Abstract: In the illustrative embodiments of the invention disclosed, a neutron porosity well logging tool includes a D-T neutron accelerator, a fast-neutron monitor for monitoring accelerator output, near and far epithermal neutron detectors for deriving a primary porosity measurement, a neutron detector array intermediate to the near and far epithermal detectors for providing enhanced spatial resolution, and a far-spaced thermal neutron detector. The epithermal neutron detectors are shielded and spaced relative to the accelerator to optimize tool response to formation porosity, whereby the tool affords accurate porosity sensitivity over the entire range of porosities of interest. The source monitor may additionally be employed to measure capture gamma ray intensity vs. time between neutron bursts for deriving formation sigma. Also, the detector array may include both epithermal and thermal neutron detectors to afford an epithermal/thermal comparison measurement.

Abstract: In the illustrative embodiments of the invention disclosed, earth formations surrounding a well bore are irradiated with pulses of fast neutrons, and gamma rays resulting from the ensuing thermal neutron capture interactions with nuclei of the formations are detected, from which measurements of the thermal neutron decay times characterizing the respective formations are derived and the gamma ray energy spectra of the respective formations are analyzed. Gating of the gamma ray detection periods is automatically controlled, both for the decay time and the spectroscopy functions, in accordance with the measured values of the decay times. The duration and repetition rate of the neutron pulses are also controlled as a function of the measured decay times to provide an overall optimized decay time-spectroscopy operating cycle. Spectroscopy outputs representative of formation lithology, salinity, porosity and shaliness are developed to supplement and improve decay time log interpretation.