Induced gamma ray spectroscopy well logging system

Abstract

A system for use in cased well boreholes for measuring gamma ray spectra induced by bombardment of earth formations in the vicinity of the borehole with fast neutrons. Monoenergetic fast neutrons emitted in repetitive pulses penetrate the borehole structure and enter earth formations in the vicinity of the borehole and are moderated by elastic and inelastic scattering to thermal energy and captured by the nuclei of elements in and about the borehole. A scintillation detector optically coupled to a photomultiplier detects gamma radiation induced by the neutrons and produces electrical pulses with amplitudes representative of the energy of gamma rays. A pulse height analyzer connected to the output of the detector and photomultiplier separates the electrical pulses into a spectrum representative of the gamma ray energy spectrum. A timing circuit generates time gate signals to separate the electrical pulses into those of fast neutron induced gamma rays, thermal neutron capture gamma rays, and background gamma rays prior to sending the pulses to the pulse height analyzer. The timing cycle has a fast subcycle in which pulses representative substantially only of fast neutron induced gamma rays and thermal neutron capture gamma rays are sent to the pulse height analyzer and a slow subcycle portion in which pulses substantially representative only of thermal neutron capture gamma rays and background gamma rays are sent to the pulse height analyzer.