in the approximate frequency range of the Larmor frequency. Borehole and shallow formation protons precessing at the high frequencies resulting from the permanent magnet are not detected by the receiver. Therefore, the receiver only detects signals from greater depths in the earth's formation.